Reducing phosphorus (P) losses from drained agricultural fields with iron coated sand (- glauconite) filters.

In north-west Europe, agricultural diffuse P losses are a major cause of eutrophication problems in surface waters. Given that the Water Framework Directive (WFD) demands fast water quality improvements and most of the actual P mitigation strategies tend to work on the long run, new short-term mitigation measures are urgently needed. We here report on the entire process of developing small scale field filters to remove P at the end of tile drains, starting from the screening of potential P sorbing materials (PSM): iron coated sand (ICS), acid pre-treated natural minerals (biotite, glauconite and olivine) and bauxite. Initial batch (ad)sorption experiments revealed following order in both, P sorption capacity and speed: ICS > bauxite > glauconite > olivine = biotite. Because of the presence of significant amounts of lead and/or nickel, we excluded bauxite and olivine from further experiments. Subsequent lab scale flow through systems were conducted with P filters containing mixtures of ICS and glauconite (100/0, 90/10, 80/20, 70/30 and 60/40%, respectively, on weight basis). We found a significant relationship between Ksat and the filter mixtures particle size distribution and bulk density, and a significant effect of the filter mixture composition on P removal efficiency and stability of Ksat. During the 10 week field trials, the pure ICS filters were capable of processing all drainage discharge rates (up to 6 m3 day-1) with a P removal efficiency of ≥74%. The 90/10 ICS/glauconite filters could process up to 4 m3 water day-1 with a P removal efficiency of 57%. As saturated ICS filters can easily be replaced and recycled for other applications, this is a promising sustainable technique to drastically cut back diffuse P losses and to tremendously improve surface water quality in the short term.