Environmental Impact of End-of-Life Tires: Life Cycle Assessment Comparison of Three Scenarios from a Case Study in Valle Del Cauca, Colombia

Life Cycle Assessment methodology has been applied to estimate diverse environmental impacts of different usage alternatives for worn-out tires at the end of their useful life in a case study at the Department of Valle del Cauca, Colombia. Different real scenarios were compared, which allowed for the assessment of the best environmental option for the management of worn-out tires. A method developed in the Institute of Environmental Sciences at University of Leiden, better known as CML-2001, was used to calculate the environmental impact indicators. The results show that the incineration of whole tires in cement plants, and the activities of grinding and floor manufacturing from granulated rubber, exhibited the best indicators, especially in terms of environmental load avoidance through the recovery of materials. Finally, the categories of depletion of the ozone layer, acidification, global warming potential, depletion of abiotic resources, and photochemical ozone formation revealed that the strongest environmental impacts are associated with retreading and the production of multipart asphalt. This is due to the use of synthetic rubber in the former alternative, and of liquid asphalt, gravel, and diesel consumption in the latter.


Introduction
The rapidly growing populations, as well as the rapidly growing economies, bring consequently an increase in waste generation, which becomes a challenge for health authorities and the environment of any city. Inappropriate management of municipal solid waste (MSW) can lead to contamination of water sources due to the percolation of leakage on public health [1].
As a developing country, Colombia has been adopting international legislative guidelines regarding the recovery and final disposal of products in their post consumption stage. According to the Colombian Ministry of Environment and Sustainable Development, the sustainable production and consumption policies of the government have made significant progress in the development of normative systems for the recovery of batteries, hospital debris, technological equipment (e.g., mobile phones, computers), car batteries, and worn-out tires. The last ones were regulated through a recent resolution [2].
Recovered worn-out tires have been used for several industrial and commercial purposes, among which their applications in civil engineering and as fuel are particularly significant. The former use has been mainly associated with the production of granulated rubber for oven heating in the cement and paper industries [3].
(crushing), where it is removed by many steel wheels. In the third fine grinding process, the textile fiber is extracted along with the remaining amount of steel, obtaining ground rubber.
Some scenarios provide positive and negative outputs. The positive ones stand for net emissions to the environment whereas the negative ones imply a good benefit, since these recovered materials would actually avoid using virgin materials.
Therefore, in the present Colombian case study, we focus on the environmental impacts from a life cycle point of view. It was done using the LCA methodology, in order to provide consistent sustainability information and criteria that facilitate adequate decision making to reduce, reuse, and recycle towards sustainable development.

