Long-term phosphorus accumulation in stormwater biofiltration systems at the field scale

Abstract

Previous research has demonstrated that biofilters are an effective technology for the removal of phosphorus (P) from stormwater. However, biofiltration is a relatively new technology and most field-scale systems are still fairly young, therefore little is understood about the long-term ability of biofilters to act as a sink for P. Studies from a board range of disciplines indicate that iron (Fe) and P interactions are an important mechanism for P sequestration in soils. To investigate long-term P retention dynamics and associations between Fe and P in biofilters we collected filter media cores from six biofilters in both Melbourne and Brisbane. The filter media was subjected to a four-step sequential extraction scheme designed to measure P associated with the following phases: i) Bioavailable P; ii) P-adsorbed to iron oxyhydroxides; iii) P associated with amorphous iron oxyhydroxides; and iv) Organic P. The results suggest that P accumulation varies spatially (areally and with depth) in biofilters. P concentrations were highest in the top 10 cm of the filter media and near stormwater inlets. In all biofilters tested, surface layer P was mostly associated with the amorphous Fe and organic phase, which is largely related to the build-up of trapped sediment. P concentrated in the Fe-adsorbed phase increased at lower depths suggesting that Fe-P sorption interactions may play an important role in long-term P retention. This result emphasises the importance of maintaining good hydraulic performance in biofilters, since Fe-adsorbed P may be sensitive to changes in redox potential, leading to release under reducing conditions. These findings may influence how we design biofilters and plan system maintenance to ensure effective long-term P removal.

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