Recycle Ratio

R = \frac{Q _{r}}{Q}

Return-sludge recycle ratio is the recycle flow Qr divided by the influent flow Q. A solids balance around the secondary clarifier (neglecting effluent solids) gives R = X / (Xr − X) where X is aeration-basin MLSS and Xr is return-sludge concentration. Typical R is 0.25–1.0.

$R$: Recycle ratio Qr/Q
$Q_{r}$ (m³/d): Recycle (RAS) flow
$Q$ (m³/d): Influent flow
$X$ (mg/L): MLSS in the aeration tank
$X_{r}$ (mg/L): Return-sludge concentration

Step-by-step solved example

Q = 5000 m³/d, MLSS X = 3000 mg/L, RAS Xr = 9000 mg/L (Xe ≈ 0). Find R and Qr.

Recycle R = Qr/Q around a reactor or clarifier.

1. Solids balance
R = X/(Xr − X) = 3000/(9000 − 3000) = 0.50.
2. Recycle flow
Qr = R Q = 0.50 × 5000 = 2500 m³/d.
$R = 0.50, Q_{r} = 2500 m^{3} / d$

Answer: R = 0.50, Qr = 2500 m³/d

10 practice questions

0/10 correct

1.Q = 4000 m³/d, Qr = 2000 m³/d. R =

2.X = 2000 mg/L, Xr = 8000 mg/L, Xe ≈ 0. R =

3.Raising Xr (thicker RAS) at fixed MLSS allows

4.Typical activated-sludge recycle ratio is on the order of

5.Internal recycle in a BNR anoxic zone is often

6.Aeration-tank flow is

7.If Xr approaches X, the required R

8.Wasting (Qw) primarily controls

9.Q = 8000 m³/d, R = 0.75. Qr =

10.Secondary-clarifier underflow Xr is limited by