Wastewater Treatment
Wastewater treatment questions combine process biology with the solids and flow math that keeps a plant stable. Expect calculation items on loading, biomass inventory, and clarifier performance.
Unit processes and goals
Preliminary treatment (screening, grit) protects equipment. Primary clarifiers remove settleable solids. Secondary treatment, usually activated sludge in Colorado, converts dissolved organics and ammonia to biomass, carbon dioxide, and nitrate. Tertiary steps polish, and disinfection precedes discharge. Know that effluent limits drive everything: CBOD and TSS commonly carry permit limits, and ammonia limits force nitrification.
Activated sludge control
Master the core parameters and what moves them. MCRT, or sludge age, controls the biomass inventory; nitrifiers need a longer MCRT than ordinary heterotrophs because they grow slowly. F/M ratio relates food (BOD mass) to biomass mass. Dissolved oxygen above roughly 2 mg/L supports nitrification. SVI tells you whether sludge settles or bulks. Rising SVI, a thick blanket in the clarifier, and turbid effluent mean the clarifier is being overloaded or sludge quality has failed.
Traps
Units are the usual killer: mg/L and MGD to lb/day, and mixing up RAS (solids returned to maintain inventory) with WAS (solids wasted to control MCRT). Also remember recycle streams from digesters and dewatering return to the head of the plant and add real load.
Worked example
A 30-minute settleometer reading is 300 mL in a 1,000-mL cylinder with MLSS of 2,500 mg/L. SVI = settled volume (mL) ÷ MLSS (g/L) = 300 ÷ 2.5 = 120 mL/g, a healthy value. SVI above about 150 mL/g suggests bulking and poor settling.