Influence of different alternative organic substrates as fillings for green roofs on the quality of rainfall runoff

Green roofs can act as a source or sink of physicochemical pollutants (pH, EC, and TSS), organic substances (BOD5 and COD), nutrients (NH4+, NO3-, TN, and TP), and metals (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn, K, Mg, and Ca). This study aims to examine the effects of organic additives, such as dried sewage sludge, sewage sludge biochar, food waste biochar, wood biochar and compost containing sewage sludge, on substrates of extensive green roofs to reduce pollution in rainwater runoff. The addition of alternative organic materials could reduce the pollution concentration of water quality parameters in the runoff. The results showed that the concentration of most chemical constituents decreased in the case of addition of (i) wood biochar, (ii) sewage sludge biochar, (iii) food waste biochar, (iv) dried sewage sludge, and (v) compost containing sewage sludge, as follows, compared to the control green roof module without organic additives. The addition of wood biochar was most effective in reducing the concentration of pollution compared to the control substrate, but some increased the concentration of pollution. However, the concentrations of these chemical constituents found in rainwater runoff are greatly influenced by the types of materials used in green roofs and the amount of rainfall. Our knowledge of the dynamics of crucial processes involving nutrient recycling and pollution retention within biochar in the ecosystem of green roofs remains limited. To advance our understanding of these ecosystems, further research is essential, including comprehensive roof-scale experiments and long-term monitoring.