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1
Return Sludge Flow Control
by R. Dale Richwine, P.E.
Richwine Environmental, Inc.
Return Sludge Flow
RAS Q
What is the correct return sludge flow rate?
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Return Sludge
What’s in it? » Water and solids » Settled material » Bacteria and other microorganisms » Viable organisms
Secondary Clarifiers
Three functions: » Clarify secondary effluent » Thicken the solids » Remove floatable material
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Return Sludge
Why do we return sludge? » Remove sludge from the clarifier » Seed the aeration tank
Return Sludge Flow
The correct return rate? » Optimum sludge quality » Optimum effluent quality
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Control F/M and MCRT?
“The F/M ratio and the SRT or MCRT are controlled through wasting.”
Return Sludge Flow
» Percentage of influent flow.
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Al West Study – Part 1
Al West Study
» RAS concentration changed rapidly. » RAS concentration changed inversely to flow
changes. » RAS Conc. decreased as flow increased. » RAS Conc. increased as flow decreased.
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Al West Study – Part 2
Al West Study
» The RAS Q increased from 15% to 127% with only a small increase in MLSS concentration, 1,230 mg/L to 1,470 mg/L.
» The RAS Q was reduced from 127% to 37%; MLSS continued to rise to 2,400 mg/L.
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Al West Study
Conclusion: » F/M, Sludge Age, MCRT, MLSS concentration are
not normally affected greatly by RAS Q adjustments unless the system is badly out of balance.
What Happens If?
Shutting off the RAS Q: » Clarifier fills up with sludge » Solids go over the weir » MLSS would decrease » BOD conversion reduced » Clarifier sludge anaerobic » Effluent quality reduced
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What Happens If?
RSF up full blast: » All solids forced to the clarifier » RSC thins out » Begin pumping water » Decrease detention time in both the aeration tank
and clarifier » Effluent quality reduced
Return Sludge Flow Control
Objectives: » Maintain good settling MLSS
– good effluent quality » Keep microorganisms in system longer than
water. » Maintain F/M balance » Solids balance: keep solids in aeration tank.
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Return Sludge Flow Control
Objectives: » Keep blanket level low in clarifier
– No solids over the weirs » Save energy on pumping
Return Sludge Flow Control
The basic ways for returning sludge to the aeration tank:
1. Constant rate • Independent of influent flow rate
2. Constant percentage of influent flow rate 3. Varying rate
• Optimize concentration • Optimize detention time
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Techniques for RAS Q Control
» Sludge blanket level control » Settleabilty » Secondary clarifier mass balance » Aeration tank mass balance » Sludge quality
Mass Balance
Example: » Influent flow: 2.4 mgd » Current RAS Q: 1.4 mgd » Current RAS %: 58% » MLSS: 2,400 mg/L » RAS Conc.: 8,000 mg/L
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Mass Balance by Centrifuge
RAS Q =MLSS,mg L x Flow to Clarifier
RAS Conc, mg L
RAS Q =2400 mg L x (2.4+1.4)mgd
8000 mg L
Mass Balance by Centrifuge
RAS Q =2400 mg L x (2.4+1.4)mgd
8000 mg L
RAS Q,mgd =1.14 mgdRAS Q = 48%
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Mass Balance Results
Result: » Original RAS Q: 1.4 MGD or 58% » New RAS Q: 1.14 MGD or 48%
RAS Q and Sludge Quality
Goal: adjust the RAS Q in response to the actual sludge characteristics.
» Starting point: calculate RAS Q – Mass Balance – Fine tune: sludge quality – SSV/SSC curves
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RSF Tune Up
SSC curve: » Optimum: reduce
RAS Q to lowest rate – Save money on
energy – Increase detention
time in aeration tank
RAS Q Tune Up
SSC curve: » Return sludge when
it is finished concentrating.
– Save volume pumped – Save volume in
digesters
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RAS Q Balance
Balancing two opposing pressures: » Biological time sludge is in the clarifier: anoxic » Hydraulic detention time:
– Aeration tank – Clarifier
RAS Q Control
As long as the MLSS stays constant (you are wasting properly), proper adjustments in the RAS Q will decrease the sludge blanket.
– Better effluent
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General Guidelines
» There is a limit to how low you can reduce the RAS Q and still keep the sludge moving.
– Always keep the sludge moving
General Guidelines
» If large changes in RAS Q rate are required, make smaller adjustments over time.
– 1 – 2 hours apart – 1 – 2 sludge cycles
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General Guidelines
» RAS Q has been reduced too low if the RAS Conc remains constant while the clarifier sludge flow and/or the wastewater flow rate changes significantly.
General Guidelines
» RAS Q has been reduced too low if the MLSS is lowered substantially after the RAS Q has been reduced.