State of the Art
During the last decade, some studies in developed and developing countries have resorted to LCA in order to estimate the environmental impact of tires along their lifecycle, and of different final disposal technologies (EOL treatments).
For EOL treatments, Molino et al. [23] evaluated the energetic potential of an alternative to the recycling of tires in the field of environmental protection. Such a study concluded that the steam gasification of scrap tires was found as sustainable and cost effective alternative compared to disposal in landfill.
Regarding the LCA methodology, a study done by Van Beukering et al. [24] analyzed the complete truck tire lifecycle, and determined the impact associated with each of its stages. They concluded that tires inevitably produce a negative environmental impact, which is further increased by inadequate final disposal strategies. Additionally, they found a clear need for harmonizing worn-out tire management environmental policies in Eastern and Western Europe. Ferrão et al. [14] affirm that the adoption of environmental policies as an extended responsibility of manufacturers forces the development of an end-of-life product management system in charge of promoting the collection, reuse, and recycling of worn-out tires. Such a system requires the participation of governmental, private, and educational institutions, together with manufacturers, traders, recyclers, and retreading companies. Making use of Eco-indicator 99, Ferrão et al. [14] compared the environmental impact of EOL technologies, and found that the incineration of rubber material in cement factories produces remarkably lower impacts. Likewise, recycling and reuse also have positive impacts, since the benefits correspond to the fact that the recovered materials would actually replace raw materials, which is extensively promoted in Portugal.
Still on the line of analyzing EOL treatments through LCA, Corti and Lombardi [18] compared the environmental impact produced by conventional combustion in cement ovens to that resulting from substitution with worn-out tires. This process was also compared to other alternatives of material recovery, namely mechanical and cryogenic powdering. From an environmental standpoint, the best result corresponded to substitution in cement ovens. In turn, Li et al. [16] resorted to Eco-Indicator 99 to compare several alternatives such as grinding, de-vulcanization, pyrolysis, and oil extraction from inner tubes. Pyrolysis was found to be the most eco-efficient option in the long term. Oil production is the most harmful alternative due to its elevated energetic consumption and air pollution levels.
Fiksel et al. [3] developed a comparative LCA as a tool for the design of products with worn-out tires. Thus, they identified environmentally friendly EOL alternatives to be compared with the traditional manufacturing of a series of evaluated products. According to these authors, the main uses of worn-out tires as raw materials correspond to asphalt and artificial lawn production in civil engineering and to the manufacturing of molds for diverse products. Comparative studies of the reduction of CO 2 , NO X , SO X , and heavy-metal emissions have revealed that the most environmentally friendly alternative is synthetic lawn production, followed by manufacture of molds. The mentioned study found that asphalt production has a negative impact due to the many additives and special raw materials that need to be incorporated for the products to reach specified technical requirements.
Pieragostini et al. [17] have found that the use of LCA is currently growing on the part of public and private entities, who are obtaining several benefits from it. Among the latter, we can count environmental development strategies for the selection of industrial practices economic and cultural comparisons of processes, products, and activities across regions; identification of environmental improvement opportunities, and process and product optimization (design and innovation). The latter issue is capable of linking environmental impacts to operational and economic aspects of processes, to the point that in certain European countries, the environmental aspects have been integrated to corporate strategies that were traditionally centered on cost reduction. LCA has been used in many multi-purpose optimization models for the design of green supply networks. In designing worn-out tire recovery networks [15], which are applicable to hospital debris [20], all these authors have resorted to eco-indicators for the purpose of quantifying the environmental impacts generated by supply chain networks [20]. In both studies, this measurement was part of the objective function, which involved minimizing environmental impacts and optimizing economic objectives.
Uncertainty is present in any LCA study. This is the result of combined effects such as variability, error in measurement, poor estimation, non-representative, data, missing information, and assumptions of the model. Uncertainty in models can be classified according to three categories: (1) parameter uncertainty due to lack of precision in measurements data scarcity, (2) uncertainty of the scenario due to its construction, and (3) actual model uncertainty due to the mathematical methods employed for LCA calculation [25].
Numerous methods have been employed to conduct LCA uncertainty analyses. These usually evaluate the soundness of the results through mathematical techniques, e.g., sensibility analysis, which assesses the way the results are affected by changes in the adjusted inputs of the model [26]. The most common example is the analysis of scenarios, which resorts to probability distributions and random sampling, usually applying Monte Carlo simulations for intervals of analysis [27]. Another commonly used technique is the theory of Fuzzy Sets, in which the ranges of the determining variables are represented by probability distributions and/or fuzzy functions according to the degree of certainty of each variable or parameter [28].

Materials and Methods
LCA was applied according to the international guideline ISO 14040:2006 [19]. We divided the study into four parts: (1) definition of the goal and scope, (2) inventory analysis, (3) environmental impact evaluation, and (4) interpretation of results.

Goal and Scope Definition
In the context of LCA, scope and objective allow for the definition of the purpose of the study, the limits of the system to be evaluated, and its functional unit. The aim of this study was to estimate the environmental impact of worn out tires at the end of their useful life in the Department of Valle del Cauca. It is an area located in the western part of Colombia, facing the Pacific Ocean. It is considered one of the most important departments in the country economy due to the privileged location in the Pacific Ocean. Besides, the department has great social-economical influence mainly centered on agriculture, e.g., sugar cane, coffee, cotton, soy, and sorghum crops.
The relevance of this study is supported by national and local data, which shows that in 2014, the number of cars in the country increased by 10.5% with respect to the previous year. The National Industry Association reported that in 2015, Colombia had approximately 48 million inhabitants that made use of 4.2 million vehicles. That is, nine vehicles per 100 inhabitants. In 2012, the production at the national level was 0.74 times larger than in year 2010. In 2012, the country consumed 4.5 million tires, each of which lasted 18 months on average. Regional data from the same year shows that in the Department of Valle del Cauca, there were 32,900 tons of worn-out tires stored in uncontrolled spaces such as houses, public spaces, and open dumps [29]. Figure 1 illustrates the system and its scope, detailing the activities that have been included, classified as primary and secondary strategies. The primary strategy includes retreading, grinding, and incineration in cement factories, all featured by the use of whole tires. In turn, secondary strategies refer to the use of grinded components of the worn out product such as rubber, steel, and textile fiber. Particularly important are the production of wire, floors, and modified asphalt.
Energies 2017, 10, 2117 5 of 13 strategies refer to the use of grinded components of the worn out product such as rubber, steel, and textile fiber. Particularly important are the production of wire, floors, and modified asphalt. An important activity that was also taken into consideration was transport. This analysis focused on the flow of the tires, from their collection in different municipalities, through their transport to collection centers, to their shipment to recycled product manufacturing plants in different areas of the department. The municipalities in question were classified as follows: 18 municipalities in the North-Eastern zone, 13 in the center of the department, 10 in the South, and one in the Pacific zone.
One (1) ton of worn out tires was defined as the functional unit (FU) for the processes of gathering, selection, separation, grinding, use in cement factories, and retreading (primary strategies). To the outputs from the secondary strategies as flows that are produced from 1 ton of worn out tire were assigned as follows.
The production of floors and modified asphalt (secondary strategies) were assigned one ton of ground rubber, while the production of textile fiber had 0.2 tons of the same material. Finally, 0.229 tons of recovered steel were assigned for wire production.
Transportation at all stages of the chain is done in 35-ton load capacity vehicles. Electric power generation corresponds to an energetic mix of 78% hydro-electrical plants, 7% carbon, 12% gas, and 3% other sources, adapted from [31]. Transport includes the distance per kilometer (ton-km) and includes emissions produced by burning fossil fuels [32].

