Sewage Soakaway Drainfields
Sewage Soakaways Functions
A sewage water soakaway is not the same as a surface water (roof water) soakaway. The only function of a surface water soak away is to disperse the water into the ground, but a sewage soakaway has two functions.
The first is to soak the septic tank liquid away. The second function is to treat the effluent to remove suspended solids, bacteria and viruses, nitrogen and phosphorous using natural aerobic bacteria which grow within the soak away. Only then can the waste be soaked away into the ground. This is to avoid groundwater contamination, as much of our drinking water is sourced from it.
Sewage effluent soak aways must conform with Section H of the Building Regulations - Sewage and Drainage issues and BS 6297 : 2007 + A1 2008
Sizing of a soak away
|The size of the soak away, (i.e. the area in square metres), is determined by the number of bedrooms in the house and the porosity of the soil. Currently, The Environment Agency requires this to be one person per bedroom plus 0.5 persons per house. The porosity of the soil can only be determined by a percolation test.|
Minimum soak away site distances
Construction of soakaways drainfields
Sewage effluent soak aways are carefully designed constructions consisting of trenches containing pipes laid on gravel or stone beds, covered with more stone and geotextile membrane and backfilled with topsoil.
Percolation tests to determine length of pipework
A soil percolation test is required to be carried out on the land where the septic tank or treatment plant soak away is intended to be built. It is a test that determines the rate in which soil absorbs a known volume of water. This is to ensure that the land (soil) is suitable for a soak away, and to ensure that the soak away is designed properly.
A percolation test is always required for septic tanks.
A percolation test should not be carried out during abnormal weather conditions and if done in summer, then the times should be increased by 30% to 50%, depending on the recent weather.
The percolation test method
Drainage field soak away disposal should only be used when percolation tests indicate average values of V of between 15 and 100 and the preliminary site assessment report and trial hole tests have been favourable. A percolation test is deemed to have failed if the results are less than 12 or more than 100.
This minimum value ensures that untreated effluent cannot percolate too rapidly into ground water. Where V is outside these limits effective disposal is unlikely to take place in a soak away drainage field.
* Some Councils insist that percolation tests are carried out by 'qualified persons' and do not accept your own test results. You may be required to contact a contractor to carry out the percolation test. Please check with your Local Authority.
Calculating the size of the soak away
The calculation gives the area (A), in square metres, required for the soak away trench.
V = The time in seconds for the water in the test hole to drop by 1mm. p = The maximum number of persons that the unit is designed to serve.
For sewage treatment units Area (A) = Vp X 0.20
For septic tanks Area (A) = Vp X 0.25
Further calculations are required to give the length of pipes required depending on the width of the trench, e.g. for a 600mm (2ft) wide trench, the area would be divided by 0.6.