Assessment of disposal options for treated wastewater from single houses in low permeability subsoil settings

The project is investigating sites where the on-site effluent is being discharged into highly permeable sandy soils where, although permitted in current Irish EPA guidelines, a feeling exists amongst local councils and practitioners that the effluent could pose a subsequent risk to groundwater - a major source of drinking water in Ireland - if it percolates at an excessive rate. Hence, this project is particularly concentrating on the attenuation of microbiological parameters (bacteria and viruses).

The domestic wastewater of over one third of the population in Ireland is treated by on-site treatment systems. In these systems the soil attenuation that takes place while the effluent percolates through the subsoil is playing an important role and protects the groundwater from pollution. The effectiveness of treatment however is highly dependent on the thickness and permeability of the subsoils. If the effluent loading on the subsoil is too high, the permeability of the subsoil very high or there is an insufficient depth of subsoil then the groundwater beneath a percolation area is at risk of pollution. Alternatively, if there is insufficient permeability in the subsoil to take the effluent load - the main focus of this research - surface ponding may occur with associated health risk and there will be a risk of effluent discharge and runoff of pollutants to surface water. The nutrient load in the effluent can contribute to eutrophication in sensitive water bodies, whilst contamination of water sources by human enteric pathogens can promote the outbreak of disease. The project aims to not only assess the pollution risk by old septic tank soakaway systems but moreover to investigate possible effluent disposal options in low permeability subsoil settings on the basis of both cost-benefit and environmental impact/sustainability principles.

Within this project the performance of existing septic tank soakaways in different subsoil settings with high, moderate and low permeability is being monitored to establish the degree of contamination to groundwater and/or surface water. Alternative methods of on-site effluent distribution, such as drip irrigation and low pressure pipe systems, will then be assessed for their suitability to improve effluent percolation and the natural attenuation of nutrients and pathogens. Willow evapotranspiration systems are seen as one potential effluent disposal option in areas of very low subsoil permeability. To investigate their performance under different Irish climatic conditions two systems have been constructed and will be monitored in the west of Ireland in order to augment the information being gained during an ongoing research project with trial systems in Co. Wexford. Using geospatial modelling different options for the treatment and disposal of wastewater effluent in areas of low subsoil permeability will be explored. This will be part of the development of a decision making prototype toolset for Local Authority planners and managers to evaluate alternative strategies and reduce the risk posed by current domestic septic tank treatment solutions.