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Causes and Control of ActivatedSludge Settleability Problems

Randal W. Samstag

Civil and Sanitary Engineer

Outline

• Why is settleability important?

• Causes of poor settleability

• What can be done about poor settleability?

• A word about foaming

• Some case studies

• Conclusions

• Credits

Why is Settleabilty Important?

• All activated sludgeprocesses requireorganism separation

• Settleability ofactivated sludgedramatically affectsplant capacity

• Filaments canimprove membranefilterability

Maximum Overflow Rate as Function of SVI(Initial TSS = 2,000 mg/L)

0

1000

2000

3000

4000

5000

6000

0 50 100 150 200 250 300 350

SVI (mL/g)

Ma

xim

um

Ov

erf

low

(gp

d/s

f)

Wahlberg - Keinath Original Daigger-Roper

Revised Daigger-Roper Pitman

Aberdeen, SVI = 225 Renton, SVI = 200

Bellingham, SVI = 200 CWS Rock Creek, SVI = 85 mL/g

West Point, SVI = 225

Primary Causes of Poor Settleability

• Filamentous Organisms

• Slime

• Low floc density

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Conditions Typically Associated with SpecificFilaments

• Low DO

• Short SRT (High F/M)

• Long SRT (Low F/M)

• Elevated VFA

• Septicity

• Nutrient Deficiency

• Low pH

Low DOType 1701

• Straight, smoothly curved, orbent

• 10 to 150 μm long / 1.0 μm width

• Gram-negative

• Neisser-negative

• Cell septa

• Encouraged by– Complete mix

– Readily biodegradable substrates(rbs)

– Low DO!

• SRT 2 – 20 days

• Selectors reported effective

Short SRT (High F/M)Type 1863

• Oval cells

• 10 – 50 μm long by 0.8 to 1.0 μm diameter

• No sheath

• Gram-negative and Neisser-negative with Neisser-positivegranules

• SRT < 2.5 days

• Like oil and grease

• Selectors not effective

Long SRT (Low F/M)Microthrix Parvicella

• Coiled growth

• 50 to 200 m long / 0.8 m wide

• Gram-positive

• Neisser-positive granules

• Encouraged by– Alternating aerobic / anoxic

conditions

• Grow in unaerated zones

• Controlled by PAX

• SRT 8-50 days

• Anoxic selectors don’t work onthem (Can denitrify?)

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Elevated VFANostacoida limicola II

• Bent and irregularly coiledfilaments

• 100 – 200 μm long / 1.2 –1.4 μm diameter

• Cell septa

• Gram and Neisser variable

• Anaerobic selectorseffective

SepticityThiothrix II

• Sulfur-oxidizing aerobe

• 50 to 200 μm in length, 0.8 – 1.4 μm diameter extending from floc surface

• Gram-negative, Neisser-negative

• Intracellular sulfur granules

• Anaerobic selectors can becounter-productive

NutrientDeficiency

• Type 021 N

• Thiothrix I and II

• N. limicola III

• H. hydrossis

• S. natans

• Funghi

Low pHSlime Bulking

• Nutrient imbalances cancause organisms to exudeexo-cellular slime

• Slime interferes withsettling

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Floc Density

• Compact flocs settle fasterthan highly filamentoussludges

• Phosphorus accumulatingorganisms (PAO) have ahigher density than typicalzoogleal or filamentousorganisms

A Word about Foaming

• Typically caused byNocardioforms

• SRT > 2 days

• Aerobic selectors cancontrol

• Anaerobic and Anoxicselectors may help if nofoam trapping

• Selective wasting

• Chlorine not effective

• Cationic polymer can be

Nocardioforms

• Irregularly shaped true-branching

• 5 to 30 μm long and 1.0 μm wide

• Gram-positive and Neisser-negative with Neisser-positive granules

• Many genera – Nocardia,Gordona, Skermania

Types of Control for SettleabilityProblems

• Chemicals– Chlorination– PAX– Polymers– Nutrients

• Selectors– Aerobic– Anoxic– Anaerobic

• SRT Control• DO Control

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Chemical Control Examples

• Chlorine for general filament control

• PAX for M. parvicella control

• Polymers for Nocardioform control

• Nutrient addition for slime bulking control

Selector – A Definition

• A tank upstream of the main aerobic portion of the activatedsludge process that is designed to control sludge settleabilityby metabolic or kinetic means– Metabolic control – Due to the way the organisms get food and energy

– Kinetic control – Due to the growth rate of the organism underdifferent conditions of growth (SRT)

Main BasinSelectorInfluent Clarifier Effluent

Metabolic Control

• Designed to encourage acertain organism byproviding the rightmetabolic conditions for itsgrowth

• Examples– PAO in anaerobic selectors

– Anoxic selectors for S. natans

and others

Kinetic Theory of Selection(Chudoba, 1973; Jenkins, 1975)

• Filaments have a competitive advantage overfloc forming organisms under conditions oflow substrate (food or electron acceptor)concentration gradient (change).

