Drainage Filter Technologies to Mitigate Site-Specific Phosphorus Losses in Agricultural Drainage Discharge.

Losses of phosphorus (P) in drainage waters contribute an estimated 33% to the total agricultural P load in Denmark. Mitigating agricultural P losses is challenging, as critical P losses comprise only a very small fraction of actual soil P contents and are not directly related to fertilizer P input. Targeting high risk areas of P loss and applying site-specific measures promises to be a cost-efficient approach. The Danish Commission for Nature and Agriculture has, therefore, now called for a paradigm shift towards targeted, cost-efficient technologies to mitigate site-specific nutrient losses in drainage.

The Danish strategic research project “SUPREME-TECH” (2010-2016) (www.supreme-tech.dk), aims at providing the scientific basis for developing cost-effective filter technologies for P in agricultural drainage waters. The project studies different approaches of implementing filter technologies including drainage well filters and constructed wetlands (CWs). Various natural and industrial P filter substrates (granulated Fe-oxides (CFH), crushed seashells, Filtralite-P, granulated lime, calcined diatomitic earth (CDE)) have been tested for their affinity and long-term capacity to reduce inlet P concentrations to below environmental threshold values (<0.05 mg P L^-1) at variable P loads and flow regimes. Intragranular diffusion made a substantial contribution to P retention and was an important filter material property. During long-term P-loading the sensitivity of flow-rate on P retention increased and further indicated the occurrence of surface-induced precipitation processes.

The P-retention efficiency of granular drainage filters and surface-flow constructed wetlands was compared for treating drainage water, and a subcatchment analysis illustrated the potential of implementing such measures.