Willow Based Evapotranspiration Systems for the on-Site Treatment of Domestic Wastewater in Areas of Low Permeability Subsoils.

Ireland has over one third of its population using on-site wastewater treatment systems, mostly consisting of septic tanks discharging effluent into a subsoil percolation area. However, across wide areas of the country the subsoil is of too low permeability for such systems. The aim of this research was to evaluate the use of evapotranspiration systems using willow trees in closed basins such that if sized correctly, they should produce no discharge either to ground or to surface water. 11 full-scale willow bed systems have been constructed as pilot trials at houses around the country in areas of heavy clay subsoils to treat the domestic wastewater produced. The systems were designed with variations between key parameters (effluent type, willow species, plan area, aspect ratio and effluent distribution), in order to determine sensitivity to these in terms of overall performance. The effluent flow into the basins, water level, rainfall and evapotranspiration have all been monitored over four years to determine the water budget and crop factors at each site.

Evapotranspiration results varied greatly between sites, with some sites exhibiting excellent willow tree establishment and correspondingly high evapotranspiration rates, while other sites showed the opposite performance. In general however, there was overflow from almost all the systems at different periods throughout the monitoring period, including some periods during the summertime. One significant problem identified was the usable void ratio in the basins that had been refilled with the excavated low permeability subsoil was much lower than expected thereby leaving little room for effluent storage over the winter periods. In addition, the evapotranspiration rates were lower when compared to other countries where these systems have been used, which was attributed mainly to the predominantly high relative humidity of the Irish climate. Water availability was determined to strongly influence the evapotranspiration rate from a system, while the addition of effluent was shown to have had a positive effect on willow tree development and evapotranspiration rates. Pollutant uptake / removal was found to be very high on the systems with well-established willow trees.

Using crop factors based upon these trial results, guidelines on the design and construction of willow systems have been prepared. As the achievement of a completely zero discharge system would appear to be difficult to achieve in an Irish climate, the guidelines were designed on the premise of minimising the number of overflow days while keeping within the boundaries of reasonable practical and financial constraints.