New hybrid green roof survives long droughts

Green roofs are one of the key solutions for adapting buildings to climate change and combating the urban heat island effect. However, to perform their function effectively, they need a sufficient and regular supply of water, which erratic rainfall cannot provide. CTU UCEEB has therefore developed and tested in operational conditions a prototype of a hybrid green roof with a wetland flume that uses pre-treated wastewater or grey water for its irrigation. During the testing, the feasibility of this solution was demonstrated and the foundations for its future commercial use were laid.

Climate change is manifesting itself in rising temperatures and intense heavy rainfall followed by long periods of drought. Green roofs make it easier for us to adapt to new climatic conditions, as they retain rainwater and then ensure its gradual evaporation, thus contributing to a reduction in temperatures not only in the buildings themselves but also in their surroundings. To do this, they need a sufficient and regular supply of water from rainfall, which they do not receive during periods of drought. 
  
Therefore, the research team of the Urban Ecohydrology CTU UCEEB, within the framework of the SIC Transfer Voucher Programme and the international NBSINFRA project, has developed a prototype of a hybrid green roof that combines the advantages of a green roof and a constructed-wetland wastewater treatment plant located on rooftop. As a result, it does not need rainwater or potable water for its regular irrigation, as it instead efficiently uses treated wastewater or grey water produced daily by households, for example when showering. In addition, the system can act as a source of pre-treated wastewater for irrigating other green areas.

Tests of the prototype under operating conditions were carried out on the roof of the UCEEB building in Bustehrad. During these trials we were able to confirm the technical feasibility of the solution, including the production of the plastic modules representing the core of the wetland system. We tested the way of their interconnection and watertightness. In cooperation with the manufacturer and designer, we selected the appropriate material design with regard to UV stability and visual requirements, tested the technology and identified the time and financial framework for production.

We created a hydraulic model in Hydrus 2D describing the path and flow velocities. We also developed a tool for dimensioning and setting the operating parameters of the hybrid green roof, allowing us to monitor and adjust the water balance, filtration capacity, evapotranspiration rates and assess the impact on cooling. We are currently working on determining how the system will react to fire. Our new technical solution is protected by utility pattern No. 38090.