Closing Water Cycles in the Built Environment through Nature-Based Solutions: The Contribution of Vertical Greening Systems and Green Roofs

Abstract

Water in the city is typically exploited in a linear process, in which most of it is polluted, treated, and discharged; during this process, valuable nutrients are lost in the treatment process instead of being cycled back and used in urban agriculture or green space. The purpose of this paper is to advance a new paradigm to close water cycles in cities via the implementation of nature-based solutions units (NBS_u), with a particular focus on building greening elements, such as green roofs (GRs) and vertical greening systems (VGS). The hypothesis is that such “circular systems” can provide substantial ecosystem services and minimize environmental degradation. Our method is twofold: we first examine these systems from a life-cycle point of view, assessing not only the inputs of conventional and alternative materials, but the ongoing input of water that is required for irrigation. Secondly, the evapotranspiration performance of VGS in Copenhagen, Berlin, Lisbon, Rome, Istanbul, and Tel Aviv, cities with different climatic, architectural, and sociocultural contexts have been simulated using a verticalized ET0 approach, assessing rainwater runoff and greywater as irrigation resources. The water cycling performance of VGS in the mentioned cities would be sufficient at recycling 44% (Lisbon) to 100% (Berlin, Istanbul) of all accruing rainwater roof–runoff, if water shortages in dry months are bridged by greywater. Then, 27–53% of the greywater accruing in a building could be managed on its greened surface. In conclusion, we address the gaps in the current knowledge and policies identified in the different stages of analyses, such as the lack of comprehensive life cycle assessment studies that quantify the complete “water footprint” of building greening systems.

Towards a Cross-Sectoral View of Nature-Based Solutions for Enabling Circular Cities

“…Within the COST Action Circular City, the NBS units and interventions (NBS_u/i) were analyzed under the perspectives of four selected urban sectors, which refer to the different fields of activities for circular management of resources in cities, namely, the built environment [14], urban water management [15], resource recovery [16], and urban farming [17]. With circularity always in focus, each sector first identified the most relevant UCCs being dealt with and then the most applicable NBS_u/i to address the relevant UCCs.…”

“…• Built environment: Pearlmutter et al [14] focused on building systems and identified the "wicked problem of water"; more provision of services by NBSs requires a higher water use, which is commonly solved by importing water from outside the city. The authors proposed to challenge this conundrum by focusing on those NBS_u/i classified as vertical greening systems and green roofs [13], and how they can be used to foster graywater reuse and capture available rainwater.…”

“…It comprises a set of 39 NBS units (NBS_u), 12 NBS interventions (NBS_i), and 10 supporting units (S_u), as well as the analysis of input and output (I/O) resource streams required for NBS units and interventions (NBS_u/i). The framework has been discussed from different perspectives that correspond to urban sectors and activities relevant for the potential of circular management of resources for the (1) built environment [14], (2) urban water management [15], (3) resource recovery [16], and (4) urban farming [17].…”

“…This can be done successfully if existing gaps in policy are filled, and planning processes include inter-and multidisciplinary approaches from the initial stages. Building system recovery, one of the UCCs defined (UCC 7 ), was not directly addressed by Pearlmutter et al [14]. Although CE itself does not distinguish among the scales of circularity, building reuse has often been agreed upon as a preferred option over material and component recycling, thanks to its higher upscaling potential.…”

“…This is of interest as those S_u can be identified as key technologies for onsite resource recovery and need to be integrated in the buildings to support circularity [16,24]. On the other hand, resource recovery did not select "vertical greening systems and green roofs" (# [13][14][15][16][17][18][19][20]. This can be explained by the applied criteria, specifically, the primary focus on nutrient recovery and usage within the city, including quantity and quality, and not on water circularity.…”