what is a drainage field

What are septic tank drainage fields?

Septic tank drainage fields are an integral part of a septic system. Often incorrectly called soakaways, a drainage field is much more than just a soakaway. A drainage field is a specifically designed arrangement of perforated pipework, laid in trenches to allow the effluent to filter into the ground. They are designed to ensure aerobic contact between effluent and the subsoil. Drainage fields need to be designed to individual requirements taking into consideration ground conditions, flows, property boundaries and many more factors.

A drainage field  has two principal purposes:-

  1. To allow infiltration of treated / partially treated effluent into the ground at a controlled rate.
  2. To allow further treatment of partially treated effluent before it reaches the groundwater level.

The phrase soakaway is often, wrongly, used in relation to septic tank and sewage treatment plant discharges to ground:

  • A soakaway is used for surface water (rainwater from roofs, paved areas etc)
  • A soakaway is typically a hole or structure filled with rubble / drainage crates)

Septic tanks can ONLY discharge to ground via a drainage field. Septic tanks that discharge to watercourses are illegal and are in breach of environmental law and will eventually leave operators liable to large fines due to the pollution of surface water.

Sewage treatment plants can either discharge to drainage fields OR to a surface watercourse / ditch dependant on flows and other factors.

Drinage field

A drainage field layout as per Building Regulations (Part H) Drainage and Wastewater Disposal

Are there drainage field regulations?

Yes. The design and construction of septic system drainage fields must be carried out by qualified and competent contractors. There are specific regulations relating to drainage fields:

  • The Environment Agency – Environmental Permitting (England and Wales) (Amendment (England) Regulations 2014
  • British Standards BS 6297:2007 A1 2008
  • The Building Regulations 2010 (Part H) Drainage and Wastewater Disposal
  • British Water COP – Drainage Fields for the disposal of septic tank and sewage treatment plant effluent

If you have been advised by a contractor about the construction of a new drainage field ensure they are working in accordance with these regulations.

These regulations cover many aspects of the design and construction procedure answering questions such as:-

  1. Is your drainage field in the vicinity of a Groundwater Protection Zone?
  2. Are the ground conditions suitable for a drainage field?
  3. Is your drainage field the correct size?
  4. Is your drainage field the correct distance from your property or property boundary?

If you have an existing drainage field that is not constructed in accordance with these regulations you may be liable to enforcement action from the Environment Agency due to the causing of pollution to groundwater. If you are having a new system installed and it does not comply with these regulations, again, you could be liable for prosecution.

Is my drainage field legal?

There are many types of “soakaway”. Only a correctly constructed drainage field will comply with the regulations.

Here are a few examples of incorrectly constructed drainage fields:

Soakaway crates

Crates or baskets are NOT acceptable for drainage fields. Many people say they are but the FACT is they are not. Beware of contractors or suppliers who suggest they are! Environment Agency  Regulations specify perforated pipework ONLY!

Drainage field

Herringbone drainage

Herringbone land drains are effective for land drainage but are NOT acceptable for drainage fields. Drainage fields must consist of perforated pipes laid in a continuous loop to ensure controlled effluent distribution throughout the drainage field.

Drainage field

Soakaway pits

No, no, no! Not even acceptable for surface water soakaways! If anyone suggests a hole filled with rubble, walk away. Quickly!

Drainage fields

Can a borehole be used for a septic tank soakaway?

The short answer is yes. However Building Regulations and BS6297 do not specifically refer to the use of boreholes for the disposal of septic or treatment plant effluent. The Environment Agency position is that these could be considered if there is no alternative. In order to consider a borehole discharge the case would require detailed investigation to ensure protection of groundwater. The point discharge poses a greater risk of groundwater pollution as well as there being significant risk of bio-fouling diminishing the hydraulic efficiency over time. If you have been advised that a borehole can be used, feel free contact us for some further advice first.

How can DJ Wilkinson help?

With over 20 years experience in drainage we can give you independent, experienced and helpful advice. We offer various services for homeowners, developers and self-builders:

  • Survey – If you are experiencing frequent septic tank problems we can survey your existing system and give you advice on the condition or any required upgrades.
  • Design – We can carry out a design of a new system for you.
  • Installation – We can repair existing soakaways and install new drainage fields for you.

Our advice is independent. We do not sell a product just a service tailored to your needs.

Contact Us for FREE no obligation advice.


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