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.

Formula A is set by Environmental Protection Agencies.

In Scotland SEPA set Formula A, In England it’s the Environment Agency.