Hybrid Green Roof System Combining Constructed Wetland and Semi-intensive Green Roof: Experimental and Numerical Study

Water scarcity and the growing need for sustainable urban water management demand innovative solutions to recycle and reuse greywater. This study explores a hybrid green roof system that integrates constructed wetlands and green roofs, enabling onsite water treatment and irrigation. This nature-based solution proposed, and recently experimentally tested by Petreje et al. (2023) additionally incorporates sustainable materials, such as recycled crushed bricks and pyrolyzed sewage sludge for increased circularity.  An extensive numerical study was done to enhance understanding of the system’s water flow and solute transport dynamics with aim to enhance and optimize the system potential in different configurations, irrigation schemes, and different climates.

The constructed wetland component was modeled using numerical modeling. First-order kinetics was assumed for BOD₅ degradation, while the green roof component was modelled using HYDRUS-2D utilizing  Richards' equation for water flow and the advection-dispersion equation (ADE) for solute transport. Input data included daily irrigation schedules, meteorological conditions, and measured outflow data from an existing experimental testbed. Validation of the model against measured outflow data demonstrated its reliability in replicating the behavior of the hybrid green roof system. Simulations further revealed that water predominantly flows through the green roof's bottom layer, which consists of mineral wool, highlighting the importance of this layer in directing water flow.

The numerical study was conducted for a number of scenarios defined by system size, irrigation schedule, and two types of climate (temperate and semi-arid). For selected scenarios, the sensitivity analysis of the model to parameters of the system (substrate and drainage depth, irrigation dose, and frequency) as well as characteristics of the porous media was conducted.

The outcome of the numerical study provides critical insights for optimizing hybrid green roof systems, including recommendations for ideal irrigation scenarios and appropriate size ratios between constructed wetlands and green roofs. By advancing the understanding of water flow and solute transport in integrated systems, this research supports the development of sustainable, scalable solutions for urban water recycling and improved green infrastructure.