Wastewater Treatment Processes
From influent to effluent
Preliminary treatment screens and removes grit; primary clarifiers settle organic solids; secondary treatment uses biology to consume BOD and convert ammonia. Activated sludge is most common: influent meets return activated sludge in the aeration basin, microorganisms form floc, and the clarifier separates them; waste activated sludge controls the population.
Control parameters that show up in questions
- F/M ratio - high F/M means young, fast-growing sludge with poor settling; low F/M means old sludge, good nitrification, possible pin floc.
- MCRT/SRT - average solids retention time. Nitrifiers need a longer MCRT and wash out first when it drops.
- MLSS/MLVSS, SVI (100–150 mL/g settles well; above ~200 suggests filamentous bulking), DO (1–2 mg/L in aeration), and sludge age interlock. More RAS returns solids; more WAS lowers MCRT.
- Nutrient removal adds anaerobic and anoxic zones: release phosphorus, then take it up; denitrify nitrate to nitrogen gas.
- Solids handling: thickening, anaerobic digestion (methane, roughly 55–60% CH4), dewatering, and biosolids reuse.
- Effluent disinfection usually requires dechlorination to protect the receiving stream.
Traps
Confusing MCRT (days of solids) with hydraulic detention time (hours), or assuming more RAS always helps - over-returning can overload the clarifier.
Worked example
SVI = (settled volume in mL/L × 1000) ÷ MLSS in mg/L. A 30-minute settled volume of 240 mL/L with MLSS of 2,400 mg/L gives SVI = 240,000 ÷ 2,400 = 100 mL/g, which indicates excellent settling.