Dry Weather Flow

Published: October 4, 2018 | Last updated: July 5, 2023

What Does Dry Weather Flow Mean?

Dry weather flow is the flow of wastewater in a sewer system during dry weather that presents with minimal infiltration. Sewer systems depend on gravity to ensure that solids and liquids continue on their way to treatment plants. During these dry spells, a "sewer drought" can occur as the amount of liquid in the sewer is reduced. This reduction in volume ensures a reduction in flow velocity.

Trenchlesspedia Explains Dry Weather Flow

The critical nature of gravity as a component of sewer design is highlighted during times of dry weather flow. Low or nonexistent sewer flow velocities increase the retention time of liquids and solids in sewer lines. This problem is exacerbated if the sewer lines are designed without sufficient slope for gravity to do its job. As these solids and liquids remain in a sewer line, they built up (sedimentation and undesirable short) and various undesirable reactions – both biological and chemical – occur. For example, the longer these solids and liquids remain in a sewer line, the potential for bacterial growth increases. The resulting effluent not only creates problems within the sewer line, but can create both health and disposal issues in treatment plants.


Sewer drought

Sanitary sewer peaking factor


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