DIGESTER FOAMINGCAUSES AND SOLUTIONSCAUSES AND SOLUTIONS

CWEA Math Operations andCWEA Math, Operations, and Maintenance for Biosolids Systems

SeminarSeminarSeptember 13, 2011

Steve WalkerSenior Operations Specialist

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LEARNING OBJECTIVESCauses• Process control

• Operational

• Process – digester internal

Control methods• Process control

• Operational awareness

• Process monitoringProcess monitoring

Physical improvements• Installed improvements

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Installed improvements

PERSONAL EXPERIENCE$3M catastrophic failure of digester roof$1M repair of “fixed” holding tank roof

Saved two other digesters from roof failuresRuined one pair of beige Dockers

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3

WHAT CAUSED FAILURE• Over-wasted secondary

– FilamentsM h l d t i– Much more volume compared to primary

• Gas mixing system piping approach

• Plugged flame arrestors – from foam

L l i di t• Level indicator wrong– From foam entrained in contents

• Didn’t shut off mixer soon enough

• After first failure, reduced active volume created

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,organic overload of other units in service (i/s)

CAUSESCAUSES

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PROCESS CONTROL CAUSES

• The blend of primary sludge to secondary sludgeThe more primary sludge the better– The more primary sludge, the better

– Measured as pound to pound, not gallon to gallon

• Excessive filaments in the secondary sludge– Nocardia and microthrix parvicella add bulk and p

structure to foam

• Feed fluctuationsFeed fluctuations– Both solids loading and volume

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OPERATIONAL CAUSES

• Inadequate or inconsistent mixing– Rapid changes create unpredictable resultsRapid changes create unpredictable results

– Gas binding of pump used to mixing

• Excessive and/or fine bubble mixing – Can change the specific gravity and “fluff” the contents

• Rapid change in internal pressure– Cogeneration demandsCoge e at o de a ds

– Moisture in gas handling system

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OPERATIONAL CAUSES • Insufficient volume

– Liquid level too low

– Processing volume taken up by grit or scum “raft”

U it t f i f i t t ti– Units out of service for maintenance or construction

– Results in solids loading rates greater than design

• Temperature fluctuations– Created by heating or feed disruptionsCreated by heating or feed disruptions

• Rapid change in liquid level

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INTERNAL CAUSES

• Microorganisms competing for food– Acid formers vs. methane (alkalinity) formers

• Surfactants– Oil and greaseg

– Overdosed polymer in thickening or dewatering processes p

There are usually multiple contributors

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POTENTIAL CONTROLPOTENTIAL CONTROL METHODS

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PROCESS CONTROL• Primary sludge and secondary sludge ratio

– Maximize primary solids removal efficiency

– Maximize primary BOD removal efficiency

Filaments in the wasted secondary sludge• Filaments in the wasted secondary sludge– Review operating conditions and take corrective

actionaction

• Feed fluctuationsIdeal is constant feed of comparable ol me to all– Ideal is constant feed of comparable volume to all digesters in service

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OPERATIONAL CAUSES

• Insufficient volume– Confirm liquid levelq

– Confirm active volume

– Understand seasonal trends of sludge production

• Rapid change in liquid level p g q– Strive for consistent dewatering feed rates

Check valving especially after maintenance work– Check valving, especially after maintenance work

– Be suspicious of the level indicators

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OPERATIONAL CAUSES

• Inadequate or inconsistent mixing– Avoid on/off control

– If problem is recirculation pump gas binding, change type

– Review directional mixing approach

• Excessive or fine bubble mixingExcessive or fine bubble mixing – Adjust equipment

• Rapid change in internal pressureRapid change in internal pressure– Strive for consistent feed

