The treatment performance of different subsoils in Ireland receiving on-site wastewater effluent

Current Irish guidelines require a comprehensive site assessment of a percolation area for wastewater disposal before planning permission is granted for dwelling in rural areas. For a site to be deemed suitable, the subsoil must have a percolation value equivalent to a field saturated by hydraulic conductivity in the range 0.08 to 4.2 m d(-1) using a falling head percolation test. A minimum of 1.2 m of unsaturated subsoil must also exist below the invert of the percolation area receiving effluent from a septic tank (or 0.6 m for secondary treated effluent). During a 2-yr period, the three-dimensional performance of four percolation areas treating domestic wastewater was monitored. At each site samples were taken at 0, 10, and 20 m along each of the four percolation trenches at depths of 0.3, 0.6, and 1.0 m below each trench to ascertain the attenuation effects of the unsaturated subsoil. The two sites with septic tanks installed performed at least as well as the other two sites with secondary treatment systems installed and appeared to discharge a better quality effluent in terms of nutrient load. An average of 2.1 and 6.8 g total N d(-1) on the sites the septic rank effluent was of an equivalent quality to the secondary treated effluency in terms of indicator bacteria (E. coli) after percolating through 0.6-m depth of unsaturated subsoil.