Biological Treatment

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Jae K. (Jim) Park, Professor

Dept. of Civil and Environmental Engineering

University of Wisconsin-Madison

Treatment Objectives

1980 to 2000Removal of toxic compounds and nutrients (N & P)

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Early 1970s to 1980Based on aesthetic and environmental

concernsBegan to address nutrient removalImproved treatment efficiency and

widespread treatment of wastewater

1900 to early 1970s Removal of suspended and floatable materialTreatment of biodegradable organicsElimination of pathogenic organisms

21st CenturyEndocrine disrupting chemicals (EDCs) and

other synthetic compounds, emerging pathogens, etc.

Minimum National Standardsfor Secondary Treatment

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Parameters Units 30-day ave. conc. 7-day ave. conc. BOD5 mg/L 30/45a 45/65 Suspended solids mg/L 30/45a 45/65 Hydrogen-ion conc. pH units 6~9b 6~9b

Carbonaceous BOD5c mg/L 25 40

a Average removal 85%b Only enforced if caused by industrial wastewater or by in-plant

chemical additionc May be substituted for BOD5 at the option of the

National Pollution Discharge Elimination System (NPDES) permitting authority

Kinetics of Biological Growth

Lagphase

log (# ofcells)

Log (exponential)growth phase

Stationary(declining)

phase Death (endogenousdecay) phase

t

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Dispersed growthCause: Microorganisms do not form but are dispersed, forming only small clumps or single cells.

Effect: Turbid effluent. No zone settling of sludge.Slime (jelly) viscous bulking; or non-filamentous bulking Cause: Microorganisms are present in large amounts of exocellular slime. In severe cases, slime imparts a jelly-like consistency to the activated sludge.

Effect: Reduced settling and compaction rates. Virtually no solids separation in severe cases resulting in overflow of sludge blanket from secondary clarifier. In less severe cases a viscous foam often is present.

BulkingCause: Filamentous organisms extend from flocs into the bulk solution and interfere with compaction and settling of activated sludge.

Effect: High SVI - very clear supernatant. Low RAS and WAS solids concentration. In severe cases, overflow of sludge blanket occurs. Solids handling processes become hydraulically overloaded.

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Activated Sludge Separation Problems (1)

Settling Problem in Activated Sludge Processes

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Activated Sludge Separation Problems (2)

Pin floc or pinpoint flocCause: Small, compact, weak, roughly spherical flocs are formed, the larger of which settle rapidly. Smaller aggregates settle slowly.

Effect: Low SVI - a cloudy, turbid effluentBlanket rising Cause: Denitrification in secondary clarifier releases poorly soluble N2 gas which attaches to activated sludge flocs and floats them to the secondary clarifier surface.

Effect: A scum of activated sludge forms on surface of secondary clarifier.Foaming/scum formationCause: Caused by non-degradable surfactants and by the presence of Nocardia spp. and sometimes by presence of Microthrix parvicellar.

Effect: Foams float large amounts of activated sludge solids to surface of treatment units. Nocardia and Microthrix foams are persistent and difficult to break mechanically. Foams accumulate and can putrefy. Solids can overflow into secondary effluent or overflow tank free-board on to walkways.

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Floc Formers

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Foaming

Nocardia spp.

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Life

1. Matter: H, O, C, N, P, S and minor elements

2. Energy• Solar radiation: Photo-synthetic autotrophs• Organics: Heterotrophs• Inorganics: Chemoautotrophs

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More plant-like(producer)

More animal-like(consumer)

Gets carbon from atmosphere

Gets carbon from organic materials

Autotroph Heterotroph

Classification of Microorganisms

AnimalsRotifers

Crustaceans

PlantsMossesFerns

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Eucaryotes: multi-cellular

Algae

Procaryotes: “before nucleus”, unicellularBlue green algae

Fungi Protozoa

Bacteria

Mic

roor

gani

sms

Redox Reaction

Organic molecule (C+H+O+N+S+P)(wastewater, hazardous chemicals,

etc.)

CO2, H+, and e-

A molecule

Oxidized

Reduced

Electron donor

Electron acceptor

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Aerobic Condition

Aerobic respirationO2 presentElectron acceptor: O2 (→ H2O)Good for large volumes of dilute

wastewater (< 500 mg BOD5/L)High growth rates, thus high

sludge production (0.3~1 lb VSS/lb BOD5)

Produce a more stable end product

O

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Anoxic Condition

Anaerobic respiration (denitrification)

No dissolved oxygenNO3

- and NO2- present

Electron acceptor: NO2- and NO3

- (→ N2 + H2O)

Relatively high sludge productionShould be avoided in the clarifier

AX

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Fermentation No O2, NO3

-, NO2-, or SO4

2- present Electron acceptor: endogenously generated

by the microorganism Good for concentrated wastes (> 1000 mg

BOD5/L) Low sludge production

Complex organic compounds

Low molecular weight fatty acids

CH4, CO2, and H2O

Anaerobic Condition AN

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Biological Treatment: Conditions Required to Remove Target

Compounds

Organics (carbonaceous energy)Aerobic, heterotrophs (aerobic oxidation)

Ammonia (nitrogenous energy)Aerobic, chemoautotrophs (nitrification)

Nitrite/nitrate (nitrogenous energy)Anoxic, heterotrophs (denitrification)

Phosphorus (nutrient)Anaerobic, heterotrophs (luxury uptake)

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SVI MeasurementOne-liter graduated cylinder30 minute settling periodSVI = (mL/L)/(g/L) = mL/g, i.e., volume

occupied by one gram of settled solids

1-L

mL

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SVI Example CalculationSettled sludge vol.: 260 mL/LMixed liquor Suspended Solids (MLSS): 2,000 mg

TSS/L = 2 g/LmL/L 130

Lg 2

LmL 260

SVI

< 75 Excellent75 < SVI < 120 Good120 < SVI < 250 Poor

> 250 Bad, bulking

t = 0 10 min 20 min 30 min

260 mL

Bulking

sludge

Well settled sludge

Aeration basin