Determining the Minimum Subsoil Permeability for Pressurised Infiltration Systems for on-Site Wastewater Treatment in Ireland.

In Ireland approximately one third of rural households depend on onsite wastewater treatment systems for their effluent disposal (CSO, 2011). The effectiveness of these systems in the prevention of water pollution from microbiological pathogens and/or nutrients is dependent on the thickness and permeability of the unsaturated layer of overlying subsoil receiving the effluent (i.e. the percolation area). In Ireland, glacial till (low permeability boulder clay) is the most widespread subsoil covering over 43% of the country area at surface, and approximately 25% beneath peat or floodplain sediments (Meehan & Lee, 2012) and is often unsuitable for traditional on-site treatment systems.

This research has evaluated two alternative technologies to the conventional gravel-filled gravity fed percolation trenches in low permeability Irish subsoils: drip distribution systems (DDS) and low pressure pipe systems (LPPS). Two sites in Ireland were upgraded from traditional septic tank soakaway systems to the two alternative pressurised infiltration systems in parallel. At each site, 50% of the effluent was distributed via a LPPS, the other 50% via a DDS. The three-dimensional distribution of the hydraulic load and resulting soil moisture profiles within the percolation area beneath each system across the seasons were monitored through the installation of tensiometers and soil moisture probes across the area and at a range of depths from the upper soil horizons down to the water tables. The timing and volume of flows to the percolation areas were recorded as well as rainfall and other meteorological data in order to understand the effectiveness of the systems across the year in the Irish climate.

Both infiltration systems had been designed to the same areal loading rate at ~ 3 L/m².d and over a two year monitoring period the soil moisture results under both the DDS and the LPPS have shown that the low permeability subsoils beneath the systems maintained unsaturated conditions and with no evidence of surface ponding. Water quality samples have also been taken of the percolating effluent in the vadose zone and downstream groundwater which have revealed excellent attenuation of on-site effluent contaminants within 1 m depth underneath the infiltration surface. The soil moisture results have then been used to calibrate a HYDRUS 2D model of the unsaturated percolation areas. These models have then been used to predict DDS and the LPPS performances on even lower permeability subsoils in order to establish design criteria limits for both the LPPS and DDS. Hence, minimum subsoil permeabilities based on the Irish falling head percolation test have now been defined for the suitability of such pressurised infiltration systems which will augment the legislative Code of Practice for Single houses in Ireland.