Formula A is a flow value (litres/day) which is the required minimum flowrate in the sewer before any spill occurs to the CSO. Formula A varies slightly depending on the type of sewer (i.e. fully combined, separate etc.)
Fully Combined System Formula A = DWF + 1360 P + 2 E
Fully Separate System Formula A = 3DWF = 3PG + I + 3E
- DWF = (Dry Water Flow (l/d) = P . G + I + E)
- P = Population (number) (contributing to flow rate)
- G = Daily Water Consumption (l/p/d) (of population)
- I = Infiltration (l/d)
- E = Effluent (Industrial Effluent) (l/d)
Mixed system Formula A = DWF + 1360Pc + 2PsG + 2Et
- DWF = (Dry Water Flow (l/d) = Pt . G + It + Et)
- Pt = Total population
- Ps = Population served by storm sewer
- Pc = Population server by combined sewer
- Et = Trade flow from the total area
- It = Total infiltration
So sewers must be able to contain at least the flow equivalent to that derived from formula A without spilling via a CSO to a water course.
If the sewer is spilling into a watercourse, and the flow passing through the sewer at the point of spill is less than the formula A value, then the CSO could be classified as operating unsatisfactorily, and changes to the sewer system may be required.
So, when if a sewer CSO ‘passes Formula A’ it means that the flowrate calculated from the Formula A passes through the sewer and the sewer is not spilling.
In Scotland SEPA set Formula A, In England it’s the Environment Agency.