Abstract
The wastewater of one third of Ireland’s population is treated on-site using domestic treatment systems (DWWTSs) that usually consist of a septic tank and soil attenuation system. Within the past four years, the legislative framework for these systems has undergone a major change with a registration and inspection regime being introduced to identify legacy sites that will require remediation work, particularly in areas of the country underlain by subsoils of very low permeability. Against this background this study aims to assess the overall sustainability of existing DWWTSs as well as alternative treatment and disposal options. The results show that main CO2eq emissions are from the methane production in septic tanks. The reduced methane production in mechanically aerated secondary treatment systems was found to counterbalance the related emissions due to the additional energy requirements. In contrast, septic tank systems have the lowest construction and operational costs representing the most economically sustainable solution. Pressurised disposal systems are slightly more expensive but have the potential to reduce environmental impact on surface water and reduce greenhouse gas emissions. Clustered decentralised treatment solutions could be environmentally and economically sustainable but ownership, management and related financial and legal issues will need to be addressed and developed.
Highlights
The domestic wastewater of approximately one third of the population in Ireland is treated on-site by domestic wastewater treatment systems (DWWTSs) [1]
While this is less than half of the per capita emissions for the operation of on-site secondary treatment systems, it is significantly higher than the emissions from the standard septic tank systems which represent most of the DWWTSs in Ireland (87%)
When operational costs are expressed as net costs and incorporate annual water and energy cost savings (€49 or €90/ca.year) operational costs for Drip Distribution (DD) and Low Pressure Pipe (LPP) systems can be reduced by 36% up to 68%
Summary
The domestic wastewater of approximately one third of the population in Ireland is treated on-site by domestic wastewater treatment systems (DWWTSs) [1]. If the permeability of the subsoil is excessive, the effluent loading on the subsoil too high, or there is an insufficient depth of unsaturated subsoil (e.g., high water table or low depth to bedrock) the groundwater beneath a percolation area is at risk of pollution, in particular from microbiological pathogens and/or nutrients. The. Code aims to define subsoil conditions that will provide an acceptable level of treatment for on-site domestic wastewater effluent in order to protect groundwater resources from contamination. Drip Distribution (DD) systems as well as Zero-discharge Evapotranspiration systems (using willow trees) in low permeability subsoil settings [7,8,9] Based on those results, design guidelines will be produced to augment the current Code of Practice.
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