Wastewater Treatment
The process train
Wastewater treatment moves from preliminary (screening, grit removal) through primary clarification, secondary biological treatment, and disinfection, then to solids handling. Activated sludge is the most tested process: influent BOD and suspended solids feed a mixed liquor, air is supplied, and solids are separated in a clarifier. Return activated sludge (RAS) maintains the biomass; waste activated sludge (WAS) controls the solids inventory.
Control parameters
Know MLSS and MLVSS, food-to-microorganism ratio (F/M), mean cell residence time (MCRT or SRT), sludge volume index (SVI), and dissolved oxygen. F/M = lb BOD applied per day ÷ lb MLSS under aeration; a low F/M means a long SRT, older sludge, more nitrification, and less excess solids. SVI = settled sludge volume (mL/L at 30 min) x 1,000 ÷ MLSS (mg/L); roughly 80-150 mL/g settles well, and above about 200 indicates bulking. MCRT = solids in the system ÷ solids leaving per day.
Traps
Reporting F/M in the wrong units; omitting the 8.34 lb/gal factor when converting mg/L and MGD to pounds; confusing RAS ratio with SRT; and assuming more air always fixes poor settling, when the cause may be filamentous bulking or a clarifier problem. Nitrification consumes alkalinity and oxygen; denitrification releases nitrogen gas that can float sludge.
Worked example
A 0.5 MG aeration tank at 2,500 mg/L MLSS holds 0.5 x 2,500 x 8.34 = 10,425 lb of solids. With 1 MGD at 200 mg/L BOD, loading is 1 x 200 x 8.34 = 1,668 lb/day. F/M = 1,668 ÷ 10,425 ≈ 0.16, a typical extended-aeration value.