Time and time again we hear the question “how do I construct a drainage field?” Whether you are a homeowner managing your own project of installing a sewage treatment plant, or a builder building a house and haven’t got a design from your architect, It is important to design the drainage field correctly as detailed in Part H of building regulations. The drainage field is a critical part of any septic tank or treatment system and plays a major role in purifying water prior to its release into the ground. It is therefore no wonder that the environment agency state you must use a correctly sized and constructed drainage field to adhere to the General Binding Rules.
A drainage field is a method of cleaning wastewater prior to it dispersing into the ground, formed from a loop of pipe surrounded in gravel, buried in the ground. It is used for the dispersal of wastewater after a septic tank or treatment plant.
Many people get confused between a drainage field and a soakaway and often the terms are used interchangeable. The difference between the two are clear:
Drainage field = Dispersal of wastewater
Soakaway = Dispersal of rainwater.
The reason behind this is also clear. A soakaway does not offer biological cleaning of water, however the correctly constructed drainage field does. We will come onto that later in the article. For the moment, we will assume you know you need a drainage field but are lost as to how to size and construct one, if so, this article is for you.
To size a drainage field, you must establish 3 key facts.
Once you have conducted the percolation test, and established your rate of percolation (VP), you can use this to size your drainage field.
The area you require is determined by the rate of percolation multiplied by the number or people in the household (the maximum occupancy, not the number of people in your family ) multiplied by 0.2
VP x Population x 0.2
Therefore in the above example
Therefore given a VP of 36 and 5 people you will have:
36 x 5 x 0.2 = 40m2
This is the metres squared you will require. Easy right?!
Yes, however I do highly recommend using the free excel calculator or paying a specialist to perform this test for you.
Once you know the area needed you will be able to use this to design the drainage field. You are required to design a loop system of pipes in the ground with the total area of the trenches equaling the area you have from your calculations. You must also make sure there is a 2m gap between your trenches. Like this:
This image is taken from the excel calculator and shows you the plan view.
A Drainage field is constructed with Slotted 110mm drainage pipe (not flexible land drainage pipe). Once you have calculated your area required you will need to do a design to work to. This is best achieved using our drainage field design calculator. It will give you a design to work to if used correctly.
Use solid 110mm pipe, not flexible land drainage pipe. Either buy the 110mm pipe with slots cut in, ot cut your own slots at 30cm intervals along the length of the pipe
Pipes should be laid on clean shingle or broken stone, graded from 20mm to 50mm
There is no building control requirement for a vent on a drainage field. However putting a vent near the end will allow air flow through the drainage system which will aid the growth of Aerobic bacteria and subsequently increase treatment quality. It is therefore highly recommended.
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