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The Biosolids Odor Reduction Roadmap (BORR): A Decision Tool to Reduce Cake Odors
1
Presented at
Dewatering 201: Technology, Evolutions, and Trends A webinar sponsored by the Water Environment Association of Texas
by
Bob Forbes, P.E.
Regional Technology Leader
CH2M HILL
Charlotte, NC
WERF Biosolids Odor Projects History/Overview
• Phase 1 (2000-2003): An extensive literature survey of odors in the municipal wastewater environment. Product: Literature Search & Review (00-HHE-5A)
• Phase 2 (2001-2003): Explored the causes of odors from biosolids based on field studies. Product: Impacts of In-Plant Parameters on Biosolids Odor Quality (00-HHE-5T)
• Health Effects Addendum (2001-2004): Reviewed evidence surrounding potential health effects of biosolids odors. Product: Health Effects of Biosolids Odors: Literature Review and Analyses (00-HHE-5C)
• Phase 3 (2004-2007): Increased our understanding by developing more quantitative odor reduction measures. Product: Biosolids Processing Modifications for Cake Odor Reduction (03-CTS-9T)
• Phases 4a, 4b, 4c (2007-2010): More laboratory-scale studies and development of Biosolids Odor Reduction Roadmap (BORR)
• ROSI (Regrowth, Odors, and Sudden Increase, 2010-2012): Connected biosolids odors with pathogen-indicator regrowth and sudden increase phenomena. Led by Bucknell University, with DC Water, Virginia Tech, CH2M HILL & others.
Phase 2 (Biosolids Odors) was a Field Study of 11 WWTPs across
North America
WAS Thickener & Storage
A
Primary Thickener & Storage
C1
Anaerobic Digester
Dewatering
Process
Conveyance System
E
B
C2
G
H F
I
D
Cake Odors are the Worst in the Biosolids Processing Chain
0
100
200
300
400
500
600
700
800
900
A B C1 C2 D F1 F2 G I1 I2
Sampling Locations
Me
an
Pe
ak
Su
lfu
r
Co
nc
en
tra
tio
n (
mg
S/m
3)
H2S Methane thiol Total sulfide species
Primary & Secondary
Clarification/Thickening
Digester
Feed
Liquid,
Digested
Solids
Dewatering
Cake
Conveyance,
Storage
Some of the Findings from WERF Phase 3
Research on Biosolids Odor Reduction: • Odors from anaerobically digested and dewatered biosolids correlate strongly with
total, volatile organic sulfur compounds (TVOSC) emissions
• Bioavailable protein is the primary precursor of TVOSC emissions from digested,
dewatered biosolids
• Odors and TVOSC emissions tend to rise and fall over time (usually 2-3 weeks)
during cake biosolids storage
• Longer sludge-residence times (SRT) in mesophilic anaerobic digestion result in
lower TVOSC emissions from biosolids
• Higher shear during dewatering results more bioavailable protein and higher TVOSC
emissions from dewatered biosolids
• Addition of metal salts in appropriate amounts pre- or post-dewatering can help bind
bioavailable protein and reduce TVOSC emissions
Case Study: Electro-dewatering Trials
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EIMCO’s Cinetik Electro-
dewatering unit was piloted on
digested, dewatered cake
from two Virginia WWTPs in
April 2010
The electro-dewatering unit
increased cake solids from 18-
19% TS range to 25-35%%TS
range in 7 minute to 10 minute
batch times
Case Study: Electro-dewatering Trials
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Tab# Sample
Log E. coli
MPN/g DS
1 Plant 1 Electro-dewatered Cake <0.97
2 Plant 1 Centrifuge Cake 6.35
3 Plant 2 Electro-dewatered Cake <0.97
4 Plant 2 Centrifuge Cake 7.91
5 Plant 2 Cinetik Cake + Plant 2 Centrifuge Cake 6.55
Pathogen Indicator Results:
•Electro-dewatered cake showed 3-4 log reduction of fecal coliform
and 6-7 log reduction E. coli
• Salmonella also < 2 MPN/ 4 gm TS meets Class A pathogen
reduction criteria
•Electro-dewatered cake was susceptible to FC and E. coli regrowth if
contaminated
Case Study: Electro-dewatering Trials
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Odor Results
•In general, the electro-dewatered cake had noticeably higher and
more persistent odors than centrifuge cake
•Odor levels ranged 50% to 100% higher from electro-dewatered cake
after one day of storage, and remained higher for 80+ days
•Hedonic tone of dewatered cake
and electro-dewatered cake were
roughly the same (-6 to -7 range)
The BORR Compiles Information from a Decade of
WERF Research on Biosolids Odor Reduction:
• Primary goal is to produce dewatered biosolids cakes that have
less odors, through reducing bioavailable protein by:
– Better Digestion
– Chemical addition
– Reduced shear during dewatering and handling
• These approaches are complementary, and several may be
needed together for significant odor reduction in biosolids
• Next slide translates these objectives into a matrix or “roadmap” of
options to consider
Decision Matrix
Potential Reduction Measures for Biosolids Cake Odors
Operational Decisions Design Decisions
Process Equipment
Chemical Addition
Anaerobic Digestion
Digester pretreatment Improve mixing
Increase VS reduction
Operate in series
Multiple phases
Increase temperature
Other?
