Wastewater Treatment Anaerobic Digester Foaming ......Wastewater Treatment Anaerobic Digester Foaming Prevention and Control Methods Water Environment Research Foundation 635 Slaters

Wastewater Treatment Anaerobic Digester Foaming Prevention and Control Methods

Water Environment Research Foundation635 Slaters Lane, Suite G-110 n Alexandria, VA 22314-1177

Phone: 571-384-2100 n Fax: 703-299-0742 n Email: [email protected]

WERF Stock No. INFR1SG10

December 2014

Wastewater Treatment Anaerobic DigesterFoaming Prevention and Control Methods

LITERATURE REVIEW AND SURVEY

Infrastructure

IWA PublishingAlliance House, 12 Caxton StreetLondon SW1H 0QSUnited KingdomPhone: +44 (0)20 7654 5500Fax: +44 (0)20 7654 5555Email: [email protected]: www.iwapublishing.comIWAP ISBN: 978-1-78040-539-1 /1-78040-539-1

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INFR1SG10 Lit Review_WEF-IWAPspread.qxd 12/8/2014 10:57 AM Page 1

WASTEWATER TREATMENT

ANAEROBIC DIGESTER FOAMING

PREVENTION AND CONTROL METHODS LITERATURE REVIEW AND SURVEY

by:

Krishna R. Pagilla, Ph.D., P.E., BCEE Bhargavi Subramanian, M.S., Ph.D. Candidate

Illinois Institute of Technology

2014

INFR1SG10

ii

The Water Environment Research Foundation, a not-for-profit organization, funds and manages water quality

research for its subscribers through a diverse public-private partnership between municipal utilities, corporations,

academia, industry, and the federal government. WERF subscribers include municipal and regional water and water

resource recovery facilities, industrial corporations, environmental engineering firms, and others that share a

commitment to cost-effective water quality solutions. WERF is dedicated to advancing science and technology

addressing water quality issues as they impact water resources, the atmosphere, the lands, and quality of life.

For more information, contact:

Water Environment Research Foundation

635 Slaters Lane, Suite G-110

Alexandria, VA 22314-1177

Tel: (571) 384-2100

Fax: (703) 299-0742

www.werf.org

[email protected]

This report was co-published by the following organization.

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Tel: +44 (0) 20 7654 5500

Fax: +44 (0) 20 7654 5555

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This report was prepared by the organization(s) named below as an account of work sponsored by the Water

Environment Research Foundation (WERF). Neither WERF, members of WERF, the organization(s) named below,

nor any person acting on their behalf: (a) makes any warranty, express or implied, with respect to the use of any

information, apparatus, method, or process disclosed in this report or that such use may not infringe on privately

owned rights; or (b) assumes any liabilities with respect to the use of, or for damages resulting from the use of, any

information, apparatus, method, or process disclosed in this report.


The research on which this report is based was developed, in part, by the United States Environmental Protection

Agency (EPA) through Cooperative Agreement No. CR-83419201-0 with the Water Environment Research

Foundation (WERF). However, the views expressed in this document are not necessarily those of the EPA and

EPA does not endorse any products or commercial services mentioned in this publication. This report is a

publication of WERF, not EPA. Funds awarded under the Cooperative Agreement cited above were not used

for editorial services, reproduction, printing, or distribution.

This document was reviewed by a panel of independent experts selected by WERF. Mention of trade names or

commercial products or services does not constitute endorsement or recommendations for use. Similarly, omission

of products or trade names indicates nothing concerning WERF's or EPA's positions regarding product effectiveness

or applicability.


Literature Review and Survey iii

About WERF

The Water Environment Research Foundation, formed in 1989, is America’s leading

independent scientific research organization dedicated to wastewater and stormwater issues.

Throughout the last 25 years, we have developed a portfolio of more than $130 million in water

quality research.

WERF is a nonprofit organization that operates with funding from subscribers and the federal

government. Our subscribers include wastewater treatment facilities, stormwater utilities, and

regulatory agencies. Equipment companies, engineers, and environmental consultants also lend

their support and expertise as subscribers. WERF takes a progressive approach to research,

stressing collaboration among teams of subscribers, environmental professionals, scientists, and

staff. All research is peer reviewed by leading experts.

For the most current updates on WERF research, sign up to receive Laterals, our bi-weekly

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Learn more about the benefits of becoming a WERF subscriber by visiting www.werf.org.

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http://www.werf.org/i/Get_Involved/Benefits/a/w/Join_WERF.aspx?hkey=a259cc40-297e-4a78-82ae-1faa4a264010

www.werf.org

iv

The research conducted to develop this report and the guidance document was funded by

the Water Environment Research Foundation and the United States Environment Protection

Agency (U.S. EPA) with additional financial support by Hazen & Sawyer (NY), Baxter &

Woodman (IL), and Depuración de Aguas del Mediterráneo (DAM) (Spain). The technical

advisory team consists of Dr. David Jenkins (University of California, Berkeley) and Dr. André

van Niekerk (Golder Associates, South Africa). Assistance to the Spanish team from DAM,

Valencia, Spain (Laura Pastor) is appreciated. The support of participating utilities which

provided survey responses and full-scale plant data is greatly appreciated. This guidance

document was prepared by the following with inputs from the whole project team members:

Research Team Principal Investigators:

Krishna R. Pagilla, Ph.D., P.E., BCEE

Bhargavi Subramanian


Project Team:

Slawomir Hermanowicz, Ph.D.

University of California, Berkeley

Ignasi Rodriguez-Roda, Ph.D.

Maria Casellas Fusté

Institut Català de Recerca de l'Aigua (ICRA)

Catalan Institute for Water Research

Robert Sharp, Ph.D., P.E.

Manhattan College

Amanda Poole

Derek Wold, P.E.

Sean O`Dell, P.E.

Baxter & Woodman

Paul Pitt, Ph.D., P.E.

Hazen & Sawyer

Alexandre Miot, P.E.

Domenic Jolis, Ph.D., P.E.

San Francisco Public Utilities – Oceanside WPCP

ACKNOWLEDGMENTS


Literature Review and Survey v

Other Participating Utilities:

Allen Deur

New York City Department of Environmental Protection

Curt Goodman

City of Marquette, MI

Gary L. Smith

City of Elmhurst, IL

James L. Huchel

City of Crystal Lake, IL

WERF Project Subcommittee Andre van Niekerk, Ph.D.

Golder Associates Africa (Pty) Ltd

David Jenkins, Ph.D.

University of California, Berkeley

Jose A. Jimenez, Ph.D., P.E.

Brown and Caldwell

William Marten, P.E., BCEE

Donohue & Associates

Innovative Infrastructure Research Committee Members Stephen P. Allbee

U.S. Environmental Protection Agency (Retired)

Frank Blaha

Water Research Foundation

Kevin Hadden

Orange County Sanitation District

Peter Gaewski, MS, P.E.

Tata & Howard, Inc. (Retired)

David Hughes

American Water

Kendall M. Jacob, P.E.

Cobb County

Jeff Leighton

City of Portland Water Bureau

Daniel Murray

Michael Royer

U.S. Environmental Protection Agency

vi

Steve Whipp

United Utilities North West (Retired)

Walter L. Graf, Jr.

Water Environment Research Foundation

Daniel M. Woltering, Ph.D.

Water Environment Research Foundation – IIRC Chair

Director of Research: Daniel M. Woltering, Ph.D.

Senior Program Director: Amit Pramanik, Ph.D., BCEEM

Water Environment Research Foundation Staff


Literature Review and Survey vii

Abstract:

This project addresses the issue of anaerobic digester (AD) foaming in different water

resource recovery facilities (WRRFs). This report is a compilation of an AD foaming literature

review, a plant survey, and findings from full-scale studies. The literature review helped to

identify gaps in knowledge of AD foaming including causes, measurement methods, effects, and

prevention and control in WRRFs. Survey responses from full-scale plants were used to

reconcile the knowledge gaps found in the literature. The survey also helped to identify plants for

full-scale study as well as the parameters to be varied in each plant studied.

The key gaps in the knowledge were investigated at full-scale in selected WRRFs. Those

results are presented in a separate report. Full-scale studies included review of historical

operational data, plant information from various publications such as technical memoranda and

engineering reports, extensive discussion with plant personnel, and full-scale modification of key

parameters to determine foaming causes and control. These findings were used to develop case

studies including critical parameters to be varied and control strategies to be implemented to

prevent and control AD foaming at each plant.

Various gaps in existing knowledge on AD foaming were identified – in causes, control,

prevention, and impacts of AD foaming. Filamentous bacteria were found to be the most

common cause of foaming in the survey respondents. Most of the causes and control strategies

reported from the survey respondents were in accordance with published literature. Full-scale

studies confirmed the existence of filaments M. parvicella and G. amarae (which are linked to

AD foaming) in most of the plants. Prevention and control options include minimizing foam-

causing materials (including filamentous bacteria) in the feed sludge, reduced mixing in most

cases, and AD process control to operate in steady state mode. A case study report of AD

foaming causes, methods of foam detection, control, and mitigation methods was developed for

each full-scale plant studied.

Benefits:

Highlights research needs and gaps to better understand the issue of AD foaming in WRRFs.

Plant survey shows current status of full-scale AD foaming.

Full-scale studies show that primary cause and supplementary factors are necessary for foam

formation – valuable for foaming control and mitigation.

Highlights that mixing is a certain factor exacerbating AD foaming when a primary cause of

foaming such as filaments exists. Mixing can be reduced without adversely affecting digester

performance while helping control the foam episodes.

Keywords: Anaerobic digestion, foaming, causes, effects, prevention, control, G. amarae, M.

parvicella, full-scale investigation, case studies.

ABSTRACT AND BENEFITS

viii

Acknowledgments.......................................................................................................................... iv

Abstract and Benefits .................................................................................................................... vii

List of Tables ...................................................................................................................................x

List of Figures ............................................................................................................................... xii

List of Acronyms ......................................................................................................................... xiii

Executive Summary ...................................................................................................................ES-1

1.0 Introduction and Methodology ..................................................................................... 1-1

1.1 Background .......................................................................................................... 1-1

1.2 Report Objectives and Tasks ............................................................................... 1-1

1.3 Scope of the Report .............................................................................................. 1-2

1.4 Methodology ........................................................................................................ 1-2

1.4.1 Task 1 – Literature Review and Identification of Gaps ........................... 1-2

1.4.2 Challenges in Existing Literature ............................................................. 1-3

1.5 Task 2 – Survey Methodology ............................................................................. 1-4

1.6 Task 3 – Full-Scale Case Study Methodology ..................................................... 1-7

1.6.1 Sampling and Experimental Procedures .................................................. 1-8

2.0 Literature Review .......................................................................................................... 2-1

2.1 Introduction .......................................................................................................... 2-1

2.2 Three Phase Foam Fundamentals ........................................................................ 2-1

2.3 AD Sludge Properties .......................................................................................... 2-4

2.3.1 Particle Size Distribution ......................................................................... 2-4

2.3.2 Sludge Rheology ...................................................................................... 2-4

2.4 Possible Types of Bubble Formation in AD ........................................................ 2-5

2.4.1 Physiochemical Characteristics of CO2 in AD ........................................ 2-6

2.4.2 Knowledge Gaps in Foam Fundamentals ................................................ 2-9

2.5 Causes of AD Foaming ........................................................................................ 2-9

2.6 Feed-Based Characteristics that Could Influence Foaming ............................... 2-10

2.6.1 Presence of Surface Active Compounds in Feed ................................... 2-10

2.6.2 Foam-Causing Filaments in Feed Sludge .............................................. 2-12

2.7 Digestion Process-Related Causes of Foaming ................................................. 2-13

2.7.1 Formation of Surface Active Agents in Digester ................................... 2-13

2.7.2 Quantity of Feed/OLR ........................................................................... 2-15

2.7.3 Formation of Biosurfactants .................................................................. 2-17

2.7.4 Gas Production ....................................................................................... 2-17

2.8 Digester Operational Characteristics that Contribute to Foaming ..................... 2-18

2.8.1 Temperature ........................................................................................... 2-18

2.8.2 Mixing .................................................................................................... 2-20

2.9 Digester Physical Properties Role in Foaming .................................................. 2-22

2.10 Discussion of Factors Causing Foaming............................................................ 2-23

2.11 Monitoring And Detection of Foam .................................................................. 2-25

TABLE OF CONTENTS


Literature Review and Survey ix

2.11.1 Applicability of Detection/Identification Techniques to

Digester Foaming ................................................................................... 2-25

2.11.2 Full-Scale Detection by Monitoring and Testing Digesters

or Contents ............................................................................................. 2-26

2.11.3 Detection Using Sensors Associated Instrumentation ........................... 2-29

2.11.4 Discussion of Monitoring and Detection of Foam ................................. 2-29

2.12 Prevention and Control of Foaming ................................................................... 2-31

2.12.1 Introduction ............................................................................................ 2-31

2.12.2 Sludge Disintegration Methods.............................................................. 2-31

2.12.3 Operational Modifications to Prevent/Control Foaming ....................... 2-33

2.12.4 Control of the Secondary Treatment Process and Associated WAS ..... 2-33

2.12.5 Control of the Feed Sludge Storage and Feeding .................................. 2-34

2.12.6 Control of the Digester Physical Features ............................................. 2-35

2.12.7 Chemical Antifoaming Agents for Foam Control ................................. 2-37

2.13 Impacts of AD Foaming..................................................................................... 2-39

2.14 Qualitative Impacts of Foaming......................................................................... 2-40

2.14.1 Performance Impacts ............................................................................. 2-40

2.14.2 Operational Impacts ............................................................................... 2-41

2.14.3 Regulatory Impacts ................................................................................ 2-42

2.15 Economic Impacts of Foaming .......................................................................... 2-43

3.0 Task 2 – Survey of Full-Scale Plants and Identification of Knowledge Gaps .......... 3-1

3.1 Background and Introduction .............................................................................. 3-1

3.2 Qualitative Analysis of Survey Responses and Discussion ................................. 3-2

3.2.1 Primary Treatment ................................................................................... 3-2

3.2.2 AS Process Configuration ........................................................................ 3-3

3.2.3 Selective Foam Wasting from Secondary Treatment .............................. 3-4

3.2.4 AD Configuration Type ........................................................................... 3-5

3.3 Causes of Foaming ............................................................................................... 3-5

3.3.1 Sludge Feed-Based Causes ...................................................................... 3-6

3.3.2 PS:WAS Solids Ratio .............................................................................. 3-8

3.3.3 Filamentous Bacteria in Feed Sludge ...................................................... 3-9

3.3.4 Digestion Process-Related Characteristics .............................................. 3-9

3.3.5 Digester Operational Characteristics ..................................................... 3-10

3.4 Foaming Control Methods ................................................................................. 3-12

3.4.1 Effectiveness of the Various Treatment Methods .................................. 3-12

3.5 Impacts of AD Foaming..................................................................................... 3-16

3.6 Summary of Survey Results............................................................................... 3-17

3.7 Identified Full-Scale Study Parameters ............................................................. 3-18

3.8 Knowledge Gaps ................................................................................................ 3-18

3.9 Next Steps – Task 3 Full-Scale Study................................................................ 3-20

3.10 Summary ............................................................................................................ 3-20

Appendix A-1.............................................................................................................................. A-1

References ....................................................................................................................................R-1

x

1-1 List of WRRFs Selected for Full-Scale Studies in AD Foaming..................................... 1-7

1-2 List of Operational Parameters/Characteristics Monitored during Full-Scale Studies .... 1-8

1-3 List of Analyses Performed ............................................................................................. 1-8

2-1 Classification of the Causes of Foaming ......................................................................... 2-9

2-2 Fundamental Causes of Foaming ................................................................................... 2-10

2-3 Common OLR for Anaerobic Digesters ........................................................................ 2-16

2-4 Typical Criteria for Anaerobic Digester Mixing Systems ............................................. 2-21

2-5 Approaches for Assessing Foaming Potential ............................................................... 2-28

2-6 List of Specific Foam Detection Methods ..................................................................... 2-29

2-7 Sensors for Foam Detection in AD ................................................................................ 2-30

2-8 Different Sludge Disintegration Methods for AD Foam Control .................................. 2-32

2-9 Case Studies of Foam Control by Sludge Disintegration Methods in WRRFs ............. 2-33

2-10 Case Studies of AD Foam Control by Modification of Operation in WRRFs .............. 2-37

2-11 Case Study of PAX-14 Use for M. parvicella Foam Control in AD ............................. 2-38

2-12 Qualitative Impacts Reported in WRRFs with AD Foaming ........................................ 2-42

2-13 Regulatory Impacts Reported in WRRFs with AD Foaming ........................................ 2-42

3-1 Comparison of Previous AD Foaming Surveys in Literature .......................................... 3-1

3-2 Occurrence and Intensity of AD Foaming Incidents in Plants in U.S.and Spain ............ 3-2

3-3 WRRFs with Primary Treatment Facilities and AD Foaming ......................................... 3-3

3-4 Digester Foaming Based on Secondary Treatment Type ................................................ 3-4

3-5 Digester Foaming Based on Selective Wasting of Foam................................................. 3-4

3-6 AD Foaming Based on Digestion Type ........................................................................... 3-4

3-7 Most Common Reported Causes of Foaming .................................................................. 3-5

3-8 Number of Plants Receiving Different Types of Hauled/Trucked Waste ....................... 3-6

3-9 Point of Introduction of Trucked/Hauled Waste into WRRF .......................................... 3-7

3-10 Sludge Holding Tanks and AD Foaming ......................................................................... 3-7

3-11 Distribution of TWAS Percent in Feed of Foaming and Non-Foaming Digesters .......... 3-8

3-12 Occurrence of Filamentous Foaming in AS and AD Processes ...................................... 3-8

3-13 Distribution of Feed Frequency of Foaming Digesters.................................................... 3-9

3-14 Distribution of Feed Frequency of Non-Foaming Digesters ........................................... 3-9

3-15 Distribution of the Different Types of Mixing in Foaming Digesters ........................... 3-10

3-16 Distribution of the Different Types of Mixing in Non-Foaming Digesters ................... 3-10

LIST OF TABLES


Literature Review and Survey xi

3-17 Frequency of Mixing...................................................................................................... 3-11

3-18 Digester Shape and Relationship to AD Foaming ......................................................... 3-11

3-19 Effectiveness of Treatment Methods Based on Total Number of Foaming AD Plants ... 3-12

3-20 Review of Effectiveness of Treatment Methods Based on Reported Cause of Foaming ... 3-13

3-21 Reported Qualitative and Quantitative Impacts and Damages Caused by Foaming ..... 3-14

3-22 Identified Knowledge Gaps in AD Foaming ................................................................. 3-15

xii

1-1 Map Showing Locations of WRRFs Surveyed in the U.S. .............................................. 1-5

1-2 Map Showing Location of WRRFs Surveyed in Spain ................................................... 1-6

2-1 Schematic of General Three Phase Foam Stabilized by Solid Particles .......................... 2-2

2-2 Conventional Three-Phase Bubbles with Polyhedral Foam Structure

Stabilized by Surface Active Molecules .......................................................................... 2-7

2-3 Growth of Bubbles in Rapid Volume Expansion ............................................................ 2-9

2-4 Possible Factors and Relationships Leading to Bubble and Foam Formation............... 2-24

LIST OF FIGURES


Literature Review and Survey xiii

AD Anaerobic Digester

AS Activated Sludge

BNR Biological Nutrient Removal

CMC Critical Micelle Concentration

CSH Cell Surface Hydrophobicity

DCS Data Collection Systems

EBPR Enhanced Biological Phosphorus Removal

EPS Extracellular Polymeric Substances

ESD Egg-Shaped Digesters

FISH Fluorescent in-situ Hybridization

FOG Fats, Oils, Grease

FSI Foam Scum Index

G Gordonia

GBT Gravity Belt Thickener

GTW Grease Trap Waste

HRT Hydraulic Retention Time

LCFA Long Chain Fatty Acid

M Microthrix

MBR Membrane Bio Reactor

MGD Million Gallons per Day

ML Mixed Liquor

MLSS Mixed Liquor Suspended Solids

MPA Microthrix parvicella (Probe)

MYC Mycolata (Probe)

MWTF Marquette Area Wastewater Treatment Facility

LIST OF ACRONYMS

xiv

ND North Digester

OLR Organic Loading Rate

OSP Oceanside Plant

PAX Poly-Aluminium Chloride Salts

PS Primary Sludge

PS:WAS Primary Sludge:Waste Activated Sludge (Ratio)

RAS Return Activated Sludge

RBC Rotating Biological Contactor

SD South Digester

SRT Sludge Retention Time

TAD-MAD Thermophilic-Mesophilic Anaerobic Digester

THMAD Thermal Hydrolysis Mesophilic Anaerobic Digestion

TPAD Temperature-Phased Anaerobic Digestion

TS Total Solids

TSS Total Suspended Solids

TWAS Thickened Waste Activated Sludge

U.S. EPA United State Environment Protection Agency

VA/A Volatile Acid/Alkalinity

VFA Volatile Fatty Acids

VS Volatile Solids

VSS Volatile Suspended Solids

WAS Waste Activated Sludge

WERF Water Environment Research Foundation

WPCP Water Pollution Control Plant

WTF Water Treatment Facility

WW Wastewater

WRRF Water Resource Recovery Facility

WWTP Wastewater Treatment Plant


Literature Review and Survey ES-1

EXECUTIVE SUMMARY

This report examines a wide range of anaerobic digester (AD) foaming aspects, with a

focus on studying foaming in full-scale water resource recovery facilities (WRRFs) in the U.S.

The following sections briefly summarize the findings.

ES.1 Task 1 – Literature Review and Gaps

Anaerobic digestion has been used in WRRFs for decades to stabilize waste sludge

and/or to release energy from sludge. However, as efficient as this process has become over the

years, the issue of foaming has plagued the AD process. AD foaming has been cited as a

significant and common problem in WRRFs. The cited causes are many, and the

prevention/control methods are numerous. The AD foaming problem is still prevalent, persistent

and is being experienced by more WRRFs than in the past. Implementation of longer sludge

retention time (SRT) processes such as biological nutrient removal (BNR) and membrane

biological reactor (MBR) processes appear to increase the incidence of AD foaming in WRRFs.

This report contains an extensive literature review to determine the causes, effects, and

control practices being implemented at numerous WRRFs in the U.S. and around the world.

Anaerobic digester foaming problems can be classified into: 1) Feed related causes (quality of

feed, presence of surface active agents and foam causing filaments), 2) Digestion process-related

causes (organic overload, formation of volatile fatty acids (VFAs), gas production),

3) Operational issues (mixing and temperature), and 4) Digester physical features (shape and

configuration). Among these groups of causes, certain factors such as organic loading rate

(OLR), primary sludge: waste activated sludge (PS:WAS) solids ratio, presence of excessive

levels of foam-causing filaments (mainly G. amarae and M. parvicella) are fundamental foam

causative agents and certain others such as mixing and digester shape are supplementary factors

or externalities that contribute and exacerbate foaming if the potential to foam already exists.

A brief summary of literature findings are: 1) Digester foaming is due to three-phase

foam formation (liquid-solid-gas) contributed by surface active materials (solids and soluble

constituents) and flotation effects of biogas produced within the digester, 2) Sludge

characteristics such as foam/foaming activated sludge (AS) in feed sludge; surface active

materials such as fats, oils, grease (FOG), polymers, surfactants, and detergents in sludge, high

organic loads in the feed sludge enhance AD foaming; PS:WAS solids ratio in feed sludge

affects AD foaming, and 3) Digester physical features such as egg-shaped versus conventional;

two-phase versus single phase; gas mixing versus mechanical mixing; lack of sufficient capacity

in gas collection/ withdrawal piping, etc. have all contributed to AD foaming at varying degrees.

In practice, severe AD foaming may be caused by a combination of several factors, initiated by

one or more factors.

Although many control technologies are practiced for eliminating/reducing AD foaming,

there is a great deal of empirical trial and error involved in implementing those control

technologies. A systematic assessment of technology maturity and feasibility, and applicability

for different WRRFs is lacking in the literature. The list of control technologies includes

physical/mechanical methods (sludge hydrolysis, froth hydrolysis, thermal lysis, physical lysis,

ES-2

etc.), chemical methods (chlorination, defoamers, polymer, ozonation, etc.) and biological

methods (acid-phase digestion, temperature-phased anaerobic digestion (TPAD), thermophilic

digestion and staged digestion). Additionally, prevention methods such as more uniform sludge

feeding (flow and loads), optimized mixing, control of filamentous foaming in liquid treatment,

consistent temperature operation and change in digester configuration have been cited as

successful methods, but the degree of success attributed to each of these practices has not been

well quantified or documented.

The impacts of AD foaming in WRRFs are multifold, namely:

Impacts on digester performance and capacity by removing active digester volume.

Create conditions that cause tank mechanical and structure failure due to foaming.

Significant maintenance required for cleaning biogas piping and foam overspills.

Potential short-circuiting of pathogens due to lower active volume in the digester.

AD foaming impacts can be classified into performance, regulatory, and economic

impacts. The most important performance impact is decrease in gas production and/or

interference with gas collection and usage. Regulatory impacts include discharge permit

violations by foam overflows and flow backups in the system causing hydraulic capacity issues

which may indirectly impact biosolids quality. Impacts such as plugged flame arrestors can lead

to structural failure with fixed cover digesters. A quantification of these effects proportional to

the degree and frequency of foaming is still lacking in the literature.

