Robust options to remove nitrate and phosphate from tile drainage

Diffuse emission from agricultural land is a major source of nitrogen and phosphorus in surface waters. In this contribution we present field experiments of a series of robust options to remove nitrate and phosphorus at field drainiage level. Nitrate removal was enhanced by stimulating naturally occurring denitrification by adding wood chips. The results showed that the initial removal rates were high (between 60 and 80%). Efficiency decreases in case of periodic presence of oxygen. The concentrations of phosphate in the effluent showed peaks during the first year at high temperatures and low water discharge. The emission of nitrous oxide to the surface above the drains was negligible, but was increased at the ends of the drains. It is estimated that the long-term efficiency depends on good control of the water levels and flow rates in the filter beds. Phosphorus removal was enhanced by using iron-oxide coated sand as filter material. Three methods were tested: drains enveloped with iron oxide coated sand, an edge of field reactor and a slow sand filter reactor at the downstream end of the drainage canal. Results show that up to 95% of the phosphorus can be removed. A reduction of the hydraulic conductivity during the experimental period reduced the efficiency of the edge of field reactor and the slow sand filter reactor. For all phosphorus treatment options tested, high removal efficiencies could be reached and no negative side-effects were found.