Foam Accumulation and Mitigationin BNR SystemsSarah Galst, PMP, PE – Hazen and Sawyer
NEWEA Annual ConferenceJanuary 27, 2016
Outline
• What is biological foam? How is it caused?
• BNR Operation and Foam
• Problems with foam
• Mitigation Strategies
• Liquid Stream
• Solids Handling
• Summary
Biological FoamFundamentals
Biological Foam Fundamentals
Filaments arehydrophobic, due tomycolic acid
Mycolic acid are found inthe cell wall
Stabilizes foam whetherfilaments are alive ordead
Biological Foam Fundamentals
1. Gas bubbles
2. Hydrophobic particles (cells)
3. Surfactants
• Chemical compounds
• Produced by cells
Three critical requirements:
Filamentous organismsprovide backbone for foam
Air
Air
cells
Air
Air
cells
BNR and Foam
Impacts of BNR on Activated Sludge Foaming
BNR operations requireincreased SRT
Nocardioforms grow at higher SRTs
Increased foaming when plants operateBNR
Impacts of BNR on Activated Sludge Foaming
BNR designs create conditions conducive tofoam trapping
Baffling to create anoxiczones
Air lift effect at transitionbetween anoxic/aerobic actsto trap foam
Digester Foaming Causes and Contributors
Classification Causes
Sludge feed characteristicsSurface active agents in feed
Foam causing filaments in feed
Digestion process-relatedcharacteristics
Organic loading aspects – overloadand inconsistent loading
VFA production - ImbalanceGas production
Digester operating conditionsTemperature, pressure, pH, alkalinity
MixingDigester configuration, shape
and physical featuresDigester shape and configurationSludge withdrawal and gas piping
The Problem withFoam is…
Foam – Problems in the Liquid Stream
Difficulty in controlling SRT for BNR processes
Increased final effluent TSS and BOD5
Recycling/reseeding issues
Unsafe work conditions
Damage to equipment
Significant clean-up costs/maintenance issues
Nuisance odors
Foam Challenges in the Solids Process
Reduction in process efficiency - Lossof digester volume
Over-topping of digester covers
Solids inversion
Poor mixing
Unsafe work conditions
Damage to equipment
Significant clean-up costs
Nuisance odors
Digester Foam Operational Challenges –Digest, Recycle, Repeat
• Recycle from the Digesterto the head of the plantcan lead to operationalissues:
• Reseed the process andcause foaming events inupstream processes
Digester Foaming is not just a Digester problem but a Plant problem.
Foam Strategiesfor the LiquidTreatment
Foam Control
Liquid treatment phase“multiple lines of defense”
Physical
Spray Water
Surface Skimming/wasting
Transport baffle walls to move frothout of anoxic zones
Chemical
RAS Chlorination
SWAS Chlorination
Polymer Addition
Froth Hoods in the aeration tank
Froth Hood
Polymer Addition
Spray Water
• Often, spray water nozzlesclog, or spray in straight streaminstead of a spray of water
• Spray water can beatfoam into solution
Surface Wasting
Difficult to preferentially waste froth forming bacteria; theyaccumulate in the foam/solids matrix.
Selectively remove froth forming bacteria by consistently skimmingwaste from the surface of the aeration tank
“Surface Wasting”
Surface wastingsystems located inplaces where foam canaccumulate/trap (i.e.,corners, at the end ofpasses)
Several ways toimplement
Hunts Point WWTP
Dipping weirs feeding a trough to a collection box
Wards Island WWTP
Battery E – Wards Island WWTP
Bell Weirs through length ofpass
Multiple locations
Tallman Island WWTP
Pass A Surface Wasting System
Surface Skimmer
SWAS Well
Bowery Bay WWTP - NYC
SWAS box at the end of Pass Awith adjustable stop-logs
100% wasting through SWAS
Surface Wasting
Include flow measurement
Provide capability to waste 100% of WAS via theSWAS system (pumps and pipes)
Compatibility with dynamic water surface levels
Concentrate SWAS locations in Pass A (if stepfeed)
Avoid systems that can create trapping/cloggingissues
Lessons Learned
Froth trapping common attransition betweenanoxic/aerated zones
‘Air lift’ effect in aerated zonescreates a higher water surfacedownstream
Can lead to froth buildup
Combat this using a systemproviding a positivedisplacement along the aerationtank
Transport baffle walls
Prevent trapping of foam inselector zones
Full-Scale Foam Control -Foam/Froth Movement
Transport Baffle Wall
Full-Scale Foam Control -Foam/Froth Movement Transport Baffle Wall
Froth Hood
The addition of oxidizing agents like chlorine can be used to killfroth forming bacteria, particularly when applied to the foam as withan enclosed froth hood spray.