Inventory Analysis
This stage of the analysis focuses on the flows of the studied processes. Here, we describe the inputs of materials and energy, and the output emissions within the limits of the system. Each scenario was adapted to the Colombian case study. Therefore, data description (input and output) and collection of each process per FU is described as follows.
2.2.1. Recollection An important activity that was also taken into consideration was transport. This analysis focused on the flow of the tires, from their collection in different municipalities, through their transport to collection centers, to their shipment to recycled product manufacturing plants in different areas of the department. The municipalities in question were classified as follows: 18 municipalities in the North-Eastern zone, 13 in the center of the department, 10 in the South, and one in the Pacific zone.
One (1) ton of worn out tires was defined as the functional unit (FU) for the processes of gathering, selection, separation, grinding, use in cement factories, and retreading (primary strategies). To the outputs from the secondary strategies as flows that are produced from 1 ton of worn out tire were assigned as follows.
The production of floors and modified asphalt (secondary strategies) were assigned one ton of ground rubber, while the production of textile fiber had 0.2 tons of the same material. Finally, 0.229 tons of recovered steel were assigned for wire production.
Transportation at all stages of the chain is done in 35-ton load capacity vehicles. Electric power generation corresponds to an energetic mix of 78% hydro-electrical plants, 7% carbon, 12% gas, and 3% other sources, adapted from [31]. Transport includes the distance per kilometer (ton-km) and includes emissions produced by burning fossil fuels [32].

Inventory Analysis
This stage of the analysis focuses on the flows of the studied processes. Here, we describe the inputs of materials and energy, and the output emissions within the limits of the system. Each scenario was adapted to the Colombian case study. Therefore, data description (input and output) and collection of each process per FU is described as follows.

Recollection
This activity starts from the collection of used tires in the different stations in each municipality and their transportation to the collection centers. The process takes into account the transport per kilometer (ton-km) and includes emissions produced by burning fossil fuels. Adapted from [16,33], see Table 1.

Retreading
This includes the transport from the store selection until retreading. In the precured tread, which was to be assembled in the process of retreading, a tire casing have been previously prepared. Adapted from [3,30]. See Table 2.

Incineration
This includes the transport of worn out tires from the cellars to the cement plant. These are downloaded and entered manually or via scroll ovens. Incineration thereof occurs and power is recovered for use for cement production process. Adapted from [18,30], see Table 3.

Production of Asphalt
The crumb rubber is transported to a plant of modified asphalt from the crushing plant. Unlike the traditional production process of asphalt that combines liquid asphalt to gravel, rubber granules are added to liquid asphalt in a mixer, in a separate process, before being mixed with the gravel in the tumble dryer, and the filtration chamber to obtain modified asphalt. Adapted from [3]. See Table 4.

