Urban metabolism: viewing cities like bodies can help reduce environmental impact
David Thorpe | 7 August 2018
A new way of reducing the environmental impact of neighbourhoods is being piloted around the world, using citizen engagement and open source data to monitor resource flows and assist citizens improve resource efficiency.
Like the human body, cities are living, ever-evolving organisms. They have their own metabolisms, converting inputs of energy and material goods into work and, in doing so, create waste and heat.
They maintain their state of high complexity in the same way as living systems, by causing a larger increase in the entropy of their environments. In thermodynamic terms, metabolism maintains order by creating disorder. The amount of disorder is a measurement of efficiency and sustainability.
Applying a framework to successfully model urban system flows will help with understanding the relationship between human activities and the natural environment.
Neighbourhoods such as Vauban in Freiberg, Germany, where such frameworks have been used in the integrated design of sustainable neighbourhoods, have achieved substantial reductions in energy intensity compared to conventional designs.
City metabolic flows can be turned into network diagrams, also known as Sankey diagrams, like this one for Vancouver by Dr Philip Mansfield of Graphical Memes:
And, like this one, most cities’ diagrams would be linear and centralised. What is needed to make cities more sustainable are more loops, which entail more reuse and recycling of materials and energy. This turns wastes into new inputs for new processes and reduces cities’ dependence on imports for resources.
Different methods have been offered to quantify resource flows: accounting approaches; input-output analysis; ecological footprint analysis; lifecycle analysis; and simulation methods. But as this is a young discipline there is no consensus yet as to which is best. The lack of standardisation of methods and guidelines on how to design a sustainable urban metabolism was addressed last year when the UN Environment Programme published an assessment of the discipline and offered advice, including that:
- all cities should assess their basic urban metabolism
- use both top-down and bottom-up approaches to capture data
- link spatial and temporal issues in urban metabolism assessments
- adopt multiple scales of analysis from household level upwards
- involve as many people as possible
This approach has been taken down to the neighbourhood level by an NGO called Ecocity Builders, which has recently implemented it in Cusco, Peru, and Medellín, Colombia in studies commissioned by the US Office of the Geographer’s Secondary Cities Initiative.
Medellín was named the most innovative city in the world in 2013 by the Urban Land Institute, due to its advances in politics, education and social development.
Ecocity Builders’ Participatory Urban Metabolism methodology permits citizens and city authorities to chart their own material flows as a first step towards transforming their communities into ecologically healthy settlements. First piloted through Ecocity Builders’ Urbinsight Global Data Initiative in Cairo, Casablanca and Lima, it uses an open source web app.
The Lima project resulted in an Urban Metabolic Information System Meta Diagram for Water in the San Isidro neighbourhood as shown below.
In Cusco, local citizens, city officials and student researchers from Universidad Alas Peruanas have been working on urban environmental accounting since 2016.
Sven Eberlein, communications strategist and community liaison for NGO Ecocity Builders, said that “the city’s historic inner city neighbourhoods, where several small study areas are located, have increasingly been feeling the need for this kind of in-depth accounting of conditions on the ground in order to find and implement a holistic solution to their garbage problem”.
Participants went on neighbourhood scoping trips before consulting with community leaders, who expressed a wish for better management of waste, a cleaner environment and healthier food.
A student team found that almost half of household waste was organic, and decided to research methods of constructing home composting modules, which they co-designed with community members and piloted in four communities.
The results were similar as for a study of waste in Bogota, which discovered that the city’s proposed formal recycling plan would produce more emissions than the current informal recycling system.
Eberlain says that city government, local universities and community groups in Cusco and Medellín are now working together to develop Neighbourhood Sustainability Plans.
Urban metabolism has also been considered in the United Kingdom. A 2015 review for the UK government’s Foresight Future of Cities Project recommended that “it is essential to assess these flows on a full life-cycle basis, in other words to take account of effects that ripple up and down supply chains whether they occur within or outside the city boundary”.
It concluded: “Strategic planning is needed to get the best results; infrastructure development should not just be left to the market,” and, “New organisational structures may be required so that innovation can keep pace in response to environmental challenges.”
It added that the material and energy flows depend on the structure and topography of a city; on the quality of its buildings and infrastructure; and on its social structure and the behaviour of its inhabitants. Input-output models have been used, for example, to assess urban-scale direct and indirect water consumption throughout the UK.
The study listed the following ways in which builders and property managers can reduce energy use for most components of urban infrastructure and material products:
- Extend service life
- Intensify use
- Increase the proportion of post-use product re-used
- Increase the proportion of post-use product recycled
- Reduce the energy required for recycling
- Reduce the energy required for re-use
- Reduce the energy required for primary material production
For the built environment, the analysis provides a basis for assessing the efficiency (or quality) of buildings and infrastructure and the benefit of retrofitting existing buildings and infrastructure (a form of re-use) and recycling components and materials from buildings demolished as shown in the diagram below:
Around the world, studies have so far taken place in the following cities:
Generally, studies show a higher consumption of food and goods in the city centres and higher consumption of fossil fuels and construction materials in the sprawling suburbs. This should alert planners to curtail such activity and adopt a more sustainable strategic urban plan.
On the energy front, heat dissipation – due to waste incineration as the predominant solid waste management strategy – is the largest contributor to energy loss. UNEP recommends capturing and reusing this heat for local electricity generation or heating.
Young cities tend to have larger inputs than outputs, as they are growing and need resources to build stocks (for example, buildings and infrastructures). Over time, this accumulation of built stocks contributes to waste flows, as it breaks down.
As cities mature, the inputs and outputs become more similar in magnitude and reuse and recycling initiatives have more impact on the city’s metabolic intensity.
The greatest gap in mainstreaming urban metabolism, particularly for cities that are growing rapidly and are still building, or have yet to build, the infrastructures needed to support their populations and industries, is the need to link studies and action to public policies.
This represents a significant opportunity for cities in the developing world to leapfrog the unsustainable infrastructure systems associated with cities of the global North, and instead embed sustainable infrastructures.
David Thorpe’s two new books are Passive Solar Architecture Pocket Reference and Solar Energy Pocket Reference. He’s also the author of Energy Management in Building and Sustainable Home Refurbishment.