• Selectors work by exposing treatmentorganisms to a high substrate concentrationgradient.

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Comparative Growth RatesDO 2.0 mg/L, 15 Degrees C

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60

Substrate, mg/L

Sp

ec

Gro

wth

Ra

te,

1/d

Zooglea ramigera

Sphaerotilus natans

Type 021N

0

1

2

3

4

5

6

7

8

0 10 20 30 40 50 60

Substrate, mg/L

Sp

ec

Gro

wth

Rate

,1/d

Zooglea ramigera

Sphaerotilus natans

021N

Comparative Growth RatesDO 2 mg/L, 25 Degrees C

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60

Substrate, mg/L

Sp

ec

Gro

wth

Rate

,1/d

Zooglea ramigera

Sphaerotilus natans

021N

Comparative Growth RatesDO 0.1 mg/L, 20 Degrees C

Aerobic Selector

• Aerobic first stage

• Classic kinetic mechanism

• SRT – 3 to 5 days

• Relies on higher substrate concentration in smaller first stage of treatment

Main BasinSelectorInfluent Clarifier Effluent

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Aerobic Selector

SelectorBOD

Storage BOD

O2

CO2 +H2O

Main Basin

Storage

O2

CO2 +H2O

Synthesis

Energy

Energy

Anoxic Selector

• Anoxic first stage

• Internal recycle

• Denitrification flow scheme

• Must nitrify! SRT - 4 to 10 days

• Most filaments don’t denitrify

• Does not control M. Parvicella, which may denitrify

Main BasinSelectorInfluent Clarifier Effluent

Anoxic Selector

Selector

BOD

Storage BOD

NO3

CO2 +H2O

Main Basin

Storage

O2

CO2 +H2O

Synthesis

Energy

Energy

N2

Anaerobic Selector

• Anaerobic first stage

• Encourage PAO and/or GAO

• SRT - 2.5 days to 5 days

• No internal recycle required

• Can encourage Thiothrix if it produces H2S

• PAO need both anaerobic and fully aerobic conditions

Main BasinSelectorInfluent Clarifier Effluent

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Anaerobic Selector – PAOPhosphorus Accumulating Organisms

SelectorBOD

Storage

PO4

Main Basin

EnergyGlycogen

Poly P

StorageO2

CO2 +H2O

Synthesis

Energy

Poly P

Glycogen

PO4

ReducingPower

Anaerobic Selector – GAOGlycogen Accumulating Organisms

SelectorBOD

Storage

Main Basin

Energy

Glycogen

StorageO2

CO2 +H2O

Synthesis

Energy

Glycogen

DO Control

• Activate sludge organisms need oxygen forgrowth

• Low DO can directly cause bulking

• Low DO can discourage PAO

• Low DO can suppress nitrifiers

• DO control is crucial

SRT Control

• Low SRT can cause outbreaks of Type 1863

• High SRT encourages Nocardioforms and M.parvicella

• Bio P organisms wash out below 2 days SRT

• Nitrifiers wash out below 3-4 days SRT

• This is the primary control for micro-organismgrowth

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Some Examples of Settleability ControlProblems

• West Point HPO Pilot – Anaerobic selectorfailure

• Bellingham – Anaerobic selector success forlow DO bulking and high VFA

• Aberdeen – Anoxic/anaerobic selector success

West Point HPO Test Facility• 4-stage HPO pilot facility

(Lotepro)

– 20-30 gpm

– SRT: 1-2 days

• Mar - Dec 1988

• 3 modes evaluated:– Plug Flow

– Contact/Reaeration

– Anaerobic Selector w/ PlugFlow

West Point HPO Test Facility: HPO Test

Facility Schematic

West Point HPO Test Facility: Settleability

Data from HPO Test Facility

0100

200300

400

500600

700800

900

Plug

Flow

Plug

Flow

Plug

Flow

Plug

Flow

Plug

Flow

Plug

Flow

Con

tact

/ Rea

erat

ion

Con

tact

/ Rea

erat

ion

Selec

tor

Con

tact

/ Rea

erat

ion

Con

tact

/ Rea

erat

ion

SV

I,m

L/g Min

Ave

Max

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West Point HPO Test Facility: The Culprits

• Microscopic evaluation ofboth the pilot test facilityand the UW bench scalefoundThiothrix II

