Trends and dynamics in material and energy ows analysis in an urban context. A case study of a city with an emerging economy

Background: Currently, most of the world’s population lives in cities, and this trend of the rapid urbanization of the population affects the increase in the demand for products, goods and services. To effectively design policies for urban sustainability it is important to know the trends of the ows of energy and materials that enter and leave the city. This knowledge is essential to recognize key elements characterizing the future urban growth and to be able to face supply challenges in the future. This paper presents the analysis of the ows of energy and materials (urban metabolism) in the city of Bogotá, covering a time span from 2001 to 2017. Urban ows are also characterized in terms of their temporal evolution with respect to the population growth, in order to compare and identify the changes in the main input ows, wealth production, emissions and waste in the city. Results: The results of the analysis are also compared with other selected large urban agglomerations in Latin America and worldwide, highlighting similarities and inferences. The results show that in Bogotá, there was a decrease in some of the material ow in recent years, such as the consumption of water and the generation of discharge, while there was an increase in the consumption of energy and cement and the production of CO 2 emissions and construction materials. Solid waste production remained relatively stable. With respect to the other large cities considered, we observe that the 10-year growth rates of the metabolic ows with respect to the population growth are lower, in particular when compared with the other urban agglomeration of Latin America. Conclusions: The ndings of this study are important to advance the characterization of the trends in material and energy ow in the city and contribute to the consolidation of a baseline that allows for the denition and evaluation of the different impacts of public policy that promotes the sustainability of the city in the coming decades. present study consists in a material ow analysis of Bogotá from household perspective, a city located in an emerging economy. Bogotá selected it is the capital of Colombia, a country with an emerging economy that contributes to a quarter of the country's internal product with a high level of industrialization, the presence of different productive activities and a city with the highest population concentration that currently faces many environmental challenges. This type of study can be an input to develop more effective programmes for sustainable development in Colombian urban areas. as a transdisciplinary study, to use a dynamic model, which allows evaluating physical and social processes as key elements in urban planning and design interventions, improving eco-eciencies, integrating urban management and development plans, and promoting social inclusiveness, among other suggestions.


