Agricultural Drainage Filters. II. Phosphorus Retention and Release at Different Flow Rates

Granular drainage filters may provide new innovative solutions for removing phosphorus (P) from agricultural tile drainage water. The influence of filter sorbent properties is well recognized; however, intragranular diffusion may be critical for the P retention efficiency in drainage filters, where convective velocities are generally high. This was investigated in granular materials (Leca, Filtralite-P (R), granulated Limestone, crushed Seashells, calcined diatomite earth (CDE), and poorly ordered Fe hydroxide aggregates (CFH)) with known solute transport behavior at variable flow rates. Phosphorus retention and release were investigated during continuous P loading with a constant inlet concentration of 0.3 mg P L-1 followed by a subsequent P release phase. A parallel batch sorption experiment ranked the materials according to strength of P retention CFH > Filtralite-P (R) > Seashells > Limestone > Leca, while CDE was a P source at low solution concentrations. Column experiments demonstrated that filters with non-equilibrium transport (CFH and Seashells) attained higher P retention rates and lower P release rates compared to filters with equilibrium flow (Filtralite-P (R) and Limestone). This behavior was even more pronounced as flow rate increased and was attributed to intragranular diffusion of P in CFH and Seashells, which would maintain concentration gradients between mobile convective water and immobile domains. This facilitated further diffusion and thus increased P retention. In addition, the release of previously retained P was limited in filters with non-equilibrium transport (<2 %) compared to the other filters (>5 %). The results demonstrated that intragranular diffusion was an important mechanism increasing P retention.