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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.

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Return Sludge Flow Control