In This Issue
Summer Bridge on Smart Agriculture
June 15, 2022 Volume 52 Issue 2
People everywhere rely on agriculture in one form or another – for food, animal feed, fiber, and other necessities. The summer 2022 articles describe precision indoor farming and alternative protein food systems, advances in food processing, genome editing, digitalization, sustainable and regenerative agriculture, the role of a circular economy, and the important role of policy.

Principles for a Sustainable Circular Economy at the Urban-Regional Food-Energy-Water Nexus: Advancing Environment, Health, and Equity

Monday, June 13, 2022

Author: Anu Ramaswami and Dana Boyer

We present an urban metabolism-based systems framework to assess resource circularity at the FEW nexus to support environmental sustainability, resilience, health, and equity.

This paper elucidates eight principles for designing a sustainable circular economy at the urban-regional food-energy-water (FEW) nexus, using a supply chain–linked urban metabolism model. Connecting in- and trans-boundary FEW consumers, producers, processes, and interactions in cities, the model enables analysis of sustainability outcomes related to environ-mental dimensions, climate vulnerability, human health, and social equity. Our design principles are elucidated by an urban metabolism systems framework and a synthesis of recent works.

What Is a Sustainable Circular Economy?

The circular economy is an amorphous concept that, overall, seeks to reduce the linear throughput of natural resources, closing material and energy loops to advance sustainability (Geng et al. 2013). The idea has received much attention,[1] yet remains fuzzy, with more than 100 definitions in the literature (Kirchherr et al. 2017).

We define a sustainable circular economy as one that leverages resource efficiency and circularity as means to advance multiple sustainability goals, which include achieving resource sustainability, reducing pollution, preserving/regenerating natural capital, and generating employment and broader benefits to human health and wellbeing, including social equity. Resource circularity is thus effectively linked with the United Nations’ Sustainable Development Goals (SDGs) addressing environment, health, and equity dimensions.[2]

A sustainable circular economy optimizes the scale and sectors engaged in resource circularity to advance environment, health, wellbeing, and equity.

Circular economy strategies are generally understood to encompass the “four Rs”: reduce, reuse, recycle, and regenerate. Efforts to “reduce” include behavioral changes and demand shifts as well as technology-level efficiency to reduce resource demand for goods or services. The reuse and recycle options can be achieved within the same sector (e.g., food) or via cross-sectoral exchanges/interactions (e.g., through society-level efficiencies across FEW and construction sectors). Regenerative solutions include renewable energy and nature-based solutions that offer environmental value.

A circular economy requires the development of analytic tools to measure multiple sustainability benefits of resource circularity. Another practical question concerns the spatial scale and sectors for implementation of resource circular interventions to optimize environmental and societal outcomes. For example, resource circularity may be designed (i) within individual industries (e.g., the use of waste heat from food processing industries in desiccation operations, or of farm-level manure to generate onsite energy); (ii) across two or more colocated industries (“industrial symbiosis”; Chertow 2007), as in ecoindustrial parks; and (iii) in multisectoral resource exchanges at societal scales, such as the FEW and construction sectors in cities and nations (Ramaswami et al. 2017a).

In this paper we focus on the rationale and potential for a sustainable circular economy at the societal scale of urban-regional systems, leveraging linkages across FEW sectors, households, commercial entities, and industries. We posit that a sustainable circular economy is one that optimizes the scale and sectors engaged in resource circularity to advance environment, health, wellbeing, and equity. We discuss emerging methods to quantify sustainability outcomes, and elucidate eight design principles for achieving a sustainable circular economy at the urban-regional FEW nexus, drawing on recent works.

Why the Urban-Regional FEW Nexus?

The urban-regional scale offers strategic opportunities for a sustainable circular economy, addressing the key sectors that provide food, energy, water, and construction materials to urban areas, where a majority of the world’s population lives. These sectors are critical since together they contribute to ~90 percent of global greenhouse gas emissions, >94 percent of global water withdrawals, and almost all the mass of materials extracted from nature globally (Ramaswami et al. 2008; -Wiedmann et al. 2013).

