Sewage sludge biochar as a sustainable and water-safe substrate additive for extensive green roofs

Article describes the application of nutrient-rich sewage sludge (SS) biochar as an innovative additive to extensive green roof substrate at application rates of 10 (SB10) and 20% (v/v) (SB20) and its long-term impact on discharge quality. In addition to a routine analysis of pH, EC, TSS, and COD in the discharge, concentrations of nutrients (total nitrogen and total phosphorus) and metals (Cd, Cu, Fe, Mn, Pb, and Zn) were used to monitor leaching from the experimental green plots. SS biochar improves the water-holding capacity and air-filled porosity of the developed substrates SB10 and SB20, where the chemical and physical parameters meet the minimum requirements for multi-layer extensive roof substrate. The addition of biochar didn't substantially increase the concentrations of nutrients in discharge compared to the conventional extensive substrate, even after 30 months. Results suggested that biochar tends to release nutrients gradually when leached out by rain and may therefore serve as a source of P and other micronutrients. Although an increase in the concentrations of metals (Cu, Pb, and Zn) in the SB10 and SB20 substrates was observed, no significant differences in metal leaching were found. Metal concentrations in the aboveground parts of Sedum plants indicated that the pyrolysis of SS transforms water-extractable forms of metals into stable forms, thereby significantly reducing their bioavailability to Sedum species and any direct ecotoxicological risk. Based on our results, we assert that SS biochar could be used as a sustainable, valuable and water-safe component of extensive substrates for green roofs.