Biodigesters overview


When building or renovating a home that has no connection to mains drainage for the disposal of sewage then alternative options have to be considered.

The UK currently has four different Environmental Regulators for surface and groundwater pollution control:

  • England – The Environmental Agency (EA). Tel: 03708 506 506
  • Wales – Natural Resources Wales / Cyfoeth Naturiol Cymru. Tel: 0300 065 3000
  • Scotland – SEPA (Scottish Environment Protection Agency) Tel: 0300 099 66 99
  • Northern Ireland – Northern Ireland Environmental Agency (NIEA) Tel: 0845 302 0008

These regulators operate in different ways and consequently generalisations have to be made.

Starting off specifically with the EA in England a document produced by DEFRA (Department of the Environment, Fisheries and Rural Affairs) entitled General Binding Rules for Small Sewage Discharges (SSDs) with effect from January 2015 is the yardstick. Refer to

For the other regulators go to their websites or ask for assistance.

The standard checking procedure starts off with the question ‘How far is the site from the nearest public foul sewer?’ If more than 30m then alternative options can be considered in most cases.

If considering pumping to a public foul sewer there need to be checks regarding the legal rights of access, connection fees, obligations in perpetuity and the technical aspects of pumping and storage.

If a property’s wastewater cannot be connected into a sewer there are three alternative options, a Cesspool, a Septic Tank or a Package Treatment Plant. Package Pumping Stations are available constructed in glassfibre (GRP) or other materials with pumps of different types.

The options

Under current naming conventions a Cesspool is a holding tank that has no overflow and from which all the effluent must be taken away by a licensed tanker firm. Compliance with Building Regulations requires a large capacity tank of several thousand gallons. Even with extreme water saving measures the cost of emptying a cesspool is likely to be several thousand pounds in a single year. The use of cesspools is therefore best avoided.

Septic Tanks are much smaller tanks than cesspools with two or more chambers that settle out some of the solids that come into them. However the liquid part remains a strong polluting liquid that may only be dispersed into a system of soakaway trenches, sometimes also called sub irrigation systems. Soil porosity tests have to be carried out in accordance with BS6297:1983 and the Amendments of 2007 & 2008 and the soakaway system sized and designed accordingly. The trenches need to be constructed with 25mm-40mm clean stone to a depth of 400mm around a rigid 100mm diameter pipe with slots or holes at the bottom. The width of each trench is normally 600mm or 900mm and there needs to be a suitable covering sheet.

Traditional herringbone soakaways are best avoided as if the main artery blocks the whole system may block. A rectangular grid allows alternative pathways. On sloping ground soakaway trenches should be constructed at a constant depth along one or more contours.

BS6297:1983 requires that septic tank soakaway trenches should only be constructed where there is at least 1 metre of soil between the soakaway and the water table at all times. This rules out the use of septic tanks in many areas where there is low lying flat ground with clay soil and a high water table.

For septic tanks there may be a restriction on their use due to underground sources used for extraction of drinking water. In England check the Environment Agency’s Protection Zones at or ask a specialist to check this for you. There may also be restrictions with respect to Sites of Special Scientific Interest (SSSI) or similar sites where installation is within and also near to such a site.

The discharge of septic tank effluent into a watercourse, including ditches has been illegal since the 1960’s. DEFRA now requires that any existing discharges are stopped by 2020.

Septic tanks should be emptied once per year to avoid failure of the soakaway system.

Package treatment plants are the modern alternative to septic tanks that produce a clear effluent suitable for discharge into a watercourse if one is available. The effluent is typically 20 times cleaner than that from a septic tank. Most new properties built in the countryside nowadays use treatment plants as they represent the environmentally friendly option.

If no watercourse is available the treated effluent still needs to be discharged into a system of soakaway trenches. The use of a treatment plant may overcome the difficulties of a high water table, underground water supply areas or sites of special scientific sensitivity.

To overcome a high water table or discharge into a watercourse that may flood it can also be necessary to use an automatic effluent pumping system that manufacturers supply with their treatment plants, normally as an option. These systems have a non-return valve that prevents any back flow.

A number of different types of package treatment plant are available. The ones with complicated mechanical and electrical components are becoming less popular and aeration of the sewage with a small air blower is now commonplace. Some of the aerated systems still have a septic settlement stage with the risk of odours and the need for emptying once or more times per year. The better systems aerate all the sewage that comes into them including all solid matter. The best systems will go for several years before they need to be emptied. The use of Linear Air Blowers provides operation that is efficient and economical with little maintenance required for the best systems. Package treatment plants should be located no nearer to inhabited dwellings than 7M.

In selecting your wastewater solution for Off Mains Drainage it is important to cover the requirements of the appropriate Environmental Regulator, BS6297:1983 as amended and local Building Regulations.


how it works

How a biodigester works

Air is blown into the biodigester by an electrically powered compressor mounted normally within 10 metres of the sewage treatment plant. The air is diffused from the bottom of the central chamber. This increased oxygen supply accelerates the activity of the naturally occurring micro-organism which degrade the solids to a clear effluent and a non toxic sludge.

The plastic media is used to provide a high surface area for the micro-organisms to adhere to and also, as it is mobile, to facilitate rapid degradation of solid matter.

The diffused air also operates as an air lift which recirculates solids from the outer settlement chamber to the inner treatment chamber. This recirculation also ensures that both chambers remain aerobic. The process runs continuously 24 hours a day.

The plant is designed to conform to the requirements of BS6297.1983 Code of practice for design and installation of small sewage treatment works and cesspools.



Got a question? Need to find out more?

My builder said I should contact you as the Biodigester product is very well made and does not need to be emptied so often.

Mrs SL