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Septic Soakaway Installation, Leach Field or Drainage Field Installation UK
What is the definition of a Soakaway?
A pit, typically filled with rubble or more frequently soakaway crates. Into which wastewater is piped so that it drains slowly out into the surrounding soil. A soakaway in its traditional sense is only legal in the UK to deal with rainwater. As of the 1st of January 2015 you are no longer allowed to use a Soakaway to discharge effluent to ground. All discharges of effluent to the ground have to be undertaken via a drainage field.
Please see the following article New Soakaway and Drainage field regulations
Is a Septic Tank Soakaway the same as a septic drain or drainage field?
The terms can be used interchangeably but from a technical and compliance perspective they are different. As discussed above a Soakaway traditionally is a hole in the ground filled with rubble or a bit or perforated pipe or such like. A Drainage field while sharing some of the characteristics is very different. As an owner of a system, you need to be clear on the legality and compliance of it under the General Binding Rules.
What is a Septic Tank Soakaway or Drainage field and how does it work?
A Septic Tank Soakaway is not a hole filled with rubble, crates or tunnels. Crates and Tunnels are not allowed to be used with a Septic Tank. Specifically designed infiltration tunnels can be used with permission in conjunction with a Sewage Treatment Plant.
A Septic Tank Soakaway or drainage field is a system of sub-surface irrigation pipes. The protected perforated pipes allow the effluent to percolate into the surrounding soil. The biological treatment takes place naturally in the aerated layers of soil which is typically the top 1 meter of your soil. Please see the image below.
The biological treatment element of a drainage field is carried out by the natural digestive process of aerobic bacteria. Aerobic digestion can only occur with oxygen. This is why the total depth of the drainage field, including gravel base, cannot be more than a meter deep. If the discharge occurs where oxygen is not present anaerobic digestion occurs. A bye product of anaerobic digestion is a black slime that will clog your soakaway and lead to failure. This can reduce the life of your soakaway from 15 to 25 years to a matter of years.
Septic Tank building regulations state that drainage fields should be designed and constructed to ensure constant aerobic contact between the liquid effluent and the subsoil. BS6297 2008 states that the maximum depth of the soakaway pipe is 700mm.
Septic Soakaway / Drainage Field
A Septic Soakaway / Drainage Field diagram
The adjacent picture shows an installed Septic Soakaway before the soil is replaced. As you can see the depth is shallow to ensure aerobic conditions. This is a large drainage field, normally domestic Soakaways are smaller.
Okay so the white pipes are perforated to slowly discharge the effluent evenly over the whole filed. These pipes are covered with protective tunnels to help prevent damage. If a pipe is broken then the system is likely to fail.
Soakaway problems, fixes and signs of failure.
Please see our Soakaway problems and fixes page for a more detailed examination. Below are the key indicators that you have a Soakaway, septic tank soakaway or septic tank problem or failure.
- The classic first sign of a problem is strong or bad odours coming from the septic tank or drains. This could also be an indicator you have left emptying your Septic tank for too long, check the tank level.
- Problems with drainage in your house. This can manifest its self as slow flushing or overflowing toilets. Internal or external drains are gurgling as they function. Unexpected water around your washing machine or dishwasher. Slow draining baths or showers.
- Septic tank effluent and waste pooling on the surface of your garden. Normally in areas where your Septic system is active.
- Ground movement or springy footing near the septic tank or soakaway drain field.
- Manholes and drain inspection chambers have standing effluent in or on them.
Installing a drainage Soakaway in clay
It is virtually impossible to soak effluent away in clay or clay-based soil. Some cowboy firms will sell you this solution; expect to be forking out for a replacement within 5 years. We will check your soil type with a percolation test; cowboy firms in the industry do not carry these out. We would recommend that if you have a clay soil type you need to install a sewerage treatment plant. A Sewerage Treat Plant emits virtually clean water, therefore, the regulations around dispersal are different.
What are the rules and regulations around septic soakaway and drainage field installations?
Please see our dedicated rules and regulations page. The key document is the building regulations part – h document which can be viewed here. As discussed above this stipulates the depth and size of any drainage field, maximum depth 700mm and the need for constant aerobic contact. If you decide against using Homeseptic’s services we recommend getting up to speed with the legislation to avoid cowboy installers. Also please read this British Water code of practice.
You also need to be up to speed with the latest legislation relating to effluent run off. Check the general binding rules. In reality, the future is Sewerage Treatment Plants. If you are going to have a whole new system installed we would recommend this option. It is highly likely that further legislation is imminent, which will make selling a house with a Septic Tank problematic. If you have a Septic Tank that discharges directly to a watercourse this will become illegal from 2020.
What is the cost of a new soakaway for a septic tank?
The cost of a new drainage field depends on three factors. Access to your property for the right sized machinery, it will take longer and require more labour if we cannot. The size of your system, this is driven by the number of people in your household. The bigger the discharge, the bigger the soakaway or drainage field needed. The type of soil, different types of soil require varying amounts of effort to complete the installation.
Generally, you are looking at between 3 and 5 thousand pounds for a domestic drainage field.
How a Septic tank Soakaway system or pit is designed
Okay so, first of all, you need to read all of the legislation, building regulations and codes of practice. The links to these documents are in the rules and regulations section above. You will also need to know the volume of discharge. To calculate this either take the size of your tank and check the manufactures maximum output or calculate your maximum output based on the numbers of people in your household.
Now you hopefully know how big your field or pit needs to be. Dig the appropriate number and length of trenches, remember not to dig deeper than 700mm. Fill the base with pea shingle and then lay the perforated drainage pipes on top. Cover with protective tunnels. Connect to an output of your Septic Tank. Backfill the trenches and re-turf.