Production of Floors
The crumb rubber is transported from crushing plant to a factory manufacturing synthetic floor. In the pressing process, it is mixed with clay and sand to achieve nonslip properties of the product. Excess water is removed in the process by drying. Tiles and/or recycled rubber mats are obtained to cover surfaces. Adapted from [30,34]. See Table 5. The recovered steel crushing plants are transported to the wire manufacturing plant. This is melted with other materials that help strengthen their metallic properties, making bullion square. After the hot rolling process, rolls of wire rod are cold drawn to reduce circulation, and the desired size of the wire section is obtained. Furthermore, this includes the selection and separation, which corresponds to the transportation from the collection centers to the store. This includes also processes such as cleaning and separation that are involved in the production of floors, asphalt, and wire. Adapted from [31,35]. See Table 6. The Ecoinvent V2.01 database was used to have the inventory data of the processes involved in the study. The software LCA-Data Manager ® has been used to validate, create, and modify the diverse scenarios under study. Material balances in the inventory have also been carried out [36]. The equation modelling used by LCA-Data Manager ® was done according to guidelines for conducting an LCA study step-by-step on the ISO standard according to the following Equation, adapted from [37]:

Environmental Impact Evaluation
The next step in the LCA methodology is the Life Cycle Impact Assessment (LCIA). The LCIA evaluates potential environmental impacts, and estimates the resources used in the modelled system. It is determined which extractions and emissions contribute to the diverse impact categories. The current research based the impact assessment upon the CML 2 method (2001) developed by the Institute of Environmental Sciences at University of Leiden [38]. The following impacts were considered: Acidification (AP, kg SO 2 equivalent.), Eutrophication (EP, kg PO 4 equivalent.), Global warming (GWP, kg CO 2 equivalent.), Photochemical ozone formation (PO, kg ethylene equivalent.), Depletion of abiotic resources (DAR, kg Antimony equivalent.), Depletion of the ozone layer (OD, kg CFC-11 equivalent.), and Human toxicity (HT, kg 1.4-DCB equivalent.).

Results
The results of the current LCA study are presented in Table 7. It shows the indicators corresponding to each of the disposal strategies at the end of the lifecycle of the tires. As it can be observed, there are positive and negative values. The first ones mean net emissions to the environment. The second ones become a benefit corresponding to the fact that these recovered materials would actually avoid using virgin raw materials. In terms of environmental impact on the global warming process, the alternatives incineration, mechanical grinding, and the manufacture of floors and wire generate benefits in terms of kg CO 2 equivalent. This is due to the recovery of tire components such as synthetic rubber and steel, whose environmental load is thus prevented. In turn, the highest impacts in the script actually correspond to retreading and modified asphalt production. The impact produced by the retreading process is associated with the use of synthetic rubber in the manufacturing of the tread band, which makes this the alternative with the strongest impact in the network. When it comes to the production of modified asphalt, the consumption of liquid asphalt, gravel, and diesel has a negative impact on this indicator. Yet, this impact is counterbalanced by the recovery of synthetic rubber.
As in the case of the previous indicator, the alternatives grinding, incineration, and floor and wire manufacturing bring about environmental benefits in terms of acidification. Contrarily, the process of retreading and the production of modified asphalt are the ones that produce the strongest environmental impacts. The latter process, in fact, is the one that generates highest amount of kg SO 2 equivalents, mostly due to diesel consumption.
In terms of eutrophication and acidification, which are measured through the amount of phosphate (kg PO 4 ) generated by each process, incineration and floor manufacturing produce the lowest of all environmental loads. In contrast, retreading has a negative impact due to the use of synthetic rubber.
Measured through the production of ethylene-equivalents (kg), photochemical ozone formation reveals that the environmental impact of the processes of gathering and selection are almost insignificant (about 10 −4 ). Except for the process of retreading, all the studied alternatives generate low or even positive environmental impacts in this category, due to the recovery of synthetic rubber, textile fiber, and steel. Therefore, incineration, and floor manufacturing exhibit the most positive impacts.
With respect to the depletion of the ozone layer, the first stages of the chain, namely gathering and selection produce insignificantly negative environmental impacts (about 10 −6 and 10 −7 respectively). The alternatives incineration and grinding, as well as the manufacturing of floors and wire, prevent environmental impacts in this category, although certainly in small amounts (around 10 −4 for the first three alternatives, and 10 −7 for wire manufacturing). Retreading produces a low negative environmental impact (about 10 −4 ), which is associated with the use of synthetic rubber during the process. Finally, a negative impact is due to the production of modified asphalt, mainly resulting from the use of diesel as its source of energy.
As to human toxicity, those alternatives that imply the recovery of steel, as is the case of retreading, incineration, grinding, and wire production, determines positive impacts that result from avoiding the environmental loads associated with this material. Besides, floor manufacturing is beneficial through the recovery of synthetic rubber. For its part, asphalt production produces negative impacts due to the use of gravel, liquid asphalt, and diesel in the process.
Finally, abiotic resource depletion, as measured in kg antimony equivalents, reveals that incineration, grinding, and floor manufacturing are positive strategies due to the recovery of synthetic rubber. Contrarily, retreading consumes this component to manufacture the tread band, and for this reason, it produces a negative impact. The strongest negative impact results in the production of modified asphalt, due to the use of liquid asphalt and diesel as its energy source.

