Dry-weather flow (DWF) in sewer design: Which of the following best describes dry-weather flow for an urban catchment, excluding any contribution from rainfall or storm runoff?

Introduction / Context:
Dry-weather flow (DWF) is a foundational term in sanitary sewer design and operation. It represents the sewage generated from households, institutions, and industries under normal conditions without rainfall contribution. Understanding DWF ensures proper sizing of pipes, appurtenances, and treatment units for routine operation and capacity checks.

Given Data / Assumptions:

• DWF excludes stormwater or infiltration due to rainfall.
• Represents typical sanitary and trade wastewater over a day.
• Used as a basis for minimum-velocity checks and treatment plant loading.

Concept / Approach:
In practice, DWF is treated as the average daily sewage flow under non-rainfall conditions. Peak factors (e.g., hourly or diurnal variations) are later applied to this base value to determine peak DWF. Separately, wet-weather flow (WWF) includes DWF plus infiltration/inflow from storms.

Step-by-Step Solution:
Identify what is excluded: rainfall-derived infiltration/inflow.Identify the period basis for DWF: daily sanitary generation.Therefore, DWF is the average daily rate of flow under dry conditions.

Verification / Alternative check:
Design manuals define DWF as the average sanitary sewage discharged per day (before applying peaking factors and without storm contributions). This aligns with utility practice worldwide.

Why Other Options Are Wrong:
Average monthly/annual rates: used for longer-term accounting, not the operational definition of DWF.Per-capita water supply allowance: a planning input, not the definition of actual sewage flow.None of these: incorrect because the daily basis is correct.

Common Pitfalls:

• Confusing DWF with peak DWF (after peaking factor).
• Including storm infiltration in DWF; that belongs to WWF.

Final Answer:
Average daily rate of flow