As it happens in nature, cities require resources and products such as food, water and energy to live; in fact, most of the resources on Earth are used for supplying urban areas. Urban metabolism provides a holistic framework for analyzing all inflows and outflows with the surrounding biophysical environment (McDonald & Patterson, 2007).
Cities require resources like nature, but meanwhile in nature there is a circular metabolism where no waste is generated, cities are based on linear metabolisms contributing to depletion of natural resources
In nature, living organisms manage its resources based on a circular metabolism, where sunlight, water, nutrients, etc. (i.e. inputs) are transformed into heat, energy and biomass, and no waste as such is generated. Instead, materials which are not needed anymore (i.e. outputs) will return into the loop and play another function within the ecosystem.
On the contrary, cities are nowadays based on linear metabolisms, extracting raw materials, manufacturing products for consumption and disposing them afterwards, contributing to different impacts such as depletion of natural resources and a high dependency on the non-renewable ones. Furthermore, most of the pollution originates from cities or from agriculture (that eventually feeds the cities) in the form of emissions and discharges of waste into the local and global environment.
A new paradigm is consequently required, turning the linear and unsustainable management of resources into circular flows, commonly known as circular urban metabolism.
How is the circular urban metabolism possible?
Closing urban cycles embraces different ways of managing loops of resources throughout cities, recognizing outputs (organic and inorganic waste) as recycled inputs capable to return to the production system.
How can this be applied in practice?
Let us consider a clear example. Generally, the urban water cycle begins with purified water being transported and entering urban settlements. After consumption along different activities (i.e. industrial processes, domestic consumption, etc.) the water quality is degraded so that effluents are discharged as wastewater, leaving the urban system.
The objective is to recognize outputs as recycled inputs
Circular urban metabolism aims at closing the water cycle rather than leaving the resource escape from the system, considering wastewater as a resource instead of a nuisance. Since waste water represents a valuable source of nutrients (e.g. nitrogen, phosphorus and potassium) and water for irrigation, both highly appreciated in agriculture, recycling technologies should recover all these elements for further reuse in farming activities. This way, urban feeding farmlands would help closing the nutrients cycle while reaching the city again. As Girardet (2008) states, wastewater treatment systems could be turned into fertilizer factories, returning plant nutrients and reclaimed water back to the crops feeding cities.
Girardet, H., 2008. Creating sustainable cities. Schumacher Briefings. Green Books.
McDonald, G.W., & Patterson, M.G., 2007. Bridging the divide in urban sustainability: from human exemptionalism to the new ecological paradigm. Urban ecosystems, 10(2), 169-192.
Rogers, R., 1996. Cities for a small planet. World Heritage Review, (3), 68-77.