Modelling phosphorus removal efficiency of a reactive filter treating agricultural tile drainage water

Phosphorus (P) leaching from agricultural to tile drainage land contributes to nonpoint pollution of surface waters. Drainage filter technologies are potentially cost-effective technologies at the field scale for mitigating P losses. The objective of this study was to evaluate the removal efficiency for dissolved P (PRE) in a porous reactive filter constructed from crushed seashells, which was part of a full-scale filter system. The hydraulic properties (hydrodynamic and hydrodispersive) were estimated by performing salt (NaCl) tracer tests at two different flow rates representative for in situ discharge conditions in Denmark. The reactive properties of the filter material were determined in the laboratory by measuring P sorption in batch experiments. The hydraulic and reactive parameters were subsequently used as input parameters in a numerical model, which was calibrated based on flow data and dissolved P concentrations collected between May 2015 and 2017. Results show a homogeneous distribution of the tracer in drainage water outside the reactive P-filter and a uniform dissolved P load into the seashell material. The in situ saturated hydraulic conductivity was one order of magnitude lower than the estimation from previous column experiments. The model described accurately dissolved P concentrations at the filter outlet (Nash-Sutcliffe index = 0.79) and the P removal efficiency (PRE) of the reactive filter was equal to 62% during the monitored period. The model presented in this work can be integrated in a larger model addressing the complexity of P sorption processes for the evaluation of the removal efficiency in full-scale filter systems.