Pilot-scale evaluation of green roofs with Sargassum biomass as an additive to improve runoff quality

Abstract

Green roofs have the potential to be used as a pollutant-sink in urban environments. Unfortunately, until now no systematic study has been conducted to enhance the runoff quality from green roofs. The present work investigates the viability of using low-cost aggregates along with a biosorbent (Sargassum biomass) to prepare substrate mix for extensive green roofs to improve runoff quality. We have used (on a volume basis) 30% perlite, 20% vermiculite, 10% sand, 20% crushed brick, 10% coco-peat and 10% Sargassum biomass to prepare green roof substrate. The developed green roof substrate was found to have low bulk density (487kg/m3), high water retention capacity (58.5%), air filled porosity (19.5%), and hydraulic conductivity (4195mm/h). Through laboratory packed column study, we identified superior sorption capacity of green roof substrate towards various metal ions such as Al, Fe, Cr, Cu, Ni, Zn, Cd and Pb. Roof-top experiments in pilot-scale green roof assemblies with Portulaca grandiflora as vegetation were conducted for several artificial rain events (unspiked and metal-spiked). Results based on unspiked artificial rain events suggested that concentrations of most of the chemical components in runoff were highest during the beginning of rain events and thereafter subsided during the subsequent rain events. Metal-spiked artificial rain events revealed that green roofs acted as a sink for various metal ions and generated better runoff, whose quality was significantly less than fresh water standards. Green roofs also showed the potential to neutralize the acidic nature of inlet water and delay runoff generation. Significant differences were also observed between non-vegetated and vegetated green roof assemblies in runoff quality and quantity, with the latter producing better results.