ES.2. Task 2 – AD Foaming Survey

After critical review of the AD foaming literature, a survey questionnaire was compiled

and WRRFs in the U.S. and Spain were surveyed to reconcile the gaps identified in the literature

and to conduct full-scale evaluations in the selected WRRFs. During the period of May through

July 2011, a total of 77 utilities in the U.S. and Spain were surveyed by the project team. Out of

these 77 plants, 54 plants were impacted by AD foaming in the past five years – 22 plants in

Spain and the rest in the U.S. Foaming events were categorized as infrequent (where the utility

experienced it as an anomaly), seasonal, intermittent or persistent. The presence of foam causing

filaments in the feed sludge was found to be the most common cause of AD foaming, reported by

19 plants. Fifteen plants attributed the foaming cause to feed sludge quality and the presence of

FOG and other surface active materials in the feed to the digester. In the control and prevention

section, antifoams were listed as effective by several utilities for control of foams. As for

impacts, over 60% of the plants in U.S. and 45% in Spain did not quantify their foaming

incidents. The rest of the facilities made crude estimates of some aspects of costs associated with

foaming. The reported impacts in both the U.S. and Spain were performance deterioration, foam

spill clean-ups, and/or major structural/equipment failures. Several factors and methods were

identified from the survey findings, and were studied at full-scale including effects of feed OLR,

feed PS:WAS solids ratio, and mixing on AD foaming.


Literature Review and Survey ES-3

ES.3. Task 3 – Full-Scale Studies

Based on the outcomes of Tasks 1 and 2, selected full-scale plants were recruited to

investigate key gaps in the knowledge on AD foaming and specifically, develop case studies that

could serve as examples for other plants seeking to study and control AD foaming in the future

in their respective plants. The results of those full-scale studies are presented in a companion

report.

The variables studied at full-scale in the WRRFs included: 1) varying mixing frequency

in digesters, 2) varying PS:WAS solids ratio in feed sludge to determine effects on AD foaming,

3) varying OLRs to establish threshold loading rates, 4) effectiveness of defoamer addition for

AD foam, and 5) use of sludge spray nozzles at the digester content surface to control foam

levels.

Synthesized information on plant background, operations, analysis of operational data,

foaming background as well as results from full-scale investigations was presented in the form of

case study reports. Based on full-scale investigations at five WRRFs, it can be concluded that

persistent AD foaming occurs when feed sludge contains foam-causing filaments at higher

levels. Otherwise, AD foaming is an occasional event and becomes a problem when other

process and operating conditions exacerbate the severity of the foam event. The role of mixing in

AD foaming seems to be certain, where mixing in excess of what is necessary to keep the

digester contents homogenous could certainly exacerbate AD foaming. Reduced mixing and

even unmixed digesters in some cases did not decrease the performance of the digesters in terms

of Volatile Solids (VS) reduction and gas production. It is possible that natural mixing due to gas

production might be sufficient to keep the AD contents homogenous in these cases. None of the

five plants studied had any significant surface active materials in the feed sludge to be the main

cause of AD foaming. Although feed sludge OLR and PS:WAS solids ratio effects were studied,

it was certain that these factors were not primary causes of AD foaming in the absence of

filaments. Use of defoamants to control AD foaming in cases of occasional foam events was

found to be feasible and effective in the full-scale plant studied.

Overall, the findings from all the plants suggest that AD foaming still occurs frequently

in WRRFs, but is often not noticed or detected when it does not become a significant operational

problem. Furthermore, due to the involvement of multiple factors responsible for AD foam,

foaming events/problems could be triggered by significant contribution of any causative factor

and difficult to detect the cause(s). The ability of a plant to proactively determine when AD

foaming becomes a problem is site specific. Some successful methods to detect or sense AD

foaming include temperature sensing of foam overflows into gas collection piping, sludge level

and pressure sensors, and manual sludge-judge monitoring. Such options to prevent AD foaming

from becoming a major operating problem should be implemented in all plants.

ES-4


Literature Review and Survey 1-1

CHAPTER 1.0

INTRODUCTION AND METHODOLOGY

1.1 Background

Anaerobic digester (AD) foaming is one of the most common operating problems in

water resource recovery facilities (WRRFs) that implement sludge processing using AD. The

cited causes of AD foaming are many, and the prevention/control methods are numerous. Yet,

AD foaming problem is still prevalent, persistent and is being experienced by more WRRFs than

in the past. Since AD is the primary energy production method from wastewater organic matter,

it is the key to the overall energy sustainability of WRRFs. Persistent or frequent AD foaming

problem is often a significant bottleneck for energy sustainability of a WRRF. In addition to AD

of sludge from municipal wastewater treatment, other concentrated organic wastes from

commercial and industrial facilities are also processed by AD to recover energy in the form of

biogas.

Occurrence of filamentous foaming bacteria (mainly nocardioforms (G. amarae) and/or

M. parvicella in a WRRF biological treatment process has been a key contributor or cause of AD

foaming. Implementation of longer SRT processes such as BNR and membrane bio reactor

(MBR) processes in WRRFs have been suggested to increase the incidence of AD foaming

during sludge treatment. Regardless of the causes, the result of AD foaming has been significant

reduction in performance, capacity, and/or operational difficulties in the liquid and sludge

treatment trains. Hence, it was critical to investigate AD foaming at multiple full-scale plants in a

systematic manner to determine the causes, the contribution of each cause to foaming,

mechanisms, prevention methods, and control practices to obtain solutions for all AD foaming

causes and develop an integrated protocol to tackle and solve any AD foaming incident.

In the current literature, there is a lack of relevant technical information resources for

operators experiencing AD foaming problems. Therefore, the main and ultimate objective of this

project is to develop a “guidance document” that will describe the strategy to evaluate foaming

causes and effects, effects of operational and engineering modifications on foaming, and present

successful measures for foaming mitigation and reduction based on full-scale data collected from

WRRFs. To the best of the project team’s knowledge, such a document does not exist to date,

and is a critical need to improve AD for enhanced energy recovery and make WRRFs more self-

sufficient in terms of energy management.

1.2 Report Objectives and Tasks

This report will address various open engineering and process problems and solutions in

AD foaming by the way of critical review of published literature from various sources and full-

scale investigations of AD foaming. To accomplish this, this report is organized into three tasks:

a critical literature review on AD foaming (Task 1), a survey of AD foaming experiencing

WRRFs (Task 2), and a full-scale investigation of AD foaming causes and control in various

WRRFs (Task 3). The literature review was conducted to understand the current state of

knowledge and determine the gaps. The survey assessed the current state of AD foaming in full-

scale WRRFs and helped reconcile the gaps found in the published literature. A list of possible

1-2

WRRFs employing AD for sludge treatment for full-scale study of foaming problems was also

gathered during the survey phase. Based on the gaps determined from the first two tasks, AD

foaming causes and prevention and control were studied full-scale in the WRRFs identified in

Task 3. This report is a compilation of two of these three tasks and a step towards the fourth and

next task of this project – a foaming prevention and control guidance document for plants.

1.3 Scope of the Report

This report and its companion report explain potential relationships between AD

mechanisms, physicochemical properties of sludge and causes of foaming. Several of the causes

of foaming described in previously published literature are classified as fundamental causes and

supplementary factors in order to establish why and how a certain cause or several of them may

contribute to foaming. Detailed explanations of the complex three-phase foaming mechanisms

are beyond the scope of this report. Merits and demerits of several control and prevention

methods are discussed from a non-site specific context. Impacts of AD foaming on WRRFs are

compiled from the literature. Specific gaps in the current knowledge of AD foaming are

determined and listed along with proposed specific steps to fill some of these key gaps in Task 3

of the project. The full-scale studies discuss the determination of foam causes, specific foam

detection methods, and prevention/control methods in each case study.

1.4 Methodology

This section summarizes the methodology for this research. Section 1.4.1 describes steps

involved in Task 1 – the literature review and identification of gaps. The Task 2 AD foaming

survey required several steps: identifying a number of WRRFs employing AD in the U.S. and

Spain, compiling the survey questionnaire based on Task 1 and reviewing the responses (Section

1.5). The Task 3 full-scale study methodologies involved determining the WRRFs for full-scale

study and the parameters to be studied in each. An overview of parameters studied in Task 3 is

discussed in Section 1.6.

1.4.1 Task 1 – Literature Review and Identification of Gaps

The Task 1 of literature review was extensive and involved peer-reviewed papers,

conference proceedings, research studies, plant operator`s reports, manufacturers` trials, and

technical memoranda pertaining to AD and foaming. In order to understand the fundamentals of

foaming, three-phase foam literature in other applications such as food foams, foams occurring

in nature, industrial foams in nuclear waste, froth flotation, oil refining, polymer applications,

etc. were reviewed. Most of the AD foaming literature focused on studies conducted in the last

20 years. All these references are listed in the References section. Though extensive literature is

available regarding filamentous AS foaming, only about 100 peer-reviewed papers and

conference proceedings were identified that discussed AD foaming exclusively.

In addition, there also exists a variety of anecdotal information which over the years has

become difficult to filter out, contributing to grey literature. Despite the large amount of

publications, there exists contradicting literature and findings with no definitive proof of the

assertions. Several of the investigations are also not systematic to deal with the complex issue of

AD foaming.



1.4.2 Challenges in Existing Literature

The following section identifies the challenges in the previous research of AD foaming

that has led to the ambiguity in the existing literature.

AD foams seem to exhibit different structure and properties (mainly stability) and are

probably formed by different mechanisms. However, neither the different mechanisms are

clearly identified nor the different types of foam properly classified. This has led to

mechanisms of foam not being established and no clear classification of foam in terms of

stability exist, creating issues for plant operators, engineers and researchers alike.

Research has first noticed foaming in AS where it was observed that filamentous bacteria,

often nocardioforms or M. parvicella have caused the foaming. However, AD foam is

formed not only by these filaments, but also by other constituents. Froth flotation (Rao,

2004) or whipped cream (Walstra, 1989; Brooker, 1993) formation demonstrate that

hydrophobic filaments are not required to cause three phase foam. Because of the AS

foaming being associated with filamentous bacteria and the fact that they are carried into the

digesters, the phenomenon of AD foaming is often only attributed to filaments, while in

reality it could be non-filamentous foaming, due to several other factors.

The foam episodes in AD are due to a series of biogas bubble formation and stabilization

processes. Not being able to identify and distinguish these processes has resulted in the

confusion between non-filamentous and filamentous AD foaming and whether AD foam can

only be of a chemical/biological nature.

The challenges regarding methods to detect and quantify foam in a WRRF, by measuring the

foaming potential of AD contents (Jenkins et al., 2004) or by determining filament thresholds

(Wilderer et al., 2002) are prevalent. Due to the inherent variability of sludge, one has to be

careful in taking values from one plant as universal and make false correlations that add to

the confusion (Hug et al., 2006).

The shortcomings in literature are also due to incomplete documentation of required AD

foaming parameters. Parameters such as OLR and loading variation in terms of both flow and

VS, composition, operational conditions such as temperature, filament presence,

instantaneous biogas production rates, sludge surface properties, and changes of foaming

properties with time are all important. In addition, each process train is unique, and the

sludge input quality is varying. Studies in one plant should be applied with caution to others

that may seem similar; especially in terms of foam mitigation. Such extrapolation to other

plants with incomplete and improper documentation has caused inconsistencies in the

literature (Hug et al., 2006).

The feed quality and environmental conditions in a WRRF are sometimes not completely

known but are usually complex and dynamic. While studies in pure cultures, AD systems fed

with synthetic wastewater, lab scale reactors fed with sludge from AD systems, or scaled

down mixing studies have all provided knowledge about some aspects of foam formation or

filament growth, it is not likely that those very same mechanisms or phenomena may be

relevant for full-scale AD foaming in WRRFs.

As the first step in identifying specific gaps in existing knowledge of AD foaming, a

systematic critical review of literature pertaining to foams in various applications was carried out

to determine analogies and differences between the types of foams in order to identify

1-4

mechanisms and types of AD foam. Critical literature review of AD foaming fundamentals,

causes, monitoring and detection, prevention and control and impacts is discussed in Chapter 2.0.

1.5 Task 2 – Survey Methodology

A survey was developed to determine the current status of AD foaming in the U.S. and

Spain. The survey questionnaire was sent to 50 selected WRRFs in the U.S. – 39 responses were

received. Out of 44 plants surveyed in Spain, 38 responses were received. A compilation of

WRRFs in the U.S. with AD for this study includes information from the local consulting firms

in the project team and EPA Envirofacs program (http://www.epa.gov/enviro/index_java.html).

The basic criteria of selection of these WRRFs are that each facility has at least one AD and

captures the biogas for beneficial reuse so that economic effects of foaming due to biogas loss

can be determined. Out of these plants, those that were willing to participate in the survey and

have experienced AD foaming issues were contacted and their responses collected.

The list of plants in Spain was obtained by the project team in Spain in conjunction with

the utilities. Out of the 38 plants in Spain, 25 are managed by the ACA (Catalan Water Agency,

http://acaweb.gencat.cat/aca/) and the rest by DAM (Depuración de Aguas del Mediterráneo,

http://www.dam-aguas.es/en/). To the best of the researchers’ knowledge, a survey of this

magnitude focusing specifically on AD foaming has not been published in the recent years. Maps

showing utilities surveyed in the U.S. and Spain are presented in Figures 1-1 and 1-2

respectively.



Figure 1-1. Map Showing Locations of WRRFs Surveyed in the U.S.

Approximate placement of

WWTPs Surveyed in USA

1-6

Figure 1-2. Map Showing Location of WRRFs Surveyed in Spain.



A copy of the survey questionnaire is presented in Appendix A-1. The survey was

structured to cover basic plant process, causes of AD foam, prevention, control methods and

impacts. In some of the WRRFs that were highly interested in participating or were unique from

a process standpoint, interviews with the operating personnel were also conducted.

The purpose of the survey responses is to make any significant observations, gather full-

scale operational information, reconcile knowledge gaps determined from literature review, and

obtain parameters/relationships to be further studied at full-scale. Survey responses were also

compared to some existing knowledge from previous publications. Task 2 – Survey responses

are discussed in detail in Chapter 3.0.

1.6 Task 3 – Full-Scale Case Study Methodology

Table 1-1 lists the WRRFs participating in the full-scale studies along with the identified

preliminary cause of AD foaming and the parameters studied in the WRRF at full-scale. These

operational parameters were varied to determine their specific impact on performance and AD

foaming. During this demonstration period, the WRRFs were monitored as described here.

Full-scale study commenced in WRRFs in Nov-Dec, 2011 and continued through Oct-

Nov, 2012.

Table 1-1. List of WRRFs Selected for Full-Scale Studies in AD Foaming.

Name of Plant Reported Cause of AD Foaming Full-Scale Study Details

Elmhurst WRRF, IL Overloading of digesters. Phase 1. Reduced mixing - Mixing only a total of three hours a day – one hour each in the morning, noon and evening. Phase 2A. Increasing OLR to ND (North Digester). Phase 2B. Increasing OLR to ND such that it was fed twice the amount of that of SD (South Digester). Mixing three hours a day as in Phase 1.

Crystal Lake WRRF-2, IL WAS levels in feed. Phase 1. Increasing PS in feed every 2 detention periods in digester, eventually feeding all PS to digester.

Marquette City WRF, MI High levels of gas production with insufficient surface area for gas to escape the liquid volume.

Phase 1. Bypassing the aerated WAS holding tank and direct feed to the thickeners before going to digesters. Phase 2. Investigating the full-scale effectiveness of the nozzle-sludge spray foam suppression system by turning it off in one digester while studying the other as a control.

Oceanside WRRF, CA Filamentous foaming. Phase 1. Mixing modifications - Digester 2 mixed 75% and Digester 4 mixed 50% of the time. Phase 2. Digester 2 mixed 50% and Digester 4 mixed 25% of the time. Phase 3. Digester 4 is unmixed and Digester 1 is the control digester (mixed 100% of the time).

Hunts Point WRRF, NY Filamentous foaming. Investigating full-scale effectiveness of ESP FC Rel® defoamant for AD foam control.

These case studies listed in the table above are explained in detail in Chapters 4.0 through

8.0. The case studies consist of a background of the WRRF, foaming history, review of

operational data, investigation of foaming causes/control, and recommendations for prevention/

control in each case.

1-8

1.6.1 Sampling and Experimental Procedures

In all of the full-scale WRRFs, PS and WAS or mixed sludge feed to digester and

digester contents were sampled. In some cases, individual PS and/or WAS were also sampled.

Sampling locations and specifications are indicated in each case study chapter. Most of the AD

monitoring and sampling was conducted by the WRRF personnel as it was being carried out in

the day-to-day operations (Table 1-2). Other specialized analyses were conducted either at the

WRRF or in the laboratory at the Illinois Institute of Technology (IIT). Table 1-3 lists the

parameters/characteristics that were analyzed, at a minimum. Additional tests, if any, are

explained in each case study chapter.

Table 1-2. List of Operational Parameters/Characteristics Monitored During Full-Scale Studies.

Parameter Item Frequency

Flow rate Digester feed

Digester contents Daily Daily

Total solids (TS) Digester feed

Digester contents Daily

3/week

VS Digester feed


3/week

Alkalinity Digester feed

Digester contents 3/week

Volatile acids Digester feed

Digester contents 3/week

pH Digester feed


Temperature Digester feed


Gas production Digester gas Daily

Table 1-3. List of Analyses Performed.

Parameter Item Frequency

Foam potential PS, WAS, digester feed, digester contents. Weekly / as required.

Surface tension WAS, digester feed, digester contents. Weekly / as required.

Filamentous bacteria identification

WAS, digester feed, digester contents. Once every change in parameter/every

detention period/ as required.

Sampling, shipping, and storage, as well as detailed methods are explained in Appendix

A-2 of the companion to this report.



CHAPTER 2.0

LITERATURE REVIEW

This chapter includes a critical review of the literature on the fundamentals, causes,

monitoring, prevention and control, and impacts of AD foaming in WRRFs. Although

information exists on several aspects of AD foaming, gaps continue to remain in the knowledge

regarding various facets of foaming. Some of the key findings of the literature review are

presented and discussed in this chapter.

2.1 Introduction

Foam generation takes place in several industrial processes such as AD of sludge (Pagilla

et al., 1998), polymeric foam production (Amon and Denson, 1984), nuclear waste processing

(Vijayaraghavan et al., 2006), and production of carbonated drinks (Liger-Belair et al., 2002;

Barker et al., 2002) and whipped creams, cakes, bread, etc. (Mills et al., 2003). The mechanisms

related to the nucleation and subsequent growth of bubbles should be fully understood for

process and engineering optimization (Chhabra, 1992). At the other end of the spectrum, bubble

nucleation also occurs in animals and humans (decompression sickness) (Kungle and Beckman,

1983; Harvey et al., 1951) and in nature as gas pressure driven eruption from volcanoes (Zhang,

1998). Though the specific mechanisms of bubble formation and stabilization are different in

various foam applications, gas/air bubbles and liquids are the primary agents in producing foams

of any kind. In spite of several decades of research, mechanisms of formation and stabilization of

foam are not clear yet and a general mechanism to describe foam behaviour does not exist

(Weaire, 1999). Consequently, this research will review the various ways in which gas bubbles

can be formed and stabilized leading to foam in ADs. One of the most complex questions about

AD foaming is to what extent digestion processes and the associated parameters as well as the

sludge properties are related to foaming and its causes. A systematic review of the relationships

between AD process as well as the parameters influencing them is carried out here to understand

AD foaming further.

2.2 Three-Phase Foam Fundamentals

The various constituents required for three-phase foaming – liquid, surface active agents

and solids, and gases (biogas) are present in all digesters at all times providing a conducive

environment for foam formation. Though gas bubbles are the primary constituent in producing

foams of any kind, surface properties of all the three phases influence foaming potential and

foam stability in three-phase foams (Vijayaraghavan et al., 2006). Mainly, particulates stabilize

foam by their attachment to the gas bubble surface, and by the flocculation of particles in the

bulk solution at increasing particle concentrations (Bindal et al., 2002; Vijayaraghavan et al.,

2005, 2009). Foamability increases with increasing particulate concentration until about 38% by

weight in solution after which particles aggregate and fewer particles attach to the bubbles

(Vijayaraghavan et al., 2006). It is not certain whether this particle content threshold is

applicable to all types of particles and all solutions since AD sludge usually contains less than

6% solids by weight. The foamability of a three-phase system was observed to be directly

proportional to particle concentration and inversely proportional to particle size (Bindal et al.,

2-2

2002). Hence, there must be a particle concentration and particle size distribution where AD

foams are more stable and vice versa. Figure 2-1 presents a schematic of three-phase foam in

which the particles and the gas bubbles are relatively of the same size order. This visualization

may be closer to AD foams than in systems where gas bubbles are much larger than the particles

such as in AS foaming and detergent foams.

Figure 2-1. Schematic of General Three-Phase Foam Stabilized by Solid Particles.

Mixing/whipping/mechanical agitation is the most widely used means of formation of

foam structures in several applications, and the bubble size and stability is maintained by careful

process control (Campbell et al., 1999). Therefore, this probable bubble creation coupled with

other sludge characteristics such as presence of surface active material, liquid-surfactant/solid-

gas bubble size, concentration dynamics could explain as why only a fraction of the digesters

experience foaming despite these conditions being present in all digesters. A certain degree of

foaming is always present in all AD systems, but becomes evident when it begins to adversely

affect the process, (popularly, termed as foaming problem). The discussion above leads to the

understanding that foam formation and stability may be a function of:

The amount of surfactants, number of particles, and the proportion of gas volume in liquid or

foam.

Gas bubble size distribution and particle size distribution.

Nature and type of surfactants (in order to adhere to bubbles).

Interfacial properties between solid, liquid, and gas (interfacial phenomenon).

Operational conditions, process parameters, and physical factors of AD.



Though the formation of foam is different from single bubble dynamics, the first step is

bubble nucleation. In general, the following non-independent steps are involved in the formation

of foam:

Bubble nucleation.

Bubble growth.

Bubble detachment/coarsening.

Bubble stabilization.

The nucleation of bubbles is the first step which must occur in foam formation (Walstra

et al., 1989). Bubble nucleation and other aspects are not discussed in depth and can be reviewed

elsewhere (Jones at al., 1999; Brujan et al., 2011). Bubble nucleation, subsequent growth, and

detachment of bubbles due to biogas production in digesters lead to undesirable foam episodes.

The presence of sludge solids with or without hydrophobic filamentous microorganisms

stabilizes and leads to persistence/accumulation of foam in AD. Since the biogas is forming

throughout the volume of the digester, the bubble size could vary throughout the depth of the

digester, with smaller bubbles at the bottom and larger ones at the top. In essence, an active AD

is a dissolved gas flotation type of system and foam stabilizers such as filaments, solids, and

soluble surface active materials make stable foam.

The particle-gas bubble system in AD is different from other three-phase foams. Most

other foams involve smaller surface active particles/molecules stabilizing larger bubbles in a

controlled manner. In AD foam, the gas phase is the biogas being produced or externally

introduced into the digesters for gas mixing and contains soluble carbon dioxide. AD foams

involve stabilization of smaller bubbles by soluble surface active molecules and larger solids, in

effect, behaving similar to a dissolved gas flotation system rather than a dispersed gas flotation

seen in traditional three-phase foams, including those found in foaming AS systems. Most

probably, a combination of both dissolved flotation and dispersed flotation exists in ADs,

depending on pH and pressure. The phenomenon of gas evolution, not dissimilar to digestion is

attributed to formation of many types of foams (Campbell et al., 1999).

Three-phase foam formation in various other systems, especially those dealing with

“desirable foams” (whipped cream, cakes, etc.) in food and other applications are fairly well

developed. Each type of foam structure is a result of proper blends of constituents,

bubble/particle sizes, air entrainment/overrun (amount of air in the foam), temperature/pressure,

blending/whipping time, etc. Most of the above-mentioned parameters that are important for

formation of foam structures are mostly uncontrolled in digesters where the foam is

“undesirable,” possibly explaining why different digester foams have been reported to be similar

to either detergent/soap lather, or gushing from a beer/carbonated beverage bottle, etc.

Non-food foams such as detergent foams are similar to wine or beer foams because of

high air content (Sadoc et al., 1999). Aerated foods such as bread, whipped cream etc. contain

much lower air content than non-food foams. Bubbles can have either one or two interfaces,

based upon constituents and process parameters. Bubble surface may separate gas on the inside

from the outside, which could itself be gaseous, liquid or solid right until the interface (Campbell

et al., 1999). Soap bubbles have two interfaces, one on the inside, and one on the outside, with

liquid in between. Bubbles in champagne or bread dough, have just one interface (Campbell et

al., 1999). The researchers hypothesize that AD foam bubbles could be of either type; depending

on whether it is a case of rapid volume expansion in the digester or conventional digester foam.

2-4

2.3 AD Sludge Properties

The following sections discuss the particle size and rheology of sludge.

2.3.1 Particle Size Distribution

Particle size of the sludge is important in the digestion process especially during

hydrolysis since a smaller particle size provides a greater area for enzymatic attack. Even though

the role of particle concentration and particle size has been explained for other three-phase

foams, similar effects in AD foaming have not been reported. In the presence of surface active

substances and absence of particles, any foam developing in digesters would rapidly collapse (de

los Reyes, 2010). Yet whether this is the still the case remains unknown, though digester foam

collapse after a rapid volume expansion event has been reported in the researchers’ full-scale

study WRRF – OSP (Chapter 7.0). Smaller particle sizes also will in effect, accelerate

hydrolysis, acidogenesis, and the production of VFA resulting in organic overload to the

digester. Reduction of mean particle size of feed sludge caused increased VFA production and

reduced methane production (Izumi et al., 2010). Finer grit particles though have an increased

ability for gas holdup compared to larger sized particles, in effect aiding rapid volume expansion

(Chhabra, 2003).

