Percolation Area

UK Soakaway Design

When installing an onsite sewage treatment system, the treated effluent that leaves the system must either go to a soakaway or to a watercourse/drain.

All Biocell sewage treatment plants are certified to allow for direct discharges of treated effluent to a drain, stream or watercourse. This is based on the high level of effluent treatment achieved in the plants.

However, on some sites, there is no access to discharge the treated effluent to a stream or drain. In this situation, the only option is to construct a soakaway onsite.

When thinking about installing and designing a soakaway, it is important to remember that the aim of this soakaway is to allow the treated wastewater from the system to infiltrate into the ground. So, in essence, the soakaway is a disposal area.

A typical domestic system with 4-8 people in the house would produce between 600-1200L of cleaned effluent per day. This is the typical volume the soakaway needs to be able to accept.

When designing a soakaway, it is important to have information on the permeability of the soil onsite. For example, if you have a very low permeability clay based soils then infiltration rates will be low and you will require a large soakaway. In contrast, if the soil is very sandy with a high infiltration rate, then the soakaway will be small.

Infiltration testing

The first step in designing a soakaway is the percolation test. As the outlet from your tank into your soakaway will generally be between 300 and 700mm BGL, we would suggest that the percolation test is carried out at this level in the subsoil. This will give a representative example of the soil infiltration rate at the depth the effluent will be entering the soakaway at.

To do a percolation test, you need to dig a small square hole into the subsoil at the level of the outlet pipe from your sewage treatment plant. The hole should be approx. 400mm deep, and 300mm x 300mm square. This hole should be filled with water. The first fill of water should be allowed to soak into the soil in order to saturate the soil. Then the hole should be filled with water again. Then you measure the time it takes for the water to drop 100mm in the hole Then this figure is divided by 4 to give the infiltration rate in minutes per 25mm.

A worked example is as follows;

1 – Hole pre-soaked overnight

2 – Hole filled with water

3 – Time recorded for water to drop 100mm – 120mins

4 – Time divided by 4 = 120min/4 = 30mins

5 – Result = 30mins/25mm = 30

The table below shows the typical amount of trench required for the number of people in a house depending on the test results achieved.

No. People Trench length for a result of 0-25 Trench length for a result of 25-50 Trench length for a result of 50-75
4 12m 24m 36m
6 18m 36m 54m
8 24m 48m 72m
10 30m 60m 90m


Warning: Just digging a pit and filling it with gravel is not a good way of installing a soakaway. A pit style soakaway has less surface area for infiltration of the effluent into the soil and has no facility for future access [rodding and maintenance]. There is also no ventilation with a pit style soakaway for bacteria in the soil to help break down any small solids that enter the soakaway. This means a pit style soakaway can block up quickly and then needs total replacement.

Below is a typical cross section of a trench. A trench is filled with gravel, with a 4” rigid sewer pipe with holes drilled in it. The gravel is covered with a geotextile before it is filled with soil. A trench is typically 300mm wide with a minimum of 300mm depth of gravel in the trench but this can vary.

A percolation trench should be a maximum of 10m long. So, if for example, you need 36m of trench, you would install 4 x 9m long trenches [total of 36m]. A typical trench layout is shown below. Trenches should be a minimum of 1m apart up to a maximum of 2.45m. Vents at the ends of trenches allow for ongoing inspection and maintenance. Below is a typical plan view of a percolation area.

A diagram of a soakaway percolation area

Please note that soakaway tunnels of crates offered by some suppliers should not be installed for sewage effluent disposal. These are only certified for use with rainwater runoff. Installing them will result in non-compliance with the building regulations.

For recommended minimum separation distances between a soakaway and a dwelling, roadway, watercourse, etc – please contact your local building control office.

NOTE: This is a general guide only and we would always recommend that a suitably indemnified engineer is involved with the design, construction and supervision of any soakaway.

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