Environmental guidance for your business in Northern Ireland & Scotland
The first step is to confirm that no connection to the foul sewer is possible. Your local council will check this first when deciding the acceptability of your plan. If a foul sewer is available you should connect to it.
You should then inspect your site to check:
If there are any problems with the site then you might need to consider alternative locations, or alternative sewage treatment systems.
You should choose a tank that has enough capacity to deal with the size of your house. This should not be the number of people that actually live there, but the maximum number who could live there. The tank must be capable of dealing with the waste water and sewage from the maximum number of people who could live in the building at some point in the future.
Check with the supplier that the tank conforms to the Construction Products Regulations.
Good drainage design is key to ensuring that sewage does not end up in groundwater. The key factor you need to consider is the availability of a suitable area of ground where you can construct a soakaway.
You should make sure that your soakaway, also known as a drainage field, conforms to the British Standard:
To protect the water environment and human health, you will need to make sure:
The area of ground needed for a soakaway depends on the number of people the septic tank is designed for (the person equivalent) and the type of soil in the drainage field. In particular it is a measure of how fast water can soak through the soil, its percolation value, which is important.
The “person equivalent” is the number of people who could live in the building (or buildings) that the septic tank serves. It is not the number who actually live there, but is based on the size of the house.
The “percolation value” is calculated by measuring how quickly water soaks into the soil under controlled conditions. This value must be calculated using a method that conforms to the standard BS926 1983. Your local council building control may want to witness the test being carried out.
The area of ground required, in square metres, is then found by the formula:
A = P x Vp x 0.25 (A = area in square metres, P = person equivalent, Vp is the percolation value)
The drainage pipes that make up the soakaway should never be more than one metre below the ground surface. The pipes should always be at least one metre, preferably more, above the water table in winter.
It is important to select a location that is away from watercourses, wells or boreholes, and does not have the potential to cause a nuisance to neighbours.
Technical guidance for constructing septic tank drainage fields can be found in the building regulations.
Also available from The Stationary Office, Belfast on 02890 238 451
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