Process Control and Operational Troubleshooting
Process control fundamentals
Process control in an activated sludge plant is a mass-balance exercise. Solids enter with influent and growth, and leave with wasting and effluent. Mean cell residence time (MCRT, or SRT) equals total solids in the system divided by solids leaving per day. It is the controlled variable that determines sludge age, and it drives nitrification, settling, and oxygen demand. F/M ratio has an inverse relationship with MCRT: high F/M means low MCRT, young sludge, poor settling, and high oxygen demand; low F/M means old sludge, possible pin floc, and risk of rising sludge.
Key tests: MLSS/MLVSS, SVI, DO profiles, settleometer, microscopic exam, ammonia, and clarifier blanket. SVI = settled volume (mL/L) x 1000 / MLSS (mg/L); 80-150 mL/g is good, over 150 indicates bulking, under 80 suggests pin floc or inorganic solids. Check for filaments before assuming a cause: low DO, low F/M, nutrient deficiency, and septic wastewater all produce filaments.
Control actions: adjust wasting to hold target MCRT, adjust RAS to control blanket, adjust aeration to hold the DO setpoint (typically 1.5-2.5 mg/L for nitrification), and check for shock loads or toxicity.
Traps: confusing HRT with MCRT; calculating wasting from volume rather than solids; forgetting effluent solids in the MCRT denominator; treating a high SVI as automatically filamentous; and ignoring that RAS rate does not change MCRT - only wasting does.
Worked example: aeration basin 2.0 MG at 2500 mg/L = 41,700 lb MLSS; clarifier 0.5 MG at 8000 mg/L = 33,360 lb; total 75,060 lb. Wasting 30,000 gpd at 8000 mg/L = 2,002 lb/day; effluent 1.0 MGD at 15 mg/L = 125 lb/day. MCRT = 75,060 / (2,002 + 125) = about 35 days - an old sludge, consistent with nitrification.