How do multilayer blue-green roofs affect the runoff water quality?

To ensure a sustainable urban development, the large-scale implementation of green roofs and, more in general, of several nature-based solutions is an essential aspect to be considered. Thanks to their multiple benefits (e.g., pluvial flood mitigation, acoustic and thermal insulation of building, urban heat island reduction, air quality improvement, increase of biodiversity and additional aesthetic value) green roofs have been widely investigated. Among them, the multilayer blue-green roofs present an additional storage layer, that enables to accumulate the rainwater that percolates from the soil when it reaches saturation. This water can potentially be used for several domestic non-potable purposes, such as garden irrigation, street cleaning or flushing the toilets. To identify the possible rainwater reuse, it is fundamental to know the physical and chemical properties of this unconventional resource and evaluate whether they respect the regulations limits. Many studies investigated the effects of traditional green roofs on the runoff quality, without reaching a complete agreement. Moreover, the influence of the additional storage layer on the water quality has not been explored yet. In this context, the multilayer blue-green roof prototype installed at the University of Cagliari has been used as case study to analyze the quality of the outflow during three artificial and three natural rainfall events, comparing the runoff with the one obtained from a traditional roof. The prototype is constituted by 8 cm of soil (classified as sand) and 10 cm of storage layer, and it is characterized by Cactacee vegetation, that does not require additional irrigation or maintenance. For each artificial event, three samples every five minutes have been collected from both traditional roof and multilayer blue-green roof, to evaluate how the water quality varies during time. For the natural events only one sample has been collected as representative of the average quality of the accumulated water. The collected samples have been analyzed, evaluating temperature, pH, conductivity, total and volatile suspended solids, Chemical Oxygen Demand (COD), most common cations and anions and heavy metals concentrations. Results showed that suspended solids and heavy metal concentrations observed in the multilayer blue-green roof outflow are lower than by the traditional roofs, underlying the beneficial effects of this Nature-based solution. On the other hand, multilayer blue-green roof outflow presents high COD concentrations, caused by the accumulation of organic matter in the additional storage layer. Hence, the collected water can be used only for irrigation either of the multilayer blue-green roof itself or of gardens. It is important to notice that results obtained in this work are limited to one single soil (sand) and vegetation (Cactacee) type: the response with different vegetation, soil type and thickness, and fertilizer should also be investigated, as well as under different climatological conditions.