Drainage Fields & Soakaways
Specialists in sewage treatment plant installation.
A drainage field is designed to disperse and treat the outflow from either a septic tank or sewage treatment plant. It is constructed of rigid perforated or slotted pipework, which is laid in a clean stone surround with a membrane layer before covering with soil. The final effluent passes through the entire length of the drainage field, seeping through the perforations and into the clean stone surround. Here, anaerobic bacteria break down the waste further, before the final liquid is absorbed into the surrounding soil. The volume of the drainage field is calculated in linear metres and is sized depending on the soil type and number of residences the property can accommodate. A percolation test is undertaken prior to the installation of the drainage field, to both ensure the ground is suitable, and to calculate the volume required.
In a Ground Water Protection Zone a septic tank and drainage field is not permitted, however, with a permit from the Environment Agency a sewage treatment plant and drainage field maybe permissible (subject to the Environment Agency’s General Binding Rules).
The Environment Agency and Building Control require a percolation test prior to the installation of a drainage field.
A percolation test consists of digging three holes at the proposed depth and position of the drainage field, filling the holes with water and timing the absorption rate. The seconds per mm recorded allows us to calculate the volume of drainage field required. A percolation test ensures the aquifer will not be contaminated with pollutants.
A soakaway is primarily designed to take rain or surface water and is generally of a pit style construction. It is usually constructed with concrete rings or plastic crates. A soakaway would not usually be considered for use with a foul water discharge, however, if a property is too small to site a drainage field, a soakaway can be considered. Soakaways cannot be used with septic tanks, as the risk of ground water pollution is too great, and a soakaway does not give any further treatment. A sewage treatment plant, however, can be discharged into a soakaway, but a permit will be required from the Environment Agency.
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