Choosing the right size sewage treatment plant is important for compliance, performance and long‑term cost reasons. We’ve previously highlighted whether you need planning permission for sewage treatment plants and how they differ from septic tanks.
To ensure you size correctly for your home or business premises, there are clear guidelines to follow.
What does the British Water Code of Practice say?
The snappily titled ‘Flows and Loads – 4: Sizing Criteria, Treatment Capacity for Sewage Treatment Systems’ is the relevant document by British Water.
This Code of Practice outlines the first step for UK homes and small businesses – calculating the ‘minimum population figure’ (symbolised as P).
This value is tied to the total number of bedrooms in the property or properties to be served by the sewage treatment plant.
The Code of Practice recommends the following:
‘For a single house with up to, and including, three bedrooms, a Minimum Population (P) of 5 people should be calculated. For each additional bedroom in a property thereafter, you should calculate for 1 additional person.’
How do I calculate my PE value?
Using the above formula, it’s straightforward to reach your ‘population equivalent’ (PE), by adding one person per additional bedroom – for example:
- Property with four bedrooms = minimum 6 PE (5+1)
- Property with six bedrooms = minimum 8 PE (5+3)
- Property with eight bedrooms = minimum 10 PE (5+5)
For a smaller house or flat, with one or two bedrooms, allow for the following values:
- Property with one bedroom = 3 PE
- Property with two bedrooms = 4 PE
And finally, for a group of properties served by the same sewage treatment plant, you add together the P values for each one, as follows:
- Two houses (with three and four bedrooms respectively) = minimum of 11 PE (5+6)
Once the total exceeds 12PE, then reductions can be made to allow for efficiency of scaling, rounding up rather than down.
So if the total is in the range of of 13-25 PE, multiply by 0.9 to give an adjusted value, such as:
- Four houses, each with four bedrooms = 24, which is then multiplied by 0.9 to give an amended value of 22 PE
And similarly, if the total PE value of a group of houses falls between 26-50, multiply by 0.8, as follows:
- Four three-bed houses and three four-bed houses = 38 PE (adjusted to 30.4 after being multiplied by 0.8)
How do I size my sewage treatment plant?
Most packaged sewage treatment plants have a rating that corresponds to your calculated PE number (e.g. 5, 10, 20 PE). Aim to select the one that comfortably equals (or even slightly exceeds) your PE value.
What if you anticipate peaks of higher domestic use during holidays, or you run a business that sees certain periods of heavier use (e.g. pubs, guesthouses)?
Simply choose a sewage treatment plant with extra capacity to prevent any potential overloading at such times. And don’t forget to allow for any anticipated changes in occupancy use (e.g. converting a larger property into flats or holiday lets).
What else do I need to know about sewage treatment plant size?
Check that any plant picked satisfies your local standards and regulatory permits in terms of discharge to surface waters or soakaways.
Always buy from reputable suppliers and ensure installation meets the relevant standards in the Flows and Loads guidance above.
And if you need further practical advice on sizing, purchasing, installing or servicing a sewage treatment plant, please get in touch.