Fosfilt filters in an agricultural catchment: a long-term field-scale

Abstract

Diffuse load mitigation is a prevailing global challenge due to the eutrophication of water bodies. We report here long-term nutrient removal performance of two on-site sand filters (F1 and F2) in southwestern Finland, established in the 1990s. The sand filters were enhanced with a layer of phosphorus binding material Fosfilt-s, a side product of titanium dioxide production. The monitoring periods were 4.5 and 3.5years for F1 and F2, respectively. F1 only worked for some months due to blockage of the crushed stone layer. After renovation (1999), the filter worked for a year but then the Fosfilt-s layer clogged and the filter increased suspended solids (SS) and dissolved reactive phosphorus (DRP) loads by 36% and 19% on average, respectively. Total P (PTOT) load was not affected. The structure and performance of F2 were more successful due to a better water distribution system. On average, 61% ofthe inflowing suspended solids, 37% of the PTOT and 45% of the DRP were removed during the monitoring period. We conclude that these filter types have the potential to be further developed into useful tools for nutrient load reduction. Development work should be focused on the treatment of subsurface drainage waters with high concentrations of dissolved nutrients, in particular. Long term field data is needed, because laboratory tests cannot fully simulate natural circumstances.