Willow-based evapotranspiration systems for on-site wastewater effluent in areas of low permeability subsoils

Six different full-scale willow evapotranspiration systems receiving both septic tank and secondary treated domestic effluent from single houses in Ireland have been investigated as a potential solution to the problem of on-site effluent disposal in areas with very low permeability subsoils. Continuous monitoring of rainfall, reference evapotranspiration, effluent flows and water level in the sealed systems revealed varying evapotranspiration rates across the different seasons with annual rates ranging from 576 to 929 mm/yr. The results showed that no system managed to achieve zero discharge in any year remaining at maximum levels for much of the winter months, indicating some loss of water by lateral exfiltration at the surface. This was attributed partly to the fact that the in-situ low permeability soil that had been excavated to form the lined basin was shown to have a much lower effective (useful) porosity when used to backfill the systems than had been assumed in the original designs. In addition, the Irish maritime climate, with its typical high relative humidity levels throughout the year (>85% on average), resulted in muted evapotranspiration performance of the systems. However, chemical and microbiological sampling of the water in the sumps and ponded water over the winter periods demonstrated that the systems were acting as excellent passive pollutant attenuation processes.