Preparation of growth substrate to improve runoff quality from green roofs: physico-chemical characterization, sorption and plant-support experiments

Growth substrate plays an important role in determining the quality of runoff from green roofs. However, no systematic research has been conducted to design a substrate to improve runoff quality. Hence, the present study aimed at designing and developing a green roof substrate using low-cost and environmentally-benign materials. The inorganic fraction of the substrate includes purosil, vermiculite, sand and light-weight clay aggregates (LECA); whereas the organic fraction includes coco-peat and Sargassum wightii. Through factorial design, 13 different substrate mixes were prepared and the optimum mix (20% purosil, 30% vermiculite, 10% sand, 20% LECA, 10% coco-peat and 10% S. wightii) was found to have high water holding capacity (67.6%), air filled porosity (21%), hydraulic conductivity (5524 mm/h) and low bulk density (495 kg/m3). The substrate also provided maximum support for the growth of Portulaca oleracea. Experiments with metal-contaminated influent from the down-flow of a packed reactor revealed that the green roof substrate possesses a high sorption capacity towards various metal ions.