In AD, the particle size distribution of feed and that of the digester contents is likely to be

different (Izumi et al., 2010). In low SRT systems (highly loaded), the particle size distribution

in the digester might be strongly influenced by feed sludge compared to that of high SRT

systems. The digestibility of the sludge also plays a role in the digester contents particle size

distribution and concentration (Zhang et al., 2013). Anecdotal reports state that digesters

receiving feed with lower PS:WAS solids ratio tend to foam more compared to those with higher

PS:WAS solids ratio might be explained by particle size-concentration effects, which is yet to be

determined.

The biogas production step (methanogenesis) itself is significant to foaming as the

concentration of gas bubbles, their size distribution as well as particle size distribution has a role

to play in foam formation. Since the foamability of three-phase suspension is dependent on the

particle concentration (directly proportional) and size (inversely proportional) (Bindal et al.,

2002), the hydrolysis of particulate organic matter and size distribution of both undigested and

digested sludge solids plays a significant role in AD foam formation. The initial steps in the AD

process, namely the hydrolysis and the acidogenesis are affected by physicochemical conditions

more than the biological factors. Kinetics of foaming varies with the physicochemical properties

of sludge due to the microbial adherence to sludges (Thaveesri et al., 1995). Surface tension of

sludge (liquid portion) plays an important part in the microbial adhesion phenomena.

Methanogenic archaea rely on extracellular polymers to bind to the sludge particles. The

methanogens and acidogens form a layer on the surface of the sludge, based on the surface

tension of the liquid. If surface tension is low, the acidogens are present in the outer layer and the

hydrophobic methanogens are present in the inner layers. If surface tension is high, methanogens

are dominant on the surfaces, and biogas bubbles produced adhere strongly to sludge, increasing

sludge propensity to foam (Thaveesri et al., 1995).

2.3.2 Sludge Rheology

Rheology of sludge refers to flow of sludge with high solids content. Digester sludge

tends to approximate non-Newtonian fluids rather than water (Wu, 2009). The thickened feed

sludge has higher solids content (up to 14% TS in some cases), so its rheology exhibits Bingham



plastic behavior with a high yield stress and shear thinning properties. Some digester sludges at

2.5-12% TS and at temperatures between 20-60°C have also been classified as a viscoplastic non-

Newtonian fluid (Achkari-Begdouri, 1992). Rheological properties can affect foaming due to

changes in yield stresses and apparent viscosity. Rheology affects stability of the foams (Prins,

1987), for instance, foams on low viscosity liquids like beer are more short-lived than those on

cappuccino. Rheology also changes during processes like aeration, making it difficult to measure

real time inside digesters. Irrespective of the type of foam bubbles formed surface tension,

viscosity and buoyancy forces impact bubble rise velocity. Sludge is very viscous and needs more

yield stress to deform. Under these conditions, small bubbles will be entrained in the digester with

zero velocity; while larger bubbles will rise (Buffiere, 1998). The former type of bubbles could be

evidence for the likes of “gas nuclei” (Blatteau et al., 2006), or “microbubbles” (Deckers et al.,

2010) in AD. Higher viscosity sludge forms stable foams due to lesser drainage of liquid from

the bubble surfaces.

2.4 Possible Types of Bubble Formation in AD

The most common visual classification of type of AD foam by plant operators is the size

of bubbles and their stability (Moeller et al., 2012). Foams with relatively large bubbles are

short-lived when compared to foams with smaller bubbles that are more stable. The properties of

these foams and their mechanisms of their formation are not available in literature.

Consequently, it is also highly unlikely that a unified theory can explain all these kinds of

digester foams. This discussion provides a brief insight into how different types of foam may be

formed and stabilized in digesters. Reviewing literature from other fields where foam formation

is well developed has helped understand the behavior of three-phase foam in general, and to a

certain extent, is the first step in adding to the understanding of the more complex digester foam.

The work to date hypothesizes those two types of foam bubbles could form in AD:

Conventional foaming – Accumulates at the gas/liquid interface in the digester. Three-phase

foam bubbles are formed in digesters with a coating of surface active material making them

stable. CO2 formed continuously diffuses into existing bubbles, entrapped during mixing

(Campbell et al., 1999) or getting attached to particles (Lieger-Blair et al., 2008). These

incorporated bubbles could serve as nucleation sites for the CO2 produced during AD. This

type of bubble formation requires high concentrations of surfactants or solids with hydrophobic

filaments; whose quantitative estimates are still unknown. These bubbles contain much less gas

and tend to have well separated spherical bubbles, mainly due to the thin films surrounding

the bubbles in the foam structure.

Rapid expansion events – Bubbles are formed by pressure difference and gas saturation in the

digester. Solid particles and cell-wall fragments present in the digester may act as nucleation

sites (Illy et al., 2011). The main difference between this type of foam and conventional

foaming is that here gas holdup finally leads to the foam. This foam has a polyhedral structure

filled with gas cells. Figure 2-2 shows a gas bubble enclosed within a network of Plateau

borders with foam lamellae that constitute the walls of the bubble. It is postulated here that foaming in the conventional sense involves the stabilization step where surface active agents

form a coating around the gas bubbles (more stable foam), whereas here, the foam is stabilized

by interacting simultaneously with surface active material and particles (foam that is short lived). The difference in the formation of these two types of foams is attributed to the heterogeneity

of the species at the interfaces in AD and subsequent adsorption and interfacial interactions.

2-6

Figure 2-2. Conventional Three-Phase Bubbles with Polyhedral Foam Structure Stabilized by Surface Active Molecules.

Both of these types of foams have been reported in full-scale digesters and major foam

episodes have been attributed to either type (Chapman et al., 2011). It is unknown if the two

types of foaming events are mutually exclusive or one can lead to another; based on filament,

particulate, and surface active material thresholds. The bubble formation is mostly the same in

both cases (gas surrounded by liquid and solids/surfactants, but in the case of rapid expansion, it

is gas holdup (due to physical factors such as increase in gas pressure in the digester, etc.) that is

finally released out of the sludge.

2.4.1 Physicochemical Characteristics of CO2 in AD

Carbon dioxide is a prerequisite to any bubbling phenomenon in the food industry, as

seen from carbonated beverages, beer and champagne (Deckers et al., 2010). In AD foaming,

similar bubble formation is categorized as rapid volume expansion where the digester contents

appear to be rapidly expanding in volume at an alarming rate, causing digester overflows. Rapid

volume expansion events could be caused by changes in biogas volumes present in the digester

liquid rather than conventional three-phase foaming. The total volume of the digester can contract

or expand according to the gas volume in the digester contents. Under the conditions existing inside

digesters, the driving force for bubble growth depends on pressure difference, surface area,

diffusion constants, and gas solubility (Blatteau et al., 2006; Liger-Belair et al., 2002; Liger-

Belair et al. 2005; Deckers et al., 2010).

During the various steps of AD, when gas is produced above the saturation levels in the

digester, the methane and CO2 is released as bubbles by nucleation (Liger-Belair et al., 2002).

Even though heterogeneous nucleation has been agreed to be the bubble nucleation process

occurring in digesters leading to bubble growth, the mechanism of nucleation for rapid volume

expansion events could be homogeneous. Homogeneous nucleation can only occur at very large

gas supersaturations; which is a possibility in AD. If the digester is undersaturated, free energy is

Surfactants



not favorable for bubbles to be thermodynamically stabilized. In case of supersaturation, the free

energy has to be supplied to achieve the critical radius of the bubble. Bubbles larger than this

radius will grow and those lesser will die. Rather than newer bubbles being generated in this

case, their mean size increases, because their nucleation is strongly dependent on supersaturation

(Slezov et al., 2004); as shown in Figure 2-3. The organic matter in the digesters and solid

impurities in the digester help bubble nucleation and growth in the case of under saturated

digesters (Leighton et al., 1996). These bubbles prevail inside digesters and could have an

immense impact on the subsequent nucleation; helping the digester gas rapidly gush out of the

digester during a rapid expansion event; not quite unlike opening of a beer can or a champagne

bottle but on a much larger scale. These types of bubbles have large gas content.

Critical Diameter Size

Bubble Size

Bubble Forming Zone

Bubble Death Zone

Explosion leading to Foam

Time

Figure 2-3. Growth of Bubbles in Rapid Volume Expansion. Increasing Size of Bubbles Rather than New Bubbles Being Formed.

2-8

Digester contents become supersaturated when the total dissolved gas pressure exceeds

the solution pressure and a gas bubble can easily form in this condition. The total dissolved gas is

the sum of the partial pressure of each gas species dissolved in solution. Based on the pH and

alkalinity in the digester, saturation conditions inside the digester dictate the type of nucleation.

For instance, in beer, supersaturation is three to six times the saturation value at 1 ATU

(atmosphere, under pressure) and only heterogeneous nucleation occurs. Homogeneous

nucleation requires saturation of carbonic acid of 103 or more at 1 ATU (Deckers et al., 2011);

which suggests that alkalinity does affect bubble nucleation.

Based on the pH inside the digester, the CO2 formed in methanogenesis could dissolve or

be released into the gas phase. Since one of the major reasons for foaming is the presence of gas

bubbles, pH fluctuations could influence bubble nucleation. Dissolved CO2 in the digester and

gaseous CO2 molecules in the headspace of the digester eventually are in equilibrium according

to Henry’s law which states that the partial pressure of a given gas above a solution is

proportional to the concentration of the gas dissolved into the solution, as expressed by the

following relation:

C = kHPCO2

where C is the concentration of dissolved CO2 molecules, PCO2 is the partial pressure of CO2

molecules in the gas phase, and kH is its Henry’s law constant. Inside the digester, dissolved and

gaseous CO2 are in equilibrium; mostly keeping the gaseous pressure at a stable level. The

excess of CO2 being generated in the digester escape as bubbles based on nucleation rate and

form foam. CO2 chemistry within a digester is governed by both Henry’s law (for CO2-dissolved

gas molecules) and the ideal gas law (for the gaseous CO2 in the headspace) (Liger-Belair et al.

2002).

Under any loss of pressure events or significant variations in gas holdup, the gaseous

volume of CO2 under pressure in the headspace suddenly expands and in several cases

overflows. After this loss of seal in the digester and gushing, the pressure of gaseous CO2

maintained inside the digester falls. The thermodynamic equilibrium is disturbed, and the

dissolved CO2 inside the digester escapes, to fulfill Henry’s law. If the excess of the dissolved

CO2 does not possess the necessary free energy to overcome the enthalpy of formation of

bubbles, then any source of external energy (e.g., mixing, shaking,) particulates or surface active

material lowers the energy barrier. The size of CO2 bubbles at this high pressure becomes critical

and forms foam explaining why rapid expansion foam events occur only in a fraction of the

digesters (Nelson et al., 2009).

The following factors that can exacerbate gas holdup potentially cause rapid expansion events:

Inconsistencies in OLR – shock loads, slug feeds, inconsistent feed rates.

Inconsistencies in gas production leading to sudden gas release.

Sudden pressure changes.

Inadequate, intermittent or non-optimal mixing which results in dead zones, pockets of

inhomogeneous material inside the digester.

All are discussed in detail in Section 3.6 – Causes of AD Foaming.



2.4.2 Knowledge Gaps in Foam Fundamentals

Although some understanding of foaming fundamentals from non-AD type of foaming

systems can be gathered, there are key gaps in the knowledge of AD foaming fundamentals.

They are:

Classifying the two different types of AD foaming episodes.

Effect of solids content threshold on three-phase foam stability.

Threshold values of solids, liquid overrun and air entrainment required to form foam.

Contribution of sludge surface properties (viscosity, particle size, etc.) to bubble formation

and stabilization.

2.5 Causes of AD Foaming

The complexity of the AD system and the foaming phenomenon makes it difficult to

correlate the type of foaming to a single parameter directly. However, the causes discussed in

this section have been popularly accepted to cause or contribute to either type of foaming to

various extents. The research team will attempt to mechanistically explain some of these causes

with respect to both types of AD foam formation.

A foaming cause is defined as a cause or group of causes that leads to a foaming episode.

In contrast, a supplementary factor is defined as one that enhances the foaming cause and favors

the foam persistence if the potential to foam or a fundamental cause already exists in a digester.

A cause may be the net effect of a supplementary factor or an externality that actually leads to

foam; for example, VFAs accumulation may be caused due to other reasons such as temperature

fluctuation, overloading, etc., finally leading to VFA production/ accumulation as the actual

reported cause of AD foaming (although there is no clear evidence that VFA accumulation in

AD causes foaming). Similarly, a supplementary factor is defined as one that sustains and

enhances foaming potential if a primary cause exists in the digester.

Based on the existing literature, the causes of foaming were classified into four groups in

this report: sludge feed characteristics, digestion process-related characteristics, digestion

operation and operating conditions, and digester configuration, shape and physical features.

Based on a critical review of the literature discussed in the following paragraphs, an attempt was

made to determine causes and the supplementary factors or the externalities that influence the

foaming. The distinction between the causes and supplementary factors is pivotal for the correct

prognosis of effective control measures.

Table 2-1 presents a list of digester foaming causes from inlet to in-digester conditions.

Table 2-1. Classification of the Causes of Foaming.

Classification Causes

Sludge feed characteristics Surface active agents in feed sludge.

Foam causing filaments in feed sludge.

Digestion process-related characteristics

Organic loading aspects – overload and inconsistent loading.

VFA production - Imbalances between the successive hydrolysis, acidogenesis and methanogenesis.

Gas production rate/withdrawal variations.

Digester operating conditions Temperature; Pressure changes.

Mixing intensity and patterns.

Digester configuration, shape and physical features Digester shape and configuration.

Sludge withdrawal and gas piping.

2-10

Table 2-2 lists the causes and supplementary factors found in literature thus far. In full-

scale digesters, the relationships between the various groups of causes have to be taken into

account. The real cause(s) of foaming cannot be completely explained without the potential links

between the groups of causes/supplementary factors. The following sections attempt to relate

these groups of causes.

Table 2-2. Fundamental Causes of Foaming.

Fundamental Cause Supplementary/ External Factor References

Classification of Fundamental Cause

Surface active agents in feed

N/A Vardar-Sukan 1998; Barber 2005; Pagilla et al., 1997; Ganidi et al.,

2009 Sludge feed characteristics

Surface active agents in digester

OLR and its variations Massart et al., 2006 Sludge feed characteristics

Surface active agents in digester

Temperature fluctuations Moen, 2003; Barber, 2005; Moeller

et al., 2010; Nges et al., 2010 Sludge feed characteristics

Surface active agents in digester, mixing

Biogas production rates Massart et al., 2006 Sludge feed characteristics;

Digester operational characteristics

Filamentous organisms

Mixing

Jones et al., 2003; Jolis et al., 2010; Westlund et al., 1998 a,b; Barber 2005; Pagilla et al., 1997

Sludge feed characteristics

Digester organic loading N/A Ganidi et al., 2011 Sludge feed characteristics

Temperature Surface active agents in digester;

filamentous organisms

Pagilla et al., 1997; Moen 2003; Marneri et al., 2009; Rimkus et al.,

2009; Nges et al., 2010

Digester operational characteristics; Sludge feed

characteristics

Foaming in AS Filamentous organisms

Hernandez et al., 1994a; Hernandez et al., 1994b;

Pagilla et al., 1997; Ganidi et al., 2009.

Digester operational characteristics

2.6 Feed-Based Characteristics that Could Influence Foaming

The principal feed based characteristics that could influence foaming include quality of

feed sludge, mainly, surface active compounds in the feed sludge and foam-causing filamentous

bacteria in the feed sludge. They are reviewed in the following sections.

2.6.1 Presence of Surface Active Compounds in Feed

Diverse surface active agents are present inside the digester. Surfactants such as proteins,

lipids (FOG), and detergents enter the digester via the feed streams. Biosurfactants or extra-

cellular polymers (EPS) are produced, possibly by filamentous and other organisms inside the

digester and are not present in feed, therefore, they are discussed in the digestion process-related

causes section. VFAs are also mostly formed in the digester and discussed in the digestion

process-related causes section.

Surface-active molecules need to be present at certain levels to form foams. A threshold

level of surfactant concentration is necessary for foam stabilization – called the critical micelle



concentration (CMC) at which they either form micelles or tend to accumulate at the liquid-gas

interfaces creating monomolecular films (Rodriguez Patino et al., 2008). The lifetime of a bubble

is increased when it is coated by surface-active material whose surface pressure reduces surface

tension, and prevents collapse by minimizing the outward diffusion of gas and provides stability

to the bubble. Surface tension and diffusion are affected by interfacial phenomena at the gas-

liquid interface, thus stabilizing the bubble. This explains how smaller bubbles do not burst due

to low surface tension.

Proteins seem to be the most important group of surfactants that are present extensively

and can contribute to foaming in several ways. One, during the degradation of proteins in the

AD, ammonium is produced. High levels of ammonium, if present, dissociate to form ammonia,

and may inhibit the digestion and cause foaming (Moeller et al., 2010). Two, higher surface

activity of the proteins present leads to gas bubble entrapment and foaming. Proteins have the

potential to stiffen, reinforce and stabilize foams provided their effective transport from the bulk

to the gas/water interface and their formation of the interfacial film can occur (Dickinson, 1992).

The protein adsorption to the interface is most rapid when the pH in the digester is closest to

their isoelectric points (pI) (Zhu et al., 1994). Due to the different types of proteins present inside

the digester and the variety of amino acids in each, the interactions at the interface could include

hydrogen bonding, electrostatic interactions, disulfide bonds and van der Waals forces (Prins, et

al., 1987). Certain proteins like casein and β-globulin can also unfold; diffuse to the surface and

spread helping stabilize the foam (Moeller at al., 2012). Proteins can also denature at higher

temperatures inside digesters and these structural changes lead to surface activity. Such

characteristics of proteins are precisely the reason they are used in stabilization of desirable food

foams, further stressing upon their importance in AD foaming.

Though inherently surface active in several ways, the foaming potential of a protein still

depends on the gas generation process (achieved by mixing or even gas production in AD)

(Wilde et al., 1996). Stable foam generation requires protein film encapsulating a gas bubble and

then packing these bubbles into the overall foam structure. Protein films by themselves are

unstable and incorporation into the gas bubble achieved by aeration makes for stable foam. The

volume of foam generated increased with gas flow rate as well as feeding rate (Merz et al.,

2011). These general observations indicate that operational parameters influence feed quality for

foam formation.

Lipids from FOG (fats, oils and grease) are also present in digesters and work by

reducing the surface tension. There is a widespread claim that communities with enforced grease

and fat intake limits in the feed sludge to AD appear to suffer less from foaming problems. While

there is no direct evidence for this statement, disposal of septage, which contains substantial

grease and oil content, through addition to small AS systems has been associated with foaming

problems. Such a statement could imply then that substantial FOG does contribute to foaming in

AD if those materials enter AD through the primary sludge (PS) route. On the other hand,

excessive FOG in the digesters coupled with other operational or process issues, might cause

foaming. The other role of lipids is that they provide a growth advantage to filaments, especially,

M. parvicella, which has good lipid substrate uptake ability. However, in the few published cases

of full-scale AD foaming due to M. parvicella, it is unclear if there was an excess of FOG in

feed. In an attempt to relate foaming to these sources of surface active materials in feed, the

survey addressed the feed composition as well as the point of introduction of these materials into

the treatment stream (Chapter 3.0).

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Detergents including ionic and nonionic surfactants are the other kinds of surface active

material that could potentially cause foaming. Most detergents contain a large amount of anionic

surfactants. The most important anionic surfactants used in laundry detergents include soaps,

linear alkylbenzene sulfonates (LAS), alkyl sulfates (AS), alkyl ether sulfates (AES), and

secondary alkane sulfonates (SAS) (Moeller et al., 2012). LAS are characterized as anionic

surfactants and are the most frequently used worldwide in both domestic and industrial

applications. It has been found by Jensen (1999) that a large amount of LAS is adsorbed onto the

particles and organic matter of sludge and is removed from the wastewater via PS. Due to the

high degradability of LAS under aerobic conditions in the AS process, PS is probably the only

AD feed stream that will contain substantial detergent concentrations. However, the amount of

LAS in the final sludge (mixture of primary and secondary sludge) is highly dependent on the

individual site-specific processes (Merrettig-Bruns and Jelen, 2009).

The most important nonionic surfactants are: alcohol ethoxylates (AE), alkylphenol

ethoxylates (APE), fatty acid alkanolamides (FAA), alkylamine oxides (AO), N-

methylglucamides (NMG), and alkylpolyglycosides (APG). Prats et al. examined the removal of

anionic (LAS) and nonionic detergents in wastewater treatment plants (WWTPs) (Prats et al.,

1997). The findings from this study revealed that during sludge settling and subsequently

anaerobic digestion of sludge, the degradation of the nonionic detergents was 27% and only 7%

for LAS. While there are no published reports of foaming being directly linked to detergent

concentrations, it is important to determine concentrations of LAS and non-ionic surfactants in

digesters (feed and contents) for threshold concentrations of detergents that cause foaming in

full-scale plants.

In digesters, a variety of these surface active materials are present at different

concentrations, undergoing various chemical and structural changes, which makes it difficult to

characterize and quantify them. Identifying threshold concentrations of these compounds for

foam stabilization has not been carried out thus far. Though the most obvious outcome of surface

activity is a reduction in interfacial tension, given this complex matrix, surface tension

measurements are not definitive indicators for foam formation. From the literature review thus

far, it can be generally observed that most surface active materials helps to stabilize the foam

while aided by other factors, in the presence of a primary cause. Specifically, proteins have a role

to play in both formation and stability of digester foams ( odr gue Patino et al., 1999;

odr guez Patino et al., 2011; Martinez et al., 2009).

2.6.2 Foam-Causing Filaments in Feed Sludge

AD foaming due to filamentous organisms such as Microthrix parvicella (M. parvicella)

and Gordona amarae (G. amarae) or nocardioforms originates in the AS systems. Several

sources report the presence of G. amarae (Pitt et al., 1990; Hernandez et al., 1994a; Pagilla et al.,

1996a; Pagilla et al., 1996 b; Tsang et al., 2008; Pagilla et al.; 1997) and M. parvicella

(Westlund et al., 1997) in WAS feed to the digester. In AS, stable foams seem to be produced by

simple floatation (de los Reyes, 2010; Soddell et al., 1998). However, given the complexity of

digester conditions, it is not simply a case of flotation and the same filamentous organisms could

cause digester foaming incidents by in several ways (i) stabilizing gas bubbles in the digester due

to their surface active nature (Jenkins et al., 1992), (ii) hydrophobic particles (the cells) adding to

particulate matter in the digester, helping to stabilize the foams by physically attaching on the

gas bubbles, and (iii) by producing biosurfactants that add to the total surface active material in

the digester (Pagilla et al., 1997). These conditions present inside the digester potentially enrich



the contribution of filaments beyond the threshold value (Frigon et al., 2006). The threshold

values of filaments measured for AS foaming is very much lowered in the digester, for both

formation as well as the stability (de los Reyes and Raskin, 2002). The quantitative thresholds of

filamentous foaming reported for G. amarae in feed WAS and the digester was in the high 105

intersections/g volatile suspended solids (VSS) (Pagilla et al., 1997). Concentrations between

0.05 and 0.1 g G. amarae / g TS resulted in severe foaming (Hernandez et al., 1994a).

There are a few reported cases of M. parvicella foam in full-scale digesters, but threshold

values for foaming are not available, unlike G. amarae (Westlund et al., 1998a; Westlund et al.,

1998b).Though not proved until recently, long chain fatty acids (LCFAs) have been found to

enhance M. parvicella growth. In most conditions prevailing in anaerobic, anoxic, and aerobic

tanks of the AS process, only a portion of LCFA is removed (Noutsopoulos et al., 2010). M.

parvicella fate in digesters seems to be influenced by surface active material present in digesters.

From the existing literature (as well as the survey results discussed in Chapter 3.0),

several plants where presence of foaming filaments was detected in the WAS have no AS

foaming but experience AD foaming. AS foaming has much higher threshold than AD due to

lower concentration of sludge in AS process. In most digesters, thickened WAS is fed to AD.

When hydrophobic filaments are present in the sludge, thickening results in more concentrated

sludge, which in turn has higher hydrophobicity per unit volume of feed sludge and hence more

propensity to foam. Furthermore, AS plant design and operational factors are attributed to

discourage foaming, even when the G. amarae exceeded the threshold limits in AS systems (Baxter-Plant et al., 1998; Baxter-Plant et al., 1999). This could also be a leading reason as why

all plants with AS foaming filaments need not necessarily experience AD foaming and vice

versa.

In investigating fate of filaments in digestion, none of the digestion systems completely

removed or inactivated all of the feed G. amarae filaments (Hernandez et al., 1994a; Hernandez

et al., 1994b). In the absence of dissolved surfactants, the foaming potential of a digested sludge

was proportional to its G. amarae filament content. Even though thermophilic conditions

resulted in a high destruction of filamentous foaming bacteria, it did not mitigate foaming

completely (Marneri et al., 2009). The foaming potential could be attributed more to the

concentration of the biosurfactant material than the filamentous bacteria itself. Filamentous

microorganisms release surface active biosurfactants when they grow at the expense of water

insoluble substrates (Pagilla et al., 2002). Extracellular polymeric substances (EPS) have been

used as biosurfactants (Irvine et al., 1997). The filaments have the ability to produce or store

surface active materials that would bring about a lowering in surface tension of the sludge, which

contributes to foaming. In this case, surface active materials (biosurfactants, EPS) seem to be the

direct cause of the foam.

2.7 Digestion Process-Related Causes of Foaming

Digestion process-related causes of foaming are discussed in this section. They include

formation of surface active agents in the digester, how quantity of organic matter effects

foaming, the role of biosurfactants, and gas production, which are all discussed in this section.

