Water Supply and Sanitary Engineering
This domain covers per-capita demand, sources and treatment of water, distribution, and the sanitary arrangements inside a building - traps, plumbing systems, septic tanks and drains.
Demand and sources
- Domestic demand is commonly taken as 135 litres per head per day; the total includes public, industrial and fire demand plus losses.
- Surface sources: rivers, lakes, reservoirs. Subsurface sources: open wells, tube wells, springs, infiltration galleries.
Treatment and distribution
- Sequence: screening → plain sedimentation with alum coagulation → filtration → disinfection by chlorination, leaving a residual chlorine of about 0.2 mg/L.
- Slow sand filters run at roughly 100–200 L per m² per hour; rapid sand filters at about 3000–6000 L per m² per hour.
- Distribution may be gravity, pumping or a combined system; a continuous supply is preferred to intermittent.
Sanitary fittings and drainage
- Every fitting is protected by a trap holding a water seal (typically 25–50 mm) to block foul gases; a vent pipe prevents the seal from being siphoned out.
- One-pipe and two-pipe systems; soil pipe carries sewage, waste pipe carries sullage.
- A septic tank retains sewage about 24 hours, settling sludge and digesting it; the effluent goes to a soak pit or drain field.
Worked example
A town of 10,000 people at 135 lpcd needs 10,000 × 135 = 1,350,000 litres per day = 1350 m³/day = 1.35 MLD. Allowing 15% for losses, the plant must supply 1.35 × 1.15 = about 1.55 MLD.
Marks-losing traps
- Writing 1 MLD = 100 m³ instead of 1000 m³.
- Claiming sedimentation makes water bacteriologically safe - only disinfection does that.
- Ignoring the vent pipe, which is the reason a water seal survives use.