Graf Sewage Treatment Plant

What Sewage Treatment Plant size for my house?

For a FREE quotation for any Septic Tank, Sewage Treatment Plant or Soakaway, replacement or installation, get in contact with us on 0800 3101092 or via our contact form.

What sewage treatment plant size for my house?

If you are thinking of installing a new sewage treatment plant at your property. Then it is important to know how big the tank needs to be. Even if you are having a professional company installing the tank it is good to know the basics.

Okay so talking about the basics, do you know the difference between a Septic Tank and a Sewage Treatment Plant? If not read this article. Right so now I am assuming you know the difference between the two.  The second caveat is this. This article is for Domestic Treatment plants only. Commercial sizing is far more complicated and you will need to speak to an expert (such as us J).

The guidance on sizing your tank is stipulated in Flows and Loads – 4 Sizing Criteria. Titled, Treatment Capacity for Sewage Treatment Systems. Created by an inter-agency panel led by British Water, you can see the full document here.

Sizing table for sewage treatment plants
Flows & Loads Regulation for waste water treatment plants

For domestic properties, the size of the tank is based on the minimum population of the property. This is calculated by the number of bedrooms the property has. For single properties, the population is very simple, see table 1 below. So a one-bedroom flat or house has a deemed population of 3, this then increases by 1 population (P) thereafter up to 10 bedrooms.

Okay if you have more than one house or flat sharing the system you have to treat each property separately and add up the P values. So, for instance, you have 3 houses, a 1 bed a 2 bed and a 3 bed, then the minimum size of the tank would be 12P. So the 1 bed = 3P the 2bed = 4P and the 3 Bed 5P. All of these rules hold up to a result of 12P.

If you have a house or group of houses that exceed 12P. Then some reduction can be made to the minimum size of the tank. Basically use the method we have used up until this point to calculate a P-value. You can then multiply this by 0.9. So, for instance, you had 3 x 3-bed houses, this would create a P value of 15, this can be reduced to give 15 x 0.9 = 13.5 P. For P values of between 26 and 50 you can use a factor of 0.8 should you wish. If you go over 50 P seek expert advice.

Regulation Plus considerations

Okay so now you have a minimum size of the tank that Building Control will allow. Further downward modification is not allowed.  But what if you are lucky enough to have an 8-bed house with only 2 of you living there for say half the year, with the rest spent in Monaco? Firstly, you shouldn’t be wasting your time reading this! The issue with this is that most sewage treatment plants are designed to run per the population size. If the tanks are under loaded they may not run correctly and the effluent quality may diminish.

The effluent quality is your responsibility. If it falls below the legal limits and you are for example discharging to a watercourse you will get fined if caught. If this is likely to be an issue then we recommend the Graf Advanced range (if ground conditions allow the installation). This has a built-in under load sensor which prevents this issue occurring.

Okay so the minimum size is sorted, all clear? Good, so would you ever look to go over the minimum? This is something you should discuss with your Building Control contact, okay caveat over. If for instance as with many houses we visit. You are thinking about building a two bedroom extension, then you should definitely consider sizing the tank on your desired property size. If you come to sell your house and the tank is below the minimum size due to expansion then you may face some problems.

Also if you have three bedrooms but you know you normally have 6 people living in the house all of the time. Then you need to apply some common sense. If this is all not clear at this stage you should speak to an expert.

Want to learn more?

We have a large collection of articles containing the answer to all of your questions. Check out our Blog Page 

Our Core Services: The Sewage Treatment expert Installers

On the 1st January 2020, all Septic Tanks that discharge to a Stream, Watercourse or Drainage Ditch will become illegal and will require replacing with a Sewage Treatment Plant. Please see here for more details.

    review rating 5  Really pleased with the whole experience with Homeseptic and the team. Working with Andy at the quotation stage, which was very competitive and Dave & James on site, who where very courteous and knowledgeable and worked with us at every stage of the installation. Weather was not great so thanks to James who gave up his Sunday to work on site to catch up. The area was left tidy and fences reinstated. We are now the proud owners of a state-of-the-art sewage treatment plant. I cannot recommend Homeseptic enough and anyone considering a sewage treatment plant or similar should absolutely give them a call.

    thumb Mark T

    review rating 5  An excellent installation. Dave and the team were fantastic!

    thumb Katharina Lopsien

    review rating 5  Actual Engineers! I had an installation carried out last year by another firm, we had problems ever since. These guys came out and got the tank working properly, fixed the faults and we have not had any problems since. It was such a relief to meet actual engineers instead of the normal sales guy followed by inept untrained guys. Great job and great value, I would recommend to anyone.

    thumb Jacob Finch

Expert wastewater treatment engineers who specialise in Septic Tank and Soakaway replacements as well as Sewage Treatment Installations. We are based in the New Forest, Hampshire but operate across the South Coast as well as Wiltshire, Somerset, Devon, Sussex, Surrey, Oxfordshire, and Berkshire.

The Home of Septic tank and private drainage system solutions. 

Based in the New Forest, Hampshire on the border with Dorset and Wiltshire we operate an expert team of off-mains drainage specialists. This encompasses Septic Tank, Soakaway and drainage system problems, installations, blockages, and servicing. In addition, any issues relating to cesspits and sewage treatment plants.

Homeseptic offers high quality, professional advice, repairs and installation for all aspects of effluent cleansing and disposal across the south of England. Whether you’re looking for septic tank installation, sewerage treatment plants or systems, piping or soakaway solutions, Homeseptic employs only the most competent engineers in the drainage and treatment industry and knows exactly which solutions suit each property.

We only install and offer quality systems we believe in, from suppliers we know and trust.

New Forest Philosophy

As we have said Homeseptic is based in the New Forest. A great location in between the cities of Southampton, Salisbury and Bournemouth. We are a group of good friends who grew up together in the area.  Individually we all have grown tired of the way other Drainage firms are run. As a result, this is why we set up Homeseptic. If you want to know more please check out our About Page

We want to actually put the customer first and turn up to do a great job that we can be proud of. Unfortunately, this is not the case in the market, with most companies pressuring their workers to sell unnecessary works on a visit. Due to this, we offer upfront quotations with any variations discussed before we start work.

We are honest hard working professionals, which we hope you will agree with. As a result, it will certainly not turn up in flash sales vehicles and give you a load of chat and then do a terrible job. Our team will come with the job in mind and the aim of giving you the best value we can in order to facilitate a quality installation. 

We cover all of Hampshire, West Sussex, Dorset, Somerset, and Wiltshire. Please get in touch and see what you think.

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What sewage treatment plant size for my house?
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What sewage treatment plant size for my house?
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If you are thinking of installing a new sewage treatment plant at your property then it is important to know how big the tank needs to be. Even if you are having a professional company installing the tank it is good to know the basics.
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