Modified Media for Heavy Metals and COD Removal from Urban Stormwater Runoff Using Pilot Bioretention Systems

The media and structural optimization in bioretention systems play important roles in removing pollutants from urban stormwater runoff. Ten bioretention basins were constructed by adding water treatment residual (WTR), green zeolite, flyash, and coconut bran to traditional bioretention soil (65% sand + 30% soil + 5% sawdust, by mass), respectively, through mixing or layering. The steady infiltration rates of modified media were 3.25 similar to 62.78 times those of plant soil. The peak flow reduction rates of plant soil (1#) and flyash (7#) basins were significantly high, ranging from 78.09% to 92.91% (median = 86.52%) and 88.01% to 96.85% (median = 93.62%). The outflow concentrations of Cu and Zn were superior to Class II limitation (1.0 mg.L-1) in surface water environmental quality standards in China. The outflow concentration was inferior to Class V for COD and Cd. COD load reduction rate decreased with the increase of the recurrence interval and discharge ratio, which increased with the increase of inflow concentration. Although load reduction rate of heavy metal Cd increased with the increase of these three influencing factors, the reduction rate of Zn and Cu in heavy metals occurred without certain regularity. The median loading reduction rates of COD were the highest for layered media structure bioretention basins (6# and 8#). The heavy metal load reduction rates of 3#similar to 6# (mixed or layered media structure, adding 10% WTR as modifier) and 8# (layered media structure, adding 10% fly ash as modifier) were higher than other basins, and the median load reduction rate was mostly above 80%.