Introduction
More people live in cities, and statistics show that, in the next 30 years, it is expected that more than 60% of people will live in cities (United Nations, 2019); therefore, it is essential to achieve sustainability and to provide policy and decision makers with instruments allowing for the reduction of the effects and impacts of current and future urbanization on the environment. In this context, it is important to know and analyse how the ows of energy and material feeding the city operate and are transformed in order to formulate and implement instruments that guarantee the quality of life and a healthy environment.
According to the United Nations Development Program (UNDP) (2012), globally, cities occupy 3% of the earth's surface, produce 80% of the gross domestic product, consume 75% of natural resources, and produce 50% of the solid waste and between 60% and 80% of greenhouse emissions.
These gures have reached a critical point, and cities have become a fundamental pivot for reducing GHG emissions and reduce the increase of the global temperature. However, as stated by the COP 21 and the following Paris agreement, cities have a potential to improve their e ciency; for example, it is estimated that although global water consumption can be reduced by 30% and energy by 30-50%, infrastructure investments years of approximately 41 trillion dollars would be required for the next 20 (UN, 2015). Moreover, the main important megacities of the world are responsible for on average 9% of global electricity consumption and 13% of the solid waste production, account for 7% of the global population and generate an important share of the gross domestic product (Kennedy et al., 2015); therefore, it is important to analyse different ows and production of goods, services and pollution to meet the requirements of the population and decrease the environmental problems and impacts.
Policies for sustainability and management of urban areas require understanding the demands of natural resources ows and the environmental impacts that are generated. The material ow analysis (MFA) allows us to demonstrate how a relationship of inputs and outputs that can promote eco-e ciency is generated, which is essential for policy makers and decision makers to formulate an urban policy that guarantees sustainability and productivity improvements and competitiveness (Moore et al., 2013).
In recent years, the analysis of urban ows, the "urban metabolism" is emerging as a reliable tool to analyse and design urban sustainability policy (Kennedy, 2007). Indeed, several studies have shown that at the urban level, knowing, analysing and evaluating the ow of materials that comprise the consumption of resources and energy are fundamental to determine the environmental effects and impacts of consumption and its transformations, possibilities of urban resilience and actions and/or options that allow for the achievement of urban sustainability over time in a differentiated way (Rosado, et al., 2017, Ferrão and Fernandez, 2013, Grimm and Wissel, 1997. Urban ows can be analysed by means of two approaches (Barles, 2010). First, there is research and decision support that evaluates the following factors: 1. Urban biogeochemistry processes, such as studies of Pataki et al. (2011) and Lin (2014) that demonstrated an integral solution and opportunities to integrate biogeochemical science in the design and evaluation of urban infrastructure and to generate synergies between environmental quality and food security through innovation and new technologies to reduce waste and pollution; 2. Effects of the material and energy demands in the cities that suggest that the present pattern of energy and resources lead to a reduction in and scarcity of natural resources, degrading ecosystems and affecting the quality of life in urban areas, where the objective is to promote energy and resource e ciency, to improve urban systems, and to increase and sustain economic growth (EEA, 2015, Song et al., 2016); 3. Biogeochemical and social operation interactions that analyse how natural cycles and society could disturb the urban ecosystem, understanding that the urban ecosystem and circular economy, among others, are important to formulate governance and policies that promote sustainable development in cities. Various studies have analysed these elements, such as Prendeville et al. (2018), who analysed these issues in six cities, and Czamanski and Broitman (2018), who suggest the importance of incorporating new innovation and structures that generate interurban migration patterns to achieve sustainable development.
The second approach is a tool to promote sustainable development challenges and requests to achieve the following goals: 1. Decoupling (to decrease the dependence between growth and use of natural resources), where studies have demonstrated that a decoupling effect seems to have occurred in G20 countries with a strategies for emissions, energy consumption, and economy decoupling supported by diversi cation, sustainable energy consumption mix and stable economic growth maintenance, and circular economy seeks to decouple global economic development from nite resource consumption (Pao and Chen, 2019, Giampietro, 2019); 2. Dematerialisation (consumption of fewer materials), which implies that the decrease in the intensity of material utilization depends on the increase in material circulation and social overall level of cleaner production and more effective use of the existing inputs and raw materials suggest to "close the loop" in the whole waste recovery mind-set, and strategies such as industrial symbiosis, resource recovery or re-use can be good approaches to achieve materials loop closure and decrease dependence on external sources. These studies are important because they allow one to design indicators, to determine sustainability targets and to establish new strategies to achieve resource e ciency policies and programmes.
The present study consists in a material ow analysis of Bogotá from household perspective, a city located in an emerging economy. Bogotá is selected because it is the capital of Colombia, a country with an emerging economy that contributes to a quarter of the country's internal product with a high level of industrialization, the presence of different productive activities and a city with the highest population concentration that currently faces many environmental challenges. This type of study can be an input to develop more effective programmes for sustainable development in Colombian urban areas.
The research questions that guide this paper are: i. What are the trends of material and energy ows in the city selected and how do they change over time?; and ii. What is the performance of material and energy ows in comparison with other Latin American Countries?. To resolve these questions, this study applies different ows analysis based on inputs and outputs, relationship with gross domestic product and comparison of performance of other cities through normalized data.
Here trends and dynamics over time are analysed with the aim to determine the temporal evolution of input and output ows over 10 and 17 years, with a focus on 2017. Data are also compared with the 10 years temporal evolution of a selected number of Latin America large urban agglomerations. Results show that Bogotá has generated sustainable processes by increasing its GDP while maintaining an environmental pressure lower with respect to the other cities as input to design adequate policies according to international trends.
The main contributions of this study are the following: i. To analysis the trends and dynamic of material and energy ows in a city of emerging economy through novelty indicators in different periods; ii. To evaluate the relationship the results of Bogotá in comparison with other Latin American countries to determine sustainable performance; iii. To contribute with analysis of ows and effects on environment from different approaches taking into account that studies related to energy and material ows are limited in emerging economies. This paper is structured in the following sections: following the present introduction, the second section describes the main characteristics of Bogotá, section three presents the data and variables used in this study, the fourth section presents the results and their analysis. Finally, conclusions and future direction are presented in Sect. 5, while Appendix A shows the main environmental policies implemented in Bogotá. and, in recent years, has managed to improve its social performance by reducing poverty and inequality. These indicators show and emphasize the importance of carrying out analyses of the sustainability of the city, especially for its importance and contribution to the country. Table 1 shows the important role that Bogotá plays for the Colombian economy: the city concentrates about the 16% of population and contributes to the 26% to the national GDP, and more importantly shares the 44% of national value added while importing the 48% of goods of the entire nation. The poverty index comparison also de nes Bogotá as the main engine of the Colombian economy

