Effects of Substrates and Plant Species on Water Quality of Extensive Green Roofs

Green roofs have multiple environmental benefits, such as saving energy, improving air quality, mitigating noise, providing habitat, beautifying the landscape, and improving urban hydrology. Among the environmental benefits of green roofs, improving runoff quality is a controversial issue. Substrate and plant litter may be transported with green roof runoff, which would increase the risk of urban nonpoint source pollution. However, the soil substrates on green roofs can filter out pollutants that can then be taken up by plants, which would improve water quality. In the present study, experimental containers with 2 substrates and 8 plant species were constructed, and data were collected from 7 real rainfall events to analyze the effluent water quality. This outdoor site was unfortunately terminated by a strong typhoon, but the idea and some results of the study might give some lights for the future study. The total solid (TS), chemical oxygen demand (COD), and total nitrogen (TN) concentrations were greater during the earlier stage and gradually decreased. However, the total phosphorous (TP) concentrations did not change with time. The limited data did not show the expected effects of the combinations of substrates and plants, but the effect of substrates was significant on the COD and TP concentrations.