Wastewater Treatment
Wastewater Treatment
This domain tests the biological and physical train - preliminary, primary, secondary (usually activated sludge), disinfection and solids handling - plus permit compliance. Expect solids-inventory math, process control decisions (wasting, return rates, dissolved oxygen), and questions on how NC NPDES limits are demonstrated and reported.
Core rules to hold onto
- Activated sludge control is a solids inventory problem: MCRT (sludge age) = solids in the system ÷ solids leaving daily. Conventional plants run roughly 5–15 days; nitrifying plants need longer, often 15–25 days.
- F/M ratio (food-to-microorganism) around 0.2–0.4 lb BOD per lb MLSS per day is the conventional target; low F/M with low dissolved oxygen is a classic filamentous bulking setup.
- Settleability: SVI near 100–150 mL/g is healthy; above about 200 means bulking and solids washing over the weirs.
- Aeration: hold dissolved oxygen at roughly 1.5–2.5 mg/L for nitrification; below about 1 mg/L nitrifiers are outcompeted and ammonia slips through.
- Permit math: limits are usually 7-day and 30-day averages, so a single low day does not fix a monthly exceedance. Reports go to NC DWR, and nutrient-sensitive basins (Neuse, Tar-Pamlico, Cape Fear) impose total nitrogen limits near 6 mg/L and total phosphorus near 2 mg/L on larger plants.
Worked example
Aeration tank 0.5 MG at 2,500 mg/L MLSS holds 0.5 × 2,500 × 8.34 = 10,425 lb of solids. Wasting 0.008 MGD at 8,000 mg/L removes 0.008 × 8,000 × 8.34 = 534 lb/day; effluent at 2 MGD and 15 mg/L TSS carries 2 × 15 × 8.34 = 250 lb/day. Total out = 784 lb/day. MCRT = 10,425 ÷ 784 = 13.3 days - adequate for nitrification but with little margin in cold weather.
Traps that cost marks
Using feed flow instead of mixed liquor volume in F/M; forgetting effluent suspended solids in the MCRT denominator; chasing a low pH with more polymer when the real cause is nitrification consuming alkalinity; and reporting as if weekly averages satisfy a 30-day limit.