{"id":442,"date":"2024-06-19T10:55:12","date_gmt":"2024-06-19T09:55:12","guid":{"rendered":"https:\/\/www.thebuildchain.co.uk\/news\/?p=442"},"modified":"2025-02-20T13:23:31","modified_gmt":"2025-02-20T13:23:31","slug":"drainage-field-sizing-for-sewage-treatment-plants","status":"publish","type":"post","link":"https:\/\/www.thebuildchain.co.uk\/news\/drainage-field-sizing-for-sewage-treatment-plants\/","title":{"rendered":"Drainage Field Sizing for Sewage Treatment Plants"},"content":{"rendered":"\n

This blog post is brought to you by our manufacturer partner Marsh Industries<\/a>.<\/em><\/p>\n\n\n\n

If you have access to a suitable area of land, discharge from your septic tank or treatment plant to a properly designed and sized drainage field is the best environmental option as the treated effluent recharges groundwater, nutrients are retained in the soil, and nutrient loads on surface waters are reduced.<\/p>\n\n\n\n

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Learn More About The Build Chain >><\/a><\/div>\n<\/div>\n\n\n\n

The most common form of drainage field is a subsurface percolation area comprising perforated infiltration pipes laid in shingle-filled trenches \u2013 normally within 1m of ground level to allow the micro-organisms in the soil to break down the organic matter, and at least 1.2m above the winter water table.<\/p>\n\n\n\n

The drainage field has two principal purposes:<\/p>\n\n\n\n

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  1. To allow percolation of partially treated\/treated effluent to ground at a controlled rate<\/li>\n\n\n\n
  2. To allow further treatment of partially treated effluent before it reaches the groundwater level<\/li>\n<\/ol>\n\n\n\n

    Before you can dispose of effluent via a drainage field you first need to assess whether such a route is appropriate, i.e. you have a good depth of well drained, well-aerated soil away from watercourses, wells\/boreholes, dwellings, and avoiding sloping sites and areas prone to waterlogging.<\/p>\n\n\n\n

    Trial hole and percolation test method<\/h2>\n\n\n\n

    To calculate the exact area of land required for effective disposal an \u2018assessment\u2019 is required, usually by performing a percolation\/water table test as outlined in BS6297 (Code of Practice for the Design and Installation of Drainage Fields for use in Wastewater Treatment) and the latest version of Building Regulations: H2<\/a>.<\/p>\n\n\n\n

    A trial hole should be dug to determine the position of the standing groundwater table a minimum of 1m2 in area and 2m deep, or a minimum of 1.5m below the invert of the proposed drainage field pipework.<\/p>\n\n\n\n

    The groundwater table should not rise to within 1m of the invert level of the proposed effluent distribution pipes. If the test is carried out in summer, the likely winter groundwater levels should be considered.<\/p>\n\n\n\n

    A percolation test should then be carried out to assess the further suitability of the proposed area. A hole 300mm square should be excavated to a depth 300mm below the proposed invert level of the effluent distribution pipe. Where deep drains are necessary the hole should conform to this shape at the bottom, but may be enlarged above the 300mm level to enable safe excavation to be carried out.<\/p>\n\n\n\n

    Fill the 300mm square section of the hole to a depth of at least 300mm with water and allow it to seep away overnight.<\/p>\n\n\n\n

    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 150. The answer gives the average time in seconds (Vp) required for the water to drop 1mm.<\/p>\n\n\n\n

    The test should be carried out at least three times with at least two trial holes and 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 when percolation tests indicate average values of Vp of between 12 and 100. This minimum value ensures that untreated effluent cannot percolate too rapidly into groundwater. Where Vp is outside these limits effective treatment is unlikely to take place in a drainage field.<\/p>\n\n\n\n

    Drainage field construction<\/h2>\n\n\n\n

    Drainage fields should be designed and constructed to ensure aerobic contact between the liquid effluent and the subsoil using perforated pipe laid in trenches:<\/p>\n\n\n\n