Wastewater Treatment
Wastewater Treatment
This domain tests biological treatment control, solids handling, disinfection, and permit compliance. Expect activated sludge math, SRT/F:M, oxygen and mixing, clarifier solids balance, and effluent limits.
Core rules and concepts
- Secondary treatment under 40 CFR 133.102: 30 mg/L BOD5 and TSS as monthly averages, with at least 85% removal, and pH 6–9.
- Activated sludge is controlled by solids retention time (SRT), food-to-microorganism ratio, dissolved oxygen (typically 1–2 mg/L in the aeration basin), and mixed liquor concentration.
- Nitrification consumes alkalinity (about 7.14 mg alkalinity per mg NH3-N) and oxygen (about 4.57 mg O2 per mg N), and is temperature-sensitive.
- Clarifier failure shows as a rising sludge blanket, high effluent TSS, or low return sludge concentration. Check RAS/WAS rates and hydraulic loading.
- Disinfection must meet permit limits; chlorine demand varies with nitrified effluent, so measure residual and adjust dose.
Worked example
Aeration basin: 1.5 MG, MLSS 2,500 mg/L. Solids = 2,500 × 1.5 × 8.34 = 31,275 lb. WAS = 0.05 MGD at 8,000 mg/L = 0.05 × 8.34 × 8,000 = 3,336 lb/day. Effluent solids = 2.5 MGD × 10 mg/L × 8.34 = 208.5 lb/day. SRT = 31,275 ÷ (3,336 + 208.5) = 8.8 days. The effluent term is small but belongs in the calculation.
Traps
Forgetting effluent suspended solids in SRT; using gpm where the formula needs MGD; confusing BOD with CBOD; and assuming a clarifier problem is biological when the cause is a plugged RAS line.