Water demand
A water supply scheme is sized for the demand at the end of its design period, often about 30 years for the main works, so the first step is forecasting population and per-capita use.
Population forecasts use methods such as arithmetical increase, geometrical increase and incremental increase, chosen to suit how the town has grown. Per-capita demand covers domestic use plus allowances for institutions, commerce, industry, fire-fighting and unavoidable losses. Indian planning norms vary with the size of the town and whether it has a sewerage system; the governing figures are in IS 1172 and the CPHEEO manual.
Demand is not constant. It varies by season, by day and through each day, with morning and evening peaks. Intake and treatment works are usually sized for the maximum daily demand, while distribution mains are checked for the peak hourly flow, which can be two to three times the average depending on population.
Fire demand is added for distribution design. Losses, from leaks and unauthorised connections, must be measured and reduced rather than simply allowed for.
Arithmetical increase: P_n = P₀ + n x̄Geometrical increase: P_n = P₀ (1 + r)ⁿAverage demand Q = P × q
Problem. A town had 40 000 people; growth averages 3 % per decade compounded. Forecast after three decades.
- P = 40 000 × 1.03³ = 40 000 × 1.0927 = 43 709.
Answer. About 43 700
Quick quiz
1What demand sizes treatment works?
Maximum daily demand.
2Name two forecasting methods.
Arithmetical and geometrical increase.
3Which standards give per-capita norms in India?
IS 1172 and the CPHEEO manual.
- IS 1172 (India): Basic requirements for water supply, drainage and sanitation, including per-capita demand.
- CPHEEO Manual on Water Supply and Treatment (India): Central government manual for planning, design and operation of public water supply.
Cited by number and clause only. We do not reproduce clause text, tables or figures (BIS Act 2016, s.10(5) and s.11). Check the current edition and amendments before use.
References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- S. K. Garg, Water Supply Engineering (Environmental Engineering Vol. I), Khanna Publishers, latest edition.
- MWH (J. C. Crittenden et al.), Water Treatment: Principles and Design, Wiley, 3rd edition.
- H. S. Peavy, D. R. Rowe, G. Tchobanoglous, Environmental Engineering, McGraw Hill, 1985.
Free and official sources
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