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How to work out what size sewage treatment system I need

Sizing the correct sewage treatment system can be complicated. But if you know all of the details of the job you can do it easily. This is a guide for sizing a sewage treatment system.


You may expect the size of the system to be based around how many people it will serve. However, you should carefully consider if this number of people is expected to increase in future.


The first thing to determine is the scale of the project; a single dwelling or multiple homes. Secondly, you should consider the number of bedrooms in the property or properties that the system will serve.


Domestic Sewage Treatment (6- 12 people)


For one or two homes, the calculation is as follows:


• Base your calculation for a single home with up to three bedrooms on a minimum population of 5 people (5PE).

• For a single home with over 3 bedrooms, add 1PE for each extra bedroom. For example, a home with 5 bedrooms requires a minimum 7PE system (5 + 2).

• For a group of 2 homes with 3 and 4 bedrooms, respectively, the system should be a minimum of 11PE (5 + 6, as per the guideline above).


Domestic Sewage Treatment (12 to 50 people)


For any Sewage Treatment System designed to suit over 12 people, the calculation is a little different. You can make slight reductions in size to allow for the balancing effects on the daily flow of the group of houses, as follows:


• When the total number of people living in the property or properties is 13 to 25PE, multiply it by 0.9 to give an adjusted P-value. For example, for 6 four-bedroom houses, the total PE based on the first part of the calculation would be 36PE (6 x 6). The adjusted PE would then be 33PE (36 x 0.9 = 32.4). Please bear in mind that the PE-value should always be rounded up, not down.

• When the total populace is 26 to 50PE, multiply it by 0.8. For example, for 4 three-bedroom and 3 four-bedroom houses, the total PE would be 38PE (4 x 5 and 3 x 6). The adjusted PE would then be 31PE (38 x 0.8 = 30.4).


If you are baffled, don’t worry. Even the experts need a helping hand every now and then. It’s always advisable to check your sizing against British Water Flows and Loads – Sizing Criteria, Treatment Capacity for Sewage Treatment Systems.


If you require any additional assistance to work out what size sewage treatment plant to buy, call our friendly team of experts on 01388 537 050. Alternatively, email us sales@direct-drainage.co.uk


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