In this section, the trends and analysis of the flow of materials in different periods of time in the city of Bogotá are presented, taking into account the main inputs and outputs at a general level and as components where the dynamics of growth and reduction are observed according to different pressures or actions on the environment or the greater awareness of the inhabitants of the city. Following this analysis, the 10-year growth of Bogotá is compared with other large urban agglomerations, pointing out differences and analogies.
4.1 Temporal variation of the flows in the period 2001–2017
During the study period, the flows of materials and energy in the city of Bogotá have been increasing, especially due to population growth from 6302880 in 2001 to 8064000 in 2017. This represents an annual increase of approximately 100000 inhabitants.
Figure 1 shows a comparison of the flow balance of materials and energy per capita for the period 2001-2017, and depicts the increase in certain flows, especially energy, cement, CO2 emissions and construction waste, reflecting the growth of the city. On the other hand, from the design and application of public policy instruments at the urban level, e.g. dissemination and awareness campaigns [BIB], the reduction of flows 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 flows, figure 1 also shows that input and output flows of energy and water are in equilibrium, indicating that the distribution and collection infrastructures are managed in an efficient 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 flows are divided in three categories:
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Significant increase (more than 50%): GDP, Debris and cement, both in the 10 year and 15 year view.
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Stability (between 0 and 50%): Natural gas, total energy, CO2 emissions, solid waste, and electricity consumption.
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Reduction (less than 0%): Water consumption, wastewater, Particle PMx.
Considering the first category, it is interesting to point out that GDP experienced a significative 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 significantly reduced. The only flows 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 significative growth in 15 years, reaching an increase of 150%.
Considering the population as the main driver for urban expansion and urban flows, we show in Fig. 3 the elasticity of the metabolic flows 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 reflect the important increase observed in Fig. 2. With regards to the other flows, 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 specific 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 flows, ensuring a comparison for all cities, in both developed and developing countries, to analyse spatial and temporal issues in urban flows, 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-efficiencies, 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 flows, highlighting the main actions implemented by the municipality to reduce the pressure of the city on the environment.
4.2 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 61014567 m3, while in 2017 the consumption was 57811956 m3, indicating a decrease of 3202611 m3 during these years. A similar situation is observed with wastewater; in 2001, 65642718 m3 was produced, and in 2017 its production was 53988449 m3.
Figure 4 shows the trends in water flow and discharge per capita for Bogotá in different years of the study period, where there is a tendency to a more efficient use of this resource, which generates reduced discharge with consequent benefits for the environment.
These results are products of various public awareness campaigns to generate less waste and the adoption of saving technologies that have resulted in a rational and efficient use of water. These results concur with OECD (2015, 2018) and Torjada et al. (2019) in the context of Spain, where according to the specific context, it is fundamental to formulate and apply a combination of measures and action plans that integrate pricing and non-pricing measures and educational campaigns to achieve the more efficient management of water resources at the household and industrial levels. Moreover, it is important to constantly evaluate the results and impacts of implemented measures to guarantee applicability and effectiveness.
In addition to the implementation of a communication campaign oriented towards the voluntary saving of water, in critical situations of drought due to climatic variability or maintenance of treatment plants, user associations state that in recent years, the gradual dismantling of subsidies and the increase in the costs due to the greater consumption have limited the payment capacity of the low socio-economic strata 1, 2, 3 (CRA, 2016), which has influenced a lower consumption of water and, therefore, less dumping and emissions.
4.3 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 efficient construction design, socio-demographic and physical dwelling characteristics, network planning and strategic instruments, regulations, socio-technical structures, and energy efficiency 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 CO2 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.
Table 3
Air pollutant emissions in Bogotá 2001–2017
Air pollutant emissions | 2001 | 2005 | 2010 | 2015 | 2017 |
CO2 emissions per capita Ton/inhab | 64.45 | 65.51 | 68.33 | 61.21 | 67.11 |
Annual average sulfur dioxide µg/m3 | ND | 0.013 | 0.020 | 0.019 | 0.013 |
Annual average nitrogen dioxide µg/m3 | ND | 0.026 | 0.038 | 0.035 | 0.024 |
Particulate matter PM10 µg /m3 | 65 | 74 | 59 | 44 | 41 |
Note: NA not available |
Source: http://oab.ambientebogota.gov.co/es/listado-indicadores-por-recurso-natural
These results are consistent with the statement that improving urban air quality is one of the most demanding tasks facing policy makers worldwide, where it is important to determine the costs and benefits of measures that could include reductions in inefficient fossil fuel combustion and improvements in energy efficiency, which should reduce air pollutant emissions (Hewitt et al., 2019, WHO, 2017).
4.4 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 difficult to recover and/or deal with the consequent problems of landfill 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 modification of those existing in urban densification 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 financial subsidy on waste recycling, and increasing waste landfilling charges, which is an opportunity for extracting economic and environmental benefits from waste as reductions in CO2 emissions, energy use, natural resources and illegal landfills (Islam, 2019, Hao et al., 2019).
4.5 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, definition of waste management scenarios, identification 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 efficiency, sustainable and socially supported.
These results show the importance of knowing the dynamics of the different inflows 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 efficiency 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 flows 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 flows elasticity with respect to population. Indeed, population is selected as guiding principle for the comparison of the metabolic flows.
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 flows 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 significant increase. These figures are reflected 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 findings are also valid for the total energy consumption. Table 4 shows values in the interval 18–55%, showing Bogotá in the lower part of the interval.
Under the economic point of view GDP shows a consistent variability among the observed cities. GDP of Bogotá grew in line with Lima and Rio de Janeiro, while the elasticity values are significantly different: Bogotá shows 5, while the other cities (excluding Lima) show higher values between 9 and 21, indicating that the population growth in Bogotá triggered in a reduced way the economic growth of the city.
Table 4
10 year (2001–2011) growth of selected Latin American Cities
| GDP | Population | Electricity | Total Energy | Water |
Mexico City | 65,8 | 3,0 | 3,6 | 32,6 | NA |
Sao Paulo | 179,5 | 10,9 | 49,0 | 42,2 | 23,9 |
Rio de Janeiro | 98,0 | 9,6 | 103,6 | 51,5 | 41,5 |
Buenos Aires | 109,3 | 11,7 | 39,2 | 18,2 | NA |
Lima | 91,99 | 17,64 | 135,14 | 55,11 | 32,83 |
Bogota | 92,1 | 18,5 | 8,4 | 25,6 | 14,6 |
Table 5
10 year (2001–2011) population elasticity of selected Latin American Cities
| GDP | Electricity | Total Energy | Water |
Mexico City | 21,8 | 1,2 | 10,8 | NA |
Sao Paulo | 16,5 | 4,5 | 3,9 | 2,2 |
Rio de Janeiro | 10,2 | 10,8 | 5,4 | 4,3 |
Buenos Aires | 9,3 | 3,3 | 1,6 | NA |
Lima | 5,2 | 7,7 | 3,1 | 1,9 |
Bogota | 5,0 | 0,5 | 1,4 | 0,8 |