Solids Retention Time (Sludge Age)

θ_{c} = \frac{V X}{Q _{w} X _{w} + Q _{e} X _{e}}

θc (SRT, sludge age) is mass of solids in the reactor divided by mass wasted per day (wasting + effluent solids). Typical conventional ~5–15 d. Related to F/M and to effluent quality.

$θ_{c}$ (d): Solids retention time
$V$ (m³): Aeration volume
$X$ (kg/m³): MLSS

Step-by-step solved example

V=500 m³, X=3000 mg/L = 3.0 kg/m³, wasted solids 150 kg/d (neglect effluent X). θc?

θc = mass in reactor / mass wasted per day.

1. Mass in reactor
VX=500×3=1500 kg.
2. θc
θc=1500/150=10 d.
$θ_{c} = 10 d$

Answer: 10 d

10 practice questions

0/10 correct

1.θc is also called

2.If wasting increases, θc

3.HRT t = V/Q is

4.Neglecting Xe, θc ≈

5.Longer θc generally

6.X is typically

7.Units: VX in kg, waste in kg/d → θc in

8.If Qe Xe is not neglected in a poorly settling plant, θc

9.Typical conventional activated-sludge θc is on the order of

10.θc and F/M are