Water Supply, Sanitation and Environmental Engineering
Water Supply and Sanitation is the public-health branch of civil engineering. The exam asks for per-capita demand, treatment trains, distribution layouts, plumbing, and sewage collection and disposal, with numericals on demand and sewer velocity.
Demand
IS 1172 gives 135 litres per head per day for domestic use in towns with full flushing, and 40-45 lpcd where only standpipes exist. Add industrial and commercial demand, fire demand from the Kuichling or Freeman formula, and 15-20 percent for losses. A design period of 30 years is usual for water supply schemes.
Treatment
Screening, then plain sedimentation for 2-4 hours, coagulation with alum, flocculation, rapid sand filtration at about 4.8-6 m/h, and disinfection with chlorine. A residual free chlorine of 0.2 mg/L should remain in the distribution system.
Worked example
A town of 50 000 people at 135 lpcd has an average demand of 6 750 000 litres per day, that is 6.75 MLD. Applying a peak factor of 1.5 gives 10.125 MLD, which is about 421 m3 per hour for sizing the pumping main.
Sewerage
Self-cleansing velocity is 0.6 m/s and the maximum permissible velocity is 3.0 m/s. Manholes are provided at every change of direction, gradient or diameter, and at 100-150 m intervals on straight runs. BOD5 is the oxygen consumed in five days at 20 degrees Celsius; COD is determined chemically and is always higher.
Traps
- Quoting 135 lpcd as the peak rather than the average demand.
- Confusing BOD with COD, or the chlorine dose with residual chlorine.
- Describing a septic tank as an Imhoff tank.
- Forgetting the peak factor and the allowance for losses.