Water Pollution Control and Wastewater Treatment
What this domain tests
Expect calculations plus process selection: which train meets a given effluent limit, and whether the biology can actually nitrify and denitrify at the design temperature.
Targets
- GB 3838-2002 grades I to V define receiving-water uses; drinking-source and fishery reaches demand higher protection.
- GB 18918-2002 一级A: COD 50, BOD5 10, SS 10, NH3-N 5(8), TN 15, TP 0.5 mg/L. 一级B: 60/20/20/8(15)/20/1. Values in brackets apply at low water temperature.
- GB 8978-1996 uses three tiers keyed to receiving-water class and discharge year; industry standards override it.
Process trains
- Pretreatment: coarse and fine screens, grit removal, primary settling, equalisation.
- Biological: conventional activated sludge; A/O for denitrification; A2/O (anaerobic–anoxic–aerobic) for combined N and P removal; oxidation ditch; SBR/CASS; MBBR; MBR. Anaerobic: UASB, IC, hydrolysis-acidification for high-strength streams.
- Tertiary: coagulation-sedimentation, filtration, granular activated carbon, ozonation or Fenton, disinfection.
- Sludge: thickening, dewatering to about 80 percent (belt/centrifuge) or 60 percent (plate-frame with conditioning), anaerobic digestion, composting, drying and incineration.
Design relations
HRT = V/Q; F/M = Q·S0/(V·X); SVI = SV30 (mL/L) x 1000 / MLSS (mg/L); return-sludge ratio R = X/(Xr − X).
Worked example
Q = 20,000 m3/d, A2/O with anaerobic 1.5 h, anoxic 3 h, aerobic 8 h; MLSS 3,500 mg/L; SVI 120 mL/g.
- Total HRT 12.5 h, so V = 20,000 x 12.5/24 = 10,417 m3; aerobic volume = 20,000 x 8/24 = 6,667 m3.
- SV30 = SVI x MLSS/1000 = 120 x 3.5/1000 = 0.42, i.e. 420 mL/L.
- Underflow concentration Xr = 1,000,000/SVI = 8,333 mg/L, so R = 3,500/(8,333 − 3,500) = 0.72, about 72 percent.
- Check nitrification: the chosen SRT must exceed the nitrifier minimum SRT at the coldest design temperature, which is roughly 4 to 5 days at 20 C and much longer near 10 C. A plant designed only on BOD loading will fail ammonia in winter.
Traps
- Confusing SV30 (percent) with SVI; SVI needs SV30 in mL/L.
- Returning nitrate to the anaerobic zone destroys phosphorus release, so the internal recycle must go to the anoxic zone only.
- Reading bracketed ammonia limits as year-round values.
- Treating 一级A as permission to discharge into any water body; the EIA may require more.