Activated Sludge Process Control
What you must know
Activated sludge control hinges on mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), food-to-microorganism ratio (F/M), sludge age (SRT), and sludge volume index (SVI). Typical F/M for conventional systems is 0.2–0.5 kg BOD/kg MLVSS·d; extended aeration runs lower. SVI = (settled sludge volume after 30 min, mL/L × 1000)/MLSS mg/L; SVI 80–150 mL/g indicates good settling, >200 suggests bulking.
Control actions: adjust return activated sludge (RAS) rate to maintain MLSS, waste activated sludge (WAS) to control SRT, and adjust DO to meet demand without over-aeration. Observe the clarifier: a rising sludge blanket, cloudy effluent, or pin floc signals problems. Filamentous bulking is treated by reducing F/M, adjusting DO, or adding selectors. Foaming may indicate Nocardia or surfactants.
Traps in questions
- Confusing F/M (food-to-microorganism) with MCRT/SRT.
- Using SVI without checking MLSS units.
- Assuming high DO prevents bulking - low F/M and selectors matter more.
- Misreading RAS vs WAS: RAS returns biomass; WAS controls inventory.
Worked example / rule of thumb
Aeration tank 2000 m³, MLSS 3000 mg/L, influent BOD load 1200 kg/d. Biomass = 2000 × 3.0 = 6000 kg. F/M = 1200/6000 = 0.2 kg/kg·d - low-rate, extended aeration range. Rule of thumb: target SVI 100–150 mL/g; adjust wasting to keep SRT stable.