Design basis for dry weather flow (DWF): It is customary to size sanitary sewers for a peak DWF equal to what multiple of the average dry weather flow?

Introduction / Context:
Sanitary sewers must carry variable flows. Even in dry weather, diurnal patterns and stochastic variations cause peaks above the average. Designers therefore apply a peaking factor to the average DWF to obtain a practical design discharge for hydraulic sizing.

Given Data / Assumptions:

• Sanitary sewer without significant infiltration or storm inflow.
• Conventional practice values for peaking factor in local conditions.
• Average DWF known from per capita flow and contributing population.

Concept / Approach:
Standard practice often uses approximately 3 times the average DWF (for small to medium systems) to cover daily peaks, upstream synchronization, and uncertainties. Larger trunk mains may justify lower factors based on empirical formulas, but 3.0 is a frequently cited benchmark in examination problems.

Step-by-Step Solution:
Compute average DWF from unit contributions.Apply peaking factor ≈ 3 to obtain design discharge.Check velocity criteria (self-cleansing and non-scouring) at both minimum and peak flows.

Verification / Alternative check:
Alternative peaking factor relations (for example population dependent) often yield values near 2.5–3.0 for many service areas, corroborating the 3.0 choice in typical exam settings.

Why Other Options Are Wrong:
1.0–2.0: may under-size upstream sewers resulting in surcharge.4.0–5.0: overly conservative for DWF and could be reserved for special cases or combined systems.

Common Pitfalls:

• Using the same peaking factor for all pipe sizes and populations without judgment.
• Ignoring infiltration and inflow which can change design basis.

Final Answer:
Thrice the average demand (3.0 times)