End-of-pipe filter to reduce phosphorus concentrations from the effluent of green infrastructure underdrains

Green stormwater infrastructure is growing in adoption across the world due to its ability to capture and treat stormwater runoff at the source; however, while green stormwater infrastructure is effective at reducing the concentration of many priority pollutants, bioretention often increases the concentration of dissolved phosphorus in the stormwater it treats. This is a significant shortcoming, as dissolved phosphorus can promote algae growth in receiving water bodies resulting in negative impacts to human and aquatic health. This study seeks to address this shortcoming through an end-of-pipe filter at the end of bioretention effluent pipes designed to reduce the concentration of dissolved phosphorus. To do so, two filters containing a mixture of activated alumina and iron filings were built and installed at the effluent point of two different bioretention systems in Milwaukee, WI. During runoff events, the filters were sampled during incoming and exiting flows and tested for water quality pollutants. Results demonstrated that the filters reduced the concentrations of both dissolved reactive phosphorus concentrations (average of 37% and 29%) and total phosphorus (average of 29% and 24%); however, once in place for several months the reductions in dissolved reactive phosphorus improved to 49% and 39%, on average. This study ultimately presents an approach to overcoming one of the most critical limitations of green stormwater infrastructure, by integrating a final treatment step.