Analysis of the effect of green roof substrate amended with biochar on water quality and quantity of rainfall runoff

Green roofs are becoming a popular ecological alternative in urban areas worldwide. In this study, we constructed two modular green roofs (commercial substrate green roof and biochar substrate green roof) and analyzed the effects that the green roof substrate amended with biochar on the runoff retention capacity, water quality, pollutants releasing characteristic, and pollution load by simulating rainfall experiment (rainfall levels 10~80 mm). Results showed that the mean retention ratio was no significant differences between the commercial substrate (72.54%) and the biochar substrate (72.08%). Both the two kinds of substrates showed that the concentrations of total nitrogen (TN), total phosphorus (TP), chemical oxygen demand (COD), and iron (Fe) decreased gradually with the extension of rainfall time. Electrical conductivity (EC) and pH, as well as mean concentrations of TN, COD, TP, total suspended solids (TSS), and Fe, showed no differences between the green roof runoff of two kind of substrate. However, the neutralizing capacity of biochar substrate for the pH of green roof runoff was stronger than the commercial substrate, and the mean concentration of the TN and COD in the commercial substrate (16.14 mg/L and 171.79 mg/L, respectively) was about two times higher than the biochar substrate (9.85 mg/L and 97.31 mg/L, respectively). Similarly, the pollution load of TN and COD in the commercial substrate was significantly higher than that in the biochar substrate. Therefore, the biochar substrate could effectively reduce the pollution load of TN and COD in the runoff of green roof. Consequently, we suggest that the biochar could be applied to green roof substrates in order to reduce the impact of city non-point pollution on receiving water bodies.