New tanks - Planning waste water and sewage treatment

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:

• Is there anything that would prevent the use of a septic tank, for example if there is a well or borehole supplying drinking water close to your site?
• Is there enough space to construct a soakaway, or drainage field that will be large enough for your needs? See the section on Calculating the area of a soakaway.
• Are the ground conditions suitable for a soakaway? Is there evidence of waterlogging or a water table close to the surface, very shallow soil or soil with very heavy clay content that prevents percolation.
• Is there a suitable slope, since the drainage will rely on gravity? If there is a low point where a soakaway could be constructed, is it prone to flooding or is the water table close to the surface?

If there are any problems with the site then you might need to consider alternative locations, or alternative sewage treatment systems.

Choosing a tank

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.

Drainage design

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:

BS 6297 Code of practice for the design and installation of drainage fields for use in wastewater treatment

To protect the water environment and human health, you will need to make sure:

• Your drainage field is at least 50m from any well of borehole supplying drinking water.
• The soil should be of a type that will allow the further treatment of the effluent from the septic tank.
• If soil conditions are unsuitable, you may want to consider a drainage mound. This can increase the distance between the disposal pipes and the water table to allow further treatment to take place. They do not overcome problems with soil that does not allow waste water to percolate through. Drainage mounds can be expensive to build and need a large area of land. They can’t go on land that has a slope greater than about 7 degrees and may need pumping equipment.

Calculate the area for a soakaway

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.

In Northern Ireland:

The Building Regulations (Northern Ireland) 1990 Technical Booklet N

Also available from The Stationary Office, Belfast on 02890 238 451

In Scotland:

Scottish Government: Technical Handbook – Domestic - section 3.9