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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
[email protected]+1(206)851-0094