2.7.1 Formation of Surface Active Agents in Digester

The metabolic steps in the AD process help break down complex soluble and particulate

sludge materials such as carbohydrates, proteins, lipids, etc., into simpler substances that become

the reactants in the methanogenesis step leading to biogas production. In this process, various

2-14

intermediates are formed. Effective and efficient AD relies on the continuous conversion of

intermediates produced in the metabolic chain. When intermediate organic acid concentration

increases (during acidogenesis), due to inefficient subsequent steps (acetogenesis and

methanogenesis), the surface-active acids concentration and foaming potential increase and/or

AD is affected. pH is an important indicator of the performance and the stability of an AD. For

example, accumulation of volatile fatty acid (VFA) may ultimately cause a pH drop once the

mineral alkalinity is completely exhausted. Many studies have reported this to be the first

indication of a digester imbalance (Hill et al., 2000; Ganidi et al., 2009).

The main source of VFAs is their formation during digestion. There are various theories

on the influence of VFA formation in digesters. Several plant operator sources claim that the

concentration of VFAs seems to depend on the biogas produced in the plant, the AD process

type, and its mode of operation. Some cases of digestion have exhibited really high levels of

acetic acid (Moeller et al., 2010). Acetic acid has been implicated to cause foaming (Ganidi et

al., 2009). Although VFA are surface-active, there is disagreement as to whether their presence

in the biogas reactor is the reason for or a consequence of the imbalance in the process that

becomes evident during foaming. One study related VFA, gas production, foaming, and

alkalinity but failed to explain the suspected cause of the increased VFA concentration (Nges et

al., 2010).

Based on the pH inside the digester, the CO2 formed in methanogenesis could dissolve or

be released into the gas phase. Since one of the major reasons for foaming is the presence of gas

bubbles, pH fluctuations could influence bubble nucleation. Even though the reasons for the pH

imbalances are different, various authors report foaming caused by lower pH that resulted from

production of VFA or feed imbalances (van Niekerk et al., 1987; Fonda 2008).

Conversely, pH imbalance has also been cited as a consequence of foaming. Some

studies also report foaming to create operational issues that create problems in maintaining pH of

the digester (Massart et al., 2006). Normally, digesters are well-buffered due to the effect of

carbon dioxide/bicarbonate and ammonia/ammonium systems formed during fermentation. The

pH in the digester is a function of the feed and the specific concentrations of products formed at

every stage.

In addition to VFA formation in the digester, septic conditions in primary clarifiers

promote excess VFA formation, which may upset the digester and cause excessive foaming. This

is observed specifically in plants that hold PS blankets in the clarifiers in order to feed VFAs for

enhanced biological phosphorus removal (EBPR) (Reusser et al., 2004). Longer hydraulic

retention times (HRTs) in sludge holding tanks and clarifiers could cause fermentation leading to

VFA formation.

Some of the VFAs produced during alkaline fermentation of PS include acetic,

propionic, iso-butyric, n-butytic, iso-valetic and n-valeric acid (Wu et al., 2010). In particular,

butyric acid was found to have the best correlated to the methanogenic population dynamics in

the system (Montero et al., 2010). The implication is that high amounts of butyric acid increase

methane production and the increased butyric acid consumption correlated with increased

foaming (Montero et al., 2010).

From the above discussion, it is yet unclear whether pH imbalances are a cause, a

consequence, or both, of foaming (Moeller et al., 2012). Alkalinity has also been thought to

contribute to foaming indirectly by aiding bubble nucleation, although it has not been established



as a primary cause (Ganidi et al., 2009). The volatile acid/alkalinity (VA/A) ratio is expressed in

equivalents of acetic acid/equivalents of calcium carbonate and generally, values between 0.1

and 0.4 are considered to indicate a healthy digester (Sánchez et al., 2005; Switzenbaum et al.,

2005). Increases above 0.4 indicate upset conditions and the need for corrective action.

In modeling risk of foaming in digesters, Dalmau et al. (2010) selected the most

important variables for AD imbalances using artificial neural networks after extracting plant

operating data from a pilot plant. They studied both foam formation and acidogenic phase

imbalances. Their results show that pH of the digester feed has a greater impact on foaming

potential than the pH of the digester. However, there is no published literature from lab or full-

scale experiments that links AD foaming and feed pH. This model assumes that pH is balanced

inside the digester. The only related literature available is a case study that discusses the start-up

of a rehabilitated digester encountering foaming because of pH falling below 5 caused by

inefficient seeding (Holden et al., 2006). This particular incident of foaming could be a

combination of various causes including equipment malfunction, slug feed and inefficient

monitoring of the digesters at start-up. To sum up, pH and alkalinity can indirectly influence

bubble formation but have not been implicated to be causes of digester foaming (Ganidi et al.,

2008).

Formation of VFAs in the digester affects the viscosity of the sludge. Any subsequent

foam that is caused also affects the viscosity. The rate of drainage of such foams depends on the

viscosity of the medium under the foam layer (Moen, 2003; Barber, 2005). Viscosity of sludge is

in-turn affected by VFAs. Therefore, prediction of foam stability becomes difficult. This could

indicate that foaming by VFAs, (digestion process-related foaming) is simply a physical surface

mechanism and foaming potential is dependent on specific sludge interaction with the VFAs.

2.7.2 Quantity of Feed/OLR

A number of researchers have stated that organic matter overloading of digesters can be a

cause of foaming (Pagilla et al., 1997; Barjenbruch et al., 2000; Moen, 2003; Barber, 2005;

Messart, 2006; Ganidi et al., 2011). Overloading could cause both rapid expansion as well as

conventional foam events. Firstly, excess organic matter not fully degraded leads to potential

accumulation of hydrophobic or surface active by-products that would promote foaming.

Secondly, an overloaded digester generates more VFAs than the methanogenic archaea can

consume readily. Thirdly, gas production rates fluctuate and can cause higher gas holdup in the

digester, leading to rapid volume expansion event. A high level of acidity is also being produced

in the digester. This causes suppression in both pH and alkalinity. If the digester is not

adequately buffered, it becomes sour. The imbalance causes foaming (Massart et al., 2006). Thus

there is a relationship between quantity of feed and alkalinity. Process imbalances and foaming

due to temperature variances were also attributed to intermittent feeding of the reactors (Nges et

al., 2010).

According to the literature, different values for OLRs for conventional AD of municipal

sludge have been reported (Table 2-3). According to Moeller et al. (2010), this value is about

4 kg /m3-day of organic dry matter. At that level of OLR, overloading of the process occurs

(Moeller et al., 2010). OLR of 2.5 kg VS/m3-day was determined to be a load threshold for foam

initiation for sludge obtained from a non-foaming full-scale digester, while 5 kg VS/m3-day

resulted in persistent foaming (Ganidi et al., 2011). It is possible that each digester has a critical

OLR threshold above which foaming appears and another above which foam persists. Case-

specific OLR thresholds signify that the digestion process is overloaded not by just the absolute

2-16

quantity of feed, but a combination of the feed quality, digestion process, as well as digester

physical properties.

In addition to OLR, three case studies of full-scale WRRFs reported foaming due to

inconsistent loading (slug feeding) of digesters (Massart et al., 2006). Excessive foaming

occurred whenever the digester feed concentration fluctuated at different levels. This has been

termed as OLR variation by others and is known to contribute to foaming at high OLR feed to

the digester (Dalmau et al., 2010). Simply monitoring and maintaining consistent feed to the

digesters at levels between ±5% and 10% variation from the average feed OLR value seemed to

predict conditions that control foaming incidents in all three cases. Feeding frequency and

patterns are other aspects of loading that have not been investigated in full-scale studies. In

digesters that are not considered to be overloaded based on OLR values, but fed only for a

certain period of time in a day and not mixed during the feeding, areas of localized overloading

near the feed inlets could occur in the digester. Feed patterns and feed frequency are as important

as OLR. The Task 3 investigation studied digester profiles for these reasons.

Table 2-3. Common OLR for ADs.

Source

OLR (kg VS m3 d-1)

OLR (lbs VS ft3 d-1)

Handbooks of UK Wastewater Practice (1996) 0.8 – 1.6 0.05 – 0.1

Metcalf and Eddy (2003) 1.6 – 4.8 0.1 – 0.3

Brown (2002) < 4.5 < 0.28

Gerardi (2003) designed: 3.2 – 7.2 (usually 0.5 – 0.6)

0.2 – 0.45 (usually 0.03 – 0.04)

Lamelot (2004) <2.5 < 0.15

Braguglia et al., (2008) 0.7 – 1.4 0.04 – 0.09

Bolzonella (2005) ~ 1 ~ 0.06

Values for Foaming Digesters in Literature

Moeller et al., 2010 4 0.25

Ganidi et al., 2011 2.5 – foam initiation;

5 – persistent foaming 0.15 – foam initiation;

0.3 – persistent foaming

The PS:WAS solids ratio fed to the digesters is another important feed based

characteristic that is related to the OLR and hence foaming. Foam mitigation in several cases has

been achieved by changing the PS:WAS solids ratio. However an optimal PS:WAS solids ratio

has not been established. Foaming could occur if the ratio of WAS to total sludge exceeds 40 %

(or feed PS:WAS solids ratio of 6:4) (Massart et al., 2006). It is unclear if this ratio was

indicated for filamentous or non-filamentous foaming. However, there is a lack of foaming data

from digesters that may have operated at this ratio. On the other hand, anecdotal reports indicate

that dedicated WAS digesters have less foaming potential due to less digester gas production

rates, because WAS cells are more difficult to digest. On the downside, it could mean more

undigested particulate matter for foam stabilization. This could be true for WAS containing no

foaming filamentous bacteria. The PS:WAS solids ratio in feed could influence foaming in many

ways. More PS in feed versus WAS could increase gas production rate and subsequently gas

holdup, resulting in more gas bubbles causing foam. It also affects the VS loading rate. Longer

retention times in holding tanks prior to digestion leads to fermentation and higher production of

VFAs, which when fed to digester in higher quantity can contribute to foaming due to reasons

discussed before.



Correlations between both OLR, PS:WAS solids ratio, and bypassing of WAS storage

tanks with high SRTs prior to digestion and each of their contributions to foaming have been

systematically studied at AD plants in Task 3 and reported in Chapters 4.0 through 6.0.

2.7.3 Formation of Biosurfactants

Production of biosurfactants inside the digester has been explained to be another cause of

foaming (Pagilla et al., 2002). G. amarae filaments produce EPS that behave like biosurfactants

(Pagilla et al., 2002). While it was thought that the operation of conventional AS plants lead to

the production of EPS and biosurfactants from filaments, this is now not found to be true as some

filaments exist in MBRs as well (Zhang et al., 2010). It seems to be the most plausible theory for

explaining why in some cases foaming occurred in plants where the biological populations were

below threshold limits, in the absence of any other foaming causes as well as increased

filamentous foaming with temperature (discussed in Section 2.8.1.2). Such a theory has its merits

but still needs to be verified by means of correlating concentration of biosurfactants produced to

the filament thresholds. EPS can be classified as hydroxylated and cross-linked fatty acids (e.g.,

mycolic and urolic acids (Baxter-Plant et al., 1998; Baxter-Plant et al., 1999)), glycolipids,

lipopolysaccharides, lipoproteins-lipopeptides and phospholipids (Saharan et al., 2011). Many

similar compounds have been used in other desirable foams, for increasing stability, as they

behave similar to surface active lipids and proteins. The mechanisms of EPS production and

release from filaments are hydrolysis at higher temperatures as well as cell lysis. Since EPS are

complex compounds, their characterization in the digester contents is difficult.

2.7.4 Gas Production

Any in-situ generation of gas, by itself, is a method of production of foam (German et al.,

1990). Biogas production is the main source of bubble nucleation and sometimes growth, but by

itself it is only a supplementary cause of foaming, exacerbated by problems in operation or based

on the digester physical properties such as gas collection and withdrawal piping. The production

of biogas by the digestion process in excess of the withdrawal rates from the digesters is

suspected to contribute to foam, mostly by rapid volume expansion. Any biogas production

upset, in addition to various other factors, could result in foaming.

When the biogas production fluctuates, gas mixing becomes an issue. Inconsistent or

insufficient gas mixing can cause foam in two ways: the gas is not stripped out of the liquid

phase completely and forms aggregates of gas that become foam, or it entraps more gas bubbles

in the liquid and causes a layer of scum to accumulate on the surface (Massart et al., 2006). In all

of the case studies and literature reviewed for this research, certain rapid volume expansion

events established only biogas production, or a sudden fluctuation in biogas production, directly

prior to the occurrence of foaming (Chapman et al., 2011).

Foaming accompanied by high variations of gas flow rates was attributed to the release of

gas bubbles from the liquid stream (Dalmau et al., 2010). This may be accompanied by further

foaming due to these gas bubbles escaping into the gas phase, increasing the concentration of the

gas phase, resulting in instability. Similar to particulate thresholds for foam (Vijayaraghavan et

al., 2006), it is possible that a gas threshold (gas holdup parameter) exists for foaming in the

presence of surface active agents. This gas threshold is substantiated by the fact that in other

desirable foams, such as beer, concentration of CO2 is maintained at a certain level to get the

desired head of foam.

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The gas inside these bubbles in a digester is predominantly methane and CO2 which is

soluble in water. Gas solubility is related to head space pressure – gases are more soluble at

greater pressures and the relationship between solubility and pressure is linear. During periods of

higher pressure more CO2 could be dissolved into digester contents, and subsequently, come out

of solution during lower headspace pressure (during loading/withdrawal, and end of the digestion

period), causing a flotation effect of solids to the surface as froth or foam. When the gas bubbles

are stabilized by surface active agents, particulates or filaments, conventional three-phase foam

could form. Mechanistically, high gas holdup will cause a rapid expansion event of digester

contents.

2.8 Digester Operational Characteristics

Some digester operational characteristics that contribute to foaming include temperature

effects and mixing.

2.8.1 Temperature

Temperature could be a contributor to foaming events in digesters because of its impact

on AD metabolism by hydrolysis and methanogenesis, and has a significant effect on other

factors such as gas transfer rates and settling characteristics of biosolids (Tchobanoglous et al.,

2003; Gerardi, 2003). The importance of temperature as an operational parameter is more

pertinent for rapid volume expansion while its effects on filaments or seasonal variations are

important with regard to conventional foam events. Temperature effects on sludge properties

such as viscosity or surface tension possibly impacts both types of foam events.

2.8.1.1 Temperature and Pressure Effects on Rapid Expansion Events

Bubble formation potential increases with temperature due to reduced Henry’s constants

and more rapid diffusion kinetics (Hikita and Konishi, 1984). Bubble drag and bubble rise

velocity are impacted by temperature. Temperature produces changes in viscosity and the

rheological behavior, which in turn affects mixing, indirectly impacting gas holdup (Gomez-Diaz

et al., 2005). Though experimental evidence is lacking, inadequate mixing contributes lesser to

foaming at higher temperatures than at lower ones (Moen, 2003). High temperature of

thermophilic digesters eliminates foaming due to viscosity changes of FOG at higher

temperatures in foaming digesters (Moen, 2003). A few other studies attribute the diminished

foaming to the effect of higher temperatures on lowering viscosity of sludge hence increasing

foam drainage and breakup (Barber, 2005; Junker, 2007).

Temperature inside a digester may also be non-uniform due to improper mixing or feed

variations, which indicates that temperature indirectly is a cause of rapid volume expansion, by

different means. In an attempt to study rapid volume expansion events in full-scale, temperature

profiles of the digesters at varying depths were determined in the researcher’s full-scale studies.

These temperature variations were related to varying mixing frequency in AD.

The solubility of CO2 in the digester contents depends on temperature (the lower the

temperature of the solution, the higher the gas solubility). Because the solubility of CO2 changes

with temperature, the partial pressure of CO2 gas also strongly depends, in turn, on the

temperature. Digesters which feed infrequently with large volumes of colder feed sludge may

experience temporary temperature fluctuations that may aid gas dissolution into the liquid phase.

Depending on the duration of the intermittent feeding, there could be changes in the gas phase

pressure due to temperatures, particularly in fixed-cover digesters.



Digester pressure has not been discussed as a cause or contributing factor in conventional

foaming. Gas phase pressure in the headspace of the digester and liquid level can impact bubble

formation and dynamics. Initial bubble size initially decreases inversely with pressure when

other parameters are kept constant (Yang, 2007). Under relatively high pressures (>290 PSI), the

effect of pressure on initial bubble volume becomes insignificant (Yang, 2007). In digesters,

pressure increases with height of the liquid above and gas system pressures may be as high as 50

inches of water column (Chapman et al., 2011). Slower bubble rise rates occur at higher pressure

in the digester.

Coalescence leading to further bubble formation is reduced at higher pressures (Clift,

1978; Yang, 2007). Smaller bubbles and lower rise velocities are prevalent at high pressures. Under these conditions, a significant volume expansion and subsequent gas release in the form of

foam from solution can occur when there is a sudden pressure drop. Therefore, it may be prudent to

operate digesters at lower headspace gas pressure, if possible, to minimize rapid expansion of sludge

during withdraw-feed periods.

2.8.1.2 Temperature Effects on Filaments and Seasonal Variations

Most published literature links filamentous foam formation and temperature in the AS

process, which is not discussed here. Temperature effects on filamentous organisms’ survival

and proliferation have several contradictory reports on optimal temperature ranges for their

growth (Dhaliwal, 1979; Seviour et al., 1990; Eikelboom et al., 1998; Soddell et al., 1995;

Soddell et al., 1998).

Fluctuations in temperature might be an effect of intermittent feeding (Nges et al., 2010).

Foaming in thermophilic digesters has been related to temperature fluctuations, but there was no

foaming observed in mesophilic digesters with the same. Thermophilic digesters in general

appear to be more sensitive to temperature fluctuations that may result in foaming. There is little

published information linking temperature fluctuations to foaming in the absence of filamentous

organisms. However, there are several published studies on the effect of thermophilic

pretreatment on sludge that report reduced or no foaming when compared to mesophilic

digestion. The reasons for the absence of foaming are again linked to the removal and/or

inactivation of foaming microorganisms or attributed to the effect of higher temperatures on

surface tension and viscosity of sludge and hence foam drainage (Moen, 2003; Marneri et al.,

2009).

The fate of G. amarae and M. parvicella in AD operating under mesophilic and

thermophilic conditions are important to be reviewed (Marneri et al., 2009). Various digester

configurations were tested for filamentous bacterial species, and the results indicate that

thermophilic digestion or a combination of thermophilic and mesophilic digestion resulted in a

higher decrease in the viability of G. amarae and M. parvicella. M. parvicella is slightly more

sensitive than G. amarae to anaerobic thermophilic conditions (Marneri et al., 2009; Pagilla et

al., 1995). Even though the higher temperature conditions induced a higher destruction of

filamentous foaming bacteria, foaming was not eliminated. Since the foaming potential and

stability of ADs is a combined effect of both the concentration of the filamentous bacteria and

the concentration of foam-stabilizing materials in the digested sludge, the concentration of foam-

stabilizing materials is expected to be higher in the thermophilic digesters due to the higher

hydrolysis of organic matter including hydrophobic filament cells, as reported in many studies

(Marneri et al., 2009). It can be hypothesized that greater destruction of filamentous bacteria

obtained in thermophilic digesters may have increased the concentration of colloidal

2-20

hydrophobic compounds due to the release of compounds such as mycolic acid, therefore

increasing the foamability of the sludge. It is interesting that while researchers have reported

reduced or almost no foaming in their thermophilic digesters in many full-scale facilities (for

instance, Rimkus et al., 1982), various other studies such as the one explained above still

experienced foam or increased foaming.

The literature on foaming at higher temperatures is very contradictory. Moeller and

colleagues suggested that thermophilic digestion lowers risk of foam formation caused by the

presence of filamentous microorganisms (Moeller et al., 2010). While it is true that thermophilic

digesters can destroy filamentous organisms effectively, it does not translate automatically to

lowered foam potential, as seen from the above-mentioned cases. These observations from

literature can help us summarize that thermophilic digestion is effective in minimizing foam

where the fundamental cause is the presence of filaments and not the presence of biosurfactants.

In summary, the role of digestion temperature in AD foaming seems to be mainly due to

indirect effects such as the gas production, biogas solubility, viscosity and surface tension of the

liquid phase in the digester.

2.8.2 Mixing

Mixing is important for good AD operation in order to maintain active digester volume,

limit short circuiting of the sludge, and to ensure gas, liquid, solids interactions. Whereas

digester configuration may affect the formation of dead-zones and grit accumulation, mixing is

critical to maintain homogeneity between the various phases in the digester. More recently, the

importance of mixing has been called into question. Improper mixing, though it cannot be given

a universal definition, is viewed as a contributor to foaming, but not necessarily a cause of the

foam. Although, the importance of mixing is clear from many studies, its effect on foaming of

ADs is still poorly understood, and there is much conflict in the literature regarding the optimal

mixing for AD.

In most digesters there are induced and natural sources of mixing. The induced sources

constitute the intended mixing system (installed mixers and associated equipment or compressed

biogas mixing). Natural mixing includes the gas evolution during digestion (Zickefoose et al.,

1976), inflow and outflow of sludge by pumping. An additional source of mixing energy in

digesters is due to recirculation mixing to heat digesters contents in heat exchangers. A fourth

source of mixing is convection currents in heated digesters (Verhoff et al., 1974). Natural mixing

by gas evolution is controlled by feeding. When loaded constantly, natural mixing will occur at

loads of 0.4 lbs/cf/day and above (Zickefoose et al., 1976). When loading can be sustained at this

level, induced mixing is not necessary. If loads drop below this level, scum blankets could form.

Increased loadings are accompanied by slower gas production and VS overloading (Zickefoose

et al., 1976). This discussion indicates that mixing and feeding are closely related to each other

and need to be closely controlled for prevention/control of foam. The above discussion also calls

into question the perceived importance of induced mixing. In a digester, solids, liquids and gas

are present at all times, providing a conducive environment for foam formation. The

phenomenon of gas evolution, not dissimilar to digestion is attributed to formation of many types

of aerated foams (Campbell et al., 1999). This evolved gas diffuses into the bubbles incorporated

by mixing/whipping/mechanical agitation thereby providing for possible nucleation sites for

foam formation (Chiotellis and Campbell, 2003). Probably a threshold level of mixing in the

presence of surface active material exists; beyond which bubbles could be stabilized in the three-

phase foam.



Mixing may contribute to AD foaming in several ways: improper mixing may cause

scum layers present in most digesters to accumulate on the surface and adhere to the rising gas

bubbles, resulting in foam (Moen, 2003). Intermittent mixing results in sudden increase in gas

holdup release. Incomplete gas stripping from solids also occurs, causing a gas flotation effect

stabilized by the particulates. Excessive mixing can increase entrapment of gas bubbles in the

liquid, similar to whipping, thereby generating foam. Popular anecdotal knowledge also suggests

that gas mixing tends to cause foaming. Comparison of foaming in a gas-mixed and a

mechanically mixed digester receiving the same feed and operated under similar conditions

(loading, temperature, etc.) found that the gas-mixed digester accumulated more foam than the

mechanically-mixed digester (Pagilla et al., 1997). The presence of G. amarae filaments also

caused thicker foam at the surface and the gas mixed digester had more propensity for foam

stabilization. Gas mixing provides favorable conditions for foam generation because it is

suspected that the presence of bubbles in the bulk phase promotes attachment of the surface

active and hydrophobic compounds found in sludge onto the bubbles (Moen, 2003; Barber,

2005). While it is not a fundamental cause of foaming, gas mixing tends to encourage and

stabilize foam when the potential to foam exists in a digester.

In further classifying gas mixing, there are small bubble gas systems (Perth, Shearfuser),

large bubble systems (Cannon), confined (draft tube induced) and unconfined bubble systems.

Each of these has different bubble hydrodynamics by virtue of how the gas is released into the

digester. Bubble hydrodynamics have been found to vary with the type of orifice releasing the

gas bubbles. Generation of these gas bubbles at a very high rate along with surface active agents

present inside digester could help enhance and stabilize foam over mechanical mixing systems

that do not have this issue (Loubiere et al., 2004). Mechanical mixing is classified into high

energy pumped recirculation (JetMix or RotaMix) or the conventional draft tubes. High energy

systems have been implicated to cause foaming due to cell lyses/destruction in biomass.

Mechanical mixing may exacerbate gas holdup leading to rapid volume expansion events

(Chapman et al., 2011). While each of these mixing systems may have their own merits and

demerits, their contribution to foaming is largely based on utility operator reports and

manufacturer trials.

Mixing performance in ADs has been evaluated using the active volume of the digester,

investigated with tracer studies and/or computational fluid dynamic (CFD) modeling (Vesvikar

et al., 2005). However, this parameter alone cannot describe all types of mixing, since some are

more effective than others in maintaining homogeneity in the digester. There are no defined

standardized performance criteria for effectiveness of digester mixing, except these typical

design parameters as listed in literature and Table 2-4.

Table 2-4. Typical Criteria for AD Mixing Systems.

Mixing Criteria Type of Mixing System Values Reference

Unit power, hp/1,000 cf digester volume

Mechanical systems 0.2 – 0.3 Tchobanoglous et al.,

2003; Appels et al., 2008

Unit gas flow, cfm/1,000 cf digester volume

Gas mixing, unconfined 4.5 – 5 Appels et al., 2008

Unit gas flow, cfm/1,000 cf Gas mixing, confined 5 – 7 Appels et al., 2008

Energy input density, W/m3 Gas mixing 8

Tchobanoglous et al., 2003; Appels et al., 2008

Velocity gradient G, s-1 All 50 – 80 Tchobanoglous et al.,

2003; Appels et al., 2008

Turnover time, minutes

Confined gas mixing and mechanical systems

20 – 30 Tchobanoglous et al.,

2003; Appels et al., 2008.

