A comparison of the treatment performance of freely-draining subsoils and horizontal subsurface-flow wetland systems receiving on-site wastewater effluent

Abstract

On-site research was carried out in Ireland to assess and compare the treatment capabilities of freely draining subsoils receiving both septic tank and secondary effluents in tandem with a treatment assessment of a horizontal subsurface-flow reed bed. This is due to recent concern being expressed as to whether a sufficient level of treatment is being achieved in the subsoil?s vadose zone coupled with doubts being raised as to the treatment capabilities of horizontal flow reed beds as stand-alone secondary treatment systems. Results over an 18-month period have shown the reed bed to remove only 52% 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 all sets of percolation trenches and, coupled with poor denitrification throughout the subsoil, has compromised groundwater quality in the form of nitrate infiltration. 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.