Customer Question: How do you calculate expected dry weather flow and Volume of effluent to be discharged per day?
Dry Weather Flow is in essence the sewage flow, from a premises without the addition of any surface water.
Dry Weather Flow (DWF) Full Definition: The average daily sewage flow entering a Sewage treatment plant or sewer measured following 7 days without rain (excluding local or Bank holidays etc) and during which on the preceding 7 days, rainfall did not exceed 25 mm on any day.
The volume of effluent is usually calculated using population equivalents (PE) 1 PE is the amount of sewage generated by one person living in a domestic dwelling and is generally taken as 200 L of flow and 60 g of BOD/day. Therefore, the DWF for a 10 PE premises would be:
10 x 200 L = 2000 L.
Full worked examples of how to calculate PE in different situations can be found in our Education Blog in Sewage Parameters 3: Population Equivalent (PE) Parts 1 & Population Equivalents (PE) Part 2. If you have any further questions please do not hesitate to contact Butler Manufacturing Services Ltd (BMS) We will be continuing to be update our education blog with helpful straightforward information on sewage treatment and the terminology surrounding it.