Towards a Dynamic Approach to Urban Metabolism: Tracing the Temporal Evolution of Brussels’ Urban Metabolism from 1970 to 2010

Urban metabolism (UM) is a way of characterizing the flows of materials and energy through and within cities. It is based on a comparison of cities to living organisms, which, like cities, require energy and matter flows to function and which generate waste during the mobilization of matter. Over the last 40 years, this approach has been applied in numerous case studies. Because of the data‐intensive nature of a UM study, however, this methodology still faces some challenges. One such challenge is that most UM studies only present macroscopic results on either energy, water, or material flows at a particular point in time. This snapshot of a particular flow does not allow the tracing back of the flow's evolution caused by a city's temporal dynamics. To better understand the temporal dynamics of a UM, this article first presents the UM for Brussels Capital Region for 2010, including energy, water, material, and pollution flows. A temporal evaluation of these metabolic flows, as well as some urban characteristics starting from the seminal study of Duvigneaud and Denayer‐De Smet in the early 1970s to 2010, is then carried out. This evolution shows that Brussels electricity, natural gas, and water use increased by 160%, 400%, and 15%, respectively, over a period of 40 years, whereas population only increased by 1%. The effect of some urban characteristics on the UM is then briefly explored. Finally, this article succinctly compares the evolution of Brussels’ UM with those of Paris, Vienna, Barcelona, and Hong Kong and concludes by describing further research pathways that enable a better understanding of the complex functioniong of UM over time.     

African Urbanization: Assimilating Urban Metabolism into Sustainability Discourse and Practice

Shaping sustainable, equitable African cities requires strengthened investigations into the cities’ current resource flows, infrastructure systems, and future resource requirements. The field of urban metabolism (UM) offers multiple forms of analysis with which to map, analyse, and visualize urban resource profiles. Challenges in assessing UM in African cities include data scarcity at the city level, difficulty in tracking informal flows, lack of standardized methods, and the open nature of cities. However, such analyses are needed at the local level, given that city practitioners cannot rely purely on urban planning traditions of the global North or the typically broad studies about urban Africa, for supporting strategies toward sustainable urban development. This article aims to draw together the concepts of sustainable development and UM and explore their application in the African context. Further, the article estimated resource profiles for 120 African cities, including consumption of biomass, fossil fuels, electricity, construction materials, and water, as well as emissions of carbon dioxide. These resource profiles serve as a baseline from which to begin assessing the current and future resource intensity of these cities. It also provides insights into the cities’ relative resource impact, future consumption trends, and potential options for sustainability interventions.     

The Changing Metabolism of Cities

Data from urban metabolism studies from eight metropolitan regions across five continents, conducted in various years since 1965, are assembled in consistent units and compared. Together with studies of water, materials, energy, and nutrient flows from additional cities, the comparison provides insights into the changing metabolism of cities. Most cities studied exhibit increasing per capita metabolism with respect to water, wastewater, energy, and materials, although one city showed increasing efficiency for energy and water over the 1990s. Changes in solid waste streams and air pollutant emissions are mixed.
The review also identifies metabolic processes that threaten the sustainability of cities. These include altered ground water levels, exhaustion of local materials, accumulation of toxic materials, summer heat islands, and irregular accumulation of nutrients. Beyond concerns over the sheer magnitudes of resource flows into cities, an understanding of these accumulation or storage processes in the urban metabolism is critical. Growth , which is inherently part of metabolism, causes changes in water stored in urban aquifers, materials in the building stock, heat stored in the urban canopy layer, and potentially useful nutrients in urban waste dumps.
Practical reasons exist for understanding urban metabolism. The vitality of cities depends on spatial relationships with surrounding hinterlands and global resource webs. Increasing metabolism implies greater loss of farmland, forests, and species diversity; plus more traffic and more pollution. Urban policy makers should consider to what extent their nearest resources are close to exhaustion and, if necessary, appropriate strategies to slow exploitation. It is apparent from this review that metabolism data have been established for only a few cities worldwide, and interpretation issues exist due to lack of common conventions. Further urban metabolism studies are required.     

