Wastewater Treatment
Biological treatment and the numbers behind it
Wastewater plants combine physical, biological, and chemical processes. Primary clarifiers remove settleable solids; activated sludge converts soluble BOD and ammonia using a mixed culture, then secondary clarifiers separate sludge from treated water; disinfection and solids handling finish the job. Operators control the biology with dissolved oxygen, solids retention time (MCRT), food-to-microorganism ratio (F/M), mixed liquor suspended solids, return and waste sludge rates, and pH/alkalinity. Nitrification needs DO and long sludge age; denitrification needs an anoxic zone and carbon. Watch clarifier blankets and SVI for bulking.
The dominant trap is loading math: lb/day = concentration (mg/L) × flow (MGD) × 8.34. Percent removal is (in − out) ÷ in × 100. SVI = settled sludge volume (mL/L) × 1,000 ÷ MLSS (mg/L). Confusing MCRT with hydraulic detention time, or recycle ratio with waste rate, is common. Also know permit limits for CBOD, TSS, ammonia, and pH, and that toxicity and temperature affect results.
Worked example: influent 250 mg/L BOD at 2 MGD = 250 × 2 × 8.34 = 4,170 lb/day. If effluent is 20 mg/L, removal = (250 − 20) ÷ 250 × 100 = 92%.
Rule of thumb: SVI near 100 mL/g settles well; above about 150 suggests bulking and poor settling.