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Septic Tanks & Drainage Fields

A septic tank and drainage field (soakaway) still remains the simplest and most cost effective way of treating sewage from domestic properties where the site characteristics allow.

Septic tanks and drainage fields are some of the most widespread and least understood of private drainage systems.

A septic tank is a settlement tank in which raw incoming sewage is retained long enough for suspended solids to settle out as sludges and for liquid organic matter to undergo anaerobic decomposition. A scum forms on top of the tank consisting of fats and soaps, and sludge settles at the bottom of the tank. A stand-alone septic tank will reduce the strength of raw incoming sewage by 30-50% under ideal conditions. The relatively clear liquid, called septic tank effluent, which lies in between the scum and sludge, leaves the tank and then undergoes secondary treatment.

In England and Wales, septic tanks are still permitted subject to the presence of an adequately sized drainage field in permitted areas.  Some septic tanks and drainage fields require an Environment Agency permit.

A septic tank must be supported by a drainage field (sometimes called a soakaway although soakaways are technically for rainwater).  A percolation test must be carried out to determine the porosity of the soil (the ability of the ground to absorb water). This data is used to calculate the size of drainage field required. A drainage field 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.

Drainage field design and installation is subject to Building Regulations Approved Document H (which reference BS6297:2007 + A1: 2008 Code of Practice for the design and installation of drainage fields for use in wastewater treatment).

Common soakaway problems

WCI receives many calls a year regarding failed soakaways. The correct design and installation of a soakaway will avoid these problems. However, here is a list of common faults.

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. This can be prevented by only installing soakaways away from shrubs and trees and ensuring the soakaway includes a geotextile covering during installation.

Insufficient porosity

A soakaway is only as good as the ability of the surrounding ground to soak up the water it receives. 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’. In other words, the soakaway must be designed according to the actual soil characteristics.

Incorrect installation

Soakaways laid at incorrect gradients, incorrect depths and with incorrect layouts will not function correctly. For example, 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. Further, if a soakaway is not laid at the correct gradient, water will pool at one end of the soakaway resulting in outcropping or septic effluent rising to the surface.

High water table

WCI usually 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 in 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 could be installed to prevent such a problem.