# How to Size a Sewage Treatment Plant

Please refer to British Water Flows and Loads Guide for further information.

**Single House**

A treatment system for a single house with up to and including 3 bedrooms shall be designed for a minimum population (P) of 5 people.

The size of a treatment system for a single house with more than 3 bedrooms shall be designed by adding 1 P for each additional bedroom to the minimum single house value of 5 P

- House with 3 bedrooms = minimum 5 P system
- House with 4 bedrooms = minimum 6 P system (5+1)
- House with 6 bedrooms = minimum 8 P system (5+3).

**Groups of Houses**

A treatment system serving a group of houses shall be designed by adding together the P values for each house calculated independently, eg: for a group of two houses (3 and 4 bedrooms, respectively) the system shall be for a minimum of 11 P (5+6)

If the calculated total for a group of houses exceeds 12P, then some reduction may be made to allow for the balancing effects on daily flow of a group of houses, like this:

- Where the total is 13-25P, multiply the total by 0.9 to give an adjusted P value e.g. if there are four 4 bedroom houses, the total P would be 24P (4 x 6) and the adjusted P would be 22P (24 x 0.9 = 21.6). Note that the P value should always be rounded up not down.
- Where the total is 26-50P, multiply the total by 0.8 to give an adjusted P value e.g. if there are four 3 bedroom houses and three 4 bedroom houses, the total P will be 38 P (4 x 5 and 3 x 6) and the adjusted P will be 31P (38 x 0.8 = 30.4)

- Where there are larger groups of houses, the P should be estimated using both the expected total load and the flow, considering both peak and total flow.
- These are minimum recommended population (P) loads, they should not be modified downwards, upward modification may be necessary because of particular characteristics of each property or groups of properties.
- The above assessments of population (P) should be used for both existing and new properties