Case study of extensive green roof with growing media amended with recycled materials and hybrid constructed wetland-extensive green roof

Implementation of a green roof can require large amounts of natural resources such as water and natural components of the substrate. Therefore, green roof system that uses the principles of a circular economy has been developed and tested. Two studies were performed to evaluate performance of substrate for green roof amended with a recycled crushed brick and pyrolyzed sewage sludge (biochar). In order to design and select a suitable substrate, 8 substrate variants were prepared and tested. Physical properties such as maximum water capacity, retention curves, bulk density, grain size and pH were analyzed in order to selected suitable substrates for case studies.

First case study was performed on green roof size 7x5 m². The aim was to evaluate the performance of the substrate in real conditions and to compare it with a commercially available substrate. To assess the effect of pyrolyzed sewage sludge, only part of the green roof contained biochar (9.5 vol. %), whereas the crushed brick was part of both substrates (37.5 vol. %).

Second study was performed on two elevated experimental beds in order to verify performance of the novel concept of combination of constructed wetland and extensive green roof irrigated with pre-treated grey water which we call hybrid green roof. The substrates amended with the same recycled materials as in the first study were used.

In hybrid green roof system, greywater is first pumped into the constructed wetland to be treated and then is used for irrigation of green roof. Performance of this hybrid green roof system was assessed on the basis of water balance measurements, laboratory analyses of water samples taken from various parts of the experimental beds, temperature and water content measurements along the experimental bed´s layers height. The hybrid green roof system is viable. There are relatively low concentrations of nutrients (phosphorus and nitrogen) in the leachate from test beds, namely because the irrigation provides the water directly to the drainage layer and nutrient rich substrate enriched with biochar isn't leached by irrigation water. Concentrations of nutrients increases only in response to precipitation. The constructed wetland part of the system proven a high potential to reduce the nutrients concentration in pre‑treated grey water.

The vegetation formed by Sedum spp. carpets is prospering well on both elevated experimental beds in the second study as well as on green roof in the first study. Nutrients from biochar are apparently available for the vegetation. Therefore, the vegetation on the bed and green roof with biochar amended substrate shows more vigorous growth and higher evapotranspiration. Substrates amended with recycled materials developed in the study had comparable properties (maximum water capacity, bulk density, pH) with commercial substrates.

This research has been funded by research projects: TN01000056/03, GA22-25673S