Evaluating the efficacy of short rotation willow coppice in attenuating phosphorus from small-scale wastewater treatment works’ effluent

Phosphorus (P) discharge from sewage treatment works remains a significant source of P pollution in freshwater systems. While a wide range of P-removal technologies have been successfully implemented at large wastewater treatment works (WWTWs), their application in small-scale systems is often constrained by high capital and operational costs, technical complexity, and highly variable influent flows. Consequently, P releases from small treatment works can have disproportionate and localised impacts on receiving freshwaters, leading to severe ecological degradation. These impacts are likely to intensify under climate change, particularly during prolonged dry periods associated with low or zero-flow conditions given reduced or zero discharge dilution. Nature-based solutions offer a potential treatment option for P removal in small-scale systems while delivering additional environmental benefits. However, the suitability of these approaches has not yet been extensively studied. Therefore, this study evaluated the performance and applicability of zero-discharge willow biofiltration system for attenuating P discharged from two small-scale WWTWs in Ireland. The experimental design comprised approximately 1.5 ha of mixed variety willow plantation at each site, irrigated with primary-treated wastewater using automated, sequential time-dosed irrigation systems. Using a before-after approach, stream P concentrations, measured as soluble reactive phosphorus (SRP), were monitored upstream and downstream of WWTW discharge points using automated samplers with hourly sampling over a period of 24 hours. Sampling was conducted following three rain-free days each month between March and November over different years. The results showed that, prior to wastewater diversion to irrigate the willow plantation, downstream SRP concentrations were substantially higher than upstream concentrations at both sites, with the highest concentrations observed during the summer months. Following the diversion of wastewater, the difference in SRP concentrations between upstream and downstream sites became negligible, indicating more than 95% of P attenuated through the willow biofiltration system. Ongoing work includes studying the fate of irrigated P to evaluate soil P saturation and P uptake in biomass.   

 

Keywords: Short rotation willow coppice, Effluent, Phosphorus, Wastewater treatment works