Rural Non-Mains Drainage

Guidance Notes on Drainage Provision in Rural Areas

Before you can install a domestic drainage system you must apply for Building Regulation approval. Where an application is made and the plans do not show that satisfactory provision has been made for drainage, they will be rejected in accordance with the Building Act 1984 Section 21.

Due to the nature of the subsoil within many areas of Worcestershire, we regularly receive complaints about drainage systems that do not perform satisfactorily. These complaints often result in formal action being taken, and in some cases can lead to prosecution. The majority of these cases are where septic tanks have been installed with an inadequate soakaway system, due either to poor design, or increased volume not catered for from the outset. With this in mind the following Guidance Notes are intended to provide advice on the correct choice of drainage system, and the tests required to ensure that the system should function satisfactorily.

Our preferred drainage options are a gravity connection to a public sewer, or the installation of a pumped system to public sewer where there is sufficient capacity. Permission must be obtained from Severn Trent Water Ltd prior to any connection to a public sewer. Where this is not possible, there are various alternative non-main drainage options (not in order of preference):

  • Installation of a septic tank and soakaway system or septic tank and Reed-bed System
  • Installation of a mini treatment plant with a permit or an exemption (awarded by the Environment Agency)
  • Installation of a sealed cesspool


Septic Tanks

Prior to selection of a septic tank system it is ESSENTIAL THAT SOIL PERCOLATION TESTS ARE CARRIED OUT in accordance with the British Standards Code of Practice. (BS6297:2007).

(WARNING : This has often been overlooked in the past and has resulted in further failure of drainage systems and additional expense to correct them.)

Septic tank systems have two main elements – a watertight underground tank and a drainage field. Raw sewage flows into the tank. The solids separate from the liquid and form into sludge at the base of the tank. The remaining liquid (effluent) discharges to the sub-surface irrigation system in the drainage field. DIRECT DISCHARGE TO A DITCH OR WATERCOURSE FROM A SEPTIC TANK SHOULD NOT OCCUR.

Where the discharge is to a permeable subsoil, such as sand, gravel, or chalk at a level well above the winter water table, a traditional soakaway pit may be acceptable. In large areas of Worcestershire the subsoil is less permeable, consisting of clay, marl etc and a subsurface irrigation system will be more suitable. To ascertain whether the subsoil is suitable, soil percolation tests are required. The BS6297 standards state that if the level of the water table rises in the winter to within 1m of the proposed invert level (bottom of pipe) of the irrigation system, it is not normally advisable to use subsurface irrigation.

It is important that the tests are representative of annual weather conditions, it is therefore not wise to carry them out during times of heavy rainfall, severe frost or drought.


Percolation tests

Excavate three holes 300mm square to a depth of 250mm below the proposed invert level of the soakaway system. Where deep drains are necessary the holes should conform to this shape at the bottom, but may be enlarged above the 250mm level to allow safe working. All works should comply with Health and Safety Requirements, and any hole deeper than 1.2m should be adequately shored.

Fill the 300mm square section hole to a depth of at least 250mm with water and allow to seep away overnight.

Next day refill the test section with water to a depth of at least 250mm and observe the time, in minutes, for the water to seep away completely. Repeat this test at least three times.

The table below indicates the minimum length of soakaway required to serve a specified number of persons. This should also correspond to the septic tank capacities shown. If the average time taken for the water to seep away completely is longer than 3 hours then it may be more cost effective to consider one of the drainage options described below. If the water takes more than 10 hours to seep away completely, soakaways are unsuitable, and one of the other drainage options detailed below will be required.


Design criteria for soakaway systems

Septic Tanks should ideally be situated at least 15m away from a habitable dwelling and all soakaways should ideally be 10m from any ditch or watercourse. If a non-mains water supply is situated within 50m of a proposed drainage system, it is important that you notify Environmental Services prior to carrying out any works.

The older herringbone systems tend to lead to saturation around localised areas, and should no longer be considered. The preferred pipes suitable for soakaways are rigid perforated plastic, 110mm in diameter, which should be laid to a maximum gradient of 1:200. Flexible coiled land drainage pipe is NOT suitable, as a constant gradient is not possible. The branches of the pipes should be laid at least 2m apart.

The pipe should be surrounded by 150 mm of clinker, clean gravel or broken stone of 20-50mm grade, covered either by polythene or water permeable matting which will prevent the covering topsoil clogging the media. The trench should ideally be 450mm wide and no more that 400mm deep. However the depth will be dictated by the invert level of the septic tank. Where the septic tank is installed to a depth in excess of this, and the sub soil is unsuitable following percolation tests, consideration may be given to pumping the effluent to a higher level and then to a soakaway system.

After carrying out the porosity tests, it will be evident whether a septic tank and soakaway system will be satisfactory, if this is not the case, one of the following two
systems should be considered.

Mini treatment plant

There are many brands of mini treatment plant available on the market; we cannot recommend any specific brand. There are two main types of plant, a rotating biological filter that lifts the effluent to allow aerobic treatment or a plant with a compressor that aerates the effluent. The effluent passing through mini treatment
plants is treated to a higher standard than with septic systems. Due to this extra treatment, direct discharge can be made to a ditch or watercourse. Consideration needs to be given to maintenance and running costs, etc. At present, all mini treatment plants require an electrical connection in order to operate.

Any discharge from a mini treatment plant to a watercourse, ditch etc requires a permit or an exemption from the Environment Agency. It is important that the Environment Agency is contacted regarding this before ordering the plant. You can contact the local officer via the general enquiry number 08708 506506.


Sealed cesspool

Where the above option is not practical another option is a cesspool, which is a sealed tank with no outfall, for which the minimum storage capacity is 18,000 litres under the Building Regulations. The table below shows the storage period for different sizes of cesspool depending on the number of persons served. This is based on each person using 137 litres of water per day.

Whilst the initial installation costs may be cheaper, the ongoing emptying costs make this the most expensive long-term solution.


Reed beds

These are becoming more frequently used as an alternative to soakaways. A reed bed is a method of purifying polluted water as it passes through an artificial wet land containing the common Reed. Reed beds can be used in conjunction with septic tanks, where soakaways are unsuitable. Maintenance and running costs can be low,
and the beds can provide a habitat for wildlife.

Approximately 2 square meters of reed bed are required per person equivalent.