Design, analysis and problem solving
The design loop assessors look for
Good answers follow a visible sequence: fix the design basis (flows, loads, effluent or ambient target, site constraints, operating hours), generate at least two options, size the preferred one, close a mass or energy balance, then compare options on capital cost, O&M, energy and carbon, land take and risk. Hong Kong sites are tight and often close to sensitive receivers, so footprint and odour buffer frequently decide the selection before cost does.
Turning standards into design inputs
- The WPCO TM effluent standard sets the treatment target; the licence then fixes the conditions you must meet.
- TM-EIAO noise criteria set the receptor limits, while the NCO permit fixes when you may work.
- AQO and odour criteria set stack height, control efficiency and buffer distance.
- Sludge goes to the Sludge Treatment Facility at Tuen Mun; large sewage works follow the Harbour Area Treatment Scheme approach, with chemically enhanced primary treatment at Stonecutters Island.
- Provide n+1 redundancy on blowers, pumps and dosing, and state the peak factor you used - treatment plants are sized on peak wet-weather flow, not average dry-weather flow.
Worked example: aeration tank volume
Flow 10,000 m3/d; influent BOD5 250 mg/L = 0.25 kg/m3, so BOD load = 10,000 × 0.25 = 2,500 kg/d. Choose F/M = 0.15 kg BOD per kg MLSS per day (a value compatible with nitrification) and MLSS = 3,500 mg/L = 3.5 kg/m3. Then V = load ÷ (F/M × MLSS) = 2,500 ÷ (0.15 × 3.5) = 4,762 m3. Check it: hydraulic retention time = 4,762 ÷ 10,000 = 0.48 d, about 11.4 hours - plausible. Then verify the sludge age is long enough for nitrification at the design temperature, and check aeration capacity against roughly 1.5 kg O2 per kg BOD removed with a peak factor applied.
Traps that cost marks
- Mixing mg/L and kg/m3, or using influent load without subtracting what leaves in the effluent.
- Sizing on average flow and then discovering the peak doubles the tank.
- Recommending membrane bioreactors for footprint alone without acknowledging the energy penalty.
- Designing the process and forgetting odour control, noise enclosures and sludge handling - the parts most likely to generate complaints and permit breaches.
- No commissioning or proving-test plan, so nobody can demonstrate the plant meets its licence.