Design assumption for sewage quantity: For sewer design, what percentage of water supply is commonly assumed to appear as sewage discharge?

Introduction / Context:
In preliminary sewer design, the expected sewage quantity is often estimated as a percentage of the water supply. Accounting for losses (outdoor use, evaporation, leakage, consumptive use) determines the proportion that reaches sewers.

Given Data / Assumptions:

• Municipal conditions with typical indoor plumbing and fixtures.
• Conventional design practice references.

Concept / Approach:
Historically, 75–80% of the supplied water is assumed to return as wastewater (sanitary sewage), allowing for non-sewered uses and system losses. This rule-of-thumb aids early sizing prior to detailed metering or modeling.

Step-by-Step Solution:
Recognize loss mechanisms: landscape irrigation, leakage, consumption.Apply the standard planning fraction for sanitary flow: approximately 0.75–0.80 of water supply.Select the choice consistent with practice: 75 to 80%.

Verification / Alternative check:
Many municipal manuals adopt 0.8 of supply as an upper bound for sanitary contribution, with I/I (infiltration/inflow) added separately for peak wet-weather analysis.

Why Other Options Are Wrong:
65–75% ranges may understate flows in many urban contexts.85% or higher usually overstates returns unless special conditions exist.

Common Pitfalls:

• Failing to add separate allowances for infiltration/inflow and industrial discharges when applicable.
• Using a single percentage for all land uses without refinement.

Final Answer:
75 to 80%