Consider the following statements about sewage discharge and design periods:

Question

Consider the following statements about sewage discharge and design periods:

I. Per-capita sewage generated is generally taken as about 75-85% of per-capita water supplied.

II. The average daily sewage flow is sufficient for sizing all components of the sewerage system without any peak-factor consideration.

III. Design periods for sewers are usually longer than those for treatment-plant components.

Which of the statements given above is/are correct?

1. A. I and II only
2. B. I and III only (Correct answer)
3. C. II and III only
4. D. I, II and III

Correct Answer

Option B — I and III only

Detailed Solution & Explanation

The correct answer is I and III only.

Key Points

• Statement I is correct. Per-capita sewage generation is conventionally taken as 75-85% of per-capita water supplied, the balance being consumptive losses.
• Statement II is incorrect, and it is the deliberate error. The average daily flow does not size every component. Sewers and pumping mains are designed for peak flow, while self-cleansing velocity is checked at minimum flow — three different flows for three different checks.
• Statement III is correct. Design periods differ by component precisely because the cost and disruption of later augmentation differ: long for buried sewers, shorter for expandable treatment units.

Additional Information

• The peak factor falls as contributing population rises, because individual peaks in use no longer coincide. A small colony may need a factor near 3, a large city closer to 1.5.
• Sizing on the average flow would under-design the sewer, causing surcharging and backflow into house connections during the morning peak — a failure that shows up immediately in service.
• Conversely, sizing everything on the ultimate peak would leave early-year flows too shallow and slow to scour the invert, so solids deposit and generate hydrogen sulphide and crown corrosion.
• Statement-based questions of this shape usually hide the error in an absolute word; here it is "all components".