Simultaneous Nutrient Removal from Urban Runoff Using Bi-layer Bioretention Cells with Novel Media Additives

Nitrogen and phosphorous compounds are significant pollutants in urban stormwater runoff. In this study, three lab-scale bioretention cells, namely a control reactor CM, and reactors M1 and M2 containing Scrap Iron Filings (SIF) with granulated brick (M1) and pumice pellets (M2), respectively, were used to evaluate the simultaneous removal of nitrate, nitrite, ammonia, total nitrogen, phosphorous, and COD using simulated runoff. Under unsaturated conditions, M1 with the ZVI-brick combination removed 91.37% TP, while M2 with the ZVI-pumice combination removed 89.76% TP. Under saturated conditions, M2 removed 72.02% TN, and M1 removed 66.1% TN. It was found that the presence of saturation zones benefitted TN removal which can be attributed to the creation of anoxic conditions within saturation zones, which favoured denitrification, as well as the prolongation of influent retention and reaction time, while it hindered TP removal. TP removal percentages for CM, M1, and M2 declined from 86.77%, 91.37%, and 89.76% in unsaturated conditions to 63.99%, 83.67%, and 71.74% in saturated conditions due to the propensity of soil-bound P to leach in anoxic environments. The media amendments were further characterized using Scanning Electron Microscopy (SEM) and X Ray Diffraction analysis (XRD), as well as adsorption and leaching tests. Significantly, the highest pollutant leaching was observed in the assessed conditions for CM, underscoring the usefulness of including media like ZVI, brick powder, and pumice pellets. This incorporation not only heightened the effectiveness of pollutant removal but also fortified their retention in potential future stormwater events. In consideration of this, M1 emerged as the preferred design option, as its non-leaching characteristics were verified through flushing with distilled water after post-stormwater influent loading cycles when compared to traditional designs.