Environmental Engineering: Water Supply, Wastewater, Air and Solid Waste Management
This domain mixes design calculations with standards: water demand and treatment trains, sewer hydraulics, BOD kinetics and effluent limits, plus air-quality and solid-waste fundamentals.
Core rules
- Per capita demand is commonly taken as 135 lpcd; peak factors are 1.5 for distribution mains and 3 for trunk mains; fire demand follows Kuichling's formula.
- Settling velocity from Stokes' law is vs = g(ρs − ρ)d²/(18μ); the overflow rate of a clarifier equals the settling velocity of the slowest particle removed.
- BOD exertion is first order: BODt = L₀(1 − e^(−kt)); the standard is 5 days at 20 °C, with k ≈ 0.23 per day for domestic sewage.
- Activated sludge: F/M = Q S₀/(V X); sludge age θc = V X/(Qw Xr + Qe Xe).
- Stream self-purification is assessed by the Streeter–Phelps oxygen sag.
- Air quality is judged against the National Ambient Air Quality Standards; composting is governed by moisture, C/N ratio and calorific value.
Worked example
A sample has a 5-day BOD of 200 mg/L and k = 0.23 per day at 20 °C. Ultimate BOD L₀ = 200/(1 − e^(−0.23×5)) = 200/(1 − 0.3166) = 293 mg/L. At 25 °C the rate constant rises roughly by 1.047^(T−20), so the same waste would show a higher BOD₅ for the same L₀.
Traps
Treating 5-day BOD as ultimate BOD; forgetting that COD includes non-biodegradable matter, so a BOD/COD ratio below about 0.4 signals poor biodegradability; mixing hours and days in detention time; and assuming complete mixing when computing the downstream deficit.