A NEW ASSESSMENT MODEL FOR POLLUTANT REMOVAL USING MULCH IN BIORETENTION PROCESSES

Bioretention is a novel low impact development (LID) technology for urban storm water management, which uses vegetation, a mixed-media of sand and clay, etc., and a cover material such as mulch to achieve good pollutant removal efficiency in stormwater runoff. To determine the best mulch material for removing multiple pollutants wih bioretention systems, seven pollutants and eight types of mulch were tested using synthetic stormwater in Beijing, China. In addition, an assessment index system and a criterion for selecting the best mulch for multiple pollutant removal were developed and tested with experimental data collected. A new assessment model, known as the set pair analysis-improved analytic hierarchy process (SPAIAHP), was first established. The weights were determined by an improved analytic hierarchy process, while the assessment grades were designated by set pair analysis. The results of the SPAIAHP model indicated that the best mulch for overall pollutant removal was the vermiculite mulch. For specific pollutants, the metal removal efficiency (cadmium, copper, lead and zinc) by vermiculite was excellent (>90%). Vermiculite was also found to have the highest efficiency for ammonium removal (ca. 78%). Also, SPAIAHP yielded more precise results than those obtained by the set pair analysis-analytic hierarchy process (SPAAHP) and is therefore recommended for complex assessment problems.