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Sizing your septic tank

To determine what size of septic tank you require, you need to do some basic calculations.

First, work out the population of the property, which is based on the number of bedrooms:

2-bed house = 4 population
3-bed house = 5 population
4-bed house = 6 population
5-bed house = 7 population
Etc.

The above sizing is based on the British water flows and loads – it is important that you buy the correct-sized tank for the number of bedrooms your property has (or if you plan to extend the number of bedrooms in your property in the future, choose a tank suitable for this purpose). The septic tank must be installed to correspond with the bedroom population.

Once the population has been established, the following calculation will give you the correct size of septic tank to be supplied:

Population x 150 litres per person, plus 2000 litres sludge retention = size of tank required

For example: 4 population house x 150 litres per person = 600l + 2000l sludge retention = 2600l (or next size up) tank required

The 2000l sludge retention is a constant figure in all calculations. If the calculation for your property is slightly higher than a tank threshold, we always recommend choosing the next size tank.

When you have your calculation and have selected the tank you wish to purchase, you might also want to consider having it installed by our qualified engineers. Septic tanks will not give the required water quality to allow discharge into streams/rivers/surface water drains, and must have a soakaway installed as per building regulations. Percolation tests must also be carried out on the area where the soakaway is to be installed, to allow your installer to quote for the correctly-sized drainage field. To discuss these options, give our team a call on 01388 537050.