Type 021N

• Sulfur oxidizing aerobes

West Point HPO Test Facility: Predicted

BioWin P04 Profile

2.5 Primary Influent2.669 Selector

2.403 HPO Cell 12.287 HPO Cell 2

2.26 HPO Cell 32.265 HPO Cell 4

2.275 ML Channel2.273 Secondary Clarif ier

2.273 Effluent2.275 Waste

BioWin Chart

TIMEPrimary Influent HPO Cell 2 HPO Cell 4 Effluent

CO

NC

EN

TR

ATIO

N(m

g/L

)

2

1

0

Primary Influent EffluentHPO Cell 2 HPO Cell 3 HPO Cell 4 ML ChannelHPO Cell 1

Waste

Selector

West Point HPO Test Facility: Predicted

Biomass Distribution: No PAO

Zbh

Zba

Zbp

Zbpa

Zbam

Zbhm

Ze

BioWin Chart

TIMEML Channel

CO

NC

EN

TR

AT

ION

(mg

/L) 600

500

400

300

200

100

0

ML Channel

ML ChannelML ChannelML ChannelML ChannelML ChannelML Channel

West Point HPO Test Facility: Conclusion

• The anaerobic selector failed because theplant was operated at SRT values that werebelow the washout SRT for PAO.

• It’s the PAO, stupid!

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The Bellingham Post Point Plant

• 20 mgd capacity high purityoxygen (HPO) activated sludgeplant

• Average SVI of 170 mL/g overperiod from 1999 to 2004

• Typical filaments causingsettleability problems: Type 1701(Low DO) and Type 1863 (LowSRT)

• Periodically high VFA feed frominfluent sewer and from solidsdewatering operation 3-4 daysper week leading to slime bulking

Idea for Improvement – AnaerobicSelector

• Provide zone for uptake of VFA

• Encourage growth of phosphorus accumulating organisms(PAO)

• Increasing population distribution of PAO increases flocdensity

• PAO have a compact morphology and higher density thanother typical activated sludge bacteria (Schuler and Jenkins)

• Experience at three other HPO plants (SE Essex SD, Hyperion,SE San Francisco)

Simulation of Anaerobic First Stage

Primary Effluent Secondary Clarifiers OutfallCl2 Contact

WAS

Aerobic AerobicAnaerobic

Simulations of Population Distributions

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Summary ComparisonPerformance

DescriptionAerobic(2006)

Anaerobic(2007)

Difference(%)

OxygenConsumption(tpd)

7.2 6.7 -6.8%

Sludge VolumeIndex (mL/g)

155.71 126.77 -22.8%

Estimated MixerPower (hp)

222.2 125 -77.9%

Influence of SRT and Temperature onPAO (Jenkins)

0

2

4

6

8

10

12

14

10 15 20 25 30

Temperature (Degrees C)

SR

T(D

ays)

Nitrifier Washout Nocardia Washout PAO Washout

EPBR

EPBR and Nitrification

No EPBR

Aberdeen Anoxic / Anaerobic Selector

• Mechanically mixedactivated sludge aerationtanks

• Upgraded in 2002 for finebubble aeration withanoxic/anaerobic selectors

Aberdeen WWTP Schematic

From Influent Pumps Primary Sed Anaerobic AerobicAnoxic Secondary Sed

Main Digester

Gravity Thickener

Grit Cyclone

Secondary Digesters

Return Flows

Grit

Screens

Screenings

Rotary Thickener

Biosolids

Chlorine Contact

Screw Press

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Selector Operation Has Improved SVI

Influence of Selector Operation on SVI and SRT

0

2

4

6

8

10

12

14

2002 2003 2004 2005 2006

SR

T(d

ays)

an

dN

H3r

(mg

/L)

0

50

100

150

200

250

300

350

400

SV

I(m

L/g

)

SRT (days) NH3 Removal (mg/L) SVI (mL/g)

Selector

Implemented

Conclusions

• Settleability problems result from many different andinteracting conditions

• The solutions to these problems are as varied as theconditions that cause them

• No one solution will cure all problems– Anaerobic selectors can can encourage PAO but also Thiothrix

– Anoxic selectors can control many filaments, but not M. parvicella orType 0041

– Chlorine doesn’t correct nutrient deficiency

• SRT and DO control are crucial

• The first step in a cure is a proper diagnosis

Credits – My Teachers

• David Jenkins, UC Berkeley

• Larry Bateman, City of Bellingham

• Mike Myers, City of Aberdeen

• Dick Finger, King County

• Rick Butler, King County

Questions?Randal W. Samstag

rwsamstag@stanfordalumni.org+1(206)851-0094