Foam Accumulation and Mitigation in BNR SystemsDigester Foam Multiple Barrier Approach Chemically...

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?