Design peaking factor for large sanitary conduits: For trunk sewers with diameter greater than 1.25 m, what ratio of maximum daily sewage flow to average daily sewage flow is generally assumed for design?

Introduction / Context:
Sanitary sewers are sized using peaking factors to accommodate diurnal variations. Larger trunk mains exhibit lower relative peaking than small laterals because flows are time-averaged over large contributing areas. Designers therefore use smaller peak-to-average ratios for large conduits.

Given Data / Assumptions:

• Trunk sewer diameter > 1.25 m.
• Need the ratio: maximum daily flow / average daily flow.
• Conventional municipal design practice (separate sewer system).

Concept / Approach:

Empirical peaking factors reduce with increasing service area or conduit size. Typical guidance: small laterals may take ratios of 2.0–3.0 (or more for peak hourly), while large trunk mains commonly assume about 1.5 for the maximum daily to average daily ratio. This reflects attenuation and storage within the system that smoothes peaks at the trunk level.

Step-by-Step Solution:

Verification / Alternative check:

Many planning manuals tabulate decreasing peaking factors with rising population served and pipe size, with 1.5 commonly adopted for large trunks.

Why Other Options Are Wrong:

2.0–3.5 are more appropriate for smaller sewers or for maximum hourly peaking; they overstate daily peaking for trunk mains.

Common Pitfalls:

Confusing daily peaking with hourly peaking; applying small-lateral peaking factors to trunks without attenuation.

Final Answer:

1.5