Synthesis Report - ERTDI Report 27 - Gill et al
Summary: The results from four separate field trials at separate sites covering a range of different subsoil types, whereby two sites were discharging septic tank effluent into conventional percolation areas and the other two were discharging secondary treated effluent from a peat filter into similar percolation areas.
The safe disposal of on-site wastewater is essential for the protection of groundwater in Ireland and has come more into focus recently due to the publication of the EPA guidance manual Treatment Systems for Single Houses (EPA, 2000).
This project reports the results from four separate field trials carried out on separate sites covering a range of different subsoil types, whereby two sites were discharging septic tank effluent into conventional percolation areas and the other two sites were discharging secondary treated effluent from a peat filter into similar percolation areas. Stratified sand filters were also constructed, one receiving septic tank effluent, the other receiving secondary treated effluent for comparison.
The short duration of the research (the maximum amount of time that each site was monitored was 12 months) and the limited numbers of sites investigated mean that only tentative conclusions can be made about on-site wastewater treatment at this stage which will hopefully be corroborated by further research in the future. The project showed that the 1.2 m of unsaturated subsoil receiving septic tank effluent (or 0.6 m for secondary treated effluent), as specified in the guidelines, provided good removal of both chemical and microbiological pollutants.
The majority of degradation of septic tank effluent occurred in the first 300 mm below the percolation trenches and the quality of the percolating effluent after 1.0 m depth was found to be acceptable for discharge to groundwater. There was a much better removal of nitrogen in the subsoil from the septic tank effluent than from the secondary treated effluent which remained largely untouched in its nitrified form of nitrate.
Phosphate removal on the sites seemed to depend mainly on the mineralogy of the subsoil. The project also showed that the biomat development seemed to be a function of the organic load applied to the subsoil. The sites receiving secondary treated effluent (having a much lower load) did not develop any significant biomat over the duration of the project and resulted in a more concentrated plume of effluent loaded onto the front of the trenches.
The biomat seemed to regulate the hydraulic capacity of the subsoil and no discernible difference could be made with regards to treatment performance between subsoils of different permeabilities. The results from the two stratified sand filters showed that each treated their respective effluents to just as high quality as an equivalent percolation area.
However, when the sand filter is used as a secondary filter (to treat the septic tank effluent) it should be designed for a hydraulic loading of no more than 30 l/m2 per day whereas, when used as a polishing filter (to treat secondary effluent), it can be designed for a hydraulic loading up to 60 l/m2 per day. Both filters also showed excellent removal of phosphorus which was attributed to the mineral content of the type of sand used in their construction.
Finally, the project showed that domestic water consumption was much lower than the 180 litres per capita per day (lcd) forecast and that a figure of 120 lcd would seem to be more reasonable for on-site treatment in Ireland.
