UK Soakaway Design

The effluent leaving an onsite sewage treatment system must discharge to a soakaway or to a watercourse/drain.

All Biocell sewage treatment plants can discharge direct to a drain, stream or watercourse (due to the high standard of effluent treatment) but if a site has no access to a stream or drain, you must construct a suitable soakaway. When planning a soakaway, it is important to remember that the aim is to facilitate the filtration of final effluent through the ground.

A typical domestic system serving a household of 4-8 people produces 600-1200l of treated effluent per day. This is the typical volume the soakaway needs to accommodate. When designing a soakaway, it is important to have information on the permeability of the soil onsite. For example, if you have low permeability clay based soil then the infiltration rate will be low and you will require a large soakaway. Whereas if the soil is very sandy with a high infiltration rate, you can install a small soakaway.

Infiltration Test

The first step in designing a soakaway is to conduct a percolation test. The tank to soakaway outlet is generally 300-700mm below ground level, so we suggest carrying out the percolation test at this level in the subsoil. This gives a representative example of the soil infiltration rate at the depth the effluent will be entering the soakaway.

How to Do a Percolation Test

To do a percolation test, begin by digging a small square hole (300mm x 300mm x 400mm) into the subsoil at the level of the sewage treatment plant outlet pipe. Then fill the hole with water; allow the first fill of water to soak into the soil in order to saturate it. Once saturated, refill the hole with water.  Measure the time it takes for the water level to drop 100mm in the hole. Divide this 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
result of 0-25
Trench length
result of 25-50
Trench length
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 blckly and then need 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.

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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