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In Ireland, the most prevalent domestic wastewater treatment application in unsewered areas is the conventional septic tank system with percolation area. However, concern has been expressed over on-site effluent discharging into highly permeable soils which, although permitted in current Irish EPA guidelines, has generated debate as to whether an insufficient level of treatment in the subsoils unsaturated zone is being achieved. In situations where a septic tank installation is considered unsuitable according to a rigorous site assessment, some form of secondary treatment system can be installed to improve the effluent quality before discharge to the subsoil. Horizontal subsurface-flow reed bed systems are one such technology receiving significant attention recently, being deemed an effective and low-cost alternative for secondary treatment applications. On-site research was thus carried out to assess and compare the treatment capabilities of freely-draining sandy subsoils receiving both septic tank and secondary effluents in tandem with a treatment assessment of a horizontal subsurface-flow reed bed receiving heavily loaded septic tank effluent. Results over a 12-month period have shown the reed bed to remove only 47% of the organic load but achieve 2-3 log removal in total coliforms and E.coli. The majority of nitrification is seen to occur in the first 0.3m of subsoil for both the trenches receiving septic tank effluent and secondary effluent respectively. The research also shows that in general the septic tank effluent has received a comparable quality to the secondary effluent in terms of E.coli by the time the point of discharge to groundwater is reached in the subsoil.