Wet and Dry Weather Flows in Wastewater Design

Sewage flows within a sanitary sewer system are divided into two major categories: wet weather flows and dry weather flows. 

Wet weather flows include sewage flows and runoff that infiltrate into the sanitary sewer systems during a storm event.  Wet weather flows also include groundwater flows that enter through defective pipe joints, connections and/or manhole walls.

Dry weather flows represent all flows within the sewer pipes on a typical day without precipitation and the results of lower flow to separate sanitary sewer systems.  During dry weather flow, insufficient flow velocities can increase the retention time within the pipes and channel.  Therefore, these flows could result in undesirable sedimentation of solid particles and biological/chemical reactions.

When determining wastewater flow within the City of Austin, residential single-family units shall be assumed to produce an average wastewater flow of 245 gallons per day and industrial wastewater flows will be evaluated on a case-by-case basis.  External contributions are accounted for by including 750 gallons per day per acre served for inflow and infiltration.  For sewer systems in the Edwards Aquifer Zone refer to the Texas Commission on Environmental Quality (TCEQ) requirements (Chapter 213). Per TCEQ, strict attention shall be given to minimizing inflow and infiltration.

Peak Dry Weather Flow

The peak dry weather flow is derived from the formula:

Qpd = [(18+(0.0206 × F)0.5)/(4+(0.0206 × F) 0.5)] × F

where: F = 70 gal./person/day x population/1440
= average dry-weather flow in gpm

Peak Wet Weather Flow

The peak wet weather flow is obtained by adding inflow and infiltration to the peak dry weather flow. When designing for an existing facility, flow measurement shall be used in lieu of calculations for the preexisting developed area.

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