Water Quality Evaluation of Extensive Green Roof with Limestone as Growth Media

Vegetated roof application has been increasing as a sustainable development system component that can mitigate peak stormwater input. The water captured by a vegetated roof is usually redirected and stored in a storage tank(s) before released into storm drains. However, most of the studies that assessed the drained water found that these waters are rich in nitrogen and phosphorus due to the leaching from the vegetation. This can influence algal growth in the tank and the subsequent piping system, in the long term due to the accumulation in the system. Thus, this research looked at specifically the design characteristics of extensive green roof substrate, by comparing limestone of different sizes as porous media using a physical model. This study used two sizes of limestone: fine (1.18–2 mm) and coarse (5–10 mm) limestone. The permeability of coarse limestone is higher compared to fine limestone, thus allowing better infiltration of rainwater. On the infiltrated water quality, several parameters were tested to determine the efficiency of limestone in treating the rainwater. The parameters were turbidity, color, pH, chemical oxygen demand (COD), iron, zinc, Kjeldahl nitrogen, and phosphorus. Using limestone, pH of the initially slightly acidic rainwater was increased to pH of 6.3–7.6. From this study, it can be concluded that fine limestone was more effective in improving the harvested rainwater quality compared to using coarse limestone due the larger surface area and longer contact time between the limestone and the rainwater itself.