An evaluation of engineered media for phosphorus removal from greenroof stormwater runoff

Greenroofs are increasingly being recognized as an effective site level best management practice (BMP) to reduce the volume of stormwater runoff in urban environments. For some water quality constituents, greenroofs can improve runoff water quality but recent studies demonstrate greenroofs are sources rather than sinks of phosphorus (P). Accordingly, further research is required to evaluate treatment technologies that improve the performance of these BMPs. This study examined the use of two engineered media types to reduce phosphorus loadings from a greenroof located on the Archetype Sustainable House at Kortright in Vaughan, Ontario. A treatment system was installed to capture and remove P in stormwater runoff using sorptive properties of an engineered media. A mass balance approach was used to evaluate pre and post-treatment water quality. Pre and post-treatment water samples were collected for 25 rainfall events from July 11, 2009 to August 22, 2010 and analyzed for soluble reactive phosphorus (SRP), total phosphorus (TP), suspended solids (SS) and total dissolved solids (TDS). Storm events ranged in return frequencies from < 2 years to 35 year periods. The results show that the greenroof was a consistent source of P. The volume weighted mean concentrations were 0.769 mg/L and 0.630 mg/L for 2009 and 2010 events, respectively. The media used in 2009 reduced SRP loadings by 32.0% and TP loadings by 25.4%. The media evaluated in 2010, reduced SRP loadings by 82.4% and TP loadings by 86.6%. The greater P removal demonstrated by the 2010 media is attributed to a higher specific surface area and increased P sorptive capacity. Results of this study will help inform the use of sorptive materials in greenroof applications and a wider range of best management practices for stormwater quality treatment.