Wastewater Treatment: Solids, Loading, and Biology
Process train and the numbers behind it
Preliminary (screens, grit removal) → primary settling (removes settleable solids and a share of BOD) → secondary biological treatment (activated sludge or fixed film) → disinfection. For activated sludge, the controls a candidate must know are MLSS and MLVSS, food-to-microorganism ratio (F/M), mean cell residence time (MCRT or SRT), hydraulic retention time, return and waste sludge rates, dissolved oxygen, and sludge volume index (SVI).
Relationships worth memorizing
- F/M (lb BOD per lb MLVSS per day) moves opposite to MCRT: high MCRT gives low F/M, more nitrification, less sludge production, and higher oxygen demand.
- SVI = settled volume (mL/L) × 1000 ÷ MLSS (mg/L). Roughly 80–150 settles well; above about 200 signals bulking, often filamentous organisms from low DO, low F/M, or septicity.
- Nitrification consumes about 4.6 lb O2 per lb ammonia-N and destroys roughly 7.1 lb alkalinity as CaCO3 per lb N.
Traps
MCRT arithmetic: solids in the clarifier and return line count as system solids, not just the aeration basin. Also separate BOD from CBOD (nitrification inhibited) and remember permit limits are usually monthly averages with weekly or daily maximums.
Worked example
Flow 1 MGD at 200 mg/L BOD: 1 × 200 × 8.34 = 1668 lb BOD/day. Aeration basin 0.5 MG at 2000 mg/L MLVSS: 2000 × 0.5 × 8.34 = 8340 lb MLVSS. F/M = 1668 ÷ 8340 = 0.20 lb/lb·day, a low-rate nitrifying operating point.