Effect of Combinations of Green Roof Substrate with Vegetation Coverage on Rainwater Quality Improvement

The significance of green roofs in stormwater management cannot be overlooked, as they have become an integral component in the creation of sustainable cities. This study not only examined the purifying effects of substrates, but also explored the interplay between substrate and vegetation through a combination planting approach, resulting in an innovative roof greening and planting method that incorporates both substrate and vegetation. In the experiment, 16 planting combinations of green roofs were created by cross-planting four proportioned substrates and four proportioned turfgrass. Subsequently, the outflow water quality indexes including ammonium nitrogen (NH4+-N), suspended solids (SS), chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) were analyzed individually. The study findings indicate significant variations in the rainwater quality improvement capacity among the 16 green roof planting combinations, with both exhibiting effective purification of NH4+-N and SS. However, all 16 planting combinations were identified as sources of pollution for TP, TN and COD in rainwater. Among the four proportioned substrates, the combination of planting and B substrate (field soil: turfy soil: vermiculite: perlite: zeolite = 5:2:1:1:1) exhibits a superior purification effect with minimal variance. Therefore, it is recommended to promote and apply B substrate in large-scale green roofs. The A2 combination (field soil: turfy soil: perlite: vermiculite = 5:3:1:1 and Poa pratensis: Agrostis matsumurae: Lolium perenne = 5:4:1) exhibited the highest effluent water quality among the 16 combinations tested, while other planting combinations utilizing substrate A did not perform as well. In summary, green roofs have the potential to purify rainwater but also pose a certain risk of pollution.