The quantity of sewage generated in a town is generally estimated as approximately:

Question

The quantity of sewage generated in a town is generally estimated as approximately:

1. A. 20-30% of per-capita water supplied
2. B. 50-60% of per-capita water supplied
3. C. 75-85% of per-capita water supplied (Correct answer)
4. D. 100% of per-capita water supplied

Correct Answer

Option C — 75-85% of per-capita water supplied

Detailed Solution & Explanation

The correct answer is 75-85% of per-capita water supplied.

Key Points

• Sewage generation is estimated as a fraction of water supplied, because nearly all water delivered to a household returns to the sewer after use. The accepted design figure is 75-85%, commonly taken as 80%.
• The shortfall of roughly 20% accounts for water that never reaches the sewer — lawn and garden watering, evaporation, cooking losses, industrial consumption in the product, and leakage in the distribution system.
• The remaining options fail on physical grounds: 20-30% and 50-60% understate return flow badly, while 100% ignores consumptive losses entirely.

Additional Information

• The design flow is not simply this average. Sewers are sized for peak flow, obtained by applying a peak factor to the average — typically about 3 for small populations and falling to around 1.5-2 for large ones, since flows even out as the contributing population grows.
• Working the other way, sewers must also stay self-cleansing at minimum flow, usually taken as about one-third of the average, so that solids do not settle and septicise in the pipe.
• Added to sanitary sewage are infiltration through joints and defects and, in older or partially combined systems, stormwater inflow — both of which the designer must allow for separately.
• CPHEEO's *Manual on Sewerage and Sewage Treatment* is the governing Indian reference for these coefficients.