Methods And Data Description
This study evaluates material and energy ows in household of Bogotá over time using secondary information reported for different local statistics institutions. The selection of data was determined from availability, consistency and integrity to provide and guarantee reliable estimations of ow changes in the period selected.
Data include both input and output ows, also considering the relationship between inputs (such as food, water, construction and other materials, raw materials, energy, capital, information, and people) and outputs (such as industrial products, goods, services, knowledge, waste, and emissions). Inputs are important because they support and allow societal activities, urban functions within a city, formation of urban stocks as infrastructure, housing, building, public space and green areas and parks and production of products and services and the whole urban management. Outputs determine the magnitude, distribution and internal interactions and feedbacks regulated by governance, policy, culture and individual and collective behaviour of the city as an urban system. study are selected from the available information of household that the city has and are used to search for a relationship between inputs and outputs over time. In addition, in order to carry out a study taking into account the growth of the city, the intensities or measurements per capita are calculated from the population data for each of the inputs and outputs analysed and normalize variables.
Inputs and outputs include the following data, which are compared and analysed trends in different period times taking into account the relationship between ows absolute and normalized by inhabitant and household sector: Together with these ows data on policies on water saving, energy, and mobility have been collected in order to better understand the temporal variation of the ows, focussing on 10 years and 15 years growth in Bogotá, to determine how policies have helped to improve performance of the city.
Moreover, temporal variation of data has been compared with other Latin American Large urban agglomerations, data used for the present study come from previous urban metabolism studies (see Kennedy  , the reduction of ows such as water and wastewater generation, were achieved, while the production of solid waste remained relatively stable. With regards to the in/out equilibrium of ows, gure 1 also shows that input and output ows of energy and water are in equilibrium, indicating that the distribution and collection infrastructures are managed in an e cient way.
A more detailed view is offered by Fig. 2, where the 10 years and 15 years growth rates are compared. It is interesting to notice that ows are divided in three categories: 1. Signi cant increase (more than 50%): GDP, Debris and cement, both in the 10 year and 15 year view. 2. Stability (between 0 and 50%): Natural gas, total energy, CO2 emissions, solid waste, and electricity consumption.
Considering the rst category, it is interesting to point out that GDP experienced a signi cative increase in 15 years (230%) and 10 years (92%) while in the same period both population and the use of resources and output experience a different increase, while water (-5,2% in 15 years), wastewater (-17,8% in 15 years) and PMx emissions (-9,2% in 10 years and − 37% in 15 years) have been signi cantly reduced. The only ows that are both over 50% in both 10 and 15 years are those relative to the construction sector, with debris and cement. In particular, debris experienced the most signi cative growth in 15 years, reaching an increase of 150%.
Considering the population as the main driver for urban expansion and urban ows, we show in Fig. 3 the elasticity of the metabolic ows with respect to population. As expected, both 10yrs and 15yrs GDP values are well above 1, 5 and 8, respectively. Such values are in line with other large urban agglomerations observed worldwide (Kennedy et al, 2015), while debris and cement elasticity re ect the important increase observed in Fig. 2. With regards to the other ows, is interesting to notice that natural gas, total energy, CO2 emissions and solid waste show elasticity values between 1 and 1,4, while electricity, water consumption (15yrs) wastewater (15yrs) and particle PMX show negative values or values lower than 1.
These results indicate the importance of understanding the speci c issues related to a sustainable urban transition at the local level that implies a link between urban activities and urban planning. Musango et al. (2017) and U4SSC (2017) recommend the design and application of methods to calculate energy and material ows, ensuring a comparison for all cities, in both developed and developing countries, to analyse spatial and temporal issues in urban ows, to assess different effects of society such as a transdisciplinary study, to use a dynamic model, which allows evaluating physical and social processes as key elements in urban planning and design interventions, improving eco-e ciencies, integrating urban management and development plans, and promoting social inclusiveness, among other suggestions.
In the following subsections the paper discussed in deeper detail the temporal variation of the urban metabolism ows, highlighting the main actions implemented by the municipality to reduce the pressure of the city on the environment.

