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

Abstract

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 3.25~62.78 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#~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%.

The Common Approaches of Nitrogen Removal in Bioretention System

“…In addition, the bilayer media concept is also used to enhance the bioretention system; it involves different layers of modifier media with various mechanical and chemical properties. The wide range of layer properties including porosity, permeability, particle size, water holding capacity, moisture content, bulk density, CEC, and pH would provide adsorption, nitrification, and denitrification conditions [7,12,39,75,118,119]. The bilayer of bioretention forms an anaerobic condition and increases nitrogen removal by applying a low-porosity layer in the lower portion of the media, which results in best nitrogen removal [39,105].…”