What is dry weather flow?
Dry weather flow (DWF) is the amount of domestic and trade wastewater that flows through the sewer system even in dry weather, separate from rainwater. In modern separate sewer systems the dry weather flow is kept apart from rainwater so the treatment plant runs more efficiently.
How does dry weather flow work?
In a separate sewer system there are two pipes:
- The dry weather flow (DWF) pipe carries wastewater from toilets, showers, kitchens and businesses to the treatment plant
- The storm water pipe carries rainwater separately to surface water or to infiltration
Because rainwater is kept out of the foul sewer, the flow into the treatment plant stays roughly constant. Even in heavy rain, the system remains stable without combined sewer overflows discharging foul water into rivers.
Application
- New developments — separate systems are usually required by building regulations
- Renovation and remodelling — downpipes are disconnected from the DWF system
- Industrial sites — trade effluent is connected separately to DWF
- City centres — to limit flooding during heavy rain
- Low-lying areas — to reduce pumping station loads
Points of attention
- Never connect rainwater to the DWF pipe — it overloads the system
- Maintain a minimum gradient of 1:200 on horizontal runs
- Use the correct pipe diameter; undersized pipes cause smells
- Provide rodding access for cleaning
- Clean groundwater or spring water should not enter the DWF
Related terms
- Wastewater
- Drainage
- Sewerage
- Fall (slope)
- Building regulations
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