Evaluating these sectors at the urban-regional scale is important for many reasons:

  • More than 80 percent of the US population and two thirds of the world’s population will live in urban areas by the year 2050, drawing vast amounts of resources (food, water, energy, and materials) (ACERE 2018).
  • The concentration and colocation of homes, businesses, and industries in urban areas provide cost--effective opportunities for exchange of “waste” resources such as industrial waste heat, municipal -solid waste, and wastewater (Lu et al. 2018; Ramaswami et al. 2017a[3]).
  • Globally, a substantial percentage of food production occurs within 20 kilometers of urban areas (Thebo et al. 2014), allowing for regional exchanges between cities and surrounding farmlands.

Not all regional food production serves local consumption, so a transboundary supply chain perspective (Ramaswami et al. 2017a) is needed, highlighting material-energy flows and associated opportunities for circularity across the supply chain of key provisioning systems.

Ramaswami figure 1.gif
FIGURE 1 Transboundary multisector framework to analyze environmental impacts of the provisioning of agrifood (F), -energy (E), and water (W) to homes, businesses, and industries in a city. The framework connects community-wide FEW demand with in- and transboundary FEW production. It also incorporates in-boundary cross-sector interactions shaping resource exchange and recovery, such as energy inputs to water use (E®W), water inputs to energy production and use (W®E), and FEW storage within and outside the city. Connecting in- and transboundary interactions informs the city’s systemwide water, energy, and greenhouse gas (GHG) impacts. Reprinted from Ramaswami et al. (2017a). © IOP Publishing Ltd.

Figure 1 illustrates a metabolism framework of FEW flows into and out of cities. The traditional metabolism framework tracks the direct flows of materials, water, fuels, and waste in and out of cities (Kennedy et al. 2011; Wolman 1965). It has been expanded mathematically and theoretically to track resource use and emissions embodied in supply chains serving cities, resulting in different types of environmental footprints (Chavez and Ramaswami 2013), such as those for resource use (water, land, nutrients) and environmental emissions (pollution, greenhouse gases [GHG], waste). The transboundary framework shown here integrates both metabolism and footprint concepts, and further incorporates FEW linkages.[4]

Figure 1 shows the administrative boundary (dotted circle) of a city or urban area with homes, businesses, and industries that collectively exert community demand for FEW. This demand is shaped by local household diets and socioeconomic status, FEW requirements of exporting industries, and consideration of pre- and postconsumer wasted food. Only some of the local FEW demand is locally produced; the rest is imported from regional and larger supply chains (outside the circle). These supply chains include water embodied in the production of food and energy, energy embodied in the production of water and food, and second-order effects (e.g., energy used to provide irrigation water for agrifood production, and water for the energy used to operate farm machinery).

The systems framework enables quantitative evaluation of in- and transboundary linkages across the FEW sectors, from farm production to consumption.

The systems framework enables quantitative evaluation of in- and transboundary linkages across the FEW sectors, from production at the farm level to consumption in urban homes and businesses. Mapping these flows and associated footprints is important for assessing the potential and sustainability of the resource circularity pathways.

Analysis of Circularity Potential and Sustainability

Circularity Pathways

Opportunities for a circular economy can be identified by quantifying the different flows in figure 1 to explore actions that can be taken by society. For example, application of the framework to Delhi, India, has yielded several insights (Boyer and Ramaswami 2018; Ramaswami et al. 2017a). First, the water embodied in food production was found to be a factor of five greater than direct water requirements in the community. This information highlights that dietary shifts (e.g., see the impact of rice in figure 2), opportunities to reduce wasted food, and ways to reduce wasted water in irrigation are larger “reduce” levers compared to other actions in the municipal water sector. Second, knowing the amounts of cooking fuel and gas used in the energy sector makes it possible to match food waste–to-energy projects with local energy requirements to help create a local circular economy. Knowledge of the magnitude/acreage of local agriculture is also useful for assessing the nutrient mass that can be recycled from wastewater treatment to local farms (Miller-Robbie et al. 2017).

