The attenuation capacity of different subsoils receiving domestic wastewater effluent

Abstract

With the domestic wastewater effluent of approximately 400,000 dwellings in Ireland being discharged to groundwater annually and about 25% of all water supplies in the country provided by groundwater a risk of contamination of groundwater resources by domestic wastewater effluent exists. Treatment Systems for Single Houses, a wastewater treatment manual produced by the EPA, is aimed at reducing that risk. It recommends a rigorous site suitability assessment procedure, comprising a desk study and on-site assessment, to determine the potential of the subsoil for effluent assimilation and attenuation. It is currently considered that a site with 1.2m of unsaturated subsoil below the invert of the proposed percolation trenches with a Tvalue falling in the range 1min/25mm and 50min/25m is suitable for the installation of a septic tank treatment system. If less than 1.2m, but greater than 0.6m, of unsaturated subsoil is present, some form of secondary treatment is also required prior to discharge.