Comparison of rainwater management performance of modified extensive green roof substrate layer with different additives in rainstorm events

Abstract

The rainwater management performance of extensive green roof (EGR) is unsatisfactory in rainstorm events and cannot be effectively improved by optimizing the type and ratio of substrate materials. Our previous study demonstrated that the addition of 2% polyaluminum chloride (PAC) can construct stable aggregates in the substrate layer to significantly enhance the pollutants controlling the performance of EGR on rainwater but may introduce aluminum pollution, which inhibits vegetation growth. Hence, some environmentally safe additives for the substrate layer that can alleviate the poor rainwater management performance of EGR in rainstorm events in a general method are worth exploring. In this study, PAC, polyferric sulfate (PFS), polyvinyl alcohol (PVA), methylcellulose (MC), carboxymethyl cellulose sodium (CMC), and hydroxypropyl methylcellulose (HPMC) were selected as the additives with a ratio of 2% added in the substrate mixture of 20% peat soil and 80% vermiculite. Six experimental columns of the modified substrate layer and one control group without additive were built. The rainwater management performance of the experimental columns in three kinds of rainstorm events was evaluated through delayed outflow time (DOT), rainwater retention rate (RRR), and load reduction rate (LRR). The results showed the additive addition in the substrate layer significantly improved its DOT (p = 0.009, R² = 0.181), NO₃⁻–N (p = 0.014, R² = 0.169), and TN (p = 0.023, R² = 0.156) interception performance in rainstorm events. The effects of rainfall depth on pollutants' interception performance depend on the tradeoff between the reduction of adsorption efficiency and the inhibition of leaching risk brought by additives. A larger proportion of additives occupying the aggregate but not the aggregate surface would improve the stability of the aggregate and reduce the nutrient leaching in rainstorm events. PFS could improve and stabilize the rainwater management performance of the substrate layer for EGR in rainstorm events compared to PAC to a certain extent for ensuring environmental safety.