Removal of phosphorus from agricultural subsurface drainage water with woodchip and mixed-media bioreactors

Woodchip bioreactors that stimulate denitrification are proposed to reduce nitrogen (N) loads emanating from subsurface agricultural drainage. However, agricultural drainage water contains both N and phosphorus (P), frequently in concentrations exceeding environmental criteria. A mixed-media bioreactor containing woodchips plus reactive media that binds soluble P could concurrently lower both N and P loads in subsurface agricultural drainage. This study evaluated the P concentration and load reductions achieved with woodchip and mixed-media bioreactors in full-scale field installations in the cold, humid climate of Quebec, Canada, during a three-year period. Bioreactors contained either woodchips-only or woodchips plus an activated alumina/gravel mixture (mixed media). Total P, particulate P, soluble P, soluble reactive P, and soluble organic P concentrations and loads for the bioreactor influents and effluents were assessed weekly and also following > 10 mm rainfall events. In the first two months of operation, both woodchips-only and mixed-media bioreactors released P but became either a negligible source or net sink for P retention within the first year of operation. While the mixed-media bioreactor had 9% lower total P concentration and about 19 times greater reduction in total P load than woodchips-only bioreactors during the study period, the total P concentration was not reduced to the critical environmental threshold level of 0.03 mg L-1 with either bioreactor type. Both configurations were effective at reducing the soluble P fraction, especially the soluble reactive P species that are bioavailable and thus contribute directly to eutrophication. A woodchips-only bioreactor can adsorb P from subsurface agricultural drainage; however, a woodchips bioreactor containing the activated alumina/gravel mixture has greater P sorption capacity and should be considered as part of an integrated system for concurrent N and P removal from subsurface agricultural drainage.