Urban Metabolisms

The world is becoming increasingly urbanized. Since 2007, more than half the world’s population has been living in cities, and that share is projected to rise to 60% by 2030. While urban areas are the powerhouse of economic growth and contribute 60% of global GDP, they also account for over 70% of global carbon emissions and over 60% of resource use.

Sustainable cities are designed with consideration for the triple bottom line of social, economic, and environmental impact. They provide a resilient habitat for existing populations without compromising the ability of future generations to experience the same. Cities must overcome challenges in the fields of environmental degradation, traffic congestion, inadequate urban infrastructure, and in some cases the lack of basic services including water supply, sanitation, and waste management.

Sustainable cities and communities draw on the resources of local, regional, and global environments without compromising their ecological, social and economic boundaries. A sustainable city is an urban center engineered to improve its environmental impact through urban planning and management.

Solutions for sustainable cities

• Different agricultural systems such as agricultural plots within the city reduce the distance food travels from field to fork.
• Renewable energy sources, such as wind turbines, solar panels or bio-gas created from sewage to reduce and manage pollutions. Cities provide economies of scale that make such energy sources viable.
• Reducing the “heat island effect” caused by large hard surfaces including tarmac and asphalt make urban areas several degrees warmer than surrounding rural areas during the day. As the stored heat in these surfaces is released during the evening, the temperature difference can climb to as much as 6°C during the evening. Methods to reduce the need for mechanical air conditioning and its large energy demand include planting trees and lightening surface colors, natural ventilation systems, increasing water features, adding green spaces.
• Improved public transport and an increase in pedestrianization to reduce car emissions. Sustainable transport in terms of fuel economy, occupancy, electrification, pedal power, and urbanization.

Circular cities

Circularity can be applied to several systemic levels of the built environment, ranging from materials and products, to buildings, to whole cities and regions. Retaining the existing material stock and making it available to designers and construction practitioners is key to unlock the full opportunity of circular economy. The opportunities of circular metabolisms need to be unlocked at city and regional level through adequate policies and a systemic approach.

“Mining” for building materials and products from decommissioned buildings is a practice that values precious existing resources and existing embodied carbon without putting additional strain on the system. Selective dismantling, such as the one performed by the pioneers Rotor, and whenever feasible, shall be preferred to demolition. Despite the hurdles posed by an aged building stock, that is old and not disassembly-ready, entire buildings can be built from salvaged materials, with an exceptional degree of quality and variety of textures, colors, and architectural character.

The main enablers towards building an urban metabolism of materials, are transparency of material declaration and building up digital networks of materials, or marketplaces, that can be used by architects, engineers and planners in their designs and material specifications for projects. Ideally these value webs would work as marketplaces, by mapping the location, amount, and time availability of each potential construction resource.

An interesting example of circular metabolism, paired with digital application and applied to water streams is achieved in Beyond Circularity, in Los Angeles, USA. Through satellite imagery, digital terrain models and geotechnical datasets, water availability is mapped for the city. Contextually to the results, strategic actions are mapped to optimize rainwater infiltration and resource recovery and improve resource aware decision-making processes in a highly water-stressed territory.

Measuring material and energy flows

In a world where data is ubiquitous, it might seem easy to monitor material and energy flows through urban systems. Policymakers and society rarely acknowledge resource, energy, or water scarcity. Often, they are not properly measured, even though a misleading indicator can be as detrimental as no measure at all: both can steer us in the wrong direction.

The urban scale applications of circularity can significantly scale up the results and bring this opportunity to the maximum fruition. Urban metabolisms are constituted by primary flows of water, energy and waste that are managed in a closed loop system, on a neighborhood or urban scale, and ideally bring to net-zero benefits. Which in turn creates extraordinary resilience towards climate change scenarios, material resources squandering and can also prevent unwanted society impacts like energy poverty. Rapid urbanization has brought with it numerous difficulties such as the growing development of slums, the inadequacy of basic services and infrastructure as well as uncontrolled urban growth, all of which increase the vulnerability of cities to natural disasters.