Trends in water consumption and wastewater production
In the urban sphere, water consumption is essential for different personal and business processes, and its use generates wastewater; thus, these types of consumption maintain a direct relationship for the study period. It is observed that in 2001, the total water consumption of the city was

Trends in energy consumption and emissions generation
Energy consumption is vital for cities and is synonymous with development and progress. In the case of Bogotá, the two most important sources of energy for residential use are electricity and natural gas. The consumption of these two resources was 7240701.31 TJ in 2001 and 9646171.83 TJ in 2017, which is an increase of 33%. Figure 5 shows the per capita energy consumption for Bogotá as a total of electricity and natural gas. From 2001 to 2017, there was an increase in consumption, while in recent years it has been reducing the same per inhabitant.
In the case of electricity, per capita consumption has remained almost constant, especially in recent years with a downward trend. In the case of natural gas, increases in consumption are evident until 2010, and then a reduction in consumption is observed, generating lower per capita consumption for this fuel and with slight increases in recent years. These results may be due to a new migration of users to electricity, mainly due to natural gas rationing or accidents that occurred with them.
In general terms, it could be said that energy consumption in the residential sector of Bogotá has remained relatively stable with minor increases in electricity consumption, reductions in natural gas consumption and a slight increase in the last three years. These results may also be due to technological changes and increased awareness of the rational use of energy.
Policy makers and decision makers should evaluate electricity demand to determine the main drivers of e cient construction design, sociodemographic and physical dwelling characteristics, network planning and strategic instruments, regulations, socio-technical structures, and energy e ciency technologies and to capture the diversity of infrastructure characteristics, which could generate potential energy reduction and cost savings (Roberts et al., 2019, Satre-Meloy, 2019).
As a result of energy consumption, emissions that generate air pollution increased. Table 3 shows the pollutant loads in the air for the different periods. It is observed that CO 2 emissions have increased slightly on average in recent years, sulfur dioxide and average nitrogen have shown a similar trend of maintaining or decreasing, and PM10 particulates have tended to decrease due mainly to improvements in the quality of the city fuels. However, in Bogotá, the need to improve the quality of fuels, especially in transport, is already observed due to the alerts for air quality that have occurred in 2019, indicating the importance of analysing the trends in emissions, taking the cycle of value of the energy sources used by sector and determine alternatives to improve air quality in the city of Bogotá.

