Improvement in pollutant controlling performance by modified aggregate structure in extensive green roof substrate layer

Abstract

The leaching and control of pollutants in extensive green roofs (EGRs) during rainfall events is crucial to their improvement and promotion of their application, in which their substrate layer plays a key role. The addition of soil amendment to accelerate and strengthen the formation of aggregate structures (AS) has wide applications for nutrient preservation and leaching control in farming. In this study, 0.5%, 1.0%, 1.5%, 2.0% and 2.5% (v/v) of polyaluminium chloride (PAC) and bentonite were added to the EGR substrate layer as modifiers. The EGR without a modifier was employed as the ‘control group’ (CN) and 11 EGR experimental devices were constructed. The event mean concentration (EMC) and load of NH₄⁺–N, NO₃⁻–N, TN and TP in 10 simulated rainfall events were used to evaluate the pollutant controlling performance of the modified AS in EGR substrate layers. Scanning electron microscopy (SEM) and nitrogen and phosphorus distribution analysis were used to characterize the surface topography of the modified AS. The results showed that the EGR experimental devices with PAC exhibited better pollutant controlling performances than that with bentonite except for NH₄⁺–N. The characterization results showed that the AS modified by PAC had significant aggregate forms, porosity and surface roughness compared with that with bentonite. Therefore, bentonite is not recommended as a choice for the EGR substrate layer modifier. Compared to CN with no modifier, the addition of 2.0% PAC to the EGR substrate layer resulted in an increase of 204.50%, 148.36%, and 38.00% and 135.76%, 99.74%, and 23.85% in the NO₃⁻–N, TN, and TP removal rate and load reduction rate, respectively.