2-22

CFD modeling of the mixing patterns in ADs can potentially be used to predict foaming

behavior in mixed digesters. Over the past years, considerable research has been conducted in the

simulation of mixing in ADs using CFD technique (Fleming, 2002; Vesvikar and Al-Dahhan,

2005; Vesvikar et al., 2006). CFD models of the digesters thus far only deal with digester mixing

efficiency and selection of an appropriate type of mixing. There is no published report

specifically relating mixing models and foaming. CFD modeling showed that the solids

concentration within the digester would reduce the active volume by approximately 40%. This

reduction in active volume, if not addressed would significantly increase the effective VS

loading rate in the digester and cut the HRT, with risk of process upset or failure (Marx et al.,

2006).

The frequency of mixing is as important a consideration as the mixing power. Plants that

feed continuously, but mix intermittently, have a higher propensity for localized areas of high

concentrations of VFAs/LCFAs and/or excessive gas production, where feed may enter the

digester. It is important to estimate the number of content turnovers in the digester to understand

the efficiency of mixing and decide a mixing operation strategy accordingly.

Based on the above discussion, the most important function of mixing seems to be to

maintain a homogeneous environment in the digester, both from a digester performance and

foaming standpoint. Mixing seems to contribute to both types of foam formation in AD. In this

direction, in Task 3 full-scale studies, modifications to both the mixing configuration as well as

the mixing frequency were evaluated in full-scale digesters. Subsequent digester performance

and foam presence/absence were monitored by following the digester temperature and TS

profiles within these digesters. Gas production and VS reduction were also monitored and are

detailed in the full-scale studies. Unmixed digesters were also studied as a control to compare

mixing effects in AD, both on performance and foaming.

2.9 Digester Physical Properties Role in Foaming

The role of digester shape and configuration on AD foaming is uncertain. Anaerobic

digestion tanks may be rectangular, cylindrical, or egg-shaped. The common configuration of

ADs is cylindrical with floating or fixed covers. ESD are being used in AD due to their perceived

advantages. ESDs claim superior sludge treatment due to their shape that helps enhance mixing

and minimizes upsets and operational problems, reduces the need to take the digester out of

service for grit cleaning, and saves energy required to maintain uniform conditions (Wu, 2010).

A CFD modeling study that compared two mixing methods and two digester shapes indicated

that the ESD provides for more efficient mixing than the cylindrical shape (Wu, 2010). Foaming

potential may be lowered in ESDs because of the higher gas flux in a smaller surface area

(Moen, 2003).

Cylindrical digesters have a relatively big surface area compared to egg-shaped digesters

(ESDs) allowing large volumes of gas to be stored and facilitating the accumulation of scum and

foam. On the other hand, ESD have a very limited surface area above the bulk phase of the

digester reducing the scum and foam accumulation potential. Poor mixing and grit accumulation

has been observed in cylindrical digesters creating dead spaces and short circuiting of sludge.

From a mechanistic point of view, there is no explanation as to why ESD may be better

with respect to prevention of foam formation and persistence (Tchobanoglous et al., 2003).

However, anecdotally, it has been thought for several years that ESDs are prone to much lesser



incidence of foaming compared to conventional digesters. There are comparatively fewer

published reports of process upsets in an ESD. A combination of G. amarae in the secondary

system along with operational issues with pumping and mixing in the digesters as well as

excessive grit and sand accumulation, caused an ESD upset and foaming issue (Jones et al.,

2003). However, the report still stresses that the versatility of the ESD design allowed the

maintenance staff to clear grit through the bottom withdrawal piping and the staff was able to

complete the cleaning job in less than three weeks (480 man-hours), which has been claimed to

be considerably less than what has been observed from other utilities using cylindrical digesters

for incidents of this magnitude.

ESDs at the City of San Francisco’s Oceanside WPCP have experienced foaming

problems caused by presence of G. amarae in the feed sludge to the digesters (Jolis, 2010). They

have also experienced non-filamentous rapid volume expansion events. A detailed analysis of

foaming in ESDs is discussed in a companion report. From the research survey results, there was

only one case of non-filamentous ESD foam that reported the cause of foam to be an inherent

one (high biogas production). These survey results are discussed in Chapter 3.0 but do not

provide any concrete evidence for correlating foam formation with type of digester

configuration. From this discussion, it is possible to hypothesize that digester configuration

influences the operation, control and mitigation of foaming but is not a cause of foaming. At

best, shape could be a factor for bubble nucleation. Certain kinds of crevices and bends provide

nucleation sites for bubbles (Loubiere et al., 2004).

Digester physical properties do not constitute just the geometry; it also is comprised of

factors such as sludge withdrawal mechanisms and positioning of piping. Rate of sludge

withdrawal’s influence on foaming lacks experimental data; however, it is widely thought that

certain sludge withdrawal processes can trap scum and debris in the digester, creating dead zones

that may lead to upset. Hydraulic overflow may be a better method than valve operated

withdrawal methods as the former does not impact the active volume of the digester (Massart et

al., 2006). Sludge removal cones also have to potential to help bubble nucleation and growth

(Deckers et al., 2010; Legair-Blair et al., 2002). In general, rapid volume expansion events seem

to be impacted more by digester design than conventional foam events. Digester headspace

volume and operational levels are the main concerns for rapid expansion events.

2.10 Discussion of Factors Causing Foaming

By review of existing literature, it is possible to identify suspected causes and

contributors for each type of AD foaming, some with proven evidence. Both feed-related and

digester process-related factors can cause foaming and/or contribute to both types of foaming

conditions in AD. The role of digestion temperature in AD foaming is mostly from indirect

effects such as the gas production, biogas solubility, and surface tension of the liquid phase in the

digester. pH and alkalinity are not possible direct causes of foaming. Much of the fundamental

phenomena involved have been extended from three-phase foam literature, but no direct

investigations have been reported on mechanistic understanding of foaming in AD using the

digester content matrix and under AD conditions. Figure 2-4 shows the role of all of the factors

discussed here, extrapolated from other three-phase foams. The relationships between the various

groups of causes/contributors and constituents of the digesters are illustrated but correlations are

not possible to predict. Some of these have been theoretically and quantitatively developed for

other desirable foams.

2-24

Based on the literature discussed in the foam fundamentals and causes sections, the role a

cause may play (whether fundamental or supplementary) seems to be specific to the digester

conditions. However, the information that exists in literature unduly places emphasis on some

factors for both enhancing foaming and minimizing foaming and some of them have been

carefully investigated in the full-scale studies. The popular opinion that all foaming has to be

filamentous seems to be unfounded. While it can be agreed it is the most common cause of

foaming, it is not conclusive if it is the only established primary cause of foaming. There is

insufficient full-scale literature to support this. In this regard, the digesters which have the

propensity to foam even without any foam-causing filaments in the feed have been studied

systematically in the Task 3 full-scale study.

Figure 2-4 depicts the relationship between the different factors causing/contributing to

foam:

The adsorption of components at the bubble interface.

The structure of the interface.

Processes occurring at the interface: The surface rheological properties of all the constituent

phases. The formation, stability, and mechanical properties of foams depend on the

interfacial physico-chemical characteristics and the process properties.

Bubble nucleation.

Foam formation.

Figure 2-4. Possible Factors and Relationships Leading to Bubble and Foam Formation.



2.11 Monitoring and Detection of Foam

Monitoring and detecting digester foam is a practical challenge. Effective tools are not

available to detect the presence of foam in ADs prior to or as soon as it occurs, in order to reduce

extreme consequences. From the discussion in this chapter thus far, foam is always present to

some extent in the digester. Early detection of foam is important so that it does not become a

major problem event and to reduce the impacts of foaming incidents.

There is widespread notion among various utilities that foam cannot be sensed reliably.

Conditions inside digesters make it difficult to sense foam. The nature of the foam also creates

problems with detection. Over time, a thick layer of conducting material builds up in the head

space. Such build-up causes fouling of sensors that may be installed in digesters, indicating the

need for robust components in the digesters. In the absence of instrumentation, utility personnel

have observed patterns in operational data or other signs in digester operation that serve as an

indication of foam, as identified from the researchers’ specific full-scale case studies.

2.11.1 Applicability of Detection/Identification Techniques to Digester Foaming

The most widely used technique for assessing the foamability of sludge is to measure

foam potential. Two approaches have been adopted to generate foam in AS: 1) Air is passed

through columns via diffusers at controlled rates to form uniform sized bubbles (Pagilla and

Jenkins, 1998) or 2) employing Alka-Seltzer tablets which foam when added to the sludge (Ross

et al., 1992). Both tests generate foam formation which can then be measured, and are commonly

used for digester foaming samples as well. These tests generate information on two of the most

important aspects of foaming – the ease of foam formation or ‘foamability’ and foam stability

(Pagilla et al., 1997). These techniques were used extensively in the food industry and borrowed

for use in AS foaming (Fryer et al., 2012). However, both of these methods are ex-situ

measurement of sludge foaming potential, not in-situ foaming potential.

Practical difficulties exist in measurement of foaming potential/stability as it quickly

loses its stability once removed from the tank. Investigation of foaming in samples requires foam

that remains stable long enough to be measured under conditions that mimic as closely as

possible to those experienced within the AD. This being said, the aeration foam potential method

does not represent digester foaming accurately for various reasons. Firstly, the aeration method is

dispersed aeration in the test while in digesters it is possibly a combination of dissolved and

dispersed gas flotation. Secondly, several of the phenomena occurring in the digester (i.e.,

mixing, gas release, etc.) have been established to be foam formation processes. Thirdly, the

biogas causing foam bubbles in ADs is soluble in liquid phase compared to poor solubility of air

dispersed in sludge foaming tests. This leads to laboratory samples from full-scale digesters not

indicating foam potential while the full-scale digesters foam during operation; in turn, making it

necessary to determine a detection method based on the specific foaming cause of each digester.

In spite of several shortcomings, the aeration test is one of the few available methods to

detect foaming potential/stability of sludge. The test has been reported to be most suitable for

biological foaming of AS. Seasonal foaming events in the AS process were represented suitably

by increasing foam potential of AS (Oerther et al., 2001). By observing the increase in foaming

potential, threshold values of foam potential were determined for foam initiation and

stabilization (de los Reyes et al., 2002). In these cases, these foam potential thresholds and

foaming scales were developed for specific plants and their use across other plants requires

observing these conditions in each case for a considerable period of time and determining foam

2-26

potential values that may effectively represent non-foaming and foaming conditions. More

recently, Fryer et al. (2012) established a process known as the Foaming Scum Index (FSI) to

measure foaming risk based on several foam characteristics such as foam stability, foam

coverage, foam suspended solids content and biological composition. This FSI is only a

comparative method to characterize foams already present in AS and more data needs to be

collected to use it as a foam detection tool. It nevertheless could provide a powerful tool to

classify foaming in various plants based on the above mentioned characteristics rather than just

the foam potential. However, the tool is limited by the specificity of each plant`s construction

and operation (Fryer et al., 2012). A list of the different techniques that have been implemented

to measure foaming in AS and their applicability to digester foaming is given in Table 2-5.

Based on this information it is evident that there are currently only limited tools available to

predict/detect foaming events as discussed in the following section. Considering these factors,

detection techniques can be classified in two groups and are described in detail in the following

sections:

Full-scale detection by monitoring and testing of digesters or its contents.

Detection using sensors and associated instrumentation.

2.11.2 Full-Scale Detection by Monitoring and Testing of Digesters or Contents

Some of the monitoring methods to assess AD foaming in the literature include volatile

acid/alkalinity (VA/A) ratio or alkalinity of the digester, feed patterns, and WAS content or

PS:WAS solids ratio. The VA/alkalinity ratio is expressed in equivalents of acetic

acid/equivalents of calcium carbonate and generally, values between 0.1 and 0.4 are considered

to indicate a healthy digester (Sánchez et al., 2005). Increases above 0.4 indicate upset and the

need for corrective action. If the ratio exceeds 0.8, pH depression as well as inhibition of

methane production can occur and the process can fail (Zhao and Viraraghavan, 2004). Though

these numbers have been reported to vary, this parameter has been used to predict foaming by

several utilities by simply looking out for widely variant values. In an instance of an acid

digestion system, acetic acid concentrations dropped dramatically with each foam incident, while

propionic or other acid concentrations did not drop as much (Reusser et al., 2004). Such patterns are

not noticed in all utilities. In the full-scale case studies, with severe and persistent foaming, no

inconsistencies were observed in the VA/A ratio, which remained consistent and well within the

levels mentioned.

Monitoring of fluctuations in feed patterns or simply monitoring feeding patterns has helped

understand and reduce foam incidents. AD plants can also predict their foaming incidents by

monitoring the WAS in digester feed. The foam incidents are linked to the PS:WAS solids ratio in

digester feed and is currently being studied further in the project. Seasonal temperature variations

help in predicting foaming due to filaments where the episodes are fairly predictable with

foaming in winter or spring.

In the course of this project, the literature review, and subsequent discussions with

several plants, brought to light various interesting monitoring methods. Such methods are listed

in Table 2-6 and discussed in detail in a companion report. In the Oceanside Plant (OSP) in CA,

foam usually enters the gas lines and disrupts gas collection. Usually, operators would have no

idea that foam was reaching the gas line unless they were visually monitoring the consistency of

the gas scum separator drain. However, the utility found an easy and reliable way to

automatically detect when foam enters the gas line.



Another indicator based on gas production is large variability in operating pressure, as

observed in Marquette WRF, MI. The pressures would fluctuate drastically with severe foam

spilling out of the digester covers. Pressure release occurred with dozens of pressure swings

occurring each day. During normal operation, the digesters were operated at 8 inches of water

column with only mild foaming.

Other reported observations for foam indication include change in appearance of digester

supernatant, spikes in gas production and sudden drops in gas production in various utilities.

Other strategies include use of sight glasses on digesters. While an economical and simple

option, conditions inside digesters make it difficult visually to spot foam.

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Table 2-5. Approaches for Assessing Foaming Potential. Method Use Applicability to Digester Foaming

Foam potential (Aeration method)

Diffused air is supplied to simulate tank conditions of gas flotation in order to evaluate foam potential.

Test does not necessarily represent full-scale digester conditions; However, by and large, high foam potential values are an indication of digester foaming than not (especially in cases of filamentous foaming). These values are not representative across all plants and specific values must be determined for each plant.

Staining procedures

Gram and Neisser stained slides are observed under a microscope for presence of filaments. Their presence can be quantified by counting methods and expressed as counts/gVSS. The relative density of dominant foaming filaments is called FI (Filament Index) and is estimated by subjectively ranking them on a scale rating from 0 (none) to 6 (excessive) (Pitt and Jenkins, 1990; Jenkins et al., 2004).

FI indices will be different for digester foaming. In full- scale digester foaming investigations filament abundance of FI≤3 did not result in foaming at the full-scale (Ganidi, 2008). Westlund et al. (1998) found that M. parvicella partitioned into foam and its abundance in foam corresponded to FI of 5 while in sludge to FI of 0-1 at the full-scale.

Fluorescence in situ Hybridization (FISH)

The FISH technique involves the use of special oligonucleotide probes designed for 16S RNA to detect complementary sequences inside specific cells.

FISH can only detect viable cells because the technique is based on ribosome levels in cells. This is an issue because non-viable cells also cause foaming with their presence by floating and attaching to gas bubbles because they retain their hydrophobicity; which is an important factor in digester foaming. (Davenport et al. 2000; Oerther and De Los Reyes, 2001).

Cell Surface Hydrophobicity (CSH) (MATH and EI assays)

These techniques potentially predict if the filaments are likely to stabilize foams or remain in suspension within the tanks.

These tests directly measure the actual cell surface hydrophobicity (CSH) of biomass and have been correlated with the capacity of the sludge samples to foam (Torregrossa et al., 2005). Foaming sludge samples can possess higher degrees of hydrophobicity compared to non-foaming sludge samples. In evaluating foam properties of several filamentous strains, foaming ability did not always correspond to a high MATH hydrophobicity values (Petrovski et al., 2011). Increased biosurfactant production was also not detected. Such tests have not been found to be appropriate for full-scale foaming measurements (Eikelboom, 1991).

Surface tension Surface tension of the liquid has been used to identify values below which foaming might occur.

Rather than simple surface tension, the CMC of sludge supernatant might be better in order to monitor and foam formation as a lowering in surface tension. The formation of micelles takes place above CMC and been widely applied to test if biosurfactants are produced and contribute to foaming in various pure culture and sludge studies; but not in full-scale monitoring.

The use of a sludge-judge is also a viable option to sample different lengths of a digester

to detect foam. It is important to ensure that the sludge-judge is used to sample at different

intervals above the liquid level to detect foam.



Table 2-6. List of Specific Foam Detection Methods. Utility Foam Monitoring/Detection Method

OSP, CA (see Chapter 7.0 for details) Temperature differential between foam separator and condenser.

Marquette City WRF, MI (see Chapter 4.0 for details) Level transducer monitoring.

Crystal Lake WRRF, IL (see Chapter 6.0 for details) Level transducer monitoring.

2.11.3 Detection Using Sensors and Associated Instrumentation

One of the main challenges in the detection of foam in ADs is the presence of the foam

layer at the surface. Typical instruments only measure the level of liquid in the digester, and will

not be effective during foam events where the foam layer increases while the liquid layer

underneath remains unchanged or increases very little. Most commonly used sensors such as

ultrasonic level, pressure measurement; floats, level switches, etc. only detect liquid level and/or

are susceptible to solids present in the digester environment. One way to address this option

would be to use a combination of two sensors – one installed at the top and the other at the

bottom (mostly a pressure sensor) to work in conjunction with each other to determine foam

levels.

Table 2-7 lists the common foam sensors available commercially for potential

consideration in ADs.

2.11.4 Discussion of Monitoring and Detection of Foam

ADs face problems of sensor reliability due to the harsh environment they are installed

in. This poses an interesting challenge for monitoring and detecting foam episodes in digesters.

Careful monitoring of digester performance is necessary. Foam potential data needs to be

collected long term to evaluate threshold foam ratios and to determine if high foam potential

values represent full-scale digester foam episodes. Full-scale studies from this project have led to

investigation of specific foam monitoring and detection methods in each plant, as discussed in a

companion report.

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Table 2-7. Sensors for Foam Detection in AD.

Sensor Technology

Possibility of Use in AD Foaming

Full-Scale Installations

Comments

Impedance measurements. Foam sensor operates by passing a small alternating current through the foam under test and measures impedance.

Yes. The impedance of the material being sensed is used to determine when foam or liquid is present.

Unknown The pricing is dependent on the length of the sensor required (determined by the design of the digester) and the required fitting for inserting the probe.

Acoustic waves of high power and low frequency.

Unknown. Presence of gases in digester headspace will affect speed of sound & cause ineffective detec-tion (Vanrolleghem and Lee, 2003).

N/A Manufacturers detail possibility of use in ADs.

RADAR measurement sensor. Yes. Full-scale installations report effective sensing of digester foam.

1. IQ160 installed in Joint Water Pollution Control Plant (JWPCP) located in Carson, California. 2. SIEMENS Sitrans LR200 installed in Truckee Meadow Water Reclamation Facility in Reno, Nevada (Gray, 2012).

RADAR liquid level indicator was installed in addition to pressure level indicators installed on the bottom of the digesters. Under no foam conditions the two liquid level indicators would read the same level. During a foam event the radar indicates the foam as liquid level and would show the foam level increasing, and the pressure level indicator remains unchanged. Thickness of foam layer on the sludge is the difference in the pressure indicator and the radar liquid levels.

Ultrasonic detection for foam. Only detects liquid levels. Absorption of the signal by the CO2, CH 4 and H2S gases in the headspace has been reported. (Ackman et al., 2006).

Prior to RADAR, Joint Water Pollution Control Plant (JWPCP) had ultrasonic detectors in their digesters.

Presence of dust, vapor and gases etc. interferes with performance.

Liquid level measurements – conductivity switches; differential pressure transducers; capacitance measure-ments. (Vanrolleghem, Lee, 2003).

Differential pressure and ultrasonic equipment give a continuous signal, the latter being more precise but also sensitive to foam.

Most common sensors present in digesters.

Again, the issue is sensing the foam layer on top of the liquid layer, which can be overcome by using a pressure sensor at the bottom of digester.

Float. It contains a small ball which floats, indicating foam level. It also provides a seal on the outlet of gas collection valve. The float ball prevents foam entering the gas line eliminating need to drain the foam.

Used in beer fermenters and large storage kegs. Possibility of use in digesters.

N/A Fairly simple equipment. Moving parts may need special construction for service inside digester.

Laser Possibility of use in digester. N/A Presence of solids can scatter laser beam and backscatter the laser light (Vanrolleghem and Lee, 2003).

Thermal Possibility of use in digester. N/A Thermal conductivities of interfaces involved need to be vastly different so that equipment can be calibrated to sense level of foam layer.

Phase Difference Sensors/ Vibrating. Switches/Tuning Forks.

Possibility of use in digester. N/A Material buildup can cause fouling and prevent operation.



2.12 Prevention and Control of Foaming

In the practice of AD foaming control and prevention, various methods have been

used, sometimes without a strong scientific basis. Merely tweaking operational parameters

such as feed rates and temperatures have worked in a few cases and failed in many. The most

common and possibly the most effective of the methods used are modifications to operation of

processes with respect to AD and secondary treatment facilities.

2.12.1 Introduction

As the causes of foaming outlined in this document have been classified to be related

either to feed characteristics including presence of foam-causing microorganisms, or

operational factors, the control methods address these factors. Broadly, the most commonly

used methods are classified into three categories, each addressed in detail in this chapter – feed

sludge disintegration (physical/chemical/mechanical methods), modification of operational

conditions, and addition of chemical antifoaming/defoaming agents.

Control strategies can be further classified as specific or non-specific methods (Martins

et al., 2004). These definitions were originally developed for filamentous bulking, but they can

be extended to filamentous and non-filamentous foaming as well. The non-specific methods

comprise sludge disintegration techniques such as application of hydrogen peroxide or

chlorination. Any method whose effects are temporary and do not remove the root cause of the

problem are termed non-specific. Specific methods are preventive methods that can offer long-

term control of foaming by removing the root cause of the problem. ATICLEIN

2.12.2 Sludge Disintegration Methods

In the field of sludge treatment, the terms sludge disintegration, physical/mechanical

pretreatment, disintegration, and hydrolysis usually refer to processes which are combined

with the main biological sludge treatment process (Carrère et al., 2010). Some of these

methods help in enhancing gas production and AD performance. Certain types of sludge

disintegration could help in mitigating digester foaming if the feed sludge characteristics are

the primary cause of AD foaming. In case of filamentous foaming, these methods might prove

to be completely specific, as most of these methods can lyse and hydrolyze the cells

completely. Different sludge disintegration methods are listed in Table 2-8 and explained here

with respect to their applicability to control AD foaming.

Sludge disintegration can be mechanical, physical, or chemical. Conditioning of

thickened WAS (TWAS) using ultrasound before AD has been implemented in a few

treatment facilities in both Europe and North America to achieve more complete digestion and

to enhance VS reduction (Massart et al., 2006). Increased VS reduction translates to increased

biogas generation and decreased quantities of stabilized biosolids for disposal. Many of the

installations that use ultrasonic conditioning have also made references to a decrease and in

some cases complete elimination in foaming incidents (Massart et al., 2006).

Sludge disintegration by steam has also been investigated (Hoyle et al., 2006). It was

observed that nocardioforms became less abundant with increased levels of steam pressure and

exposure time combinations. However, the method was ineffective for M. parvicella

filaments; it was specific towards G. amarae. Foam potential is unaffected but foam stability

is greatly affected by the treatment. Again, the reason could be the release of biosurfactants at

high temperatures of the steam treatment. This plant experienced AS foaming, however, it is

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unclear if this plant actually experienced AD foaming. The study simply mentions that it could

be a viable method for digester foaming control. Apart from vibrational methods discussed in

Table 2-8, all of the other sludge disintegration methods described here are concerned only

with filamentous foaming. Table 2-9 lists case studies of WRRFs where sludge disintegration

methods were used for foam control.

Table 2-8. Different Sludge Disintegration Methods and Applicability for AD Foam Control.

Mechanical Methods

Application Type

Successes/Failures or Limitations

Suggested Mechanisms

References

Thermal hydrolysis (commercial manufacturers Cambi and Kruger)

Solubilizes the organic fraction of the sludge by elevated temperature and pressure.

No full-scale thermal hydrolysis installations in the U.S.

Cell destruction is achieved through a sudden pressure drop.

Sundin et al., 2008

Direct steam injection Mixes precisely metered amounts of steam directly with a liquid or slurry, providing an instantaneous transfer of heat from steam to the liquid.

It is uncertain whether M. parvicella filament destruction will be achieved with direct steam injection. No full-scale installations in the U.S. but one in Norway.

Destruction of the cell structure of nocardioforms as a result of exposure to steam pressure.

Hoyle et al., 2006; Sundin et al., 2008

Pastuerization (commercial manufacturers Eco-Therm (Ashbrook) and BioPasteur (Kruger)

Both systems heat thickened sludge to 70°C for a minimum of 30 minutes to provide pathogen reduction.

Uncertain if M. parvicella filament destruction will be achieved. One full-scale installation in the U.S.

Deactivation by heat. Sundin et al., 2008

Electric-pulsing (OpenCel)

Focused high-voltage pulses of electricity to rupture the cellular membranes.

The only successful full-scale installation is at the Mesa Northwest Treatment Plant (Mesa, AZ).

Cell rupture by electric pulses.

Sundin et al., 2008

The Crown sludge disintegration system (Siemens) and Micro Sludge (Paradigm Environ. Technol.)

Cavitation through different means.

Both manufacturers claim the destruction of foam-forming filamentous organisms. No full-scale installations of Crown Sludge in the U.S. Micro Sludge has full-scale installations in New Zealand and Germany.

