The Difference Between Septic Tank Soakaways & Drainage Fields

It’s easy to get confused when trying to understand the rules and regulations surrounding the dispersal of sewage effluent from a septic tank. That’s partly because the terms ‘soakaway’ and ‘drainage field’ are still being used almost interchangeably in many places, despite new laws introduced in 2020 that make it very clear what each is and what they can be used for.

Soakaways used to be the means by which sewage effluent from a septic tank could be dispersed into the local environment. However, as of January 2020, they are now illegal for this purpose and can only be used for taking excess and surface rainwater away from a property. Instead, the wastewater from a septic tank should be discharged into a drainage field, where it can be further treated before being released into the soil.

This month, we’re examining the difference between soakaways and drainage fields, finding out why the law was changed, and explaining some of the regulations surrounding the installation of a drainage field.

The difference between septic tank soakaways and drainage fields

A soakaway was traditionally just a hole in the ground filled with rubble, but has more recently been developed into more complex plastic boxes known as attenuation crates. In both cases, voids in the soakaway store the water being discharged into it before releasing it into the soil. 

The main drawback with using soakaways with a septic tank, and one of the key reasons why they can no longer be used for this purpose, is that they can become easily blocked, which can lead to the effluent backing up. They also don’t treat the effluent any further, meaning that there is a serious risk of the soil and any nearby watercourses becoming polluted.

A drainage field, on the other hand, is made up of a series of pipes laid in trenches. These allow the effluent to be treated through aerobic digestion, an oxidisation process that makes the effluent safer by converting it into water and carbon dioxide, as well as nitrate, sulphate and phosphate.

How deep should a drainage field be?

When installing a drainage field, the pipes should be laid no deeper than 700mm from the surface to ensure that the aerobic digestion that is such a fundamental part of the process can occur successfully.

How large should a drainage field be? 

The size of your drainage field will be determined by a formula using the maximum occupancy of the property it is serving (this would be five for a three-bedroom house) and by conducting a percolation test on the soil to determine its porosity. The percolation test basically comprises seeing how long it takes water to seep away through the soil. 

By multiplying the property’s occupancy figure, the result of the percolation test (expressed as the number of seconds it takes 1 mm of water to drain away) and 0.25, you get the required size of your drainage field in square metres.

Legal requirements for a drainage field

Given what your drainage field will be managing, it’s understandable that there are strict regulations that govern how and where it should be installed. These can be found in the Building Regulations 2010, while the design of your drainage field should also conform to British Standard BS 6297:2007 + A1 2008. 

Regulations state that all parts of your drainage field should be:

• At least 10 metres from a watercourse
• At least 15 metres from any building
• At least 50 metres from a water abstraction point
• At least 2 metres from a boundary

The drainage field area should also:

• Be downslope of any groundwater sources
• Not contain any water supply pipes, underground services, access roads, driveways or paved areas 
• Not allow rainwater system to enter the system

Who is responsible for land drainage?

Although regulations are quite complex, the default position is that the landowner is obligated to ensure that drainage is carried out according to current legislation. This would even apply where the water on your land has drained there from higher neighbouring land, as long as such drainage is natural and not as a result of changes (such as the introduction of paving) carried out by the neighbouring landowner.

If you’re the owner of a septic tank, you don’t just need to make sure that it is draining according to strict government regulations. You also need to stay on top of its maintenance so that it can effectively manage the wastewater being discharged into it from your property.

Here at AA Turner Tankers, we provide comprehensive and reliable septic tank emptying and sewage tank cleaning services for customers located in and around Colchester, Essex and Suffolk. Whether you are interested in arranging regularly scheduled visits or are in need of emergency help, our professional team has the knowledge and experience to help.

Get in touch with us now to find out more or to book your next service visit.