Trends in cement consumption and generation of construction waste
In the urbanization process, a large amount of construction material is required, which in parallel, generate construction waste that in many cases, due to their mixture, is di cult to recover and/or deal with the consequent problems of land ll occupation and pollution. In 2001, the consumption of grey cement in the city was 962,000 tons, and in 2017, 1461,000 tons were consumed, which was an increase of 52%. Regarding the generation of debris, in the year 2001, 6132000 tons, and in 2017, 15487802 tons, were generated, which was an increase of three times compared to the base year. Figure 6 describes the trends in cement consumption and construction waste generation per capita in the city of Bogotá for the study period.
Cement consumption per inhabitant has remained relatively constant, while the generation of construction waste per person in the city has increased, mainly due to the dynamics of new construction and the modi cation of those existing in urban densi cation and expansion processes.
Additionally, in recent years, in the city, the demand for materials for periodic housing remodelling, industrial and commercial areas have increased, which may suggest an increase in this consumption pattern and a greater added value to the construction.
Moreover, construction waste can be recycled or reused as building materials through integrated waste management systems that include waste sorting, reducing illegal dumping behaviour, promoting the government's nancial subsidy on waste recycling, and increasing waste land lling charges, which is an opportunity for extracting economic and environmental bene ts from waste as reductions in CO 2 emissions, energy use,

Trends in the production of solid waste
Another fundamental element to ensure sustainability in urban areas is the management of solid waste. In Bogotá, in the year 2001, 1794430 tons of solid waste was generated, while in 2017 the generation was 2295821 tons, which is an increase of 28%. Figure 7 shows the per capita generation of waste in Bogotá for the study period indicating that between 2001-2010 there was an increase in per capita waste generation and that in the last year, a downward trend has been achieved that could be attributed to greater awareness of the importance of reducing waste generation and contributing to recycling.
According to the World Bank (2019), managing waste is essential for achieving sustainable and liveable cities, especially in cities of developing countries. However, effective waste management is expensive (on average 20%-50% of municipal budgets) and complex. The main objectives of solid waste management include (Das et al., 2019) description of new technologies, strategic innovations and monitoring tools, de nition of waste management scenarios, identi cation of the role of life cycle assessment and other modelling and simulation tools and alternatives for sustainable recycling and use of solid waste to achieve an integrated system characterized by e ciency, sustainable and socially supported.
These results show the importance of knowing the dynamics of the different in ows of inputs and outputs in urban areas in order to determine how actions can be achieved when the population takes responsibility for their environmental impacts. These results also illustrate how the population can be aware of the pollution and environmental problems and how local actions take precedence to impact global indicators, improve the environment and contribute to the sustainability of the city; these actions take into account that urban activities and processes are managed at a sub-national level by municipal or city governments, that should formulate effective action and processes to simultaneously improve and strengthen resource e ciency and productivity and achieve sustainable development (Musango et al., 2017).