Thus the urban FEW metabolism model with in-boundary detail and transboundary supply chains enables assessment and prioritization of the most significant circularity options.

Ramaswami and Boyer figure2.
FIGURE 2 Mass, nutrition, and environmental dimensions of annual food supply to Delhi, India, derived from the systems framework in figure 1. Distribution of various foods by (a) food mass, (b) nutritional energy (calories), (c) greenhouse gas (GHG) footprint (including energy use and agriculture-based emissions, tons of CO2 equivalents), (d) consumptive water-loss footprint (cubic meters), (e) land footprint (hectares), and (f) distribution of these impacts as in- or trans-boundary (in, solid; trans, shaded). veg. = vegetables. Reprinted with permission from Boyer and Ramaswami (2017). © American Chemical Society.

Environmental Impacts, Trade-offs, and Cobenefits

The systems framework in figure 1 also enables assessment of the baseline environmental footprints of urban FEW provisioning by combining the mass of urban FEW demand with life cycle analysis of FEW production across the supply chain. Multiple environmental impacts (see figure 2) can be assessed—for example, on water -resources, energy, GHG emissions, and land—relevant to SDGs 6 (clean water and sanitation), 7 (affordable and clean energy), 13 (climate action), and 15 (life on land). Figure 2 illustrates quantification of the multiple environmental impacts of various foods consumed in Delhi, in the base case. Methods are also evolving to quantify nutrient footprints of urban FEW provisioning by assessing nitrogen and phosphorus flows (Leach et al. 2012; Singh and Bakshi 2013).

Such baseline data on environmental impacts enable quantitative assessment of potential future circular economy interventions, as shown in figure 3 for Delhi, including the impact of household dietary shifts, urban agriculture, and technologies that reduce, reuse, or enable recycling and valorization of food waste (Boyer and Ramaswami 2017).

Ramaswami figure 3.gif
FIGURE 3 Scenario analysis of several circular economy “4R” interventions in Delhi’s food system, showing impacts on various environmental outcomes. Percent reduction of annual systemwide food-related water (blue), greenhouse gas (GHG; red), and land (green) impacts shown as a result of 100% adoption of food system scenarios. Expressed in terms of 1st order (solid bars) and 2nd order (shaded bars) impacts. *n/d = insufficient data to determine a result. Error bars represent range of intensity factors and technologies for each scenario. AD = anaerobic digestion; HH = household; LPG = liquefied petroleum gas; VFT = vertical farming technologies. Reprinted with permission from Boyer and Ramaswami (2017). © American Chemical Society.

As seen in figure 3, there often are trade-offs among the different environmental outcomes (water, GHG, land), requiring prioritization by decision makers.

Supply Chain Vulnerability, Climate Risks, and Resilience

Supply chain analysis, aided by spatial delineation (figure 1), enables assessment of the resilience of FEW supplies to cities. For example, spatially delineated supply chains in Delhi (Boyer et al. 2019; Ramaswami et al. 2017b) revealed the high-water vulnerability of all transboundary regions supplying food to the city as well as local agriculture, all occurring in conditions of severe ground water overdraft. And supply chain analysis during drought events in the United States over a 200-year period found that urban areas with diversified food supply chains were more resilient (Gomez et al. 2021). Similar supply chain analyses have been developed to assess the vulnerability of energy supplies to cities in light of projected climate change, which affects precipitation patterns relevant to hydropower generation and cloud cover relevant to solar power generation (Miara et al. 2019). Such analyses are critical to SDG 11, which calls for developing sustainable climate-resilient cities.

