A domestic wastewater treatment system treats the foul drainage from a house that is not connected to a public sewer, before the treated effluent is discharged to a percolation area in the ground. Systems must be designed, installed and maintained in accordance with the EPA Code of Practice.
In unsewered areas, each house must treat its own wastewater. The system typically comprises primary settlement, secondary treatment using aeration or a filter media, and then discharge of the treated effluent to a percolation area where the soil provides final treatment before the water reaches groundwater.
The EPA Code of Practice governs how these systems are selected, sized and located, based on a site characterisation report that examines soil, subsoil, water table, bedrock depth and proximity to wells and watercourses. The planning authority relies on that report when deciding an application.