Urban Metabolism

Urban metabolism studies have been established for only a few cities worldwide, and difficulties obtaining adequate statistical data are universal. Constraints and peculiarities call for innovative methods to quantify the materials entering and leaving city boundaries. Such methods include the extrapolation of data at the country or the region level based, namely, on sales, population, commuters, workers, and waste produced.
The work described in this article offers a new methodology developed specifically for quantifying urban material flows, making possible the regular compilation of data pertinent to the characterization of a city's metabolism. This methodology was tested in a case study that characterized the urban metabolism of the city of Lisbon by quantifying Lisbon's material balance for 2004. With this aim, four variables were characterized and linked to material flows associated with the city: absolute consumption of materials/products per category, throughput of materials in the urban system per material category, material intensity of economic activities, and waste flows per treatment technology.
Results show that annual material consumption in Lisbon totals 11.223 million tonnes (20 tonnes per capita), and material outputs sum 2.149 million tonnes. Nonrenewable resources represent almost 80% of the total material consumption, and renewables consumption (biomass) constitutes only 18% of the total consumption. The remaining portion is made up of nonspecified materials.
A seemingly excessive consumption amount of nonrenewable materials compared to renewables may be the result of a large investment in building construction and a significant shift toward private car traveling, to the detriment of public transportation.     

Complexity in Urban Development and Management

Systems dynamics, cellular automata, agent-based modeling, and network analyses have been used in population, land use, and transport planning models. An overview of complex systems science as applied to urban development is presented, and examples are given of where the problems of housing people and anticipating their movements have been addressed with complex approaches, sometimes in concert with deterministic, large-scale urban models. Planning for cities today has additional environmental and social priorities in common with many topics that concern industrial ecology. The research agenda suggested here is that this, too, can be enriched with complex systems thinking and models to complement the often static assessment of environmental performance and better inform decision processes.     

Downscaling Aggregate Urban Metabolism Accounts to Local Districts

Urban metabolism accounts of total annual energy, water, and other resource flows are increasingly available for a variety of world cities. For local decision makers, however, it may be important to understand the variations of resource consumption within the city. Given the difficulty of gathering suburban resource consumption data for many cities, this article investigates the potential of statistical downscaling methods to estimate local resource consumption using socioeconomic or other data sources. We evaluate six classes of downscaling methods: ratio‐based normalization; linear regression (both internally and externally calibrated); linear regression with spatial autocorrelation; multilevel linear regression; and a basic Bayesian analysis. The methods were applied to domestic energy consumption in London, UK, and our results show that it is possible to downscale aggregate resource consumption to smaller geographies with an average absolute prediction error of around 20%; however, performance varies widely by method, geography size, and fuel type. We also show how mapping these results can quickly identify districts with noteworthy resource consumption profiles. Further work should explore the design of local data collection strategies to enhance these methods and apply the techniques to other urban resources such as water or waste.     

A High‐Resolution Approach to Mapping Energy Flows through Water Infrastructure Systems

Using data from the water service area of the East Bay Municipal Utility District in Northern California, we develop and discuss a method for assessing, at a high resolution, the energy intensity of water treated and delivered to customers of a major metropolitan water district. This method extends previous efforts by integrating hourly data from supervisory control and data acquisition systems with calculations based on the actual structure of the engineered infrastructure to produce a detailed understanding of energy use in space and time within the territory of a large‐scale urban water provider. We found significant variations in the energy intensity of delivered potable water resulting from seasonal and topographic effects. This method enhances our understanding of the energy inputs for potable water systems and can be applied to the entire delivery and postuse water life cycle. A nuanced understanding of water's energy intensity in an urban setting enables more intelligent, targeted efforts to jointly conserve water and energy resources that take seasonal, distance, and elevation effects into account.     

Surveying the Environmental Footprint of Urban Food Consumption

Assessments of urban metabolism (UM) are well situated to identify the scale, components, and direction of urban and energy flows in cities and have been instrumental in benchmarking and monitoring the key levers of urban environmental pressure, such as transport, space conditioning, and electricity. Hitherto, urban food consumption has garnered scant attention both in UM accounting (typically lumped with “biomass”) and on the urban policy agenda, despite its relevance to local and global environmental pressures. With future growth expected in urban population and wealth, an accounting of the environmental footprint from urban food demand (“foodprint”) is necessary. This article reviews 43 UM assessments including 100 cities, and a total of 132 foodprints in terms of mass, carbon footprint, and ecological footprint and situates it relative to other significant environmental drivers (transport, energy, and so on) The foodprint was typically the third largest source of mass flows (average is 0.8 tonnes per capita per annum) and carbon footprint (average is 2.1 tonnes carbon dioxide equivalents per capita per annum) in the reviewed cities, whereas it was generally the largest driver of urban ecological footprints (average is 1.2 global hectares per capita per annum), with large deviations based on wealth, culture, and urban form. Meat and dairy are the primary drivers of both global warming and ecological footprint impacts, with little relationship between their consumption and city wealth. The foodprint is primarily linear in form, producing significant organic exhaust from the urban system that has a strong, positive correlation to wealth. Though much of the foodprint is embodied within imported foodstuffs, cities can still implement design and policy interventions, such as improved nutrient recycling and food waste avoidance, to redress the foodprint.     

