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Technical guidance - drainage fields

Information you will need to know when installing septic tank soakaways and drainage fields for waste water treatment plants

Drainage fields

This gives guidance on the design and construction of drainage fields to provide secondary treatment to the discharge from a septic tank or package treatment plant.

Siting

A drainage field serving a wastewater treatment plant or septic tank should be located:

The disposal area should be downslope of groundwater sources.

No water supply pipes or underground services other than those required by the disposal system itself should be located in the disposal area.

No access roads, driveways or paved areas should be located within the disposal area.

Ground conditions

Well drained and well aerated subsoils are usually brown, yellow, or reddish in colour. Examples of subsoils with good percolation characteristics are sand, gravel, chalk, sandy loam and clay loam. It is important that the filtration characteristics are suitable in both summer and winter. Poorly drained or saturated subsoils are often grey or blue in colour. Brown and grey mottling usually indicates periodic saturation. Examples of subsoils with poor filtration characteristics are sandy clay, silt clay, and clay. A first assessment should be carried out including consultation with the Environment Agency and local authority to determine the suitability of the site. The natural vegetation on the site should also give an indication of its suitability for a drainage field. A trial hole should be dug to determine the position of the standing ground water table. The trial hole should be a minimum of 1m2 in area and 2m deep, or a minimum of 1.5m below the invert level of the proposed drainage field pipework. The ground water table should not rise to within 1m of the proposed distribution pipes. If the test is carried out in summer, the likely winter groundwater levels should be considered. A percolation test should then be carried out to assess the further suitability of the proposed area.

Filtration test method - A hole 300mm square should be excavated to a depth of 300mm below the invert level of the effluent distribution pipe. Where deep drains are necessary the hole should confirm to this shape at the bottom, but may be enlarged above the 300mm level to enable safe dig to be carried out. Where deep digs are necessary, a modified test procedure may be adopted using a 300mm earth auger. Bore the test hole vertically to the appropriate depth taking care to remove all loose debris. Fill the 300mm square section of the hole to a depth of at least 300mm with water and allow it to seep away overnight. Next day, refill the test section with water to a depth of at least 300mm and observe the time, in seconds, for the water to seep away from 75% full to 25% full level (i.e. a depth of 150mm). Divide this time by 150mm. The answer gives the average time in seconds (Vp) required for the water to drop 1mm. The test should be carried out at least three times with at least two trial holes. The average figure from the tests should be taken. The test should not be carried out during abnormal weather conditions such as heavy rain, severe frost or drought.

Drainage field disposal should only be used where percolation tests indicate average values of Vp of between 12 and 100 and the preliminary site assessment report and trial hole tests have been good. This minimum value ensures that untreated effluent cannot filter too rapidly into groundwater. Where Vp is outside these limits effective treatment is unlikely to take place in a drainage field. However, provided that a different form of secondary treatment is provided to treat the waste from septic tanks, it may still be possible to send the treated waste to a soakaway.

Design and construction

Drainage fields should be designed and constructed to ensure air contact between liquid waste and the subsoil. Drainage fields should be constructed using perforated pipe laid in trenches of a uniform gradient which should not be steeper than 1/200.

Pipes should be laid on a 300mm layer of clean shingle or broken stone graded between 20mm and 50mm. Trenches should be filled to a level 50mm above the pipe and covered with a layer of geotextile to prevent the entry of silt. The remainder of the trench can be filled with soil; distribution pipes should be laid at a minimum depth of 500mm below the surface. Drainage trenches should be from 300mm to 900mm wide, with areas of undisturbed ground 2m wide being maintained between in line trenches. An inspection chamber should be installed between the septic tank and the drainage field. Drainage fields should be set out as a continuous loop fed from the inspection chamber. To calculate the floor area of the drainage field (At in m2), the following formula should be used: At = p x Vp x 0.25 Where p is the number of persons served by the tank, Vp is the percolation (filtration) value (secs/mm) obtained as described above.