Water tools

Return activated sludge ratio and flow

A solids balance around the clarifier fixes how much sludge must be returned to hold the mixed liquor concentration. Enter what you know and read the rest.

I know the

Influent flowm³/d

MLSSmg/L

Return sludge SSmg/LTypical 8,000 to 12,000 mg/L; varies strongly over the day.

Return ratio Qr/Q

0.45

Return flow

682m³/d

Design pumping capacity (R = 1.5)

2,250m³/d

How it works

A steady-state solids balance around the clarifier, with influent and effluent solids neglected, fixes the return flow:

(Q + Q_{r}) X = Q_{r} X_{r}

(1)

R = \frac{Q_{r}}{Q} = \frac{X}{X_{r} - X} X_{r} = X \frac{R + 1}{R}

(2)

where

$Q$: influent flow, m³/d
$Q_{r}$: return sludge flow, m³/d
$R$: return ratio
$X$: mixed liquor suspended solids, mg/L
$X_{r}$: return sludge suspended solids, mg/L

where X is the MLSS and X_r the return sludge concentration, both in mg/L.

Raising R does not raise X on its own. A higher return flow dilutes the underflow, so the ratio cannot be solved for a new target MLSS from a fixed X_r. Recirculation changes the sludge age, not the hydraulic retention time. Provide pumping capacity for a ratio of about 1.5 regardless of the operating value.

Return activated sludge ratio and flow

How it works

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