Guide to selecting the size and type of sewage treatment plant


If your property is unable to connect to the mains sewerage infrastructure, a sewage treatment plant is an ideal choice. It is considered the most cost-effective and environmentally friendly “off mains” drainage option. When it comes to selecting the size and type of the sewage treatment plant though there are a number of things to consider. Here’s a useful guide.

How does a sewage treatment plant work?

A sewage treatment plant, in the most simplistic of terms, is a tank that is located underground and is designed to treat incoming effluent to an environmentally acceptable quality. Once the water has been treated, it is then legally discharged to flowing surface waters or in-ground soakaways following applicable exemptions or consents.

How to size a sewage treatment plant?

First, you need to understand how many people the property can accommodate. This is not how many people that plan to live in the dwelling at that time but the maximum number of people that could potentially live in it. According to British Water Flows and Loads document the following sizes apply:

  • 3 bedrooms property must be sized at 5 people
  • 4 bedrooms property must be sized at 6 people
  • 5 bedrooms property must be sized at 7 people
  • 6 bedrooms property must be sized at 8 people

Multiple properties are sized individually and added together. From 13-25 population, you can reduce the figure by multiplying the population by 0.9. From 26-50 population, you can reduce the figure by multiplying by 0.8.

You can view the latest version of British Water Flows and Loads document here.


Calculation Example

Example A:

5 bedrooms = 7 people

Example B:

4 bedroom + 6 bedroom property = 6 people + 8 people = 14 people.
Then multiply by 0.9 & round-up: 14 x 0.9 = 12.6 (13 rounded up)


What is the difference between a pumped and gravity outlet?

Some sewage treatment plants will come with a pumped outlet and some with a gravity outlet. The preferred choice of sewage treatment plant is one with a gravity outlet. If your dwelling is on a slight incline or hill though, then a pumped outlet will be required. The pump is designed to lift the treated water out of the tank and into the ditch or stream.

What is a shallow sewage treatment plant?

A sewage treatment plant with a “shallow dig” profile is pretty much what it says on the tin – it won’t need anywhere near as deep an excavation as other sewage treatment plants. It is ideally suited for sites where hard rock conditions make excavations more difficult and can therefore end up being much more costly and time-consuming.

How can we help?

We are specialists when it comes to sewage treatment solutions, including sewage treatment plants, septic tanks, cesspools and sewage pumping stations, with over 25 years experience of supplying and installing these systems. We can supply you with a wide range of different brands and models to suit your specific requirements and if needed we can also provide you with an installation package. Please feel free to contact us for expert advice on 0121 351 3230.

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