Soakaway/Drainage Field
The principle of a septic tank soakaway is to disperse septic effluent (liquid from a septic tank) as it is displaced by new incoming waste water entering the septic tank, into the sub soil at the site.
How does a soakaway work?
Soakaways can also be used to disperse treated effluent from a sewage treatment plant and/ or reed bed. The size of the soakaway will be dependent on the porosity of the sub soil material, the number of residents it is serving and the type of primary treatment i.e. a septic tank or a sewage treatment plant. A percolation test will need to be carried out to ascertain the porosity of the soil (the ability of the ground to absorb water). This data is used to calculate the length of soakaway needed. A soakaway might not be the best secondary treatment solution, especially in places where the ground is made up of primarily clay soil that absorbs water very slowly, where the level of the natural water table is very high and/or the land area available is limited or too steep.
Soakaway installation comes under Building Regulations and there are various design standards and minimum distances that must be adhered to. Please note that a septic tank soakaway is not the same as a rain water soakaway.
Common Soakaway Problems
Roots entering and choking the pipe work
Water thirsty roots will often find their way into soakaways and over time roots can block the soakaway pipe work. The picture below shows such a blockage in a field land drain. Quite often the only solution is to clear the soakaway of the blockages. This might include re-siting the soakaway away from trees. If planting around your soakaway area choose only shallow rooted, non water loving plants. Species such as willow and poplar are a definite no. The best option is laying lawn over the soakaway area.
Soil Characteristics
As stated in Section H of the Building Regulations `drainage field disposal should only be used when percolation tests indicate average values of Vp of between 12 and 100 and the preliminary site assessment report and trial hole tests have been favourable`. A soakaway should not be installed on heavy clay or clay-based soil as the septic tank liquor cannot percolate well enough through the soils and therefore treatment and dispersion will be limited. Suspended soilds carried over from the septic tank will also add to the soakaway failure by blocking available air spaces within the soil.
Depth of the soakaway
The deeper a soakaway is installed, the less oxygen is available for the aerobic bacteria in the soakaway to break down the septic tank effluent. Often the deeper the dig the more chance of hitting the water table and impermeable clay subsoils or parent rock which are all detrimental to soakaway performance. Section H of the Building Regulations 1.39 states that `Drainage fields should be designed and constructed to ensure aerobic contact between the liquid effluent and the subsoil`.
High water table
WCI often has more calls to our offices in winter regarding soakaway failure. This is because the water table is much higher in the winter months and can result is the backing up of wastewater towards the property as the septic tank effluent cannot be dispersed through the soakaway. The solids within the septic tank mix and when the water table drops, solids from the tank are distributed in the soakaway adding to its failure. If a watercourse or flowing ditch is available to you a sewage treatment plant or reed bed could be installed to prevent such a problem.
Septic tank problems
If a septic tank is incorrectly sized and is too small for the application then solids do not have enough retention time within the tank to allow for settlement and can pass out into the soakaway. Broken dip pipes and baffles also allow solids to enter the soakaway. If roof water enters the septic tank then solids are mixed during a storm and can carryout to the soakaway. All the aforementioned situations involve the blocking up of the soakaway with suspended solids from the septic tank.
Inadequate Soakaway Construction
Often soakaways are not constructed correctly in accordance with Approved document H in Building Regulations and as a result they fail. Examples include soakaway pits which consist of a hole often filled with stones or builders rubble. If the porosity for the entire holes depth is poor, then the septic tank effluent will not be able to disperse into the environment and wastewater will back up towards the property.
For further information please contact us on 01984 623404.