Municipal drainage design: If the ratio of drainage flow to sanitary sewage flow is 20, the peak dry weather flow (PDWF) will be approximately what fraction of the overall design discharge for the combined system?

Introduction / Context:

When designing combined sewer systems, engineers compare the magnitude of storm (drainage) flows to sanitary sewage flows. The ratio of drainage to sewage often reaches large values during storms, meaning the sanitary (dry-weather) component is a small fraction of the overall design discharge. This question checks whether you can translate a drainage-to-sewage ratio into the approximate fraction represented by the peak dry weather flow (PDWF) within the total combined design discharge.

Given Data / Assumptions:

• Drainage-to-sewage ratio = 20 (i.e., storm drainage ≈ 20 times the average sanitary sewage).
• Design discharge is for the combined system during storm conditions.
• PDWF is slightly larger than average sewage flow due to peaking factors from diurnal variations.

Concept / Approach:

If storm flow is 20 times the sanitary flow, then the sanitary component alone is 1/21 of the total. However, the system must pass the peak dry weather flow, which incorporates a peaking factor (typically 1.2–1.8 or more depending on context). Thus, PDWF is just over 1/20 of the overall design flow.

Step-by-Step Solution:

Verification / Alternative check:

Even if the peaking factor is modest (say 1.1), the fraction remains slightly above 5%. Therefore, PDWF is not less than 5% but rather slightly more than 5% of the combined design discharge under the stated ratio.

Why Other Options Are Wrong:

• 20% of the design discharge: Far too high; storm flow dominates.
• Slightly less than 5%: With kp > 1, this underestimates PDWF.
• None of these: Incorrect because the “slightly more than 5%” estimate is appropriate.
• About 10%: Overshoots typical PDWF fractions for a 20:1 ratio.

Common Pitfalls:

• Confusing average sanitary flow with PDWF; peaking matters.
• Ignoring that the “20” ratio refers to storm dominance in combined systems.

Final Answer:

Slightly more than 5% of the design discharge