Sludge pre-treatment technologies that rely on cavitation for cell destruction. Pressure drop causes cavitation and cell destruction. WAS is mixed, homogenized, pressurized, and forced through a disintegration nozzle, causing the cell structure to rupture.

Sundin et al., 2008



Table 2-9. Case Studies of Foam Control by Sludge Disintegration Methods in WRRFS.

Cause Control Measure

Effectiveness of Control/ Additional Comments References

AS filamentous bulking, AD filamentous foaming, and digester design and operational issues

Digester foam solutions have included tank and plant upgrades, additional instrumentation, and changes to operational procedure.

N/A Jacobs et al., 2008

Filamentous foaming Ultrasonic sludge disintegration of sludge. N/A Vera, 2006

Filamentous foaming Ultrasonic sludge disintegration of sludge. Plant also reports mitigation of foam in AS due to absence of filaments in its recycle.

Vera, 2006

M. parvicella Conversion to acid phase digestion was found to be the best option over various alternatives analyzed.

N/A Sundin et al., 2008

Filamentous foaming

MicroSludge disintegration method was tested full-scale by feeding pretreated WAS and WAS+PS to a digester. Radar level and flow instruments installed in digesters to control and monitor flow to prevent overflows.

N/A Ackman et al.,2006

G. amarae N/A N/A Pagilla et al., 1997

G. amarae Surface foam removal. N/A Jolis et al., 2003

M. parvicella Lower level operation, anti-foam agent (Poly-Aluminium Chloride Salts (PAX) -21), mixers in foam layer.

Worked in all WRRFs except one.

Westlund et al., 1998

Filamentous foaming Pulsed electric field treatment, thermal hydrolysis.

N/A Kopplow et al., 2004

2.12.3 Operational Modifications to Prevent/Control Foaming

Operational modifications to either the AD processes or the liquid process streams are

typically methods for prevention rather than control. Generally operational modifications are

the first means attempted and preferred before chemical and physical control methods are

employed. Whereas chemical and physical methods may target a single foaming factor,

operational modifications are generally more robust and specific. Modification of operational

parameters gives room for a trial and error approach, and many utilities have known to

experiment with SRT, and feed rates, among others, in an attempt to mitigate foam in the feed

sludge and/or AD. Therefore the success of AD foaming mitigation through similar methods

could be anticipated at other plants. However, it is important to determine what works and

what does not, and why it works in order to implement these methods at other plants. These

operational modifications can be classified into three specific sub-groups, mainly:

Control of the secondary treatment process and associated WAS foaming.

Control of the feed sludge storage and feeding.

Control of the digester physical features.

Each of these sub-groups is discussed in the following section.

2.12.4 Control of the Secondary Treatment Process and Associated WAS Foaming

The most important of the foam control methods is the use of selector mechanisms in

AS foaming control. Selectors are not explained here in detail, as that information is available

elsewhere (Jenkins et al., 1992; Wanner et al., 1998; Martins et al., 2004; Parker et al., 2003).

2-34

As AS systems evolved and nutrient removal came into effect, “selectors” became anoxic or

anaerobic to achieve both filament control and nutrient removal. The reported successes of

selectors in certain foaming cases and their relative low cost to construct and operate have

resulted in their wide-spread use around the world. With a classifying selector, all foam

causing organisms are preferentially removed first due to their higher concentrations in the

surface foam (Pagilla et al., 1996). In a review that discussed several case studies of

implementing selectors in WRRFs, only one case mentioned digester foaming (Parker et al.,

2003). In this pure-O2 AS plant, G. amarae counts were reduced but not completely removed,

thus failing to mitigate digester foam. To study the effectiveness of selectors on AD foaming,

the plant survey discussed selectors as a control method. The results are presented in Chapter

3.0.

Selection against foam-forming microorganisms through operational control becomes

more complex in BNR systems where it is important to keep the system under strict balance to

promote the growth of nitrifying organisms. Reducing the SRT can inhibit nocardioforms and

M. parvicella growth but can also wash out nitrifying bacteria. In most cases where AD

foaming was found to be caused by filamentous organisms from AS, mitigation measures were

applied in the aeration tanks such that the growth of the foam-causing organisms was

controlled. In many cases, selective removal of the foam is also carried out, hence removing

organisms that caused the foaming problems. This in effect, reduced the SRT of the foam-

causing filaments while maintaining a high enough SRT to achieve nitrification by bulk of the

non-foaming AS.

There have also been many cases where selectors have often proved ineffective at

controlling filamentous organisms in AS (Gabb, 1988; Gabb et al., 1991; Daigger and

Nicholson, 1990; Davoli et al., 2002; Labek and Rosenwinkel, 2002). Anoxic selectors do not

appear to significantly control filamentous organisms in long-MCRT (mean cell residence

time) plants (Wanner et al., 1998; Martins et al., 2004). It was discovered then that selectors

were ineffective for M. parvicella foam, because of its high storage capacity for lipids from

LCFAs (Rossetti et al., 2005). In spite of this, there are a few reports of the successful

application of aerobic contact zones for controlling the growth of M. parvicella (Pujol et al.,

1994). A control measure for wastewaters of high lipid content would be to remove some of

these compounds by a pretreatment method such as flotation.

Similarly, removal of LCFA in selector tanks to achieve M. parvicella control has

shown success. As discussed earlier, LCFA has been found to enhance M. parvicella growth.

LCFA removal efficiency is inversely proportional to the applied OLR (Noutsopoulos et al.,

2010). Based on these findings it can be stated that under the conditions prevailing in

anaerobic, anoxic and aerobic selector tanks, only a portion of LCFA is removed. The

remaining LCFA under the completely mixed conditions prevailing at the aeration zone would

stimulate M. parvicella growth. This study could be useful in explaining why selectors do not

help M. parvicella foaming/bulking in many cases and subsequently creating foaming in the

downstream AD. Hence selector methods can be specific control methods for certain types of

filamentous foaming.

2.12.5 Control of the Feed Sludge Storage and Feeding

The PS:WAS solids ratio in feed sludge could influence foam in three ways: 1) If

filaments are present, more WAS results in more filaments fed to the digester; 2) More PS in

feed versus WAS could increase gas production rate resulting in more gas bubbles causing



foam; and 3) Longer retention times in holding tanks lead to fermentation and higher

production of VFAs, when fed to digester in higher quantity can contribute to foaming. It

appears that both PS and WAS should be present to cause foaming because the gas production

predominantly is due to PS and surface active materials such as filamentous foam-causing

organisms or recalcitrant biomass solids are needed to create stable foam are due to WAS. The

optimum PS:WAS solids ratio for foaming to occur is a function of the biodegradability of the

solids in each stream and operating SRT of the digester.

Foaming is reported to occur more frequently if the ratio of WAS to total sludge solids

exceeds 40% (Massart et al., 2006). However, there is a lack of foaming data from digesters

that may have operated at this ratio. A staged configuration is one with primary and secondary

digesters which receive PS and WAS, respectively. Staged operations have been reported in

several facilities as well (Carrère et al., 2010). It is necessary to see if PS or WAS feed would

contribute to foaming or if there exists an “ideal” mix of WAS/PS that does not influence

foaming. Full-scale operations with dedicated WAS digesters are in use in many WRRFs. In

cases of combined feed with larger PS content, higher solids residence time in the primaries

can cause fermentation and high VFAs. In such cases, control of sludge blankets in primary

clarifier systems may be more effective in controlling the load to digesters (Kobylinski et al.,

2009). The researchers’ survey had a question about the PS:WAS solids ratio of feed to the

digester and collected responses from plants that operated at various ratios (Chapter 3.0).

However, a consensus on the ideal ratio for non-foaming plants could not be reached from

these results.

In the case of combined sludge, the holding tank residence time seems to be one of the

responsible factors for foaming. One such case is reported where it was suspected that blended

and hauled sludge in the holding tank was causing foaming (Fonda, 2008). The hypothesis

behind this cause of foaming is that increased HRT leads to higher fermentation which in turn

leads to enhanced production of LCFA that may cause foaming. Holding combined sludge in

primary clarifiers may cause washout of acids/intermediates to liquid treatment whereas

holding in sludge holding tank feeds them into the digester, potentially causing foaming.

These factors need to be investigated in full-scale plants. Bypassing a WAS storage tank of a

high HRT plant before the digester was studied full-scale to assess these effects (Chapter 4.0).

Other aspects of feed control to digesters include feed frequency to the digester, was

discussed in the causes section. Regulating feed frequency can help in establishing a constant

level of OLR to the digesters and equalize gas production rate that can mitigate foaming to

some extent.

2.12.6 Control of the Digester Physical Features

This section discusses modifications to the digesters that may be mechanical (addition

of foam breakers, sprays, etc.) or changes to digester design (gas collection piping changes or

sludge withdrawal changes, etc.) to mitigate/control foaming or its effects.

Design parameters such as the sloped gas piping and condensate removal from gas

removal piping are instrumental in controlling foam formation and removal (Massart et al.,

2006). Water and/or foaming solids in the gas piping can cause blockages that increase the

pressure in the system. When the blockage is removed, foaming can occur in the digester

because of sudden biogas release from the liquid phase. Maintaining a constant pressure by

allowing condensate and biogas to be quickly and effectively removed can contribute to foam

2-36

mitigation. Thus, it is imperative that the system is designed to allow the condensate to be

quickly and effectively removed from the gas collection piping system.

Foam separation is another means of control. Mechanical foam removal by drawing

gas/foam from the top of the digester and pushing it down using a series of water spray

nozzles seems to have success in managing the foam levels in AD (Kobylinski et al., 2009).

Such equipment seems to be an improvement over the water sprays where the volume of water

added in the digesters’ active volume has a significant impact on the HRT (Moen, 2003;

Barber, 2005). An alternative to the water sprays was digested sludge sprays over the foam

layer at the top of the digesters through large bore nozzles. This method was successful when

applied at WRRFs but generated operational concerns due to blockages of the nozzles (Barber,

2005). Marquette WRF, MI operates digested sludge sprays for foam suppression with the

help of only one nozzle very effectively and is explained in a companion report.

The mitigation of foaming problems involves the consideration of multiple objectives

of AD simultaneously. Even though the foremost objective is to mitigate foaming while

achieving effective digestion, it is imperative that the alternatives be economical and energy

efficient. In this report, only a review of the available methods has been presented. More full-

scale demonstrations of these methods are required before any concrete recommendations of

foam control can be made. Table 2-10 lists some WRRFs that have modified operational

procedures to control AD foam.



Table 2-10. Case Studies of AD Foam Control by Modification of Operation in WRRFs.

Cause

Control Measure

Effectiveness of Control/ Additional Comments

Reference

Extensive ATAD foaming. Increased HRT in the blended sludge tank lead to increased fermentation, that in-turn increased LCFA production and caused foaming.

Blended sludge storage tank level was monitored, HRT reduced and heating turned off. Prescreening of haulers was implemented. Chemical toilet waste was limited.

10 events occurred over a period of 3 months. 2 events have occurred since these changes were implemented.

Fonda, 2008

Inconsistent organic loading. Monitoring the digesters and controlling feed rate and influent mass of VS.

Yes. Massart et al., 2006

Inconsistent organic loading. 1. Maintaining influent solids concentrations between 4.5% and 5.0% to optimize digester mixing; 2. Limiting the daily variation in VS load (organic load) to the digesters at levels between ±5% and 10%; 3. Maintaining lower sludge blankets in the primary clarifiers to avoid generating uncontrolled VFA. 4. Comparing VS reduction to actual digester gas production.

Once VS loading was controlled, excessive foaming ceased.

Massart et al., 2006

Denitrifying AS created foaming issues in the digesters.

GPS-X modeling determined that the plant could achieve the solution for the foam and scum issue by converting to an anoxic/aerobic system.

N/A Fonda, 2008

In BNR operation mode, M. parvicella growth was attributed to the AD foaming events, which were observed primarily in winter.

Using antifoams at the onset of an event, altering DO in aeration basins, installation of a sludge disintegration system as well as modifying gas collection and mixing in digesters.

N/A Fonda, 2008

Insufficient mixing and excessive HRT provided by operating the digesters in parallel.

Stable operation was restored when the digesters were placed in series mode of operation and the gas mixers were turned on continuously.

The plant has continued operating in this mode and foaming has not occurred since.

Fonda, 2008

Investigation of the digestion system revealed that the digester was under loaded and the digesters were being over mixed.

Modifying the digester mixing rate reduced the amount of foam to manageable levels.

N/A Fonda, 2008

Prolonged solids detention times in the primary clarifiers led to fermentation causing lower pH.

By maintaining the pH level in the PS above 5.4 and the digester alkalinity level below 4,800 mg/.L, the plant has been able to eliminate foaming problems in their digesters.

Yes Fonda, 2008

Effects of deteriorating headworks and limited operational volumes.

Replacement of the covers, mixing systems, heating system, and gas handling system.

Foam problems at start-up after rehab.

Holden, 2006

2.12.7 Chemical Antifoaming Agents for Foam Control

Where physical/chemical and/or operational means have been ineffective in control of

foam, often antifoaming chemicals have been employed, with varying success. Anti-foaming

or defoamers could be a potential solution for plants that experience very few and infrequent

occurrence of foaming problems in AD.

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2.12.7.1 Antifoams

Antifoams and defoamers are fundamentally the same in what they aim to achieve

(Pelton, 2002). Antifoams are strongly surface-active substances which replace foam-forming

components at the water surface and lower surface tension of liquids and act by altering the

surface interactions between the foam bubbles and the solid-liquid interface (Junker, 2007).

In most cases, M. parvicella foaming does not seem to be mitigated by antifoam

dosages. With respect to G. amarae, antifoams can also be used initially to control the

foaming, but it is not recommended for continuous use as they are traditionally mixtures of

hydrophobic liquids and solids that may lose effectiveness if continually applied. Since there

is a lack of substantial full-scale literature on the use and success of antifoams for AD

foaming, the effectiveness of an antifoaming agent was tested full-scale in the Task 3 study.

2.12.7.2 Coagulating Salts and Polymers

Certain coagulants and flocculating polymers are specific to mitigating M. parvicella

foam but non-specific to foam caused by G. amarae mycolata (MYC). These findings are in

acceptance with several published studies where PAX-14 was effective in treating only M.

parvicella foam but not G. amarae foam (Westlund et al., 1998a, b; Rossetti et al., 2005).

There has been much speculation on the mechanism of M. parvicella removal by PAX salts. A

reduction in enzyme activity of M. parvicella was observed by PAX-14 addition (Nielsen at

al., 2005). Several possible reasons were suggested including the formation of dense flocs,

thereby embedding the M. parvicella, and not allowing it to access the substrate. PAX-14 also

inhibited the activity of enzymes but not their production.

Addition of PAX-14 caused no changes in the hydrophobicity of the bacterium surface

(Eminovski, et al., 2010). Since it has been suggested that M. parvicella may preferably store

LCFAs under anaerobic conditions and subsequently use it for growth, PAX is a specific

chemical method that seems to affect LCFA uptake by these mechanisms to mitigate M.

parvicella foam. This is possibly how PAX preferentially treats M. parvicella foam. While

dosing with PAX causes only specific removal of M. parvicella, no such polymers had been

identified to be effective for G. amarae. Reports of polymer dosing in AS systems wherein

they were inefficient for MYC foaming are abundant (Mamais et al., 2007; Kragelund et al.,

2010). Table 2-11 lists the summary of a case study of PAX-coagulant use for M. parvicella

foam control in AD.

Table 2-11. Case Study of PAX-14 Use for M. parvicella Foam Control in AD.

Coagulant/ Chemical

Application Type/ Cause of Foaming

Success/ Failure

Comments

Reference

PAX-14 Digester foaming/M. parvicella.

PAX-14 was used at three WRRFs but was successful only at one of the three.

Specific removal of M. parvicella.

The failure of the antifoam in controlling foaming in the digesters at the other two plants was attributed to poor mixing of the antifoam with the sludge stream.

Not found to be effective for G. amarae MYC.

Possible mechanism may include substrate inhibition and LCFA uptake.

Westlund et al., 1998



Cationic polymers have been used in controlling foam caused by G. amarae in aeration

basins (Shao et al., 1997). It was hypothesized that the polymer coagulates the free-floating

filaments and incorporates them into the floc, thereby reducing their tendency to produce

foam. While reports such as this are available for polymer treatment of mixed liquor (ML) for

AS foam control, no published reports are available discussing foam control in digesters using

polymers. An unpublished account of polymer addition to a floating cover digester reports that

it was effective in collapsing the foam. Very little is known about the behavior of these

polymers in digesters in terms of their digestibility and fate in the digested sludge.

These antifoaming chemicals and coagulating polymers are the most effective when

complete mixing of the sludge and these chemicals is ensured. In a study that treated M.

parvicella foam in three WRRFs, it was found that the PAX-14 treatment was not effective in

one of the WRRFs where mixing was poor (Westlund et al., 1998).

2.12.7.3 Chemical Oxidants

Chlorination of WAS has been carried out in an attempt to eliminate AD foaming

(Pagilla et al., 1998). The study found that chlorination was not only ineffective in controlling

foam but it increased foaming and foaming potential. The number of filaments also increased

in the chlorinated foam due to breakdown of sludge flocs.

The main point of difference between the mechanisms of chlorination and chemicals

addition described earlier is that the polymers may have immobilized the G. amarae within the

flocs, increasing the mass transfer resistance and inhibiting substrate uptake in both M.

parvicella and G. amarae while chlorine breaks down cells and releases EPS or biosurfactants

produced by the microorganisms. In cases of polymer addition, filamentous foam potential

reduced because of greater floc formation. In chlorination, increase in chlorine dose only

increased foam potential, due to increased breakdown of the filaments from the sludge floc

structure by the chlorine.

Other chemical treatment methods include addition of oxidizing chemicals such as

hydrogen peroxide (H2O2) which attacks the filament sections that are outside the floc

structure. Dosage must be controlled closely to prevent damage to the other microorganisms.

This method can be used to provide a temporary solution during foaming episodes but cannot

be considered as a long term solution. Though there seem to be pilot studies of H2O2 used in

WRRFs, there are no reports of full-scale usage of H2O2 for AD foaming control.

2.13 Impacts of AD Foaming

The focus to mitigate/control AD foaming in WRRFs has not been as significant as it

should be due to multiple reasons related to sludge processing. First, AD foaming is not

visible until it becomes a severe problem with sludge overflowing from the digesters. Second,

the digested sludge does not need to meet any specific regulatory requirements as long as it

undergoes standard AD to produce Class B sludge. Thirdly, the energy value of the AD has

not been a key concern until recently due to cheap external energy (electricity, natural gas,

etc.) available for WRRF use, at least in the U.S. Lastly, the impacts of foaming in AD has not

been determined in quantitative terms as lost gas production, lost capacity or active volume,

actual costs for control by means of chemical addition, operating expenses for clean-up of

foam overflows, etc. As more WRRFs move toward AD of sludge to meet more plant energy

needs through the biogas produced and sustainable plants, AD has regained focus in WRRFs

as an important process. Hence, control of AD foaming has gained significant attention as

2-40

increased number of WRRFs are experiencing foaming or have begun to notice a problem

which was in existence before. However, a systematic qualitative and quantitative

determination of impacts caused by AD foaming is still lacking based on full-scale plant

investigations. In general, the main operational difficulties caused by AD foaming incidents

are:

Impacts on digester performance by removing active volume resulting in lowered gas

production and VS destruction.

Create conditions that cause tank mechanical and structure failure due to foaming.

Significant maintenance required for cleaning biogas piping and foam overspills.

Potential short-circuiting of pathogens due to lower active volume in the digester.

Once foaming incident occurs, the magnitude of impacts can be very significant. Most

foaming incidents bring about both qualitative and quantitative impacts.

2.14 Qualitative Impacts of Foaming

Qualitative impacts are impacts that are assessed qualitatively and consist of

performance-related aspects of the digestion process, specifically impacts on gas production,

VS removal rate, and biosolids quality, among others. They could also be operational impacts

such as gas collection issues, mixing problems, etc. An indirect impact of this is regulatory

and sustainability concerns. Though qualitative and economic impacts are discussed as

separate sections, economic impacts due to loss in biogas production and damages to

equipment are brought about by impacts that are assessed qualitatively quite often at WRRFs.

Qualitative impacts can be further classified into performance impacts, operational impacts

and regulatory impacts. It is required to quantify all qualitative impacts in a direct or indirect

way in the future.

2.14.1 Performance Impacts

In general, many studies have reported ineffective digester performance during

foaming periods (Pagilla et al., 1997; Massart et al., 2006; Ganidi et al., 2008). Qualitative

impacts reported include inverse TS profiles in the digesters leading to reduced sludge

stabilization and decreased gas production (Pagilla et al., 1997; Ganidi et al., 2008).

Decrease in VS reduction due to foaming usually causes a decrease in gas production

in most cases (Pagilla et al., 1997). However, a reverse of this effect has also been observed

during foaming, with abnormally high gas-production rates (Ganidi et al., 2008). The reason

for foaming was an overloaded digester where VS overloading caused the foam to entrap large

amount of solids fraction, which may have caused higher than average gas production. The

difference in gas production as an impact may be directly related to the cause of foaming

itself.

Odors and aesthetic impacts generally do not become a concern unless the plant is

located within the vicinity of a neighborhood which is impacted by such occurrences.

Unsightly conditions within the plant and odors due to foam overflows from severe AD

foaming incidents are known to occur in WRRFs.



2.14.2 Operational Impacts

Foaming episodes create issues with mixing and gas collection. Excessive foam causes

sludge to overflow the digester and enter the biogas collection piping, blinds flame traps, and

interferes with gas-mixing operations (Massart et al., 2006). Excessive foaming also causes

increase in gas pressure (Fonda, 2008). Other problems caused by foaming incidents are

blockages of gas mixing devices, foam binding of sludge recirculation pumps (Moen, 2003),

fouling of gas collection pipes due to entrapped foam solids (Massart et al., 2006), foam

penetration between floating covers and digester walls, and tipping of floating covers during

foam expansion and collapse (Fonda, 2008).

In cases of extreme foam incidents, failure of gas collection systems and high pressure

alarms have caused digester covers to explode resulting in structural failure (Fonda, 2008).

This is a critical problem especially for concrete-roofed digesters that have no visual access to

inside the digester. There have been cases where foam pressure caused roof to break its seal

and stop operations for weeks (Massart et al., 2006). In fixed cover digesters, foam could

potentially plug the pressure/vacuum relief valves and lead to structural failure of the digester.

Anecdotal reports claim that digester rehabilitation has become necessary in many cases after

foam incidents. Most structural damages are very expensive and are part of huge cost

implications due to foaming incidents.

Additionally, the occurrence of foam overflows from the digesters due to foaming

could also be considered a safety hazard to plant personnel during clean-up of such overflows.

The sludge and foam overflows cause slippery conditions on the digesters, on the floors, and

on the walkways around the digester, creating hazardous conditions for the plant personnel.

Qualitative impacts commonly reported are summarized in Table 2-12.

2-42

Table 2-12. Qualitative Impacts Reported in WRRFs with AD Foaming.

Cause of Foaming Impact Reference

Inconsistent organic loading. Sludge overflowed the digester walls and entered the biogas collection piping, blinding flame traps and interfering with gas-mixing operations. Facility also reported foam entrapped solids, resulting in abnormally higher gas-production rates.

Messart et al., 2006

Although excessive grease and insufficient blending were contributing to the foaming problems, operators determined that they were not the primary cause. The primary cause was VS overloading.

Reduced gas production and increased difficulty in maintaining pH of digesters.

Messart et al., 2006

Liquid stream filamentous foaming, AD foaming, liquid stream pass-through events, and digester design and operational issues.

Operational problems ranging from the need for increased operator intervention, to structural damage (digester dome) and reduced solids handling capacity.

Jacobs et al., 2008

Foaming due to operational/digester issues. Operational problems, biogas production issues. Fonda, 2008

Filamentous foaming. Operational issues, inefficient biogas collection. Vera, 2006

Filamentous foaming in AD. Varying foam layer of several inches to several feet. Blocked valves or run off lines could cause foam layer expansion and collapse.

Ackman et al., 2006

Extensive operational issues in the WRRF. Extensive equipment damage and operational issues.

Ackman et al., 2006

2.14.3 Regulatory Impacts

Foaming incidents may cause discharge permit violations by overflows and flow

backups in the system when excessive foaming creates a hydraulic capacity issue. Table 2-13

reports different regulatory impacts due to foaming.

Table 2-13. Regulatory Impacts Reported in WRRFs with AD Foaming.

Cause of Foaming Impact Reference

Inconsistent organic loading. Effluent discharge violation due to foam. Messart et al., 2006

Upstream liquid treatment process problems and reduced active volumes due to grit and other debris accumulation.

Digesters were contributing to the reduced performance of the WRRF which has led to numerous effluent discharge violations and the imposition of Consent Decree by the West Virginia Department of Environmental Protection (WVDEP).

Ackman et al., 2006

Biosolids quality, however, may be indirectly affected due to operational problems in

digesters caused by AD foaming. In a report that discussed digester cleaning requirements

after foaming, it was stated that the worst case scenario impact of foaming episodes involves

very poor VS destruction, coupled with low detention times, results in not meeting Class B

vector attraction and pathogen content requirements (Massart et al., 2009). Biosolids then

cannot be applied on land if Class B criteria are not met. This again leads to indirect economic

implications of additional costs to landfill the solids or a backup biosolids treatment system

(Massart et al., 2006).



