How to size a domestic sewage treatment plant

The vast majority of homes in the UK are attached to a mains sewer via a network of pipes. For these people, there is no concern about how they dispose of their wastewater.

But around five per cent of the UK’s homes, mainly those in remote rural areas, aren’t connected to the mains sewer. They need an alternative way of disposing of the wastewater from their homes safely and efficiently. Wastewater includes water from baths, showers, sinks, dishwashers, washing machines and toilets.

The preferred method for most is a domestic sewage treatment plant, which is the most environmentally friendly way of dealing with wastewater. With a sewage treatment plant, the water is treated within the system and produces a clean, non-polluting effluent, which is safe to release into the environment.

As the waste leaves the final chamber of the sewage treatment plant to be discharged, the effluent can be as much as 95% clean, posing no threat to the environment. In addition, sewage treatment plants produce a small amount of sludge, which needs removing by a contractor once a year.

While the initial outlay on a domestic sewage treatment plant is quite high, it has many advantages over the alternatives (such as septic tanks), including lower running costs.

Sewage treatment plants come in a variety of sizes and styles, and you need to find the one which best meets your needs, so that your wastewater can be treated efficiently.

It is important to ensure you have the right size of domestic sewage treatment plant before you go ahead and order one. It is a big cost, so you can’t afford to make a mistake.

Sizing a domestic sewage treatment plant

The British Water Flows and Loads Code of Practice makes recommendations on sizing a domestic sewage treatment plant.

The key thing is to focus not just on the number of people living in a house now, but on its full potential occupancy. For example, it’s not unusual to have only two people living in a three-bedroom house, but that house actually has capacity for five people.

So a sewage treatment plant for single domestic households of up to three bedrooms should always be installed for a minimum of five people (5P). This is sometimes known as the Minimum Population (P) figure. (The only time you would consider a smaller sewage treatment plant would be for a small flat. A one-bedroom flat should be sized for three people and a two-bedroom flat sized for four people.)

After that, the recommendation is for an extra person for each extra bedroom – so six people for four bedrooms, seven people for five bedrooms etc.

To calculate for either a single house or pair of houses using the same sewage treatment plant, start with the minimum of five people (5P). Then add 1P for each extra bedroom, so a six-bedroom house is 5P + 3 = 8P – meaning you need a system for eight people.

But two three-bedroom houses would be 10P – 5P + 5P. And if a house with three bedrooms is sharing a sewage treatment plant with a house with four bedrooms, that is 5P + 6P = 11P – a system for 11 people.

If you know that you are going to extend your three-bedroom house with an extra two bedrooms, you should of course size it as a five-bedroom house. Likewise, if you have eight people living in your four-bedroom house, you should ensure your sewage treatment plant is big enough for eight people.

For 13-25 people

Where the sewage treatment plant is to be shared between a group of houses totalling more than 12 occupants, there is a slightly different calculation. This is based on the fact that homes with higher use will balance out those with lower use.

For a group of homes with a potential occupancy of 13-25: Start with the existing P value used for homes/ groups of homes of up to 12 people eg a four-bedroom house is for six people (6P), but then multiply the total by 0.9 to give an adjusted P value for a larger number of homes.

For example, four homes with four bedrooms would have a total P value of 24.

Multiply this by 0.9 to get a total of 21.6. This is then rounded up to get an adjusted P value of 22. (You should always round the adjusted P value up rather than down).

For 26-50 people

Where a group of homes has a total potential occupancy of 26-50 people, the P value should be multiplied by 0.8 to give the appropriate adjusted P value.

For example, if a development has four three-bedroom houses and three four-bedroom houses, that has a total P value of 38.

Multiply this by 0.8 to get a total of 30.5. This is then rounded up to get a total adjusted P value of 31.

While this calculation is recommended and is a good rule of thumb, it is possible that individual homes or developments may have particularly high water demands, in which case it makes sense to have a larger sewage treatment plant. Things like spa baths will increase the amount of wastewater a property produces, if they are used regularly.

In the unlikely event that the population using a sewage treatment plant is greater than 50, it is essential to consult a qualified engineer to ensure the design is suitable for such a large number of people.

If you come to sell your house and your sewage treatment plant is below minimum size, it may cause problems with the sale.

What if your sewage treatment plant is the wrong size?

If your sewage treatment plant is too small, it won’t be able to cope with the amount of waste your home or group of homes is generating, and won’t be able to process it properly. If this happens, it can lead to smells, blockages and overflows.

But should you just opt for a bigger sewage treatment plant than you need to be on the safe side? This isn’t a good idea either. If the system is too big, the correct amount of wastewater won’t flow through it and can cause the system to malfunction. This can also lead to blockages and drainage problems.

Once you know what you are doing, it isn’t too difficult to work out the right size of domestic sewage treatment plant for you. But it is very important to get it right. If you are in any doubt, always talk to your supplier. Here at Tanks for Everything, we are very happy to give advice, to make sure you make the right decision about installing the correct domestic sewage treatment