– Drain moisture traps

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– Drain moisture traps

INTERNAL CAUSES

• Temperature fluctuations– No more than 1 degree F per 24 hour change – up or

down

• Avoid septic sludgeAvoid septic sludge– More acidic than fresh sludge so greater reaction with

alkalinity

• Surfactants– Review scum and FOG feed ratese e scu a d OG eed a es

– Avoid polymer overdose

O

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OPERATIONAL AWARENESS

Placing a digester in service

• Ramp the feed up slowly• Ramp the feed up slowly

• Avoid seeding with poor quality sludge

• Ramp up mixing system slowly

• Maintain heat throughout

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OPERATIONAL AWARENESS

Taking a digester out of service

• Ramp down the mixing system slowly• Ramp down the mixing system slowly

• Lower liquid level slowly as this sludge is usually transferred to other units in service

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OPERATIONAL AWARENESS

Feed strategies

• Ideal is constant loading to each unit in service

• Minimize downtime between feeding

Withdrawal strategies

• Consistent rate from all units in service

Feed disruptionsp

• Plan maintenance and construction activities

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PROCESS MEASUREMENTCriteria Measurement CommentFeed volume gallons per feed cycle Equal to all units i/s

Feed volume gallons per day Equal to all units i/s

Feed solids pounds per feed cycle Equal to all units i/s

Feed solids pounds per day Equal to all units i/s

Primary solids to pounds / pounds 50% primary sludge orPrimary solids to secondary solids ratio

pounds / pounds 50% primary sludge or greater

Volatile solids pounds/cubic foot/day 0 1 0 2 Overload =Volatile solids loading rate

pounds/cubic foot/day 0.1 – 0.2. Overload = 10% greater

Gas pressure inches W. C. 16 +/- 2

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PROCESS MEASUREMENT

Criteria Measurement Comment

Liquid level feet Determines active volumeLiquid level feet Determines active volume

Freeboard feet 6 ft or greater from top

Foam depth feet MinimalFoam depth feet Minimal

Volatile acids mg/L 50 - 330

Alkalinity mg/L 1500 - 6000y g

VA/Alk ratio mg/L / mg/L 0.1 – 0.2

Temperature degrees 95 – 100F (mesophilic)

C fi l l d t k/t d d bli h

Grit accumulation feet (quarterly) Lost active volume

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Confirm, sample, analyze, record, track/trend and publish

HIGH-TECH MEASURING DEVICE

Works with grit, scum and foam

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MAINTENANCE

C ill t fl t if it i h th th

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Cover will not float if it weighs more than the gas can lift

MAINTENANCE – FLOATING COVERS

• Wash off overflow

• Do not store anything on the roof – such as the fan used for venting during cleaning work

• Confirm the pressure relief pallet weights

• Grease the corbelsGrease the corbels

• Inspect the ballast troughs – make sure the drain ports are openports are open

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PHYSICAL IMPROVEMENTS

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VIEWING PORTS

Use to determine the foam depth and how much freeboard is available

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is available

FOAM DEPTH GUIDE

Stainless steel; extends 9 inches away from sidewall;inches away from sidewall; 8 feet long; Z-shaped jog at every foot

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MULTIPLE LEVEL INDICATORS

Compare the bubbler level to the radar level. The difference is foam.

Bubbler works off of pressure Thepressure. The specific gravity of the digester contents

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more entrained gas.

MOISTURE SEPARATIONTrapped water or foam can stop gas flow.

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PRESSURE RELIEF HATCHES

Containment area f ll illfor small spills

Weighted to open at 2 to 3 inches WC greater than typical gas pressure

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typical gas pressure

FOAM TROUGH WITH SPRAY BAR

Allows the foam to drain out of the digester, but creates other problems whereproblems where it discharges.

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WHAT WAS DISCUSSEDCauses• Process control

• Operational

• Process - internal

Control methods• Process control

• Operational awareness

• Process monitoringProcess monitoring

Physical improvements• Installed improvements

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Installed improvements

DIGESTER FOAMINGDIGESTER FOAMINGCAUSES AND SOLUTIONS

Thank you for being a wastewater treatment professional. Be proud of your contributions.

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professional. Be proud of your contributions.