Customer Question:

How do you calculate expected dry weather flow and the volume of effluent to be discharged per day?

What is Dry Weather Flow (DWF)?

• Definition:
  Dry Weather Flow is the average daily sewage flow entering a sewage treatment plant or sewer, excluding any addition of surface water.

• Conditions for Measurement:

  • Measured following 7 consecutive days without rainfall, excluding holidays.

  • Rainfall during the preceding 7 days should not exceed 25 mm on any given day.

How to Calculate DWF:

The volume of effluent is calculated using Population Equivalents (PE).

1. Population Equivalent (PE):

   • 1 PE is defined as the amount of sewage generated by one person living in a domestic dwelling.

   • Assumed sewage flow:

     • 200 litres of flow/day

     • 60 grams of BOD/day

2. Formula for DWF:
   DWF=Population Equivalent (PE)×Flow per PE (200 L/day)\text{DWF} = \text{Population Equivalent (PE)} \times \text{Flow per PE (200 L/day)}DWF=Population Equivalent (PE)×Flow per PE (200 L/day)

3. Example Calculation:

   • For a premises serving 10 PE:
     10×200=2000 litres/day (2 m³/day)10 \times 200 = 2000 \, \text{litres/day (2 m³/day)}10×200=2000litres/day (2 m³/day)

Additional Information:

• Worked Examples:
  Detailed examples for calculating PE in various scenarios are available in our Education Blog, specifically:

  • Sewage Parameters 3: Population Equivalent (PE) Part 1

  • Population Equivalents (PE) Part 2

• Contact for Queries:
  For further questions or guidance, please reach out to Butler Manufacturing Services Ltd (BMS).

Our Education Blog is continuously updated with clear and helpful information about sewage treatment and related terminology.

About BMS

BMS is a quality manufacturer of package products for wastewater and stormwater treatment:

• Established: 1986

• Global Reach: Exported to over 50 countries.

• Family Business: A third-generation company with deep expertise.

BMS Product Range

BMS provides a wide array of surface water and wastewater treatment products, including:

• Attenuation systems

• Interceptors

• Holding tanks (chemical-resistant options available)

• Hydrobrakes

• Rainwater harvesting tanks

• Vortex silt/oil/debris separators

• Fully fitted pumping stations

• Recyclone: A plug & play vehicle wash water recycling system

View the full product catalogue here

Additional Services by BMS

• Free Design Service: For sewage treatment, pump stations, and surface water management.

• Operation and Maintenance: Full services for existing installations, including repair, advice, and testing.

• Free CPD Presentations: Available for sewage treatment and surface water management, delivered at your location.

We would be delighted to discuss any requirements you might have, and we are available to meet you at your convenience.  Contact us today.

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