Dewatering & Cake Conveyance/ Storage
Decrease centrifuge bowl speed
Decrease centrifuge torque
Increase cake moisture content Consider BFPs
Belt Conveyors
Cake storage/aging
Other?
Pre-Digestion
Aluminum
Iron
Other cations
pH control CEBAs
Other?
Pre- or Post-Dewatering
Aluminum
Iron
Other cations
pH control Polymer dose and type
CEBAs
Other?
Anaerobic Digestion
Digester pretreatment Better mixing
Longer SRTs
Egg-shaped
Acid-Gas (multi-phase) Thermophilic
Temperature-phased
Other?
Dewatering & Cake Storage
Belt Filter Press
Rotary Press
Variable-speed centrifuges
Adjust cake solids
Minimize conveyance
Belt conveyors
Enclose cake storage areas
Other?
Each decision
must consider
any other direct
or indirect
impacts of
design and
operational
changes on
wastewater
treatment and
solids handling
processes
Secondary Treatment
Anaerobic Digestion
Dewatering
Final Handling
Pre-Secondary
Treatment System
Nutrient
Removal?
VSR >50%
?
High Shear
?
On-site
Storage?
Al/Fe
Addition?
Al/Fe
Addition?
Al/Fe
Addition?
YES
NO
NO
NO
YES
YES
NO
NO
NO
NO
YES
YES
YES
YES
Digestion pre-treatment
Pre- or post-dewatering
Fe/Al addition
Polymer dose and type
adjustment
Optimize cake solids
content
Digestion pre-treatment
Improve digester mixing
Reduce digester loading
Decrease centrifuge speed/torque
Consider BFPs or rotary press
Pre- or post-dewatering Fe/Al
Optimize cake solids content
Consider extending storage time
Digestion pre-treatment
Improve digester operation
Multi-phase digester operation
Minimize dewatering shear
Pre- or post-dewatering Fe/Al
addition
Optimize cake solids content
Consider extending storage time
Optimize chemical addition
Improve digester operation
Multi-phase digester operation
Provide pH control
Minimize dewatering shear
Optimize cake solids content
Consider extending storage time
Al/Fe
Addition?
YES
NO
Raw Wastewater
Decision Tree for Reducing Biosolids Odors
Decision Tool • The decision tool prompts the user to the following
subject areas for guidance: – Biosolids odor sampling and testing
– Treatment and handling options for reducing odors prior to digestion
– Effects of digestion process variables on odors
– Effects of dewatering process options on odors
– Chemical addition approaches
– Other operational suggestions
– Design suggestions
http://borr.werf.org
Case Study – Sydney Australia
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•Problem – Digested, dewatered biosolids causing odor
complaints when hauled through a resort area
•Causes Identified
•Digestion SRT <20 days
•High %VS in digested solids
•High-shear centrifuges
•Shear in screw conveyors
•Cake storage of 3-5 days
Case Study – Sydney Australia
15
Recommended Odor Mitigation Measures – Short Term
•Improve sludge digester feed - Give it more consistent quality
(mostly %TS and %VS) and more consistent feed rate (improved
results)
•Decrease the duration of dewatered cake storage: Odors increase
in 3-5 day period, so load and transport biosolids after < 1 day storage
time when possible (improved results)
•Reduce the speed of centrifuge and optimize polymer dose: So
far these adjustments have shown mixed results
Case Study – Sydney Australia
16
Recommended Odor Mitigation Measures – Mid-term
•Improve digester-feed thickening process: An optimization project
is proposed to improve the reliability and performance of the Rotary
Drum Thickeners of the digester feed
•Recuperative thickening digestion: This process aimed to
increase the digester solids retention time by increasing the solids
content in the digesters (in the planning phase)
•Increase capacity of screw conveyors to reduce conveyor
speed and shear: A belt conveyor system was desired but is not
possible due to space constraints. However, the existing conveyer
system is being replaced with a higher capacity system so it can run
at slower speed
Case Study – Sydney Australia
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Recommended Odor Mitigation Measures – Long Term
These measures target improvements to long term efficiency,
reliability, and redundancy of anaerobic digestion. They would
likely provide the most substantial reduction in frequency and
potential of odorous biosolids. They include:
•Build an additional mesophilic digester
•Convert to temperature-phased or two-phased (acid-gas) anaerobic
digestion
Both options are costly and not being considered at this stage.
The Biosolids Odor Reduction Roadmap (BORR): A Decision Tool to Reduce Cake Odors
18
Presented at
Dewatering 201: Technology, Evolutions, and Trends A webinar sponsored by the Water Environment Association of Texas
Questions?
