Sewer flow variation: The ratio of minimum hourly flow to average sewage flow is typically closest to which value?

Introduction / Context:

Sewer design requires knowledge of diurnal variation in sanitary flows. Minimum hourly flows govern issues such as odor control, sulfide formation, and maintenance of self-cleansing velocities during off-peak periods.

Given Data / Assumptions:

• Domestic sanitary flow exhibits morning and evening peaks and overnight lows.
• Typical design guides assume empirical ratios for peak and minimum hourly flow relative to the average daily flow.
• Industrial contributions may alter the profile but household-dominant systems follow common rules of thumb.

Concept / Approach:

Common guidance sets peak hourly flow at roughly 1.5–3.0 times the average, while the minimum hourly flow can be around 0.5 times the average for typical communities. This accounts for human activity patterns and building plumbing behavior.

Step-by-Step Solution:

Verification / Alternative check:

Multiple design manuals and empirical curves for domestic systems show minimum-hour factors near 0.4–0.6. Selecting 1/2 matches the central tendency and is widely used for preliminary design.

Why Other Options Are Wrong:

• 1/4 or 1/3: Too low for typical domestic diurnal curves, may apply only to very small or intermittent populations.
• 3/4 or 2/3: Too high; do not reflect the pronounced overnight lull commonly observed.

Common Pitfalls:

• Using peak-to-average ratios for minimum flow checks.
• Ignoring infiltration/inflow which can slightly raise the minimum but is not the governing sanitary component.

Final Answer:

1/2