Potential of an Integrated Green Roof-Rainwater Harvesting System in Improving Stormwater Quality and Quantity

Abstract

Urban environments are increasingly challenged by flash flooding, water quality degradation, and pressure on potable water supply. This study investigated the performance of an integrated green roof-rainwater harvesting system aimed at improving both stormwater quality and runoff management. A laboratory scale model equipped with a rainfall simulator was designed and constructed to evaluate both the water retention capacity and rainwater quality enhancement. The system was tested using one grass species (Philippine Grass), two types of main filter media (biochar and activated carbon), and two roof configurations (flat and sloped roof, 15° designs) across three different flowrates, comparing them to a conventional roof without substrates and vegetation. Rainwater samples before and after passing through the integrated model were analyzed for pH, total dissolved solids (TDS), chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), iron (Fe), zinc (Zn), ammoniacal nitrogen (NH₃-N), total coliforms, and Escherichia coli. The results showed that activated carbon improved rainwater quality by achieving up to 94% COD, 60% BOD₅, 100% ammonia nitrogen, 100% Fe, 100% Zn, and 88% total coliform removal, while biochar achieved up to 93% pH neutralisation and 73% TDS removal. E. coli was not detected in any sample. In terms of hydrological performance, the combination of a flat roof with activated carbon performed the best in terms of runoff reduction by retaining up to 42.1% of runoff. This study demonstrates that the integrated green roof-rainwater harvesting system with natural filter media enhances water quality and retention, supporting non-potable water reuse and urban flood mitigation.