1. Optimization for Centrifuge Dewatering
WEFTEC New Orleans, Louisiana
Rashi Gupta, P.E.
Steve Walker, CWP
Carollo Engineers
September 28, 2016
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Questions to Answer Today…
• Why is optimization important?
• What should I consider within my dewatering
process?
• How do I systematically optimize the process?
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Reducing Polymer Can Save on Chemical $
~$20,000 between 38 lb act/DT
and 40 lb act/DT per centrifuge at
250 gpm; 2% feed
Case Study 1
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Operational Targets to Track Performance
Criteria Units Importance
Cake dryness %TS • Disposal costs
• Water equals weight and takes up
volume
Centrate
quality
%TS
or
TSS, mg/L
• Impact on liquid treatment
processes
• Re-treatment of solids (thickening,
digestion and dewatering)
• Impact of inert solids on activated
sludge mass calculations
Throughput lb/hr and gpm • Centrifuge capacity
• Operations and hauling logistics
Polymer dose Active pounds
per dry ton
• Operating costs
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Dewatering Feed Characteristics Matter
• Important Sludge Feed Characteristics
− PS:TWAS ratio in digester feed, by mass
Important because PS is MUCH easier to digest and
dewater than TWAS
The higher the PS:TWAS ratio, the better the
dewaterability
− Volatile solids content
Generally, higher VS = More difficult to dewater
− Biological phosphorous removal
− Divalent cations (Ca2+, Mg2+) vs Monovalent (Na+, K+)
− Sludge feed temperature
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Understanding Polymer Basics Necessary to
Assess Effectiveness
• “Neat” polymer
− Oil, surfactants, water, and polymer
(emulsion as delivered)
• “Active”
− What’s doing the work
− 40-50% typical for emulsion
− 90%+ typical for dry
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Many Pieces to the Polymer Puzzle
• Procure the right polymer
• Select right dilution concentration
for polymer solution
• Produce fully activated solution
• Find best polymer injection location
• Maintain req’d water pressure
• Check water characteristics (chlorine,
hardness, temperature)
• Optimize polymer dose
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• Loading rates (throughput)
• Bowl speed (G-force)
• Conveyor/scroll speed (differential
speed) and torque
− Newer centrifuges are controlled using
a % torque or pressure setpoint
− To achieve the desired
torque/pressure, the PLC adjusts the
scroll speed
• Weir plates
− Set the pool depth within the bowl
Centrifuge Parameters that Affect
Performance
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Process Optimization
• Achieve:
− Desired cake dryness and
centrate quality
− Lowest polymer cost
− Maximum throughput and
efficiency
• Balancing act: Maximize one or
more of the process
parameters, while keeping all
others within their constraints
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Parameters used for Optimization
• Type of polymer
• Polymer dose
• Polymer dilution/solution concentration
• Polymer injection point
• Centrifuge torque/pressure
• Centrifuge pond depth
• Centrifuge bowl speed
• Sludge feed (throughput)
• Sludge characteristics
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Process Optimization – Polymer Selection
• Selecting the right polymer:
− Site specific jar testing by different
suppliers
High molecular weight for
centrifuge dewatering
High charge density helps with
dewatering of secondary sludges
Cationic
− Site specific full scale tests with
promising polymers
• Full scale tests can identify more
effective polymers
• Polymer effectiveness can be
gauged through viscosity
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Polymer Contract Flexibility
• Establish Primary and Alternate supplies
− Use two different suppliers rather than two different
products from the same supplier
• Determine if seasonal constraints support
alternatives. For example:
− An emulsion polymer may be more cost effective in
winter months
− A dry polymer may be more cost effective in the
other months
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Test Methodology – Phase 1 (Polymer Dose)
• Batch polymer at preferred solution strength
• Set sludge feed rate
• Enter typical torque setpoint and bowl speed
• Set lowest polymer feed rate
• Run for a standard time (i.e., 45 minutes)
• Sample feed, centrate and cake for TS/TSS.
− Feed solids concentration used to calculate throughput
• Run through sequence for at least 5 polymer feed
rates
• Calculate associated dose and plot results
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Test Methodology – Phase 2 (Torque)
• Set sludge feed rate
• Set polymer solution feed rate for the “ideal” polymer
dose
• With sludge and polymer feed rates “locked in”,
adjust % torque setpoint
• Run for a standard time (i.e., 45 minutes)
• Sample feed, centrate and cake for TS/TSS.
• Run through sequence for at least 5 torque setpoint
• Plot results
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Further Optimization
• Choose variable to test
− Polymer solution concentration or injection point
− Throughput
− Centrate weir location
− Etc.
• Run through same process as above, holding
other parameters constant
• Run through sequence with incremental changes
to variable
• Plot results
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Optimize Upstream Processes
• PS:TWAS ratio
− If possible, modify upstream
operations to favor PS in the
digester feed
• Dewatering feed temperature
− Try minimizing storage or
feeding from digesters
• Track VS in dewatering feed
• Plot these parameters and
compare to performance
Case Study 1A Case Study 1B Case Study 1C Case Study 1D
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Outcomes of Regular Optimization
• Determine what dose is effective under what conditions
• Determine what torque is effective at what throughput
• Determine impacts of upstream processes on dewatering
• Provide general guidelines to dial in centrifuges and
understand impacts of changes
• If using more than one polymer, determine under what
conditions each is most effective
• Forecast usage to establish purchasing frequency and
budget projections
Consider a full range of testing at least every 6 months
