Using aquifers to store and treat recycled water provides a cost effective solution for management and optimal use of water resources, a necessity in light of predicted population growth and climate change. Operational wastewater disposal schemes provide a unique opportunity to develop the scientific understanding of the treatment capacity of the aquifers. Analysis of long-term datasets from these schemes and extrapolation to managed aquifer recharge schemes can facilitate future innovations in water recycling.
Eighteen wastewater infiltration sites, constructed over sand or sand/limestone, were used to evaluate the aquifer treatment performance. Water quality data was analysed to assess the removal efficiency for nitrogen, phosphorus and Escherichia coli using a probabilistic modelling approach. The treated wastewater (input) and groundwater (output) quality were considered as stochastic variables represented by probability density functions (PDFs). These input and output PDFs were used to derive a theoretical aquifer removal efficiency PDF for nitrogen, phosphorus and E. coli.
Site specific characteristics influenced removal efficiency, including wastewater treatment type, soil and aquifer characteristics and operational practices. Despite such influences, median total phosphorus removal efficiencies were typically >90%. Downgradient concentrations of phosphorus were comparable to background groundwater quality, for bores within 700 m of the infiltration basin. While overall the median total nitrogen removal efficiency was 80%, nitrogen attenuation between sites was more variable than for phosphorus. The efficiency of nitrogen removal was controlled by the propensity for denitrification at selected sites. The probabilistic modelling approach was limited for E. coli due to the low number detections in groundwater despite the high concentrations in treated wastewater.
The probabilistic modelling approach was effective in calculating the attenuation of nitrogen and phosphorus within aquifers across eighteen different wastewater infiltration sites. This approach was demonstrated as a useful management tool to assess operational performance of the aquifer treatment barrier in water recycling.