• Effective strategy tominimize re-seedingof filaments inprocess
• Dosing must becontrolled tominimize impact on“good” bacteria
RAS Chlorination
Effective when used to help control bulking organisms
Less effective than SWAS chlorination for foam/frothcontrol
Again, dosing must be controlled to minimize impact on“good” bacteria
Operational dose of 1-3 lbCl2/1000lb MLSS
Emergency dose of 5 lbCl2/1000lb MLSS
Polymer Interaction with Foam
Water surface
Gasbubble
Filament
Other cells andparticulatematter
Free filaments havelarge surface area tointeract with gasbubble
Cationicpolymer
Polymer stabilize flocstructure and bindsfree filaments totemporarily reducefoam
Full-Scale Foam Control -No Polymer Addition
Full-Scale Foam Control –Polymer Addition
Summary of Activated Sludge Foam Mitigation
Redundant control is necessary
Selective wasting of foam-formingorganisms
Elimination of foam trapping andreseeding, free flow of MLSS through theaeration basin
Polymer addition
Chlorine Spray Hoods/RAS chlorination
Surface Spray
Foam Strategiesfor the SolidTreatment
Foam Challenges in the Liquid Stream
Nocardioforms infoam
Nocardioforms inliquid
Foam Control
Again, “multiple lines of defense” strategy
Physical
Foam wasting
Surface spray
Chemical
Defoamant
Monitoring
Digester Foam Multiple Barrier ApproachFlexible Foam Wasting and Suppression
San Francisco Public Utilities Commission – Oceanside Water Pollution Control Plant
Top Ring
Foam Trough
San Francisco Public Utilities Commission – Oceanside WaterPollution Control Plant
Foam Trough
Digester Foam Multiple Barrier ApproachFlexible Foam Wasting and Suppression
Digester Foam Multiple Barrier ApproachFlexible Foam Wasting and Suppression
Foam controltop ring withnozzlesFoam/Scum
discharge
• Foam Control Mechanisms onEgg Shaped Digesters atNewtown Creek
Digester Foam Multiple Barrier ApproachChemically Enhanced Foam Suppression
New York City Department of Environmental Protection, Hunt’s Point Wastewater Treatment Plant
Control 5 mg/L 15 mg/L 35 mg/L 50 mg/L
Application of a commercial Defoamant can reduce the length and severity ofFoaming events
• Defoamant can be introduced throughthe Recirculation Piping with dosingcontinuing until foaming has ended.
• The dose for each plant is unique.
Monitoring and detecting digester foam - practical challenge.
• Limited access
• Reliance on sensors and associated instrumentation
Examples of Specific Foam Detection Strategies:
Utility Foam Monitoring/Detection Method
Oceanside Plant, CATemperature differential between foamseparator and condenser.
Marquette City WRF, MI Level transducer monitoring.Crystal Lake WWTP, IL Level transducer monitoring.Los Angeles Sanitation District Radar Level Instruments
Digester Foam Multiple Barrier ApproachYou can’t hit what you can’t see
Los Angeles County Sanitation Districts – Joint Water PollutionControl Plant
Radar Level Instruments provide accuratereadings of digester foam levels. Theinstruments are top mounted for easeof maintenance
Viewports with internal cleaning capabilityallow for direct observation of theDigester
Digester Foam Multiple Barrier ApproachFoam Detection
Summary of Digester Foam Mitigation
Redundant control is necessary
Control on liquid side
Early warning – microscopic analysis, radar
Top rings to entrain foam
Surface skim (at varying elevations)
Defoamant addition
Digester Foam Multiple Barrier Approach
Feedsludge
Grit
Sludge
Foam
Grit separator
Sludgerecirculation
and HeatExchange
Top mixingring toentrainfoam
Foamseparator
Digester gas
Polymer/Defoamant
Flexibility in Sludgesuction and return
(Top/Bottom/Middle)
Flexible surfaceskim/withdrawal
Temperatureprobe
Radar level instruments; accesshatch with view ports
Foam buster nozzles
Ability touse entiredigestervolume
Early detection/warningmechanism
Polymer/Defoamant
Summary
Multiple Lines of Defense
Most effective strategies
Liquid
– Surface wasting
– Flow transport
– Polymer addition
Solids
– Early warning/monitoring
– Draw from Surface
– Defoamant addition
Foam Trough
Acknowledgements
NYC DEP
• Keith Mahoney
Hazen and Sawyer
• Paul Pitt
• Michael Lynch
• Wendell Khunjar
Questions?
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