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Septic Tanks & Drainage Fields

A septic tank and 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 where raw incoming sewage is retained long enough for suspended solids to settle out as a sludge and for liquid organic matter to undergo anaerobic decomposition. How does it work? – A scum forms on top of the tank, consisting of fats and soaps, whilst sludge settles to the bottom of the tank. The relatively clear liquid, called septic tank effluent, which lies in between the scum and sludge, leaves the tank and then undergoes a secondary treatment.

Septic tanks can be built using traditional methods, which are slow and costly. However, by using a pre-fabricated composite septic tank the costs and project times will be considerably reduced.

These are typical examples of pre-fabricated composite septic tanks;

Typically, a stand-alone septic tank will reduce the strength of the raw incoming sewage by 30-50% when designed and installed correctly in accordance with Building Regulations.

In England and Wales, septic tanks are still permitted subject to the presence of an adequately sized drainage field.  In some cases septic tanks and drainage fields will require an (Environment Agency) permit.

A septic tank must be supported by a drainage field (sometimes called a soakaway).  This requires a percolation test, which must be carried out to determine the porosity of the soil (the ability of the ground to absorb water, and this data is used to calculate the size of the soakaway required.

A soakaway might not be the best secondary treatment solution, especially in places where the ground is made up primarily of clay soil, which absorbs water very slowly and also where the level of the natural water table is very high or the land area available is limited or too steep.

Building Regulations

The soakaway design and installation is subject to building regulations (reference BS6297:2007 + A1: 2008 Code of Practice for the design and installation of drainage fields for use in waste water treatment).