For proper design of sewerage components, the design period is normally taken as:

Question

For proper design of sewerage components, the design period is normally taken as:

1. A. 5-10 years for sewers and 1-5 years for treatment works
2. B. 10-15 years for sewers and 5-10 years for treatment works
3. C. 20-30 years for sewers and 15-25 years for treatment works (Correct answer)
4. D. 40-50 years for both sewers and treatment works

Correct Answer

Option C — 20-30 years for sewers and 15-25 years for treatment works

Detailed Solution & Explanation

The correct answer is 20-30 years for sewers and 15-25 years for treatment works.

Key Points

• The design period is the number of years for which a facility is expected to serve adequately before it must be expanded.
• Sewers are designed for the longer period, 20-30 years, because they are buried, long-lived, and enormously disruptive and costly to dig up and enlarge once roads and buildings are established.
• Treatment works take a shorter 15-25 years, since plants are modular — additional aeration tanks or clarifiers can be added on land already acquired, so there is no advantage in over-building at the outset.

Additional Information

• The principle generalises: the harder something is to augment later, the longer its design period. Trunk sewers and outfalls may be designed for the ultimate saturation population; pumping machinery, being replaceable, is often designed for only 10-15 years.
• The choice is a balance of two costs. Too long a period means capital locked up in idle capacity and, in a sewer, flows too low to be self-cleansing in early years. Too short means premature reconstruction.
• Population at the end of the design period is forecast by the arithmetical increase, geometrical increase, incremental increase or logistic curve methods, the last being most defensible for a town approaching saturation.
• Rate of interest and expected life of the material also enter the decision — a stoneware or RCC sewer may physically last far beyond its hydraulic design period.