Developing a hybrid green roof: A rooftop solution for wastewater treatment

Urban areas face increasing challenges from climate change, particularly the urban heat island (UHI) effect and water scarcity. Green roofs are effective adaptation measures, but their benefits in terms of cooling decrease during droughts without additional irrigation. Using potable water for irrigation is unsustainable; thus, recycling greywater or pre-treated wastewater represents an ideal alternative. This study presents the Hybrid Green Roof (HGR), an innovative nature-based solution (NBS) that integrates a modular rooftop constructed wetland (CW) with a semi-intensive green roof (GR).

The circular HGR system enables efficient wastewater recycling at the point of origin, reducing potable water consumption while enhancing the cooling effect of vegetation through evapotranspiration of recycled water. The research progressed from elevated experimental plots to a full-scale prototype at the CTU UCEEB (Czech Technical University in Prague, University Centre for Energy Efficient Buildings in Bustehrad). The system architecture consists of mechanically pre-treated wastewater pumped into modular plastic flumes acting as the CW. These modules are filled with lightweight ceramic aggregate and planted with wetland vegetation. Pre-treated water then overflows onto the green roof. The GR utilizes the "reBrick" circular substrate, containing 25% recycled construction waste and 10% pyrolyzed sewage sludge (biochar), significantly reducing its environmental footprint and supplementing fertilization. Water distribution from CW to GR is managed by an outflow module equipped with a pulse dosing system that supplies a hydrophilic mineral wool layer in GR, making water available to plants via capillary forces. For experimental purposes, the green roof is divided into three different sectrors that vary in substrate thickness and vegetation above the mineral wool. The following combinations are being tested: 4 cm of substrate and sedum seedlings; 4 cm of substrate and 3 cm thick grass mats; and 4 cm of substrate with 3 cm thick biodiverse vegetation mats with perennials.

The temperature and humidity are measured in all green roof sectors. A water meter is used to monitor the volume of water flowing into the CW, and the level in the last CW module is monitored to measure the volume of water overflowing from the CW into the GR. This allows the water balance of the system to be calculated.

Long-term monitoring confirmed high stability and efficiency. Chemical analysis showed average pollutant removal efficiencies of 90% for Chemical Oxygen Demand (COD), 99% for Total Nitrogen (TN), and 96% for Total Phosphorus (TP). While the CW provides primary treatment, the green roof layer acts as a crucial tertiary stage, eliminating remaining nutrients without excessive leaching. The HGR is a promising technology for sustainable urban water management, closing both water and material cycles. Ongoing research focuses on optimizing CW flume design to enhance aerobic processes and refining hydraulic parameters to ensure stability under extreme climatic conditions.