Inequality, Health, and Equity

The urban metabolism model illustrated in figure 1 enables assessment of multiple dimensions of inequality. For example, in many Indian cities, dietary intake represents a high level of inequality and deprivation relevant to the UN’s SDG 1, zero hunger: the percentage of urban populations with protein- and calorie-deficient diets is as high as 70 percent and 90 percent, respectively, in some cities (Boyer et al. 2019). Future scenarios therefore not only explore more equitable diets that enhance nutrition for the underserved but also reduce environmental impacts by both switching to low-intensive yet nutritious crops like sorghum and managing irrigation and food waste (Boyer and Ramaswami 2017; Davis et al. 2018).

Larger-scale models in the United States have tracked county-level farm fertilizer application, associated transboundary windblown air pollution emissions, and premature disease burden and mortality, finding substantial inequality across counties by income and race (Tessum et al. 2019). By assessing the distribution of burdens across social strata, the studies cited here provide the criteria for designing more equitable interventions that reduce burdens on the most vulnerable in society.

Designing a sustainable circular economy calls for careful consideration of equity. In addition to unequal air pollution exposure and access to nutritious food, who gains from new employment generated through resource circularity technologies? Who loses jobs? What is the quality of jobs lost—and created? What are the trade-offs with the increasing use of robots and artificial intelligence in precision agriculture? Will indoor agriculture displace (often lower-income) farm workers? How many jobs may be generated in other sectors, such as construction or AI?

Ramaswami box 1.gif

Eight Principles for a Sustainable Circular Economy

Based on the concepts and case studies summarized in the figures, we delineate eight principles to design a sustainable circular economy at the FEW nexus (box 1), with some examples to highlight these points.

  1. Connect consumption, production, and waste across supply chains to get a systemwide perspective of resource circularity options. Such a systemic approach can demon-strate the combined impacts of behavioral changes, adoption of economically viable vertical farming technologies, and food waste valorization technologies (figure 3). These areas represent the spectrum of 4R strategies involved in circularity, with the systems framework explicitly connecting consumption in cities to transboundary agricultural production.
  2. Evaluate opportunities for resource circularity at societal scales, both within and across sectors and city boundaries. In many cases transboundary nexus linkages, though not directly visible within cities, can contribute to better urban sustainability outcomes. For example, transboundary irrigation management with solar-powered pumps that feed excess electricity back to the grid can be very effective in advancing energy conservation, water conservation, and decarbonization in India. Likewise, application of food waste nutrients and biochar may be limited to the fewer and smaller farms in dense urban regions (e.g., -Miller-Robbie et al. 2017), but can be maximized when applied regionally to farms with greater land area for regeneration.
  3. Use per capita or regional analysis to assess potential for circularity, matching magnitude of resources from one sector to another. Recycling requires matching waste quantity and quality with suitable reuse, often with further treatment or valorization to useful products. The maximum potential for reuse and valorization requires a combination of material-energy flow analysis with life cycle assessments. Currently, life cycle analysis of food waste technologies is conducted per unit of waste to choose among valorization technologies and pathways (e.g., Hodge et al. 2016), and requires data on the mass of virgin resources that can be substituted to maximize circularity. Studies have demonstrated ways to match the supply and demand of waste heat across sectors in cities (Ramaswami et al. 2017a; Tong et al. 2017).
  4. Consider sequential resource circularity processes that can create opportunities for virtuous cycles. Biochar from wood and food waste (Lehmann and Joseph 2021), for example, can be a net carbon negative due to postpyrolysis production of fuel (oil and gas); the impact can be multiplied by beneficially applied biochar at suitable rates to selected soil types to enhance plant growth (Guo 2020). Another example of sequential circularity (being quantified in the Urban Nexus Lab at Princeton University) is the upgradation of spent coffee grounds to grow mushrooms (Dorr et al. 2021), followed by substitution of mushrooms for beef in hamburgers, significantly reducing GHG emissions (Waite et al. 2018). These virtuous cycles have a cascading effect that reflects the power of the circular economy.
  5. Pair technoeconomic analysis with evaluation of multiple sustainability outcomes. Outcomes should include environmental impacts, vulnerability/resilience, and multiple dimensions of equity, using a metabolism approach with spatially delineated supply chains (figure 1 and accompanying text).
  6. Recognize that trade-offs and cobenefits may occur within and across sustainability outcomes, including environmental and equity dimensions. For example, figure 3 illustrates that certain dietary shifts may reduce GHG emissions while increasing land or water impacts. Trade-offs can also affect equity dimensions such as nutrition access, pollution exposure, and/or -employment.
  7. Develop multiscale nationwide models of resource circularity to identify the most suitable spatial scale for operationalizing the circular economy. The most suitable scale for implementing a circular economy can be determined only by modeling exchanges at multiple scales and comparing them. For example, resource use occurs in cities, at a regional scale, and at state and national scales based on both the value added of the product and the economy or ease of transporting materials over large distances. Multiscale modeling is essential to help identify the most suitable scale for achieving a sustainable circular economy.
  8. Engage in multiscale cross-sectoral knowledge -coproduction to help design sustainable and equitable circular economy transitions. Given the unequal impacts of circular economy transitions on society, with implications for equity, the design of sustainable circular economy transitions cannot be done by scientists or researchers alone. A combination of technical analysis combined with democratic deliberation is needed (Stern 2005), enabled by the practice of coproduction (Norström et al. 2020). Coproduction—defined as meaningful, context-specific, iterative, and goal-oriented collaboration among researchers and a broad swath of decision makers (Norström et al. 2020)—can provide a framework to engage stakeholders important to enable sustainable circular economy transitions at the FEW nexus. The actors must themselves transcend scale and be appropriate to the FEW nexus context.


