Activated sludge process: F/M ratio, MLSS, SRT and SVI
How the most common sewage treatment process works, and the four numbers operators watch every day.
In the activated sludge process (ASP), settled sewage is mixed with a large mass of bacteria in an aeration tank. Air keeps them active. The bacteria eat the organic matter, then settle out in a secondary clarifier, and most of them are returned to the tank.
The flow
Primary clarifier → aeration tank → secondary clarifier → treated effluent. Part of the settled sludge is returned (RAS) and the surplus is wasted (WAS).
MLSS
Mixed liquor suspended solids is the concentration of solids, mostly bacteria, in the aeration tank. Typical conventional ASP: 1500 to 3000 mg/L.
F/M ratio
Food to microorganism ratio compares the BOD fed each day with the bacteria available:
FM = Q S₀V X
Q is flow, S₀ influent BOD, V tank volume and X the MLSS. Conventional ASP runs at about 0.2 to 0.4 per day; extended aeration at 0.05 to 0.15.
Hydraulic retention time
HRT = VQ
Typically 4 to 8 hours for conventional ASP.
Sludge age (SRT, mean cell residence time)
θc = V XQw Xr + (Q − Qw) Xe
Qw is the waste sludge flow, Xr its concentration and Xe the solids in the effluent. Typical values are 5 to 15 days. Longer sludge age means more stable sludge and better nitrification.
Sludge volume index
Settle 1 litre of mixed liquor for 30 minutes and read the settled volume (mL):
SVI = settled volume (mL/L) × 1000MLSS (mg/L)
| SVI (mL/g) | Meaning |
|---|---|
| 50 to 150 | Good settling |
| above 200 | Bulking sludge, poor clarifier performance |
Worked example
Q = 10,000 m³/day, BOD after primary = 180 mg/L, V = 3,000 m³, MLSS = 2,500 mg/L.
FM = 10,000 × 1803,000 × 2,500 = 0.24 per day
HRT = 3,00010,000 × 24 = 7.2 h
Both are within the normal range for conventional ASP.
Quick check
A sludge volume index (SVI) of 250 mL/g usually indicates: