Environmental Engineering (Water Supply, Sanitation and Public Health)
Environmental engineering covers water demand, treatment and distribution, sewage generation and treatment, solid waste and public health. The exam tests design periods, per-capita demand, treatment sequence and BOD.
Core rules
- CPHEEO guidance uses about 135 lpcd for domestic demand in Indian towns; peak factor is commonly 1.5–2.0.
- Water treatment train: aeration, coagulation–flocculation with alum, sedimentation, filtration (rapid sand), disinfection by chlorination.
- IS 10500 drinking-water standards: pH 6.5–8.5, turbidity 1 NTU acceptable, residual free chlorine 0.2–0.5 mg/L at consumer end.
- BOD is oxygen consumed by microbes at 20°C over 5 days; COD is a chemical measure and is higher than BOD.
- Sewage flow is often taken as 70–80% of water supply; self-cleansing velocity in sewers is about 0.6–0.9 m/s.
Worked example
A town of 50,000 people has a demand of 135 lpcd. Average demand = 50,000 × 135 = 6.75 × 10⁶ L/day = 6.75 MLD. With a peak factor of 1.5, design flow = 10.125 MLD. A treatment plant must be sized for the peak, while storage and source capacity are checked against average and fire demand.
Traps
Using 135 lpcd without converting to MLD; treating BOD and COD as equal; forgetting chlorination contact time; designing sewers for minimum velocity without checking self-cleansing; confusing MPN with colony count.