Comparison With Latin American Large Urban Agglomeration
The 10 years variation of energy and material ows of Bogotá are compared with other large urban agglomeration of Latin America: Mexico City, Sao Paulo, Rio de Janeiro, Buenos Aires, and Lima. Tables 4 and 5 report the percentual growth and the ows elasticity with respect to population.
Indeed, population is selected as guiding principle for the comparison of the metabolic ows.
Bogotá experienced the higher population growth rate, 18,5% (in line with Lima) and against values around 10% of Buenos Aires, Rio and Sao Paulo. between 3% (Mexico City) and 18% (Lima and Bogotá). Excluding Mexico City, the population growth of the other major cities in the region is between 10 and 20%. Despite this relative homogeneity, the observed metabolic ows show consistent differences. Focusing on the energy sector, electricity consumption in Bogotá is located in the lower bracket of the interval, while Lima and Rio the Janeiro showed a signi cant increase. These gures are re ected in the elasticity value, that for Bogotá is 0.5, the only one below one observed in the table. This is particularly relevant under the point of view of sustainability and resource conservation. Low elasticity values for resources mean that the city has been able to develop and increase its quality of life while reducing the pressure on environment. These ndings are also valid for the total energy consumption. Table 4   The results show that at a global level, most of the ows increased under pressure from population growth and city densi cation/expansion.
However, when reviewing the ows at the inhabitant level, greater e ciency is observed, which demonstrates that day-by-day, citizens are more aware of consumption and the environmental problems that they can generate. During the study period, the ows that increased in the city were the consumption of energy and cement and the generation of CO 2 and debris emissions, which is consistent with the dynamics of the city. In contrast, the reduction in water consumption and the generation of land lls and the production of solid waste were relatively stable. These ndings are also con rmed by the 10 year and 15 year growth analysis, showing that the elasticity of the metabolic ows in relation to the population increase are either reduced or in the lower bracket when compared with other large urban agglomerations in the Latin America Region.
Furthermore, the comparison of the 10 year growth of Bogotá highlighted signi cant differences, especially regarding the use of metabolic ows This study demonstrates that urban systems present different opportunities for the development and application of strategies, plans and programmes for the e cient management of resources within the framework of integral urban planning. In addition, the paper seeks to migrate from a linear behaviour of ow of materials to more circular processes, making the provision of various urban and public products and services to the inhabitants and economic activities sustainable. In this way, it is possible to promote change in citizen rationalizations and consumption patterns oriented towards the best use of resources, reducing the generation of waste to strengthen the various aspects of the urban system.
From the analysis presented here clearly emerged the prioritized role of public policy instruments that allow, at the macrolevel, impacting citizen actions in favour of sustainability and generating an adequate urban environmental management based on a baseline that allows the measurement of the impact of the different programmes that promote the care of the environment.
Future work will be devoted to a better understanding of the metabolic ows in a circular economy perspective, also making use of a novel urban model that integrates urban metabolism and Life-cycle assessment (Maranghi, 2020 Appendix A: Policies And Regulation Implemented To Reduce Environmental Impact Table 6 shows different policies and regulations developed in Colombia and specially in Bogotá with the aim to foster the rational and e cient use of natural resources and solid waste management. The policies here collected allowed for a greater awareness of population in in caring for and protecting natural resources, e ciency use and rational generation of solid waste. Regulations are based on integral management with participatory approach that involving users, planners and decision makers at all levels. Programs have been de ned from the characterization of the demand for water (qualify and quantify) by different users and the analysis of pro le consumption to undertake actions aimed at changes that optimize its use, as well as the promotion of practices that favour the sustainability of ecosystems and the reduction of contamination.
Law 373/1997: The program on conservation and e cient use of water (a set of projects and actions for e cient use of water).
National policy for Integral Management of Water Resource / 2010: with a special principle (6): "fresh water is considered a scarce resource and, therefore, its use will be rational and will be based on saving and e cient use" Decree 3570/2011: Integral management of water resources, in order to promote the conservation and sustainable use of water.
Signing of agreements with productive sectors of interest to promote e cient use of water.
Tariffs differentiated by the water consumption guarantee the minimum subsistence and average consumption. A higher consumption, higher cost by cubic meter.

Energy
Policies and regulations on energy e ciency seek to ensure energy supply, based on the adoption of new technologies and good consumption habits, in order to optimize the management and use of available energy resources. Energy e ciency is a driver to increase national productivity and competitiveness, and is one of the main strategies for mitigating environmental impacts in the energy chain. Priority programs: i. Residential: Illumination, refrigeration, and building.
Transport: Technology reconversion, good practices, transport means.

Solid Waste
Integral solid waste management has developed different programs and actions as the formulation of guidelines for waste management in different industrial sectors, subsystem of information to follow and measure of waste generation, recycling process, tariffs and other strategies to decrease waste management generation and increase recycling and potential use of these materials.

Construction Waste
Resolution 2397/2011: the entities, generators, transporters, exploiters and sites for the temporary and nal disposal of construction waste go in the same line and that this allows appropriate management to be carried out in the city, resulting in a lower environmental impact. Comparison of 10 years and 15 years growth rates for the main input and outputs of Bogotá