Urban Metabolism and the Energy Stored in Cities

Using the city of Toronto as a case study, this article examines impacts of energy stocks and flexible demand in the urban metabolism on the resilience of the city, including discussion of directions for further study of the resiliency of the urban metabolism. An important element developed is the nominal residence time of the energy stocks. This value defines how long an energy stock lasts under typical patterns of energy use. The findings suggest that the residence times of many sources of energy overcome vulnerability when energy supply shocks last on the order of hours or a few days, but that the measure is limited to assessing only certain types of commonly used energy sources in aggregate terms. Discussion is included on the uncertainty of this measure and on the metabolic and resiliency implications of new technologies intended to reduce energy use and improve sustainability of cities and the use of the urban metabolism as a means of comparison. The methodology employed highlights how waste energy could be used to increase the resiliency of the city's water supply, but also how the study of the urban metabolism would benefit from a more disaggregate form in the study of sustainable and resilient cities.     

Urban Metabolism of Intermediate Cities: The Material Flow Analysis,Hinterlands and the Logistics‐Hub Function of Rennes and Le Mans (France)

Although urban metabolism has been a subject of renewed interest for some years, the related studies remain fragmented throughout the world. Most of them concern major cities (megacities and/or national capitals) and, more rarely, intermediate, medium‐sized or small cities. However, urbanization trends show that together with the metropolization process, another one is characterized by the proliferation of intermediate cities. We have studied the metabolism of two French intermediate cities for the year 2012: Rennes Métropole (400,000 inhabitants) and Le Mans Métropole (200,000 inhabitants). To this end, we used material flow analysis (MFA) based on the methodology developed by Eurostat, adapted to the subnational level. This has been made possible by the use, for the first time, of very precise statistical sources concerning freight. We have developed a multiscale approach in order to weigh the urban metabolism of those two cities and to compare it to other cases and larger territories. This allows a better understanding of the specific territorial metabolism of intermediate cities, their hinterlands, and their logistics‐hub function. We conclude with the “urban dimension” of social metabolism, and, thanks to the multiscale approach, to the debate regarding logistical hubs, dematerialization, and territorial autonomy.     

Enhanced Performance of the Eurostat Method for Comprehensive Assessment of Urban Metabolism: A Material Flow Analysis of Amsterdam

Sustainable urban resource management depends essentially on a sound understanding of a city's resource flows. One established method for analyzing the urban metabolism (UM) is the Eurostat material flow analysis (MFA). However, for a comprehensive assessment of the UM, this method has its limitations. It does not account for all relevant resource flows, such as locally sourced resources, and it does not differentiate between flows that are associated with the city's resource consumption and resources that only pass through the city. This research sought to gain insights into the UM of Amsterdam by performing an MFA employing the Eurostat method. Modifications to that method were made to enhance its performance for comprehensive UM analyses. A case study of Amsterdam for the year 2012 was conducted and the results of the Eurostat and the modified Eurostat method were compared. The results show that Amsterdam's metabolism is dominated by water flows and by port‐related throughput of fossil fuels. The modified Eurostat method provides a deeper understanding of the UM than the urban Eurostat MFA attributed to three major benefits of the proposed modifications. First, the MFA presents a more complete image of the flows in the UM. Second, the modified resource classification presents findings in more detail. Third, explicating throughput flows yields a much‐improved insight into the nature of a city's imports, exports, and stock. Overall, these advancements provide a deeper understanding of the UM and make the MFA method more useful for sustainable urban resource management.     

Service Sector Metabolism: Accounting for Energy Impacts of the Montjuic Urban Park in Barcelona

This article evaluates, from an industrial ecology (IE) perspective, the energy performance of the services inside an urban system and determines their global environmental impact. Additionally, this study determines which are the most energy demanding services and the efficiency of their energy use per visitor and per surface area unit.
The urban system under study is the Montjuïc urban park in Barcelona, Catalonia, Spain, which can be considered a services system. In this case study we distinguished the different patterns of consumption among the service fields and, by studying each field individually, found the most efficient facilities and identified the most critical services based on energy use per visitor or per square meter. These findings are based on the use of energy flow accounting (EFA), life-cycle assessment (LCA), and the energy footprint to analyze the Park's technical energy consumption.
Electricity consumption represents nearly 70% of the total energy consumed by the services at Montjuïc Park. The forest surface area required to absorb the CO2-equivalent emissions produced by the life cycle of the energy consumed at Montjuïc Park represents 12.2 times the Park's surface area. We conclude this article by proposing the incorporation of the methods of IE within the study of parks containing multiple services to improve energy management, and as a result, to raise the global environmental performance of the service sector.     

