Biodiverse dual-purpose wetland-green rooftop design based on recyclates

A circular economy-based hybrid green roof system and green roof substrate was developed and tested to reduce the environmental impact of using natural resources such as water and components of substrates based on common primary materials. Two case studies and laboratory analysis were conducted to assess the performance of green roof substrate consisting of recycled crushed brick-based demolition waste and biochar from pyrolyzed sewage sludge. Substrates were tested for their hydro-physical properties such as maximum water capacity, retention curves, bulk density, grain size and pH and suitability for vegetation growth.

The purpose of first case study, which involved a green roof of 7×5 m², was to test two newly developed circular substrates in conditions of real green roof and to compare it with standard, commercially available, substrate. The new substrates differed in the amount of pyrolyzed sewage sludge biochar they contained (9.5 vol. % for one and none for the other), but both contained large proportion of crushed brick (37.5 vol. %). The impact of the pyrolyzed sewage sludge was the main focus of the evaluation. At the same time, the changes in hydrophysical characteristics (retention curves, hydraulic conductivity, grain size) over time were evaluated.

Second case study was conducted on two raised beds to test the newly developed substrates in the context of the novel solution combining an extensive green roof and rooftop constructed wetland that uses pre-treated grey water. This system is called Hybrid green roof (HGR). The viability of a hybrid green roof system that uses greywater for irrigation was evaluated by measuring water balance, testing water samples from different sections of the experimental beds, and monitoring temperature and water content along the height of the bed layers. The hybrid green roof system has a constructed wetland section that treats the greywater before it reaches the green roof.

Extensive green roof areas of experimental beds in both studies were planted with Sedum spp. Vegetation in both case studies is thriving. The biochar apparently provides nutrients for the plants, which results in more vigorous growth on the substrates containing biochar. In case of HGR, the nutrient (phosphorus and nitrogen) levels in the leachate from the test beds were relatively low, because the irrigation water goes directly to the drainage layer and does not wash out the nutrient rich substrate with biochar. The nutrient levels have only increased when there is rainfall. The recycled materials used to amend the substrates in this study had similar properties (maximum water capacity, bulk density, pH) to the commercial ones.

The results of the experiment show that hybrid green roof system can effectively reduce the nutrients concentrations in greywater and provide enough water for vegetation to grow, which can effectively reduce the urban heat island effect, cool the building underneath and even provide a source of good quality domestic water.

References:

• Petreje, et.al, Performance study of an innovative concept of hybrid constructed wetland extensive green roof with growing media amended with recycled materials, J. Environ. Manag. vol, 331 (2023), 10.1016/j.jenvman.2022.117151