It is estimated that up to 20% of a plant’s TS inventory can be trapped in the foam (De

Los Reyes et al., 1998). This greatly reduces a plant`s potential for solids recovery for land

application and/or fertilizer conversion. This is one of the implications of foaming affecting

sustainability. Another implication that is both an economic and sustainability issue is the loss

of biogas production due to foaming. Both sustainability and regulatory impacts are difficult to

quantify and convert to economic equivalents. Loss from biogas production and losses from

land application of biosolids could be measured and translated to a currency value.

2.15 Economic Impacts of Foaming

Quantitative impacts are mostly economic and consist of operational and financial

losses that are largely un-documented in published literature. Economic impacts considered

herein include components such as biogas loss, digester cleaning costs after a foam incident,

digester downtime/active volume loss, repair costs, and personnel costs for cleanup and

additional maintenance. AD foaming has a direct impact on the production of renewable

energy. Most qualitative impacts lead to economic losses (Pagilla et al., 1997; Westlund et al.,

1998a; Barber 2005). A WRRF in Sweden suffered from 40% biogas loss after a 10-week

foaming incident (Westlund et al., 1998b). Foaming incidents cause disrupted operations and

downtimes. While there are mentions of biogas loss due to foaming, there is a lack of literature

that quantifies and relates biogas loss to foaming incidents. Most WRRFs in a survey

mentioned that they did not measure biogas loss during the foaming events (Ganidi et al.,

2009).

Costs of cleaning, manpower, and energy costs due to digester downtime caused by

foaming will vary significantly between the various WRRFs. It was reported in a survey

though; the economic impacts were significantly reduced when the response to a foaming

incident was immediate. One WRRF in the UK reports to have spent £5,600 per week total for

cleaning and additional working hours of personnel to deal with foaming incidents (Ganidi et

al., 2009). Westlund et al. (1998b) reported a loss equivalent to 150,000 USD due to loss in

electricity production, extra personnel costs, increased oil consumption, and use of polymers

for dewatering due to AD foaming. These are the only two available reports that quantify

losses.

A recent case study attempted to compare and quantify the effects of digester cleaning

during normal and foaming events where a WRRF that had major foaming incidents and had

to be cleaned due to accumulation of grit and debris as a result of these incidents (Massart et

al., 2009). The study reports that a major part of the costs (54%) was repairs to the digester

including structural repairs, piping and painting. A biosolids grinder was also installed, though

it is not clear if it was direct impact of the foaming incident. The contractor costs for cleaning

the digesters was 32% of the total costs. Disposal was 14% of the total costs. Costs from other

WRRFs are not available for comparison. Economic impacts are more difficult to compare and

quantify on a common basis for all plants because they are case specific.

These case studies represent some of the very few published reports outlining the

economic impacts of foaming events. Costs depend greatly on a number of factors involving

the severity of foaming, manpower availability and the availability of technological equipment

for foam detection in the digesters (sensors, radars) which enable immediate response to

foaming.

2-44

Economic impacts of AD foaming have been gathered to a certain extent in the Task 3

full-scale study. This data collection addresses gathering economic cost data and estimates

from participating WRRFs during their foaming incidents, as seen from the impacts questions

in the survey (Appendix A). An economic framework for quantifying foaming impacts in

WRRFs could possibly be developed from this data.



CHAPTER 3.0

TASK 2 SURVEY OF FULL-SCALE PLANTS AND IDENTIFICATION OF KNOWLEDGE GAPS

3.1 Background and Introduction

There is a lack of full-scale AD foaming data from municipal utilities in the U.S as well

as those around the world, specifically the ones which experience AD foaming. Over the years,

there have been a number of surveys conducted in foaming AS systems, but the most recent

published full-scale AD foaming survey was conducted as far back as 1987 in the U.S (van

Niekerk et al., 1987). Other surveys conducted in AS foaming were in Germany (Soddell et al.,

1998; Eikelboom et al., 1998), Australia (Seviour et al., 1990; Blackall et al., 1991), France

(Eikelboom et al., 1998), and Czech Republic (Wanner et al., 1998). The latest AD foaming

survey published was in the UK (Ganidi et al., 2008). A summary of the results from AD

foaming surveys conducted thus far is given in Table 3-1.

Table 3-1. Comparison of Previous AD Foaming Surveys in Literature.

Location Reference No. of Plants|

Surveyed No. of Plants

Reporting Foaming

U.S. Filbert (1985) (ASCE Survey) 21 11

U.S. van Niekerk et al. (1987) 26 14

UK Ganidi et al. (2008) 17 9

Wisconsin, Illinois, Minnesota,

U.S. CSWEA, unpublished, (2010) 94 50

Wisconsin, Illinois, Minnesota,

U.S. CSWEA, unpublished, (2011) 41 15

U.S. This study 39 32

Spain This study 38 22

A survey of full-scale WRRFs which employed AD for sludge digestion and were

reportedly experiencing or experienced foaming was conducted in this study. The goal was to

obtain information beyond what was available in the published or grey literature. The survey’s

purpose was also to allow the project team to identify selected plants which could be targeted

for future full-scale studies. This section presents the survey findings, allowing the gaps/needs

for full-scale AD foaming investigations to be refined and presented for the subsequent tasks of

the study.

In the course of the literature review and in consultation with the members of the

project team, the specific gaps in knowledge regarding AD foaming were identified. The

survey was used to address gaps in the knowledge of AD foaming. The identified knowledge

3-2

gaps in AD foaming from published literature were reconciled with the survey responses, so

that the main causative factors and control/prevention options could be studied in full-scale

participating facilities.

The objectives of the survey were:

To determine the current status of full-scale AD foaming in WRRFs.

To reconcile gaps found in published literature with these full-scale plants.

To obtain a list of diverse plants with foaming issues to conduct full-scale studies.

To gather full-scale plant operational data to design full-scale studies.

3.2 Qualitative Analysis of Survey Responses and Treatment Processes and Configurations

During the period of May-July 2011, a total of 77 utilities in the U.S. and Spain were

surveyed by the project team. The survey form sent out to the plants is contained in Appendix

A-1.

Out of the 77 plants, 54 plants were affected by digester foaming in the past five years

– 22 plants in Spain and 32 in the U.S. The foaming occurrences were categorized as no

foaming, infrequent (where the utility experienced it as an anomaly), persistent (continuous),

and intermittent or seasonal. Table 3-2 presents the frequency of digester foaming.

Table 3-2. Occurrence and Intensity of AD Foaming Incidents in Plants in U.S. and Spain.

Frequency of Foaming*

Infrequent Seasonal

Intermittent Persistent No Foaming Winter Spring

14

6 1

15 7 29 Unspecified - 10

Total - 17

*Certain utility responses were classified as both seasonal as well as intermittent.

More than half of the cases with seasonal foaming were attributed to the presence of

foam-causing filamentous bacteria in the AS such as G. amarae and M. parvicella in both the

U.S. and Spain.

3.2.1 Primary Treatment

Primary treatment is for the settling of influent SS and the resulting PS is sent to AD for

stabilization. Apart from SS removal, primary settling also helps to remove FOG and other

surface active materials in the WRRF feed. Some literature claims that PS may be a causative

factor in AD foaming due to longer HRTs in primary settlers that could cause sludge

fermentation leading to VFA formation. Some of the VFAs produced during fermentation of

PS include acetic, propionic, iso-butyric, n-butytic, iso-valeric and n-valeric acids (Wu et al.,

2010). The researchers’ survey aimed to identify correlations between surface active agents in

the feed, presence of primary treatment facilities, and sludge holding tanks which might

facilitate sludge fermentation. The survey asked utilities if they had primary treatment

facilities.

Out of the 77 plants, 72 had primary facilities. Out of the five plants that had no

primary facilities, only three of them had foaming. Out of the three without primary clarifiers,



plants that experienced foaming, two reported that their foaming was due to G. amarae. It is

unknown at this point if there is a relationship between primary treatment and filamentous

foaming in these facilities or due to the presence of surface active agents in the feed. Since

both plants with and without primary clarifiers have AD foaming, there is no conclusive

relationship between primary treatment and digester foaming. Table 3-3 provides the summary

data to show the plants with and without primary treatment and AD foaming occurrence.

Table 3-3. WRRFs with Primary Treatment Facilities and AD Foaming.

Primary Treatment Facilities Number with AD Foaming

Number of Plants with Primary Treatment

72 43

Number of Plants without Primary Treatment

5 3

3.2.2 AS Process Configuration

The most common secondary treatment configuration is conventional AS followed by

high purity O2 (Pur-Ox) AS. Of the five Pur-Ox plants in survey, four reported G. amarae

foam. There are two previously published case studies of digester foaming in Pur-Ox plants –

one reports G. amarae foam and the other digester operational factors (Pitt et al., 1990; Pagilla

et al., 1996b; Parker et al., 2003; Ackman et al., 2006). It was previously reported that the

higher incidence of foaming in Pure-Ox AS process is due to foam trapping in the closed

reactor tanks and subsurface flow through the AS system (Pagilla et al., 1996a). Consequently,

the AS fed to the ADs results in foaming in ADs. One MBR and one rotating biological

contactor (RBC) plant were surveyed.

Out of all the plants surveyed, 28 plants do not nitrify and 40 plants do not remove P. A

total of nine plants used enhanced biological P removal (EBPR), out of which five reported

foaming. Out of the 32 plants that foam in the U.S., four plants attribute the root cause of their

digester foaming to their BNR operations as they did not have any foaming issues before their

startup. Four plants in Spain report AD foam after startup of BNR. It is also interesting to note

that one of the plants report AD foam after nutrient removal was started up, before which even

an AS foam episode did not cause foam in the digesters. These incidences are in line with

several reports from utility operators that have indicated that fixed film/RBC plants did not

foam or foamed much lesser than AS plants. Increased in sludge age has been implicated to be

one of the main reasons of foaming incidence in BNR plants that creates favorable conditions

for the filaments to grow in the aeration tanks, particularly, G. amarae.

Various published sources implicate the AS process itself to contribute to foaming,

mainly due to the presence of EPS such as lectin, that cause bioflocculation of the AS, which

affects the hydrophobicity and surface activity of the sludge solids, thereby influencing their

digestion properties (Park et al., 2009). One plant with MBR configuration was surveyed in

Spain that reported G. amarae foaming. Table 3-4 summarizes the AD foaming occurrence in

WRRFs as related to the AS secondary treatment type.

3-4

Table 3-4. Digester Foaming Based on Secondary Treatment Type.

Type of Secondary Process Total Number of Plants Number of Foaming Digesters

AS 69 49

Pur-Ox AS 5 4

MBR 1 1

RBC 1 0

Oxidation Ditch 1 0

3.2.3 Selective Foam Wasting from Secondary Treatment

Classifying selectors are used to control the population of foam causing organisms in

AS plants and prevent foaming in the solids treatment. Table 3-5 presents the number of plants

with classifying selectors or selective foam wasting and their relationship to foaming.

Table 3-5. Digester Foaming Based on Selective Wasting of Foam.

Classifying Selector/Selective Foam Wasting

Number of Plants with Selective Wasting

Yes 29

No 46

No response 2

In the U.S., only one plant with classifying selectors reported no AD foaming. In the

plants with classifying selectors that reported foaming, the leading cause reported was AS

filaments in both Spain as well as the U.S. It is possible that the classifying selectors can

reduce the foaming to a manageable level in the AS process, but not enough to eliminate AD

foaming or some of the plants may be wasting the foam directly to the ADs. The latter aspect

was not surveyed during this study. In depth details of the selector systems were not collected.

For example, classifying selectors need to be operated continuously to be successful in

eliminating the populations of foam causing filaments in the AS system. This occurs when the

foaming causing filaments are removed from the AS system at rates faster than their growth

rate. Hence, presence of a classifying selector or selective foam wasting from the AS process

seems to increase the likelihood of AD foaming due to seeding of filaments in the digesters

from the feed sludge.

3.2.4 AD Configuration Type

Single-stage mesophilic digesters were the most common digester configuration of

those surveyed. Two utilities had TPAD, none experienced foaming. One facility that has a

single phase thermophilic AD with the ability to go to acid phase followed by mesophilic

methane phase reported filamentous foaming. Two of the respondents had practiced staged

digestion in order to mitigate foam. A case of an acid digester foaming was caused reportedly

by various operational problems in both the liquid and the sludge treatment sides of the plant.

Table 3-6 represents the digester type and foaming data.

Table 3-6. AD Foaming Based on Digestion Type.

Digester Configuration Number of Plants Number of Foaming Digesters

Single-stage mesophilic 71 53

Single-stage thermophilic 1 1

Thermophilic/phased mesophilic anaerobic digestion (TPAD)

2 0

Acid phase followed by methane phase

4 4



While there exists contradictory literature about foaming in thermophilic versus

mesophilic temperatures, there are not enough cases of digester types in the survey to compare

these digesters with respect to foaming. The most common digester configuration surveyed

was the mesophilic digester. Table 3-6 indicates that a large number of mesophilic digesters

experienced foaming, and there is insufficient data regarding the other configurations to be

able to make significant observations. Acid phase digestion preceding methane phase does not

prevent AD foaming, and it may be a contributing cause of AD foaming in some plants where

VFAs are overloaded to the methane phase due to intermittent feed. This fact can be observed

from the data. All four acid phase followed by methane phase digester plants have experienced

AD foaming.

3.3 Causes of Foaming

As explained in Chapter 2.0, the causes of foaming can be classified into four groups:

sludge feed characteristics, digestion process-related characteristics, digestion operation and

operating conditions, and digester configuration, shape and physical features. The survey

responses for the causes of AD foaming cited by plants are discussed in this section.

Based on this classification of causes, the survey addressed the following specific

causes of foaming: G. amarae, M. parvicella, surfactants, FOG, VFA and unknown. There was

also an option to list out specific causes other than these listed choices. Many utilities took the

time to fill out their plant specific information. Table 3-7 shows the distribution of various

causes in the U.S. and Spain.

Table 3-7. Most Common Reported Causes of Foaming.

Most Common Causes of Foaming Number of Plants Reporting Comments

Filaments 19

Surfactants, FOG,VFA and feed quality 15

Other WAS feed – 3 Excessive gas production – 3

Operational issues – 5 Soilds overload – 1

Operational issues include: Poor heating – 1 Mixing issues – 3

Digester startup – 1

Unknown Cause 13

No foaming 29

The presence of foam causing filaments is a very common cause of AD foaming. The

other causes included BNR operation possibly linked to filaments as well, overfeeding/

inconsistent feed (OLR and OLR variations), biogas production (high), poor mixing, high

digester levels, bad process control, and bad digester design. Almost all of these reported

causes have been found widely reported in literature, in some capacity of contributing/causing

foam in ADs.

3-6

3.3.1 Sludge Feed-Based Causes

Feed-based causes include both the presence of surface active agents in the feed as well

as filaments. Both of these were addressed in the survey in separate questions.

3.3.1.1 Feed Quality

The utilities were asked to list their industrial inputs as well as hauled/trucked sludge,

in order to determine sludge feed quality. Table 3-8 presents the types of possible surface

active agents present in the feed in the surveyed plants. Out of the plants surveyed, 22 plants

had no trucked/hauled/industrial inputs. However, out of these 22 plants, only two plants had

no foaming. It is not possible to state if such sludge inputs may or may not be a contributing

cause of foaming though the majority of the plants not receiving hauled/trucked/industrial

sludge inputs experienced AD foaming.

Table 3-8. Number of Plants Receiving Different Types of Hauled/Trucked Waste.

Type of Hauled/Trucked Sludge Input

Hauled/Trucked Sludge Input (Number of Plants)

Septage 39

FOG 14

Food Waste 7

Landfill 12

Fats/Oils 3

Soaps/Surfactants 2

Other Industrial Waste 20

None 22

There are municipal plants suffering from foam where hardly any grease and oil input

can be detected. However, there are several plants with significant surface active material input

detailed from the survey. It is generally accepted that substantial FOG does contribute to

foaming (Ganidi et al., 2009; Muller et al. 2010). However it was not possible to correlate the

presence of G. amarae and/or M. parvicella to be due to the presence of surface active agents

in feed. Out of the 18 plants that reported foaming due to filaments, 13 plants had possible

sources of surface active agents in feed. Similarly, a majority of the plants with G. amarae

foaming had substantial sources of surface active agents in the feed.

3.3.1.2 Hauled Sludge

Out of the 77 plants surveyed, 22 plants had no trucked/hauled/industrial inputs. From

the survey responses, it was not possible to directly correlate these feed inputs with foaming

since other causes of foaming were also present. It is also significant to note that one of the

utilities that typically has a considerable amount of hauled/trucked inputs could not correlate

their major foam episodes to the feed quality, though they suspect one of their small foaming

events was caused by high carbohydrate sources in feed. Out of the nine plants in Spain, whose

reported foaming cause is feed-related, five of them cite VFA, FOG, surfactants, or some

chemical industrial input to be their cause of foaming, though this can only be verified after

testing the actual feed sludge during foaming events.

Another question in the survey related to primary treatment and surface active agents

where the hauled/trucked wastes were introduced in the feed. Based on the constituents of the

feed, it is important to know where they are introduced in the treatment train. In many cases,

the utilities directly feed their hauled wastes into their digesters, and hence there is no



secondary treatment. To evaluate such effects, a detailed analysis of the hauled/trucked

sludge is necessary. Table 3-9 details that the digesters experience foaming when

hauled sludge is fed directly or otherwise. It is not possible to conclude from this data

as the survey sample size is small.

Table 3-9. Point of Introduction of Trucked/Hauled Waste into WRRF.

Point of Introduction of Trucked/Hauled Waste

in Treatment Train

Number of Plants

Number of Foaming Digesters

Headworks of Plant/Prior to Screens 36 29

Digester 6 5

Septage Receiving Station 3 1

3.3.1.3 Sludge Holding Tanks

The literature survey determined that long HRTs in sludge holding tanks could

contribute to foaming. The fermentation of sludge in holding tanks leads to VFA formation.

Some of the VFAs produced during fermentation of PS include acetic, propionic, iso-butyric,

n-butytic, iso-valeric and n-valeric acids (Wu et al., 2010). In order to better understand this

behavior, the survey had two related questions: sludge holding tanks and mixing of PS and

WAS in holding tanks prior to feeding. Table 3-10 discusses holding tanks for both the U.S.

and Spain.

Table 3-10. Sludge Holding Tanks and AD Foaming.

Sludge Holding Tanks Number of Plants

Number of Plants with Digester Foaming

Yes No NA Yes No

Storage Tanks 39 34 4 10 23

PS and WAS Mixed during Storage

28 12 6 26 9

Out of the four facilities in the U.S. that reported feed-quality related causes (filaments,

surface active agents in feed) in the survey, two did not use hauled sludge tanks. In Spain, all

of the facilities that reported feed-quality causes used hauled sludge tanks, where sludge

trucked/hauled in separately was stored prior to feeding to the plant. In the case of non-

foaming plants, none of the plants in the U.S. had hauled sludge tanks. In Spain, half of the

foaming plants did not use sludge holding tanks.

3.3.2 PS:WAS Solids Ratio

The ratio of PS to WAS solids in the feed to digesters seems to be another important

feed-based characteristic causing or contributing to AD foaming. Foam mitigation in several

cases has been achieved by changing the PS:WAS ratio. In the survey, various utilities have

mentioned that their PS:WAS ratio was their main cause of foaming. Out of the seven plants in

U.S., whose cause can be categorized as feed-related, three of them cited that the WAS feed

was the main cause of foaming. Over all the utilities in both the countries, the percent WAS

solids in the feed sludge ranged from 0 to 80% for plants that feed mixed sludge. One plant fed

all PS and two of them fed all WAS. There seems to no optimal PS to WAS solids ratio to

minimize foaming as the nature of the sludge solids has a significant role in AD foaming.

Table 3-11 gives the distribution of % TWAS solids in the feed sludge as reported by the

foaming WRRFs in this survey.

3-8

Table 3-11. Distribution of TWAS Percent in Feed of Foaming and Non-Foaming Digesters.

TWAS Solids in Digester Feed (%)

Number of Foaming Digesters

Number of Non-Foaming Digesters

No WAS 2 0

0-10 8 4

11-20 0 0

21-30 4 2

31-40 5 0

41-50 9 3

51-60 2 0

61-70 3 0

71-80 1 0

81-90 0 1

91-100 0 0

All WAS 1 0

Unknown 21 11

Literature reports that foaming could occur if the percent of WAS to total sludge solids

exceeds 40% (Massart et al., 2006). Even though the survey results indicate that a majority of

the foaming plants operate at WAS content higher than 40%, data is not conclusive. Dedicated

WAS digesters are considered to have less foaming potential due to less digester gas

production rates, because WAS cells are more difficult to digest (e.g.,, Igoni et al., 2006). But

this aspect was not addressed in the survey and thus data was not available to verify this.

Hence, as has been discussed in previous sections, the ratio of PS:WAS solids ratio in feed

could influence foaming in many ways. The PS:WAS solids feed ratio effects have been

studied full-scale in Task 3 particularly in the plants not experiencing filamentous foaming.

3.3.3 Filamentous Bacteria in Feed Sludge

The most commonly found cause of foam was filamentous bacteria, in the U.S.,

predominantly G. amarae and to a lesser extent M. parvicella, while in Spain, M. parvicella

and G. amarae were equally present. Four utilities reported the presence of both filaments and

three reported both filaments and overfeeding; and one, filaments and mixing issues.

A majority of the digesters in the U.S. and Spain attribute their foaming to filaments

(Table 3-12). It was unclear whether the plants experienced AS foaming – only four plants in

total stated so and only two of these could correlate AS and AD foaming events. An important

observation gathered from these responses is that threshold for the filaments to cause foaming

is much lower in AD than in aeration tanks. It can be seen from Table 3-12 that only two out of

the 12 plants experiencing AD foaming due to G. amarae expressed that AS foaming was also

occurring. Microbiological data from all these WRRFs was not collected in the survey stage

but was carried out on the plants during full-scale study.

Table 3-12. Occurrence of Filamentous Foaming in AS and AD Processes.

Type of Filament

Number of Plants Reporting AD Foaming Due to Filaments

Number of Plants Reporting Foaming in AS (Corresponding to AD)

G. amarae 12 2

M. parvicella 2 1

Both 4 1

Other (or unknown) 1 NA

None 58 NA



3.3.4 Digestion Process-Related Characteristics

In Chapter 2.0, overloading of feed sludge was suggested as a cause of foaming

because it allows the presence of incompletely degraded organic material, temporary increase

in total biogas production, and probably the generation of hydrophobic and/or surface active

by-products that would promote foaming.

3.3.4.1 Feed Quantity

The distribution of the total number of plants with continuous and intermittent feeding

from Spain and the U.S. is presented in Tables 3-13 and 3-14.

Table 3-13. Distribution of Feed Frequency of Foaming Digesters. Feed Frequency Number of Plants

Continuous 24

Intermittent 26

N/A 7

Table 3-14. Distribution of Feed Frequency of Non-Foaming Digesters. Feed Frequency Number of Plants

Continuous 8

Intermittent 13

N/A 1

A total of 10 plants reported that their foaming was due to overloading of their

digesters. The exact values of their OLRs are not known at this point, as only the frequency of

digester feeding is reported. All of the foaming plants in both the U.S. and Spain were feeding

continuously. Out of the non-foaming digesters, the feeding frequency ranged between feeding

continuously to once every 24 hr in the U.S. and once every 0.5 to 3 hr in Spain.

This frequency of feeding is very important because in addition to OLR, at least three

case studies of full-scale WRRF reported foaming due to inconsistent loading of digesters

(Massart et al., 2006). OLR data was not collected from all survey respondents but reviewed in

all plants participating in Task 3.

3.3.5 Digester Operational Characteristics

The most important operational aspect with regards to foam appears to be mixing. The

survey covered mixing type and frequency of mixing. Both are discussed below.

3.3.5.1 Mixing

Though mixing has not been proven to be a direct cause of foaming, it is very clear

from previously published literature that it plays an important part in foam formation and

stability (Pagilla et al., 2002). Three U.S. utilities cited mixing problems as their cause of

foaming. Two US utilities had unmixed digesters, and both experience G. amarae foaming. In

Task 3, unmixed digesters were studied as a control for mixing studies.

Gas mixing provides favorable conditions for foam generation/stability because it is

suspected that the presence of bubbles in the bulk phase promotes attachment of the surface

active and hydrophobic compounds found in sludge onto the bubbles (Moen, 2003; Barber,

2005). It is widely accepted in the industry that gas mixing systems foam more than those that

3-10

are mechanically mixed. Contrary to this opinion, one utility mentioned that it mixed

continuously using sludge pumps, reportedly turning gas mixing on intermittently because it

aided in foam control. The reported cause of foaming in this plant is both G. amarae and M.

parvicella.

There were questions in the survey about the type of mixing and the mixing frequency.

From the responses, it was seen that there was a good distribution of gas and mechanically

mixed digesters. They have various mixing frequencies. The survey also covered the specific

type of gas or mechanical mixers. In the case of “other” mixing, most utilities in the U.S.

reported draft tube mixing whereas in Spain, it was the SCABA (mechanical submerged

mixer). In the case of non-foaming digesters, there was only one case of Cannon gas mixers in

the U.S., while the majority was pumped or jet mixers. In Spain, all of them used SCABA

submerged mechanical mixers except one which used gas lance mixing.

It is not possible to state conclusively whether gas mixing or mechanical mixing has a

higher incidence of foaming. It is possible that the dependence of foaming on the mixing type

may be a function of the mixing frequency as well as the cause of the foam, for instance,

filament presence. The distribution of the types of mixing is given in Tables 3-15 and 3-16.

