Wastewater Treatment: Process Control and Permit Compliance
Wastewater Treatment tests process control, permit compliance, solids handling, disinfection, and safety. Hawaii facilities often discharge through ocean outfalls or injection wells and produce reclaimed water for irrigation. The exam ties unit processes to BOD, TSS, nitrogen, and permit limits, so operators must explain why a change in one unit affects the whole plant.
Core rules and concepts
- Preliminary and primary treatment. Screens and grit removal protect pumps; primary clarifiers remove settleable solids. Hydraulic loading and sludge withdrawal control performance.
- Secondary treatment. Activated sludge depends on MLSS, MLVSS, dissolved oxygen, sludge age (SRT), and food-to-microorganism ratio (F/M). Nitrification needs oxygen, alkalinity, and adequate SRT; denitrification needs anoxic conditions and carbon.
- Clarification. Watch sludge blanket, return activated sludge (RAS), waste activated sludge (WAS), and sludge volume index. Bulking and foaming reduce solids capture.
- Disinfection and permit limits. Chlorine, UV, or other disinfection must meet coliform limits; chlorine may need dechlorination. Ammonia interferes with chlorination and can create chloramines.
- Solids handling. Thickening, digestion, dewatering, and biosolids management control costs and stability. Methane is an explosion hazard.
Worked example
A 1.0 MGD plant has influent BOD of 200 mg/L, an aeration tank volume of 0.5 MG, and MLSS of 2,500 mg/L. Food = 1.0 × 200 × 8.34 = 1,668 lb BOD/day. MLSS = 0.5 × 2,500 × 8.34 = 10,425 lb. F/M = 1,668/10,425 = 0.16 day⁻¹. A low F/M means older sludge, more nitrification, and less excess sludge, but possibly pin floc.
Traps that cost marks
Using COD instead of BOD; using MLSS when the question asks for MLVSS; feeding gpm into the 8.34 formula without converting to MGD; confusing SRT with F/M; ignoring ammonia's chlorine demand; and forgetting that nitrification consumes alkalinity and oxygen.
Key points
- F/M = lb BOD/day ÷ lb MLSS; SRT is a different control parameter.
- Nitrification requires DO, alkalinity, and sufficient sludge age.
- Chlorine demand from ammonia affects disinfection and dechlorination.
- Solids handling and methane safety are part of plant operation.