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Unlocking Digester Foaming Issues through Research
Presenter: Kota Nishiguchi, E.I.T.Principal Investigator: Dr. Mari WinklerProject Sponsor: King County Wastewater Treatment Division 1
Acknowledgments
Thank you to all of the members of King County who have been very helpful and supportive in the ongoing research.
Thank you to Dr. Mari Winkler for her guidance and support.
2
Overview
1. Digester Foaming Issues2. Causes of Digester Foaming3. Methods of Evaluating Foaming4. Study of Foam-Causing Bacteria and Foam Evaluation Methods5. Future Work
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Anaerobic Digester
Structural Damage
What are Digester Foaming Issues?
Overflow
Clog Gas Lines
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Decrease Capacity
Gas + Liquid + Solids Buildup
Causes of Foaming: Operation/Design
Feeding Patterns• Too much/inconsistent• Composition
• Primary sludge• Activated sludge
Mixing• Too little or too much• Gas or mechanical
mixing
Temperature• Fluctuations
Pagilla and Subramanian, 2014 5
Gas Withdrawal • Rapid changes to
pressure
Causes of Foaming: Feed CharacteristicsFoam-Causing Bacteria• Gordonia amarae (pictured left) and Microthrix parvicella• Cause foaming in secondary treatment• Hydrophobic cell-surfaces• Release hydrophobic substances
• Live or dead
Surface Active Agents (Surfactants)• “Skin” of bubbles, air-liquid interfaces• Biosurfactants
• Dead cell matter• Extracellular polymeric substances (EPS)
• Linear alkylbenzene sulfonates (LAS)• Soaps and detergents
Cells surrounded by EPS (Aquafix Inc.)
Gordonia from WPTP
Pagilla and Subramanian, 2014 6
How do we evaluate foaming?
Full-scale methods:
Liquid level (pressure sensors)• Foaming sludge less dense• Foaming liquid level readings drop
Temperature sensors• Foam attach to temp. sensors• Foaming temperature readings drop
Visual Inspection• Site glasses on digesters• Sludge on top of digester cover
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How do we evaluate foaming?
Bench-scale methods:Foam Potential Test
Unstable foam index foam production
Stable foam index stable foam production
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How do we evaluate foaming?
Bench-scale methods:Viscometer Ramp Test
AMETEK Brookfield Shear Rate
Shear Stress
High yield stress = bubbles trapped in sludge
High plastic viscosity = bubble rise slower
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Different Types of “Foaming”
True foaming = Gas at Surface Rapid volume expansion (RVE) = Gas in Liquid
M. Mahdi via YouTube
Thomas Chapman and Steve Krugel (2011) differentiate types of “foaming”
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Importance of Viscometer Tests
• Nick Bartek and Matthew Higgins –rheology vs RVE
• Relatively new approach• Easier to replicate than foam potential
test
Bartek, 2017 11
Different sludges, different potential for volume expansion!
Causes?
12Bartek, 2017
RVE Foaming
Physical Parameter
Biological Parameter
Simple, quick method to find foam causes?
Foam Evaluation Methods
Foam Causes EPS FOGLAS
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Foam-Causing Bacteria vs. Foam Evaluation Tests
• Test foam potential test and viscometer against G. amarae• Study began June 2018 (still early)• Digester samples from King County’s 3 WWTPs
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West Point Treatment Plant (WPTP)• 100 MGD • High purity oxygen• Periods of frequent
digester foaming
South Treatment Plant (STP)• 71 MGD• Anaerobic selector • Little to no foaming
Brightwater Treatment Plant (BWTP)• 18 MGD• Membrane bioreactor• Occasional digester foaming
episodes
0
0.5
1
1.5
2
2.5
0% 1% 2% 3% 4%
Stab
el F
oam
Inde
x
Gordonia Concentration
WPTPSTPBWTP
Early Results: Foam Potential Test
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ExpectedMore Gordonia, more foam potential
?
1.5
1.7
1.9
2.1
2.3
2.5
2.7
0% 1% 2% 3% 4%
Yiel
d St
ress
, Pa
Gordonia Concentration
WPTPSTPBWTP
Early Results: Yield Stress
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?
20
25
30
35
40
45
50
55
0% 1% 2% 3% 4%
Plas
tic V
iscos
ity, m
Pa-s
Gordonia Concentration
WPTPSTPBWTP
Early Results: Plastic Viscosity
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Low Gordonia, variable viscosity
Higher Gordonia, emerging trend?
Need more data!!
Summary of Early Results
• Foam potential test detects G. amarae foaming• Yield stress affected by other factors• Plastic viscosity needs more data to reveal trends
?19
Still Unknown…
Foam Potential Test
Viscometer
Which causes are identifiable by each test?
EPS
FOGLAS
??
?
?
Causes of Foaming:
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References
Pagilla, Krishna R., and Bhargavi Subramanian. 2014. “Wastewater Treatment Anaerobic Digester Foaming Prevention and Control Methods: Literature Review and Survey.” Water Environment Research Foundation.Chapman, Thomas. 2011. “Rapid Volume Expansion - an Investigation Into Digester Overflows and Safety.” In Water Environment Federation Residuals and Biosolids. Alexandria, VA. https://doi.org/10.1080/21680566.2016.1169954.
Bartek, Nick, Matthew J. Higgins, Sudhir N. Murthy, Steven Beightol, and Travis Peaslee. 2017. “Causes and Cures of Rapid Volume Expansion in Anaerobic Digesters Due to Gas Holdup Nick.” In WEF Residuals and Biosolids Conference 2017. Seattle, WA.
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