Engineering bioretention for treatment of urban storm water runoff

Abstract

Bioretention, a "Low Impact Development" urban storm water best management practice, was developed in the early 1990's. Although bioretention has been used at many areas in the United States, the impact of this technology on ground and surface water quality as well as the optimal design of bioretention media for pollutant removal, have not been systematically investigated. The objectives of this study were to investigate the effectiveness of this technology for storm water runoff treatment and finally to give recommendations for future design. The methods used included developing pollutant removal performance curves for a variety of bioretention media mixes and evaluating the effectiveness of existing bioretention facilities. Synthetic runoff, which contained oil and grease (O/G), suspended solids (SS), lead (Pb), phosphorus (P), nitrate, and ammonium, was employed in laboratory experiments and 6 on-site bioretention evaluations. Two more on-site experiments were conducted during a rainfall event to compare with laboratory investigations. Overall, all bioretention columns and on-site facilities demonstrated excellent removal for O/G and Pb. TSS removal was good in columns, but washing out of media particles was noted in field facilities, mostly from new installations. For nutrients treatment during a 6-hr experiment, the removal efficiency of Total P ranged widely and appears to be related not only to chemical properties of the media, but also to the flow behavior of runoff through the media. Results from batch P sorption tests on six media, three continuous column studies, and two repetitive 6-hr bioretention columns with total 28 repetitions showed that the medium with a higher P sorption capacity can retain more P from the infiltrating runoff after a high P loading. However, the sorption data alone is not adequate to predict the P retention through a bioretention column for a short-term experiment due to the complicated processes occurring between the runoff and media. Unless special provision were made, all media employed in this study were ineffective in removing nitrate and ammonium. The removal efficiency of both pollutants was improved by increasing the water holding capacity of the media and enhancing the development of nitrification and denitrification processes in the bioretention column.