Cities pose environmental challenges but also offer possibilities to close material and energy loops and connect multiple societal and ecologic services. This article reviews and brings together the literature on two important new research directions that address urban sustainability by integrating functions or material flows: Circular Urban Metabolism (CUM) and Multifunctional Land Use (MLU). We focus on challenges to MLU and CUM and strategies to facilitate their realization. The review shows that although MLU and CUM differ in what they integrate, they face partly similar integration challenges. In both fields, the collaboration between actors related to particular functions (water safety, recreation), high investment costs and uncertainties about costs and benefits, and legislation that hampers integration are identified as challenges. In both fields, strategies are proposed to facilitate the collaboration between actors. However, other challenges and strategies are specific. Whilst MLU scholars mostly highlight socioeconomic aspects of realizing integration, CUM scholars focus more on technical aspects. We find limited cross-fertilization between both fields so far. To stimulate discussion and knowledge exchange, we introduce 'integration of urban functions' as a shared idea for a sustainable urban system. To find further solutions for integration challenges, we propose conceptualizing MLU or CUM initiatives as processes of change, which requires connecting across previously separate 'worlds' and changing previously established monofunctional ways of working.
The metabolism of cities bears important anomalies: high levels of resources flow in cities while large quantities of (hazardous) wastes flow out of cities. In order to decrease cities' environmental footprint, Girardet [1] and Rogers [2] suggested making a transition from a linear to a circular urban metabolism. Even though the concept was coined a few decades ago, it still holds prominence among academics and practitioners (see for instance [3][4][5][6]).However, few cities have made such a transition. This shows that creating a circular urban metabolism is complex and challenging. Research so far mainly considered the creation of a circular urban metabolism as a technical endeavour. We would like to argue that organisational and institutional aspects are also of crucial importance for understanding why there is an implementation gap. However these have been understudied. This paper tries to bridge this gap by adopting a different conceptual approach. We consider that the creation of a circular urban metabolism passes through multiple innovation processes. Such processes happen in a systemic manner wherein, not only technical but organisational and institutional solutions and changes are adopted and adapted.In this paper we conceptualise the circular urban systems, as a robust concept to address the anomalies of the metabolism of cities. We discuss how this concept is different from existing one and why we think it can help in better grasping the challenges ahead. We also focus on the kind of insights that can be gained by adopting this new concept and how this can open a new agenda for future research.
Urban symbiosis is a strategy to create a more efficient metabolism of cities. However, urban symbiosis requires the integration of different systems, which is hard to achieve. Actors involved in existing systems can hardly develop 'the bridges' that are required to connect the thus far unrelated worlds of pre-existing systems. This paper analyses two cases of urban symbiosis and focusses on the process of building new actor networks. Urban symbiosis projects are generally complex and involve various stakeholders. The authors conclude that the introduction of additional actors or an additional institutional framework might be an effective means to create bridges between actors that could facilitate a more efficient metabolism of cities.
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