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.
