Percolation Test for Septic Tank


If you are planning on installing a soakaway or drainage-field for your septic tank or treatment plant system, here are a few things you will need to do:

What Percolation test method (in order to size your soakaway system):

Excavate a hole 300mm square to a depth 300mm below the proposed invert level of the soakaway pipes.

Fill the 300mm square hole to at least the 300mm mark and allow to soakaway overnight.

On the next day, fill to the 300mm mark again and observe the time it takes to soakaway in seconds, from 70% full to 25% full.

Divide the time taken by 150.  The answer gives you the average time in seconds (V).

Carry out this test at least 3 times in at least 2 different places.

Drain fields or soakaways should only be installed when the results are between 12 -100.

If the value is outside these parameters you might need to install a sewage treatment plant.

Design and Construction of your Soakaway:

Soakaways should be always constructed with perforated pipe, and laid in trenches with the same gradient not steeper than 1:200

Pipes should be laid on a 300mm layer of shingle or medium of up to 50mm.

The trenches must be filled 50mm above perforated pipe and covered with a membrane and then filled in with soil.

The trenches for the soakaway should be between 300mm and 900mm wide with a distance of 2m between trenches.

A inspection chamber must be installed between the septic tank and the soakaway.

Soakaways should be constructed in a circuit to make a continuous loop.

To calculate the size of the soakaway, use this formula:

Area = P x V x 0.25

P= the number of persons served
V= percolation test value
Remember you must register with the Environment Agency (EA) complete the relevant application so that you either get a permit to discharge or an exemption.

Installing the right size sewage treatment plant is critical. Read more here. Don’t worry Premier Drainage Solutions are here to help, give us a call on 01268 950050 or drop us a message here and we will guide you where we can.


Do you need assistance?


Find the answers to some of the most commonly asked questions.

Learn more

Find us

P4 Allen House Business Centre The Maltings, Station Road Sawbridgeworth Herts CM21 9JX

Google maps

Contact us

Discuss your project with a member of our head office team today.

Learn more

Contact us

Premier Drainage Solutions
P4 Allen House Business Centre
The Maltings, Station Road
Herts CM21 9JX

Request a call

Our charity

MSA Trust receives no government support and relies entirely on charitable donations to fund their support services as well as research into finding the cause and cure for multiple system atrophy (MSA).