[Bioretention Media Screening for the Removal of Phosphorus in Urban Stormwater]

Urban runoff is an increasingly important source of excess phosphorus (P) to local receiving waters. Bioretention, a promising technology for urban stormwater pollution treatment, was investigated to determine whether the mixture of purple soil and sand could adsorb sufficient P at low concentrations in urban stormwater. The TP concentrations of urban runoff from variously impervious areas in Chongqing City ranged from 0. 04 to 7. 00 mg . L-1 (mean ± S. D. = 0. 75 mg . L-1 ± 1. 08 mg . L-1); the TDP concentrations ranged from 0. 02-0. 46 mg . L-1 ( mean ± S. D. = 0. 15 mg . L-1 ± 0. 10 mg . L-1). The media adsorption benchmark was determined for a bioretention facility sized at 10% of the 100% impervious catchment area and having 10 years of capacity according to annual rainfall pattern and the runoff TDP range. The media benchmark for adsorption was calculated as 7. 5 mg . kg-1 at soluble P concentration of 0. 30 mg . L-1 which provided the necessary stormwater treatment. The oxalate-extractable aluminum and iron content influenced the P sorption capacity for neutral and acid purple soils. A strong positive linear relationship was observed between the oxalate ratio [OR = (Alox + Feox)/Pox] and media P sorption capacity. The media mixture of 20% purple soil and 80% sand showed excellent P removal, meeting the developed benchmark for adsorptive behavior. The media mixture in a large-scale (60 cm) column consistently produced soluble reactive phosphorus effluent event with mean concentrations <0. 05 mg . L-1. The media mixture of purple soil and sand can be used as a bioretention media to treat low-concentration phosphorus in urban runoff under various hydrologic and pollutant concentration conditions.