A Material Flow Accounting Case Study of the Lisbon Metropolitan Area using the Urban Metabolism Analyst Model

This article describes a new methodological framework to account for urban material flows and stocks, using material flow accounting (MFA) as the underlying method. The proposed model, urban metabolism analyst (UMAn), bridges seven major gaps in previous urban metabolism studies: lack of a unified methodology; lack of material flows data at the urban level; limited categorizations of material types; limited results about material flows as they are related to economic activities; limited understanding of the origin and destination of flows; lack of understanding about the dynamics of added stock; and lack of knowledge about the magnitude of the flow of materials that are imported and then, to a great extent, exported. To explore and validate the UMAn model, a case study of the Lisbon Metropolitan Area was used. An annual time series of material flows from 2003 to 2009 is disaggregated by the model into 28 material types, 55 economic activity categories, and 18 municipalities. Additionally, an annual projection of the obsolescence of materials for 2010–2050 was performed. The results of the case study validate the proposed methodology, which broadens the contribution of existing urban MFA studies and presents pioneering information in the field of urban metabolism. In particular, the model associates material flows with economic activities and their spatial location within the urban area.     

Metabolized‐Water Breeding Diseases in Urban India: Sociospatiality of Water Problems and Health Burden in Ahmedabad City

Studies on urban metabolism have provided important insights in the material and sociopolitical issues associated with the flow of water. However, there is a dearth of studies that reveal how infrastructure, as a hybrid of social and material construct, facilitates disease emergence. The article brings together urban metabolism, political ecology, and anthropological studies to examine the social construction and reconstruction of the material flow through everyday practices for addressing the water problem and its health burden in Ahmedabad city. The article georeferences the water problems and occurrence of diseases and, through interviews, documents Ahmedabad's sociospatial characteristics of water problems and health burden in two case study wards. Through a situated understanding of the everyday practices, the infrastructure is exposed through leakages, reveals the citizens desire for better water quality, and struggle to gain access to water using diverse ‘pressure’ tactics. It is these social‐material constructs of water that give structure and coherence to urban space, which spatially coincides with the occurrence of diseases. It reveals the sociopolitical drivers of the water problems and identifies different hypotheses of the hotspots of disease emergences. The methodology offers a way forward for researchers and development agencies to improve the surveillance and monitoring of water infrastructure and public health through an incremental approach that takes into consideration the diffuse interplay of power by diverse actors. It charts out avenues for building on the urban metabolism by emphasizing the importance of examining the sociospatiality of the everyday practices for improving resource use efficiencies in cities of rapidly growing economies.     

A Spatial Analysis of Residential Greenhouse Gas Emissions in the Toronto Census Metropolitan Area

Residential greenhouse gas (GHG) emissions in the Toronto Census Metropolitan Area are spatially analyzed to determine the impact of urban form on emission-causing activities. The key finding is that over the entire region, emissions from private auto use are on par with those from fuel use for building heating. Once beyond the transit-intensive central core, private auto emissions surpass the emissions from building operations. Variation in total auto- and building-related emissions is quite significant between census tracts, ranging from 3.1 to 13.1 tonnes of carbon dioxide equivalents per year. Of all tracts, the top ten in terms of GHG emission are located in the lower-density suburbs, and their high emissions were largely due to private auto use.     

Drivers of urban metabolism: Toward a framework for urban transformations

The environmental and social crises in cities call for radical future visions that can incite transformative change. Yet, urban metabolism research typically adopts an explanatory, retrospective approach to the drivers of urban flows and stocks, resulting in conservative, business-as-usual future outlooks. In this study, we present the results of a narrative literature review on drivers and futures of urban metabolism, and consequently use these results to propose and apply a framework that can be used by researchers (i) to systematically identify the drivers of urban metabolism, and (ii) to critically engage with these drivers for the development of transformative future visions. The framework comprises seven thematic categories of drivers (demographic, economic, cultural, political, technological, environmental, and infrastructural) and an eighth category (power) to be used as the lens through which the interactions between drivers, activities, and flows in the city are critically examined. Applying the framework to the case study of biowaste management in Rennes, France, we found it useful for the systematic identification of often overseen drivers. The proposed framework, allowing for a combined analysis of flows and drivers, can become a useful tool toward a solution-oriented urban metabolism research.     

