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What is a septic tank soakaway or land drain?

| Michael Quinn

This is an image of a drainage field installation, also known as a land drain.
This is an image of a drainage field installation, also known as a land drain.

At ASL Limited, we often serve customers whose homes and other properties are in rural areas. They are served by septic tanks because there had previously been no opportunities to access main drains in the vicinity.

A soakaway is the drainage system used to discharge wastewater from a private sewage system like a septic tank or domestic treatment plant. In this case, we are referring to a drainage field, also known as a land drain. 

Our experienced engineers can fix any problem that you might see with your own septic system. As such, we can also provide answers to any questions about septic tanks and soakaway systems; not a soakaway pit, which is normally used for rainwater. This is because the Environmental Agency does not usually allow soakaway pits, boreholes or deep excavations filled with rubble. If you want something like this, you must apply for a permit.

If you have found that your septic tank or your drainage field is blocked, needs repairing, or even needs replacing, get in touch with us today. We will be happy to provide you with the drainage system your property needs, and our engineers (or engineer) can be with you as soon as you need them to carry out the work.

What is a septic tank drainage field, and how does one work?

A septic tank drainage field (often called land drains) is used to remove contaminants and impurities from the wastewater leaving your tank. The wastewater which has flowed through each chamber of your system, having the solid sewage effluent separated along the way, will leave the septic tank's second chamber via pipework leading to the drainage fields. It will then discharge to drainage fields without causing any damage to the environment. 

As of January 2020, new binding rules were put in place to change the standards by which a septic tank may be kept. The most significant change to the General Binding Rules, in this case, stated that all septic tanks must not drain to a soakaway but to a drainage field, or drainage fields, definitely not to a local ditch or watercourse.

This is the reason why it is stated that septic tanks should have drainage fields in the new regulations, or else be connected to a sewage treatment plant. Septic tanks that discharge into watercourses or ditches are likely to pollute the environment, and as such now must be replaced with a treatment plant, after a search has been carried out to ensure no main drains are within 30 metres.

Septic tank drainage fields usually consist of long trenches, which contain perforated pipes and gravel. These are then sited commonly beneath a soft, landscaped area such as a lawn. They and any other connecting pipework, are the parts that make up your septic system. The septic tank itself will only perform about half of the treatment work. The rest will be performed by the drain field, which is why your septic tank drainage field must be designed and installed correctly.

Installing a drainage field for a septic tank

We pride ourselves on providing any service our customers need for their septic systems, from unblocking a septic tank drainage field to repairing or replacing the tank. Below, we have listed some of the conditions required to install or replace a septic tank and to build a drainage field for a septic tank:

  • A test hole survey must be carried out to determine the soil’s suitability.
  • The septic tank must discharge into the ground via a drainage field or drainage field.
  • Drainage fields can only be used where the soil conditions are suitable; clay soil is not appropriate for drainage fields.
  • Building Regulations must be checked to reference boundaries.
  • The Environment Agency’s Groundwater Protection Zone range must be checked.
This is a photo of a drainage field installation by ASL Limited.
This is a photo of a drainage field installation by ASL Limited.

Testing suitability

When we test your soil's suitability for a septic tank drainage field, we start with a test hole to ensure the land is suitable. If it's clay or something non-porous, we will decide on a different solution. If the ground is suitable, we will then carry out a full percolation test (conducted over two days).

This test determines the infiltration rate into the ground by measuring how long it takes water to go down in a saturated hole. The time it takes measures the suitability of the soil. Upon receiving the calculations, it tells our engineer the length and the width required to construct the drainage fields.

Ground which is prone to flooding will not be suitable for drainage fields for a septic tank. Drainage fields that are always damp or wet do not fair well because no secondary treatment will occur.

Common soils such as chalk, which can drain too quickly, must have their septic tank drainage fields designed and spread out so that the infiltration system works properly. On occasion, it may also be necessary to upgrade to a sewage treatment plant.

The percolation test’s results will determine the drainage field, but the size will be dependent on how many bedrooms there are on your property, as advised by the Environment Agency.

Rules for new and existing discharges

If your system was installed and discharging effluent before December 31, 2014, your system is known as an “existing discharge”, while any system installed and running on or after January 1, 2015, is known as a “new discharge”. You must be using a system which does not discharge into a local watercourse at this time, as it is illegal to have one which does and you may not be able to sell your property if there is an older, non-compliant model installed there.

If you are buying or selling a property, it's strongly advised your arrange to have a home buyer's survey.

You must be using the correct sewage system for your individual case, which will either be a septic tank or small sewage treatment plant, to treat all sewage before discharging the effluent to ground via a drainage field.

The difference between a septic tank (with land drainage) and a sewage treatment plant is that the treatment plant uses electrical and mechanical parts to treat the liquid to a higher standard. In contrast, the septic tank has filtration systems which filter out solid effluents from wastewater.

You can visit our articles on septic tanks and sewage treatment plants for more information.

You cannot use a rainwater soakaway, well or borehole to discharge effluent to ground. Instead, to meet the Binding Rules, it's advised you upgrade what you have to a drainage field, or apply for a permit so that the Environment Agency can assess the risks of you using your current system.

We service septic tanks and soakaways.

At  ASL, we have many years of experience handling all problems related to septic tanks, from unblocking septic tank drainage fields and pumping septic tank chambers, to installing new septic tanks or replacing old ones should no longer be used. If there is anything you need in advice or services for septic tanks and drainage fields (or drainage fields), we have the experience and required equipment and technology to get the job done. We also want you to feel secure with the drains, so we can send our highly trained and fully qualified engineer/s out to you, to take care of any problems found in your septic tank or its drainage fields.


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