Table 3-15. Distribution of the Different Types of Mixing in Foaming Digesters*. Mechanical Mixing Type Number of Digesters Gas Mixing Type Number of Digesters

SCABA (mechanical submerged agitation)

11 Perth 2

Internal draft tubes 8 Gas lances 8

Pump and jet mix 6 Bubble gun 1

Single impeller mixing with lightning mixers

1 Cannon gas mixing 4

Foam suppression mixing 1 Shearfuser 1

Heatmix (biogas injection) 2 Internal Recirculation 3

External mixing and pumping 2

No Mixing 2

Table 3-16. Distribution of the Different Types of Mixing in Non- Foaming Digesters *.

Mechanical Mixing Type Number of Digesters Gas Mixing Type Number of Digesters

SCABA (mechanical submerged agitation)

8 Diffuser 1

Mechanical agitation 1 Gas lances 4

Pump and jet mix 4 Bubble gun 1

Heat exchangers with gas recirculation

1 Cannon gas mixing 1

Draft tubes 2 Internal Recirculation 2

NA 3

*Some respondents reported more than one type of mixing.



3.3.5.2 Frequency of Mixing

All the utilities in the U.S. that experience foaming mix continuously. The unmixed

digesters did not report any foam incidents. Several utilities mentioned optimized mixing as a

general control measure for foaming irrespective of the cause. Detailed information was not

provided in the survey responses and has been collected for full-scale study plants.

A majority of the foaming digesters mix continuously, while a majority of the

intermittently mixed digesters do not experience foaming. Though the data are not statistically

significant, digester mixing frequency as well as the mixer type seems to influence AD

foaming. These distributions are shown in Table 3-17. One pertinent issue that was not

surveyed was the level of mixing intensity needed to achieve good digestion without wasting

energy for mixing and possibly contributing to foaming or its persistence, particularly, in the

case of gas mixing in AD.

Table 3-17. Frequency of Mixing.

Frequency of Mixing Number of Foaming Digesters Number of Non-Foaming Digesters

Continuous 40 14

Intermittent 1 7

None 2 0

Unknown 2 1

3.3.5.3 Digester Physical Features

The role of digester shape on AD foaming is unclear. There was a survey question

regarding the shape of the digesters. The majority of digesters surveyed were cylindrical, and

ESDs were not as common. One U.S. facility had both ESD and cylindrical digesters where

both types of digesters are heated with direct steam injection and mixed.

In Spain, all of the digesters were cylindrical. In the U.S., all of the ESDs have reported

G. amarae foam but for the utility with both cylindrical and ESD where the reported cause of

foaming was excessive biogas production. The Oceanside WPCP was a respondent in the

survey reported G. amarae foam and was a participant in the full-scale study. Table 3-18

presents the relationship between digester shape and foaming.

Table 3-18. Digester Shape and Relationship to AD Foaming.

Shape of Digester Number of Foaming Digesters Number of Non-Foaming Digesters

Cylindrical 44 29

ESD 4 0

3-12

3.4 Foaming Control Methods

As seen in earlier sections, control methods can be divided into (a) Sludge

Disintegration Methods, (b) Operational Modifications, and (c) Chemical Antifoaming Agents.

The results from the survey regarding foam control/mitigation methods are as follows:

3.4.1 Effectiveness of the Various Treatment Methods

Table 3-19 outlines the number of successes in control/treatment of foam across these

three groups of methods, including thickening which was listed as a mode of pretreatment. As

seen from the Table, several utilities have reportedly tried thickening for various causes with

some success.

Table 3-19. Effectiveness of Treatment Methods Based on Total Number of Foaming AD Plants.

Treatment Method

Number of Plants Reporting Treatment

Number of Reported Successes

Sludge Disintegration

Thickening 21 8

Staged digestion 6 2

Mechanical 7 3

Electrical 2 N/A

OpenCel 1 N/A

Ultrasonic 4 2

Thermal 7 5

Chemical lysis 1 1

Steam injection 1 1

Chemical Methods

Antifoam/defoamers 29 10

Chlorination of WAS 9 5

Coagulants, PAX 2 1

Bacteria and enzymes 1 NA

Modification of Operation

Reducing feed 1 1

Optimized mixing 17 8

Uniform sludge feed 30 19

Control of foam in liquid treatment 10 4

Biogas removal modifications 2 1

Decrease level in digesters 1 1

Use of antifoams/defoamants and defoamers is the other popular method of

treatment/control of foaming. These chemicals are typically used at the time of an event. Even

though a number of utilities reported successes with antifoams, a majority of them indicate that

frequent dosing is required. Two facilities in Spain indicated that their antifoams were effective

only for the first few days after dosing. The same was observed when they dosed with bacterial

enzymes as defoamers. This is in accordance with published literature where it is reported that

antifoams can also be used initially to control foaming, but it is not recommended for

continuous use as they are traditionally mixtures of hydrophobic liquids and solids that may

lose effectiveness if continually applied (Rossetti et al., 2005). The type of the antifoam

materials and their concentrations were not provided in the survey responses. Table 3-20

reviews the effectiveness of treatment methods reported in survey based on available literature.



Table 3-20. Review of Effectiveness of Treatment Methods Based on Reported Cause of Foaming.

Cause

Effectiveness of Control Method Review of

Treatment Method Successful Not Successful Results NA

Digestion Process-Related

Excessive gas production with insufficient surface area for gas to escape liquid volume.

Operation at reduced temperature; control of foaming in secondary treatment, optimized mixing, biogas removal modifications.

Defoamers; optimized mixing

Thickening From two of the three utilities that reported this cause, optimized mixing and biogas removal modifications worked. In the other utility, these mixing measures did not work. They are now implementing reduced temperature operation from 98 to 90°F.

Feed-Related

High carbohydrate hauled wastes.

Reducing feed, antifoams. This is based on working mechanisms of antifoams which can replace foam-forming components at the water surface and lower surface tension of liquids. However there is lack of data on antifoams used in full-scale plants for this cause.

WAS in feed. Reduce WAS to less than 25%; uniform sludge feeding and optimized mixing.

Antifoams and defoamers Staged digestion The WAS % may be a secondary cause. Some surface active components in the feed may be a primary cause. Foaming is reported to occur more frequently if WAS to total sludge exceeds 40%.

VFA, FOG, surface active agents in feed.

Mixing system was changed from internal gas recirculation to mechanical; defoamers, chlorination, PAX in biological liquid treatment.

This is in accordance with published literature that states gas mixing systems have been implicated to cause more foaming because it is suspected the presence of bubbles in the bulk phase promote attachment of the surface active and hydrophobic compounds found in sludge onto the bubbles (Moen, 2003; Barber, 2005).

Overload/ Inconsistent feed.

Defoamers, uniform sludge feed, optimized mixing, control of foaming in liquid treatment.

In three published case studies (Massart et al., 2006), uniform sludge feed, mixing alterations and limiting surface active agents were found to be effective for overloaded digesters.

Filaments

G. amarae. Lysis, defoamers, optimized mixing, thickening, uniform sludge feed, continuous feeding, biogas removal modifications.

Defoamers, Chlorination of WAS, thickening

Thickening, chlorination of WAS, control of foaming in liquid treatment

Defoamers are usually only effective for G. amarae foam breakup, but it is necessary to physically remove the foam containing most of the G. amarae, because these cells are hydrophobic and can cause foaming even if not alive. This could be a possibility of why defoamers were not effective in this case. Contrary to published literature, survey results report chlorination of WAS effective for G. amarae (Pagilla et al., 1998).

3-14

Table 3-20. Review of Effectiveness of Treatment Methods Based on Reported Cause of Foaming.

Cause

Effectiveness of Control Method Review of

Treatment Method Successful Not Successful Results NA

Generally chemical lysis using oxidants has not been successful in G. amarae foam control but it has been reported successful here. The exact method of the chemical lysis is not known.

M. parvicella Defoamers, antifoams, optimized mixing.

Thickening, thermal, defoamers, uniform feed, thermal, phased digestion, heating WAS to 170°F and holding for 4 hours to break down exocellular polymers, steam injection, and OpenCel.

Control of foaming in liquid treatment

There are contradicting cases here. A few utilities have reported that defoamers have been successful, while others have found it unsuccessful. In literature, M. parvicella was not found to be mitigated by antifoam dosages (Applied Tech et al., 2009).

Unknown Ultrasound, uniform sludge feed & optimized mixing, thickening, thermal, and mechanical pretreatment, defoamers.

Antifoam, uniform sludge feed, optimized mixing, chlorination of WAS, control of foam in liquid treatment, thickening, mechanical pretreatment, and staged digestion.

*Note: Plants were asked to list/select all methods they may have used. NA indicates results not known after the treatment method was implemented by the plants.



3.5 Impacts of AD Foaming

AD foaming impacts can be classified as qualitative and quantitative as discussed in

Chapter 2.0. Most of the utilities described qualitatively the impacts, but two utilities were able

to quantify the impacts with a rough estimate of their costs for foaming incidents.

Qualitative impacts are in agreement with the published literature, wherein utilities

report decrease in biogas production or difficulties in the collection of biogas. Decrease in

biogas may be direct – measured decrease in biogas production or indirect wherein biogas

decrease is attributed to problems due to feeding, mixing, reduced AD HRT, or gas collection

issues due to foam. Three cases of utilities reported that increased biogas production during

foaming. This relationship has also been found by Massart et al. (2006). The reason cited in the

literature was an overloaded digester where VS overloading caused the foam to entrap large

amount of solids fraction, which may have caused higher than average gas production. In the

survey responses, the reported cause was G. amarae in all of the three cases. The reason for

this increased gas production is unclear at this stage.

The utilities were asked to report economic impacts if available. There were 25 plants

in the U.S. and 16 plants in Spain that did not quantify their foaming incidents. The rest of the

utilities had made crude estimates of some aspects of costs associated with foaming. The only

utility that reported actual costs was from Spain. In this utility, there was a reduction of 10-

20% of the total plant generated energy which equates to approximately 1500kWH/day (with

0.1€/kWH) totaling to 150 € per day, for a plant of 8 million gallons per day (MGD) capacity.

The foaming events usually lasted for two to three weeks and utility estimated 2000-3000 € in

biogas losses. The plant did not provide any information about costs associated with cleaning,

maintenance, and antifoam dosing during these foaming events. The only published costs for

comparison from published literature report a 40% biogas loss after a 10-week foaming

incident (Westlund et al., 1998a). Table 3-21 presents various quantitative and qualitative

impacts as well as the major damages reported due to foaming in the surveys.

Table 3-21. Reported Qualitative and Quantitative Impacts and Damages Caused by Foaming.

Quantitative Impacts

Qualitative Impacts

Major Structural or Equipment Failures Caused by Foaming

Consumption of antifoam chemicals. Decrease of biogas production. Foaming dislodged a steel hatch that was bolted into concrete.

Contractual work for cleaning digesters.

Increase of biogas production. Broken seal on the fixed covers.

Loss of biogas. Issues of biogas collection. Gas system failures, fixed cover heaving due to foam pressure build up.

Labor time of 10 man hour/week for an intermittent event.

Operational issues of mixing, pumping and feeding due to foaming.

Broken seals and sight glasses on digester covers.

Overtime for personnel in severe foaming events.

Decrease in biogas due to decreased mixing, decreased feed and decreased digester detention times.

Digester cover structurally failed after repeated or severe foaming incidents.

Cost of fuel for facility and digester heating when digester gas cannot be captured and used.

Roof explosion due to severe foam.

Dystor gas collection system damage.

Breakdown of mixing compressors due to clogging of foam.

Blockages in the condensers caused by foam.

3-16

3.6 Summary of Survey Results

Similar to several previously conducted surveys (van Niekerk et al., 1987; Ganidi et al.,

2008) it was not possible to establish statistically significant relationships based on actual plant

data on conditions that may cause foaming. The survey responses helped compare full-scale

plants’ information with the several available sources of information on AD foaming. Most

gaps and needs from earlier knowledge based on literature still remain the same after the

survey in this study. Some of the observations are listed here.

Several of the survey respondents in both the U.S., as well as in Spain, reported that their

AD foaming episodes started or worsened when BNR processes were added to the facility.

The presence of foam causing filaments is the most common cause of foaming, in both the

U.S. (16 plants) and Spain (three plants). One of the significant observations was that the

threshold for the filaments to cause foaming is much lower in the AD than in AS.

In the U.S., the second most common reported cause of foaming was feed sludge quality

and the presence of FOG and other surface active materials in the feed to the digester.

However, it was not possible to determine relationships between surface active material in

feed sludge, point of introduction in the treatment stream and foaming, based on the survey

results. In Spain, overloading was second most common cause of AD foaming.

Most of the causes reported in Spain are similar to the ones in the U.S. and could be

classified into feed sludge and operational characteristics.

It was not possible to differentiate between the causes and contributing factors to the

foaming episodes in the plants surveyed.

A total of 13 plants across both countries report that their foaming cause is still unknown.

A majority of the foaming digesters mix continuously, while a majority of the

intermittently mixed digesters do not experience foaming. The effect of mixing intensity

and frequency on digester foaming still remains uncertain, although suggesting that more

mixing is not better with respect to AD foaming.

Defoamers, uniform loading, “optimum mixing”, WAS chlorination, and thickening are the

most popularly tried prevention/control/mitigation methods.

25 plants in the U.S. and 16 plants in Spain did not quantify economic impacts of AD

foaming.



3.7 Identified Full-Scale Study Parameters

Since gaps and needs from earlier knowledge still remain, full-scale investigation at

selected plants was deemed to be the way forward in this study. Parameters selected for full-

scale studies from survey responses studied in Task 3 are:

Modifying different OLRs for full-scale digesters in an attempt to determine the role of

OLRs on AD foaming.

Modifying WAS content in digester feed to determine the effect of PS:WAS solids ratio on

AD foaming.

Correlating effects of feed and mixing of digesters - profiling the depth of digesters to

determine their homogeneity and “localized overloading” when they may not be

overloaded from a VS perspective.

Studying foaming in full-scale utilities where the reported cause of foaming is non-

filamentous.

A considerable number of utilities were successful in controlling foaming with

antifoams, which was tested in a full-scale plant in this study.

3.8 Knowledge Gaps

In spite of the widespread nature of AD foaming problem in WRRFs and the work that

has been conducted on AD foaming control, both through limited research and in-plant

observations and practices, definitive solutions for AD foam prevention and mitigation have

not been developed. AD foaming remains a ubiquitous operational problem. Control methods

established to this point have been either site-specific or problem specific, and do not work all

the time. To obtain more insight of foam control, both of these aspects need to be integrated.

The first step in this regard is to reconcile the knowledge gaps from the critical review of data

from the surveys conducted and the reported information from the literature. The survey in the

study provided an increased and up-to-date knowledge about the relationships or lack thereof

between the process, operational parameters, and the primary and secondary causes of foaming

in full-scale plants that is needed to better understand the complex foaming phenomena in AD

and its prevention and control.

In the course of the literature review and review of survey responses, and consultation

with the operational staff of the various WRRF utilities and experts of this project team, the

following specific gaps in knowledge regarding AD foaming were identified and are described

in Table 3-22.

3-18

Table 3-22. Identified Knowledge Gaps in AD Foaming.

Identified Knowledge Gap

Reconciliation with Survey Responses Comment

Classification of transient and metastable foams based on severity and frequency.

Utilities with transient, seasonal & persistent foaming have been identified from survey responses.

Data from full-scale WRRFs is lacking that correlates these types of foams with feed and process characteristics.

Practical foam detection techniques.

Methods have to be developed based on a large sample of plants and techniques from other fields.

The aeration test continues to be used widely though not an accurate indicator of full-scale AD foam. Some in-situ methods are being used with limited success.

Effect of solids content on three-phase foam stability.

Utilities operating at different TWAS % and thickening aspects were identified and selected for full-scale study.

Effect of solids content on foam stability is based on the type of digester foam and specific mechanisms.

Determination of surface active constituent concentrations in digesters (feed and contents) in an attempt to determine threshold concentrations that may cause foaming.

Utilities with possible feed quality related causes of foaming were selected for full-scale studies.

Surface tension of samples from WRRFs that have established or suspect their foaming is due to surface active agents were identified for full-scale study.

Effect of point of introduction of hauled/trucked-in wastes in the process stream and their type and properties.

At this stage, any effect cannot be established without full-scale studies.

Estimating foaming potential of samples from WRRFs that pump hauled waste into different stages in the treatment train would be useful.

Establishing an optimum ratio for PS to WAS (if it exists) that promotes foaming.

Digesters with different PS:WAS ratios as well as digesters operating with all WAS or PS were selected for further study.

Studying relation between foaming incidents and PS/WAS ratio in the WRRFs surveyed. PS and thickened WAS characteristics, as well as total and VS concentrations were monitored, to track daily variations in OLR to the digesters.

Extent of persistent foaming problems versus episodic foaming incidents in plants.

Survey responses helped select such utilities. Full-scale data analyses are required to determine if the foaming problems are persistent throughout the year/season or just periodic episodes of short duration.

In the case of combined sludge, (a) studying effect of holding tank residence time on foaming, (b) studying the effects of mixing PS & WAS in storage tank, specifically in consideration of increased HRT and VFA production, as a cause of foaming.

It was not possible to identify significant relationships from survey responses.

Bypass of sludge holding tank was studied full-scale to study effects of increased downstream HRT and possible VFA concentrations.

Feed microbiological thresholds and generation of surface active compounds by filaments.

Microbiological analyses conducted using samples from different plants in full-scale study since most plants do not conduct such analyses normally.

Estimating foaming potential of feed in WRRFs that have established or suspect their foaming is due to filaments.



Identified Knowledge Gap

Reconciliation with Survey Responses

Comment

Investigating optimum mixing speed, frequency and type of mixing responsible for foaming.

Survey could not correlate type of mixing and frequency with foaming.

Correlating full-scale foaming WRRF data with type of mixing, frequency and consistency to establish relationships. Unmixed digesters were also studied as control.

Impact of temperature fluctuations on digester performance and foaming.

Digesters that changed their operational temperature will be studied. Full-scale data in this regard has been found to be almost non-existent from the surveys.

Temperature profiling of digesters; full-scale studies of digesters operating at different temperatures; analyzing operational data at reduced digester temperature have been carried out in full-scale studies.

Effects of biogas withdrawal rate or gas phase pressure on foaming.

Not addressed in survey. Full-scale kinetic studies on biogas production; compare VS loading to instantaneous digester gas production.

Effect of different defoamers/antifoams on foaming and digester performance.

Survey has provided certain conflicting responses in some cases with respect to antifoam efficiency for controlling foam.

Document different chemicals/doses/application rates used to control foaming and investigate through full-scale studies any toxic effects on digestion process itself. Full-scale test of effectiveness of antifoaming chemicals.

Effectiveness of in-situ foam suppression/ destruction to control/prevent foaming.

Full-scale data in this regard has been found to be non-existent from the surveys.

Full-scale testing of in-situ foam suppression has been carried out in full-scale study.

Quantify economic impacts due to AD foaming in full-scale plants, using economic data gathered from various plants, assessing costs of cleaning, biogas production loss, prevention/control costs, damage and other maintenance costs due to foaming incidents.

Very few utilities identified and estimated economic impacts due to AD foaming.

Gathering full-scale economic data from plants and developing economic metrics from plants of similar size and capacity for various effects such as biogas production, cost of mitigation etc resulting in an economic framework to estimate impacts for future study.

3-20

3.9 Next Steps – Task 3 Full-Scale Study

In order to fill the knowledge gaps, the next step of this study was to gather full-scale

operating data of the selected utilities. Utilities selected for full-scale studies were based on

their process diversity, foaming cause, availability of resources for full-scale demonstrations,

and interest in further participation. Results of the full-scale study and protocol for full-scale

testing and data collection are presented in a companion report and its Appendix A-2, Task 3

Full-Scale Study Methods.

3.10 Summary

The survey was the second step (Task 2) in a systematic review of full-scale data

available, following the critical review of published information. This survey helped gather

information on current status of AD foaming full-scale. Based on the information available on

AD foaming causes, methods of determination, impacts, and prevention and control, this report

also helped extract useful and unified knowledge to effectively identify knowledge gaps. Since

several of these gaps still exist, full-scale investigations at a select group of plants with varying

configurations and operational practices are the most effective steps forward to study critical

knowledge gaps. Such full-scale studies are discussed in the following chapters.


Literature Review and Survey A-1

APPENDIX A-1

FULL-SCALE PLANT SURVEY QUESTIONNAIRE

Note: Appendix A-2, Full-Scale Study Methods, can be found in the

Full-Scale Studies companion report.

A-2

Anaerobic Digester Foaming – Prevention and Control Research Existing Facilities Questionnaire

WERF INFR1SG10

A. Contact Information:

(This information will be kept confidential within the research team)

1. Please provide: a. Facility (WRRF) name

b. Owner/operating company Facility location (City and State) d. Your name e. Your email f. Your phone number with area code_

c. Your Job title

B. Facility Information:

2. What is the current average daily flow in million gallons per day (MGD) of this WRRF?

3. Type of Influent?

Domestic flow, MGD Industrial flow, MGD

List major industrial contributions

4. Specify trucked-in wastes accepted at

the facility: Septage

Agricultural (specify type: ) Fat, oil, and grease (FOG)

Food Processing Plants / Food waste

Landfill

Sources high in fats

and oils Sources of

soaps and surfactants

Industrial Waste

(specify type)

Other None

Where are they introduced?

Comments:

Page 1 30-Mar-11



Anaerobic Digester Foaming – Prevention and Control Research

5. Is primary sedimentation utilized at the plant?

Yes

No

6. What is the secondary biological process used at your plant?

Activated sludge

Pure Oxygen Activated Sludge

Trickling filters MBR’s

Oxidation ditch

Stabilization ponds

Other

Does the plant practice foam wasting (selective wasting) from the secondary system?

Yes

No

7. What type of anaerobic digestion system do you have?

Single stage mesophilic (may be followed by unheated secondary digesters) Single

stage thermophilic

Thermophilic/phased mesophilic anaerobic digestion (TPAD)

Acid phase followed by methane phase

Other (specify)

Quality of final sludge?

Class A

Class B

Other

8. What is the total digester detention time? (Do not include unheated secondary digesters)

1 to 10 days

11 to 20 days

21 to 30 days

>30 days

A-4


9. Does your plant remove nitrogen?

Yes – Ammonia nitrogen

Yes – Total Nitrogen

No

10. Does your plant remove phosphorus?

Yes – Chemical Iron salts,

Aluminum salts,

Other (specify)

Yes – Enhanced biological

Yes – Enhanced biological and chemical

Iron salts,

Aluminum salts,

Other (specify)

No

Comments on your Facility:

C. Foaming Information:

11. Has your facility experienced digester foaming in the last 10 Years?

Yes

No (Skip to question #20, page 5)

12. When was the most recent foaming event?

Within the last week

Within the last month

Within the last year

Longer than 1 year ago

Page 3 30-Mar-1




13. Describe the frequency of foaming.

Persistent

Intermittent

Seasonal

Other (specify)

14. Describe the severity of foaming (check all that apply):

Just a Nuisance Unsafe

Predictable Unpredictable

Manageable Difficult to manage

Insignificant cost Significant cost

15. Please state level of foam in your digester.

10% – light surface foam

25% – surface foam accumulates in the corners

50% – at least half of digesting sludge volume occupied by foam

70% – at least 2/3 of digesting sludge volume occupied by foam

100% – foam throughout the entire digesting sludge volume and overflowing digester

D. Impacts of Anaerobic Digester Foaming:

16. Is biogas generation affected by foam events?

Yes. Explain

No

Don’t know

17. Has your utility ever quantified the economic impacts of foaming events?

Yes. Explain_

No

18. Has anaerobic digester foaming caused any significant structural or equipment failures?

Yes. Explain

No

Comments on Foaming:

A-6


E. Causes of Anaerobic Digester Foaming:

19. To what do you attribute the cause of anaerobic digester foaming?

“Nocardia”

Microthrix parvicella

Surfactants Fat, oil,

grease Volatile fatty

acids

Other (specify)

Don’t know

20. Feed sludge type?

Primary Only

WAS only

Primary + WAS

%WAS on TSS basis

Other

21. Sludge feeding method?

Continuous

Intermittent every hours

22. Do you use a holding tank to store sludge prior to feeding it to the digester?

Yes

No

If “Yes”, are primary sludge and WAS mixed during storage?

Yes

No

23. Digester shape?

Cylindrical

Egg-Shaped (ESD)

Other (specify)

Page 5 30-Mar-11




24. Digester mixing?

None

Cannon gas mixing

Gas lances

Pumped or jet mixing

Draft tube mixing

Other (specify)

25. Mixing frequency?

Continuous

Intermittent

Other (specify)

26. Digester cover type?

Fixed

Floating

Comments on Causes of Foaming:

Continue to next page for pre-treatment, prevention and control.

A-8


F. Prevention and Control:

27. Have you tried any of the following sludge pre-treatment processes to mitigate anaerobic digester

foaming? (check all that apply)

• Thickening

• Thermal

• Mechanical

• Chemical Lysis

• Staged Digestion

• Electrical

• Thermal Hydrolysis

• Direct Steam Injection

• Pasteurization

• Electric-Pulsing (OpenCel)

• Crown Sludge Disintegration System

• Ultrasonic Cavitation

• High-Pressure Homogenizer

• Ball Mill

• Other (specify)

Successful Not Successful Results not known



28. Have you tried any of the following chemical additions to prevent/control anaerobic digester foaming?


• Anti-Foam (prevention)

• Defoamers (control)

• Coagulants

• Chlorination of WAS

• Other (specify)

29. Have you tried any of the following methods for preventing/controlling anaerobic digester foaming?


• Uniform Sludge Feed

• Optimized Mixing

• Control of Foaming in Liquid Treatment

• Biogas Removal Modifications

• Other (specify)

A-10


G. Conclusion:

30. Would you be willing to allow the research team to contact you for follow-up information?

Yes

No

31. Additional Comments or Suggestions:

Thank you for your time!


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