What does ‘circular’ actually mean when we’re talking about cities?

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Circular economy has achieved near-universal approval as an idea while remaining frustratingly difficult to define. Everyone — industries, cities, policymakers, NGOs — is for circularity. The concept has become so elastic that it risks meaning everything and nothing at once. In a paper just published in Resources, Conservation and Recycling, Benjamin Goldstein, Danielle Levy, Brandon Marc Finn, and I try to get more precise about what circularity actually looks like in an urban context — and why the distinction matters.

The field already has a well-developed concept for firm-to-firm resource exchange: industrial symbiosis. The classic example is Kalundborg, Denmark, where a power plant, oil refinery, pharmaceutical manufacturer, and several other facilities have built a web of exchanges — waste heat, water, gypsum, fly ash — that substantially reduce the combined waste of the industrial park. It’s elegant, well-studied, and genuinely circular in the strict sense of the term.

But when you look at what actually happens in cities — the composting programs, the community energy co-ops, the seed libraries, the repair cafes, the community gardens that turn neighborhood food scraps into growing inputs — industrial symbiosis doesn’t quite capture it. These practices involve residents, households, community organizations, and public institutions as much as they involve firms. They take advantage of something specific to cities: not just industrial co-location, but the social and spatial density that puts thousands of potential resource partners within walking distance of each other.

The concept we develop to describe this is urban symbiosis. We define it as the symbiotic sharing of material and energy between urban actors — a category that extends well beyond firms to include households and community institutions — that takes advantage of cities’ unique concentration to minimize raw material demand and keep resources in circulation. Urban symbiosis is related to industrial symbiosis, but it’s distinct: it’s citizen-engaged, place-specific, and often informal in ways that industrial symbiosis is not.

What makes this more than a terminological exercise is that urban symbiosis remains critically undertheorized. The practices it describes are widespread — go look at any active community composting program, or a tool library, or a community energy co-op — but they haven’t been studied with the rigor or consistency that industrial symbiosis has accumulated over decades. By giving urban symbiosis a clear definition and a place in the broader typology of circularity, we’re trying to open up a research agenda: where does urban symbiosis happen, what enables it, how does city policy support or constrain it, and what does it actually contribute to resource efficiency at the urban scale?

The broader typology we propose — industrial symbiosis, conventional waste management, and urban symbiosis as three distinct but complementary modes of circularity — gives cities a more complete toolkit for thinking about circular policy. Different mechanisms for different actors, different scales, different intervention points. The 15-minute city already asks what resources should be accessible within walking distance; urban symbiosis asks what resources could be shared within that same radius. Those are related questions, and I’m glad we now have a vocabulary to pursue them together.

Goldstein, B. P., Levy, D., Hawes, J. K., & Finn, B. M. (2026). A typology of resource circularity in cities. Resources, Conservation and Recycling, 227, 108743. https://doi.org/10.1016/j.resconrec.2025.108743

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