Expert Guide

What is a soakaway?

This guide will explain the different types of soakaways which are typically in use, and things which might affect them.


What is a wastewater soakaway and why is it required?

A soakaway is a generic term often used to describe anything which takes the wastewater from a septic tank or sewage treatment plant. These two types of tank do not operate in isolation, and they need an outlet for the separated wastewater to pass into in order for the wastewater to pass safely into the surrounding ground. This is typically either a drainage field, or some sort of soakaway arrangement, for example, a deep bore soakaway or rubble pit.

It is important to note that a sewage treatment plant produces a cleaner quality of wastewater than a septic tank, as it provides treatment of the waste within the tank before it exits through the outlet. Therefore, a sewage treatment plant is able to discharge straight to a watercourse or ditch (if one is nearby), whereas a septic tank is not permitted to do so.


The different types of soakaway

Drainage field

A drainage field is a network of perforated or slotted pipework, running from the septic tank or sewage treatment plant. It provides a form of treatment of the waste water, by enabling it to percolate safely through the subsoils without causing pollution. The size and structure of a drainage field is determined by a percolation test (you can find out more in our Guide to Percolation Tests here), and there must be sufficient space available at the property for it to be installed. Furthermore, not all ground conditions are appropriate for a drainage field - if the ground conditions at the property are too wet or too dry, it can prevent the proper functioning of the drainage field. A percolation test will also determine the nature of the ground conditions, and whether or not a drainage field would be appropriate.

Bore hole soakaway

A bore hole soakaway is typically a 6 inch diameter vertical perforated pipe, and is generally used when a percolation test reveals that the ground conditions at the property are not appropriate (or where there is insufficient space) for a drainage field. It is effectively a way of accessing more appropriate ground conditions at a deeper level. A trial bore hole would be required, initially to a depth of 10 metres, to confirm whether a bore hole soakaway would be suitable at the property.

Soakaway chamber

A soakaway chamber is generally used when there is a lack of space available for a drainage field, but the ground conditions are generally suitable for one. A trial excavation is required to confirm whether the ground is suitable at the required depth for a soakaway chamber.

Rubble filled pit

This is typically an excavation filled with rubble into which a solid pipe from the septic tank discharges. It is an old style of soakaway which may still be in existence, but which is no longer considered an appropriate solution for a septic tank discharge.

Bespoke soakaway arrangement

If all other options have been exhausted, and with the explicit permission of the local Building Control department, a bespoke soakaway arrangement such as a single line deep trench arrangement may be permitted.


Important considerations for wastewater soakaways

There are a number of factors which can affect the design, structure and functioning of wastewater soakaways:


There are regulations in place surrounding wastewater soakaways, largely designed to protect the environment from pollution. You can read our Guide to Regulations here, but the main consideration is that a septic tank should only discharge to a drainage field unless a permit has been sought for any other type of soakaway arrangement. A sewage treatment plant may discharge straight to a watercourse or to a drainage field, any other type of soakaway arrangement would also require a permit.

Ground conditions

Ground conditions at a property can change over time, particularly the level of ground water (also known as the water table). This can have an effect on the functioning of a drainage field or soakaway, and can cause problems.

Usage of the septic tank or sewage treatment plant

It is important to use a septic tank or sewage treatment plant carefully, because careful use of the tank will ensure you get as many years of service from your soakaway or drainage field as possible.

Read our article on what not to flush into your drainage system.

Percolation tests

It is vital that a percolation test is carried out at a property prior to any drainage field or soakaway design and installation. A percolation test is crucial in assessing the suitability of the ground conditions, and the required size of any drainage field.

Damage to the tank can cause problems for the soakaway

Septic tanks and sewage treatment plants are designed to prevent solid waste from exiting the tank and entering into the drainage field or soakaway. Damage to the tank can result in solid waste particles clogging the soakaway, resulting in the drainage system surcharging.

If you suspect that damage may have occurred to your septic tank, it’s important to get it investigated as quickly as possible.


How long does a soakaway last?

There is no defined lifespan for a soakaway or drainage field, because there are so many factors which can affect their longevity.

The ground conditions at the property, the amount of usage the septic tank gets and how often it is emptied can all have an effect. Many soakaway systems function without issue for a number of decades, whereas others may fail much sooner. Problems with your soakaway or drainage field may result in the system surcharging, or dirty water pooling in your garden above the soakaway.

If you are having problems with your soakaway or drainage system, call our team today on 0800 028 9903 for advice.

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