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How To Calculate Sewage Treatment Plant Size

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If you are living in a property that is unable to be connected to mains sewage systems, one option that you have to dispose of your wastewater is through a sewage treatment plant. If you are going to fit a sewage treatment plant, however, it is important that you get it right. Problems with the disposal of sewage can not only be unpleasant for you and your neighbours, but it can also be harmful to the local environment, expensive to fix and get you into trouble with your local authority.

A sewage water plant is an environmentally friendly, efficient wastewater system that allows you to dispose of your (cleaned) wastewater into the local environment including watercourses (unlike a septic tank).

When you are considering what sewage treatment plant to install, one of the main factors to think about is its size. The sewage treatment plant must be able to adequately process the wastewater from the house according to demand to ensure that it works efficiently.

Sizing A Sewage Treatment Plant

When the calculation regarding the size of a sewage treatment plant is carried out, British Water’s Flow and Loads code of practice stipulates that you must base it on ‘minimum population’ – or ‘p’. This enables you to understand roughly how much sewage is likely to be created by an average household. It doesn’t, however, take into consideration if there is more extensive water use such as if you have a spa bath or do home brewing.

If you engage with activities that result in a significantly higher amount of water being used, you should speak to the professional who is installing your sewage treatment plant so that they can add this into their calculations.

What Is Minimum Population (P)?

Minimum population – or ‘p’ does not directly relate to the minimum number of people who are in the property and using the sewage treatment plant. It is, instead, a calculation that is based on the number of bedrooms of a property.

When it comes to the sizing of sewage treatment tanks, the minimum population is calculated in the following way:

  • Any single house of up to three bedrooms = 5 people
  • Any additional bedrooms = 1 person per bedroom

So, a 2 bedroom house will count as 5 people, a 3 bedroom house will count as 5 people, a 4 bedroom house as 6 people, a 5 bedroom house as 7 people, and so on…

For a group of flats or small houses, the minimum population is calculated as follows:

  • 1 bedroom = 3 people
  • 2 bedroom = 4 people

So, a group of 4 x 1 bedroom flats would count as (4 x 3) 12 people, and so on…

A Reduction of Minimum Population (P) For 12+ People

In the event of more than 12 people are counted as the minimum population, there is also a reduction added to the calculation. This is due to the fact that there is likely to be slightly less water used per person in larger households – water saved on washing up or cleaning, for example.

If this is the case, the rules state that:

  • If the minimum population is between 13 and 25, the minimum population will then be multiplied by 0.9 and rounded up. So, 4 x 3 bedroom houses will be: 5(people) x 4 (No. of houses) = 20 (total population)
    20 x 0.9 = 18
  • If the minimum population is between 26 and 50, the minimum population with then be multiplied by 0.8 and rounded up. So, 5 x 3 bed houses and 3 x 4 bed houses using the same sewage treatment plant, would be: 5(people) x 5(No. of houses) + 6(people) x 3(No. of houses) = 25 + 18 = 43 (total population)
    43 x 0.8 = 34.4 (35)

Key Things to Remember

When you are calculating the minimum population value, there are some important considerations that should be made. Getting the right size sewage treatment system is very important, so you should speak to a professional if you have any doubts.

You should always round the numbers up, as these are minimum calculations, and rounding them down could result in blockages in your system. You should also consider that larger more affluent households might have higher loads, as could holiday homes as people tend to also occupy the living room. You should also note that certain characteristics of the soakaway can also influence the size of the sewage treatment plant that you require.

Monday 8th August 2022
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