Select the correct statements used in design and operation of sanitary sewers for municipal systems.
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AMaximum daily flow is commonly taken as about 2 times the average daily flow
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BMaximum hourly flow is often taken as about 3 to 6 times the average hourly flow, depending on population
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CSewers are designed to maintain a minimum self-cleansing velocity at minimum flow conditions
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DAll of the above
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EOnly (a) and (c)
Answer
Correct Answer: All of the above
Explanation
Introduction / Context:Peaking factors and self-cleansing velocity are basic to sewer design. They determine pipe sizes and slopes that avoid surcharge and sediment deposition.
Given Data / Assumptions:
- Municipal sewer system with diurnal variability.
- Typical peaking factors from empirical curves (e.g., Harmon, Babbit).
- Self-cleansing velocity threshold around 0.6–0.75 m/s (varies with grit).
Concept / Approach:Designers apply peaking factors to average flows to estimate maximum daily/hourly flows. They also ensure that at minimum flow the velocity exceeds the self-cleansing limit to avoid deposition.
Step-by-Step Solution:Step 1: Adopt max day ≈ 2 × average day (commonly used planning factor).Step 2: Adopt max hour ≈ 3–6 × average hour (population dependent).Step 3: Provide slopes to keep v ≥ v_self-cleansing at minimum flow.Step 4: Recognize that all three statements are applied together in design.
Verification / Alternative check:Standard practice manuals and codes show comparable peaking factors and self-cleansing criteria; actual values may be refined by local data.
Why Other Options Are Wrong:
- (a) and (c) only: Omits hourly peaking, which is critical.
Common Pitfalls:Using a single peaking factor for all sizes; neglecting minimum-velocity checks resulting in chronic silting.
Final Answer:All of the above