Why Install a Sewage Treatment Plant vs a Septic Tank?

Both H2 building regulations and EN6297 state that, for a septic tank to be installed, the water table must be at least one metre below a septic tank outlet. In this instance, and if your property is in a flood risk area, you will need to install a sewage treatment plant.

The choice between both options essentially boils down to the site characteristics. One of the most important factors in determining the choice of sewage treatment product is the site’s drainage properties; you must first determine its suitability for off-mains drainage.  Both septic tanks and sewage treatment plants typically require a drainage field into which they discharge effluent.

Testing the ground for its permeability is usually done by a professional installer using a percolation test; the correct procedure is described in BS 6297:2007.

To install a septic tank, your property will need to be located on highly permeable ground, otherwise, the drainage field will fail.  If instead, your plot sits on a clay substrate, then a sewage treatment plant is the only viable option.

The propensity of the site to flood and the height of the water table are also important considerations. At best, septic tanks remove about 30% of the pollutants – which is why a septic installed in the wrong place is likely to smell obnoxious and why unfiltered solids may find their way to the surface. This will, in a short space of time, create a serious health risk and pollute groundwater (the water we drink).