Sanitary sewer design: if q is the average sewage flow from a city of population P, how is the maximum sewage flow (design peak) generally expressed for sizing main sewers?

Introduction / Context:
Sewer capacities must accommodate short-term peaks above the daily average. Designers apply a population-dependent peak factor (PF) to the average flow to obtain the design peak discharge for pipes and pumping stations.

Given Data / Assumptions:

• q represents average sewage flow; P is contributing population.
• Diurnal, seasonal, and infiltration variations cause peaks.
• Empirical PF relationships (e.g., Harmon, Babbit, or local codes) are used.

Concept / Approach:

The general relationship is Q_max = PF * q. PF decreases with increasing P because larger populations “average out” individual usage variability. Additional allowances may be added for infiltration/inflow (I/I) based on local experience and standards.

Step-by-Step Solution:

Verification / Alternative check:

Compare against monitored flow data where available; confirm that resulting velocities meet minimum and maximum criteria under peak and low flows.

Why Other Options Are Wrong:

(a) Ignores peaking; (b) a fixed factor is not universally valid; (d) inverts the relationship and would underestimate the peak.

Common Pitfalls:

Using an inappropriate PF for population size; omitting infiltration allowances; not checking pump cycles or storage needs during extreme peaks.

Final Answer:

Q_max = PF * q, where PF is a peak factor dependent on P and local criteria