Together, our eight design principles can advance sustainability at the urban-regional FEW nexus, addressing environment, supply chain resilience, health, economy, and equity. Real-world codesign of solutions with multiple stakeholders in communities is essential to realize the potential of a sustainable circular economy at the urban-regional scale.

Implementing the eight principles will require concerted action by all sectors of society. Starting with individuals, consumer behaviors profoundly shape the metabolism of cities through dietary changes. For example, reducing meat consumption can dramatically reduce environmental burdens at the FEW nexus. To design systemic solutions, both academic units and professional societies should teach, research, practice, and disseminate the systems concepts outlined in the framework (figure 1) underlying the principles. Collaboration among researchers, policymakers, and industry practitioners is essential to develop practical metrics for sustainable and equitable resource circularity. Community groups must be engaged at every level to ensure that metrics relate to equitable outcomes. Engineers and scientists must embrace multisector partnered projects that transcend disciplinary boundaries, sectors, and spatial scale to harness opportunities at the urban-regional FEW nexus.


The case studies and insights discussed were informed by the US National Science Foundation Sustainability Research Network (award #1444745) and NSF-USDA Innovations at the Nexus of Food, Energy, and Water Systems (grant #2019-67019-30463). We thank Peter Nixon, Emily Eckart, and Jinjin Chen for assistance with parts of this paper.


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[1]  See, e.g., the Ellen MacArthur Foundation ( economy-introduction/overview) and the World Business Council for -Sustainable Development’s Circular Economy Program (

[2]  The goals are available at

[3]  Also see the Ellen MacArthur Foundation’s “Circular economy in cities: Project guide” ( /preview/1?o).

[4]  The framework in figure 1 can be expanded to include material flows, which intersect with the food and forestry systems. For example, research has shown that platform chemicals, bio-based plastics, and other construction materials can be produced through food waste valorization technologies (Lu et al. 2018).

About the Author:Anu Ramaswami is professor, Department of Civil and Environmental Engineering and High Meadows Environmental Institute, and the Sanjay Swani ’87 Professor of India Studies, Princeton University. Dana Boyer is sustainability manager, Cargill.