What Size Sewage Treatment Plant Do I Need?

Once the requirement for a Sewage Treatment Plant system has been established at the premises, it is important to choose the correct sized package. Manufacturers of sewage treatment plant systems offer an indication on what their options will suit; however it is purely indicative.

The sizing is governed by the ‘Flows & Loads Guide – 4 Code of Practice’ British Water: 

Sewage Treatment Plants for Domestic Housing

A treatment 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 1P for each additional bedroom to the minimum single house value of 5 P


  • house with 3 bedrooms = minimum of 5P system
  • house with 4 bedrooms = minimum 6P systems (5+1P)
  • house with 6 bedrooms = minimum 8P system (5+3)

For groups of small 1 and 2 bedroom properties

Property with 1 bedroom = allow 3P

Property with 2 bedrooms = allow 4P

Private vs. Shared Sewage Treatment Plants

A treatment system serving a group of houses shall be designed by adding together the P values of each house calculated independently, e.g.

For a group of two houses (3 and 4 bedrooms, respectively) the system shall be for a minimum of 11P (5+6)

If the calculated total P for a group of houses exceeds 12 P then some reduction may be made to allow for the balancing effects on daily flow of a group of houses (round UP not down)

Where the total is 13-25P multiplay the total by 0.9 to give an adjusted value, e.g. if there are four four-bedroom houses the total P will be 24P (4x6) and the adjusted P will be 22 P (24 x 0.9 = 21.6)

Where the total is 26-50P multiply the total by 0.8 to give an adjusted P value, e.g. if there are four three-bedroom houses and three four-bedroom houses the total P will be 38P (4 x 5 and 3 x 6) and the adjusted P will be 31 P (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 group of properties.

The above assessments of population (P) should be used for both existing and new properties.

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