What is a drainage field/soakaway, and how do they work?

A drainage field/soakaway is designed to spread partially-treated effluent over a larger area. A septic tank must connect to a drainage field, as you cannot discharge directly into a watercourse from a septic tank. A sewage treatment system can be connected to a drainage field if a watercourse cannot be located for discharge. The drainage field will provide further treatment to the settled effluent and disperse it safely into the natural ground. A suitable porosity test will ensure that the receiving ground is suitable for the installation of a drainage field.

Drainage fields typically consist of a system of sub-surface irrigation pipes which allow the effluent to percolate into the surrounding soil. Biological treatment takes place naturally in the aerated layers of soil.

If you are installing a sewage treatment system and can connect directly to a watercourse, we would always recommend this. There are several factors that can affect the suitability of a drainage field and its ongoing effectiveness, and the direct connection to a watercourses also removes any future problems associated with a blocked or failed drainage field. Installation of a drainage field also requires suitable ground conditions, and adequate space to meet Building Regulations.

The appropriate design of a drainage field is covered in BS6279, and is also well documented in Building Regulations Document H and the Environment Agency PPG4 Document (both available online).

The most common problem with a drainage field is that the drainage is backing up. This can be identified by checking the levels in the septic tank/sewage treatment system and/or upstream drainage. The cause is likely to be a failed drainage field, which could be fully saturated due to poor soil conditions or a high winter water table, or a blocked drainage field due to solids carried over from the septic tank/sewage treatment system. If you do not maintain (arrange emptying of) your system, solids can pass through the tank and therefore block the downstream drainage field.

Generally, to rectify the situation, an engineer would need to replace the drainage field – jetwashing may occasionally be an effective solution, but this is on a case-by-case basis and advice should always be sought by a qualified engineer.