Evaluation Approaches for Advancing Urban Water Goals

Urban areas (especially cities) are challenged in meeting their direct water needs from local sources. They also exert strain on global water resources through their indirect (virtual) water use. Agencies concerned with urban water management have visions and goals for managing direct water use, but indirect use is only inferred in more global visions for sustainable consumption. There is limited quantification of “urban water performance” at the macro urban scale (whole of city) to monitor progress toward these goals. It is constrained by a lack of clarity about the evaluation approaches that best serve them. We ask, How can the evaluation approaches described in literature advance urban water management goals? We reviewed the utility of eight evaluation approaches, including urban water system modeling, urban metabolism (territorial and mass balance), consumption (life cycle assessment, water footprinting, and input‐output analysis), and complex systems (ecological network analysis and systems dynamics) approaches. We found that urban metabolism based on water mass balance is a core method for generating information to inform current goals for direct urban water use, with potential for being “coupled” with the other approaches. Consumption approaches inform the management of indirect water use. We describe this in a framework for urban water evaluation to give greater clarity to this field and flag the further research that would be needed to progress this. It includes the recommendation to differentiate the evaluation of direct and indirect urban water, but to also interpret them together.     

An Approach to Integrating Environmental Considerations Within Managerial Decision‐Making

Recent environmental trends, including (1) an expansion of existing command and control directives, (2) the introduction of market‐based policy instruments, and (3) the adoption of extended producer responsibility, have created a need for new tools to help managerial decision‐making. To address this need, we develop a nonlinear mathematical programming model from a profit‐maximizing firm's perspective, which can be tailored as a decision‐support tool for firms facing environmental goals and constraints. We typify our approach using the specific context of diesel engine manufacturing and remanufacturing. Our model constructs are based on detailed interviews with top managers from two leading competitors in the medium and heavy‐duty diesel engine industry. The approach allows the incorporation of traditional operations‐planning considerations—in particular, capacity, production, and inventory—together with environmental considerations that range from product design through production to product end of life. A current hurdle to implementing such a model is the availability of input data. We therefore highlight the need not only to involve all departments within businesses but also for industrial ecologists and business managers to work together to implement meaningful decision models that are based on accurate and timely data and can have positive economic and environmental impact.     

Towards Measuring the Informal City: A Societal Metabolism Approach

The rapid growth of urban informal settlements, or slums, poses a particular challenge for balancing developmental and environmental goals. In South Africa, high levels of inequality, poverty, and unemployment contribute to widespread migration. The influx of migrant workers to cities, however, is rarely matched with adequate housing and infrastructure, resulting in the formation and growth of urban informal settlements. Despite the persistence of the slum phenomenon, very few studies provide an in‐depth understanding of the metabolic processes that link these spaces, and informal economies, to the broader urban environment and economy. This article therefore utilized a multiscale integrated assessment of the societal and ecosystem metabolism approach to examine human activity and land use in Enkanini, an urban informal settlement in Stellenbosch, South Africa. The results highlight a number of issues to be addressed through spatial, developmental, and local economic policy, such as the need for improved transport linkages. The time‐use results show that Enkanini is a net provider of labor to the surrounding area. Further, geographical mapping indicates Enkanini as a small, but vibrant, informal economy, while being grossly underserviced in terms of water, waste, and sanitation infrastructure. Key implications are discussed in terms of the theoretical, methodological, societal, and policy impact of the study, including the need for city observatories that conduct regular data collection and analysis.     

Urban Metabolism of Paris and Its Region

The article presents the results of a research project aimed at (1) examining the feasibility of material flow analysis (MFA) on a regional and urban scale in France, (2) selecting the most appropriate method, (3) identifying the available data, and (4) calculating the material balance for a specific case. Using the Eurostat method, the study was conducted for the year 2003 and for three regional levels: Paris, Paris and its suburbs, and the entire region. Applying the method on a local scale required two local indicators to be defined in order to take into account the impact of exported wastes on MFA: LEPO, local and exported flows to nature, and DMC_corr, a modified domestic material consumption (DMC) that excludes exported wastes (and imported ones if necessary). As the region extracts, produces, and transforms less material than the country as a whole, its direct material input (DMI) is lower than the national DMI. In all the areas, LEPO exceeds 50% of DMI; in contrast, recycling is very low. The multiscale approach reveals that urban metabolism is strongly impacted by density and the distribution of activities: the dense city center (Paris) exports all of its wastes to the other parts of the region and concentrates food consumption, whereas the agricultural and urban sprawl area consumes high levels of construction materials and fuel. This supports the use of MFA on an urban and regional scale as a basis for material flow management and dematerialization strategies and clearly reveals the important interactions between urban and regional planning and development, and material flows.