Discussion
An adequate management of worn-out tires implies the development of processes of gathering, selection, and separation. Gathering was observed to generate the lowest impact within the network under study. Transportation from the different gathering centers in the municipalities to the major stockpiling centers determines the strongest impacts within the collection process. Both activities produce significant amounts of CO 2 kg equivalents (global warming). The impacts associated with the selection and separation processes are relatively low when compared to the alternatives of use. However, global warming and human toxicity indicate that they are, respectively, 20 and 14 times higher than those produced by the gathering process are. The total impact of this process is also incremented by transportation from the gathering centers to the warehouses where selection takes place.
Among the primary strategies, incineration in cement factories and grinding show negative values for all the studied impact categories. This means they are beneficial activities, as they avoid environmental loads and allow for the recovery of materials for recycling. In other words, the avoided impact (recovery of materials and energy saving) is higher than the one produced by the activity itself. The global warming values for incineration are significantly lower (almost half as much) than those exhibited by grinding.
Human toxicity shows similar values for both alternatives. In turn, the process of retreading has a negative environmental impact across all the evaluated impact categories, except for human toxicity, wherein this activity records negative values (positive impacts). This is caused by the use of both natural and synthetic rubber for the manufacturing of the tread band.
Regarding the usage alternatives for grinding sub products, floor manufacturing is the one that most prevents environmental impacts in all the categories. The global warming record of this activity is the most negative value of the entire network, which reveals this activity as the most environmentally friendly alternative in this category. For its part, wire production also records important environmental benefits in almost all indicators, except for eutrophication and abiotic resource depletion. The latter result is due to the consumption of carbon as energy source during the process. Contrarily, the production of modified asphalt produces negative environmental impacts in all indicators, resulting from the traditional use of diesel, liquid asphalt, and gravel. The environmental impact generated by this alternative is not significantly different from that generated by traditional methods of asphaltic mix production.
These results coincide with those of [3], who have ascertained that the use of worn-out tires for the production of artificial lawn (a very similar process to floor manufacturing) and for their incineration in Cement Factories hold the largest environmental impact reduction potentials. Similarly, reference [18] have observed that the replacement of traditional fuels with worn-out tires in the production of cement produces interesting environmental results. In accordance with these data, the most environmentally friendly alternatives are incineration in cement factories, grinding, and floor manufacturing from granulated rubber.
While the above-mentioned studies have applied LCA analyses in Europe and China, this mode of assessment is still an important need in developing countries, especially in Latin America. This is particularly true, not only in face of the need to evaluate the environmental adequacy of specific management systems for post consumption residues, but of the great potential observed in the recycling industry in this region as well. This industry, indeed, might become a powerful source of important opportunities in face of the challenges of the future, which are specifically related to the availability of renewable and nonrenewable raw materials [39,40].

Conclusions
The alternatives floor manufacturing, incineration in cement factories, and mechanical grinding have positive environmental impacts. In turn, gathering, selection, and separation activities present negative environmental impacts of low magnitude. Finally, the retreading and manufacturing of modified asphalt generates the strongest negative environmental impacts.
The environmental impact indicators for the gathering, selection, and separation activities show positive values; therefore, they represent a negative environmental impact, mostly associated with elevated fossil fuel consumption levels. Still, these values are considered low when compared to the other usage alternatives under consideration. The global warming and human toxicity indicators are approximately 20 and 14 times smaller than those of the other activities analyzed in the studied logistic network.
The longer the traveled distance between facilities intended for gathering, separation, and usage alternatives, the stronger is the environmental impact of the transport activities. This is particularly true with regard to global warming, due to the fossil fuel consumption involved in transportation.
Wire manufacturing generates positive and negative environmental impacts. Using worn-out tires for the recovery of steel and, later, wire manufacture generates positive environmental impacts associated with global warming, acidification, photochemical oxidation, and human toxicity. Yet, negative environmental impacts result from eutrophication, the destruction of the ozone layer, and the depletion of abiotic resources, mainly due to the use of fossil fuels such as gas or carbon for the melting of steel at elevated temperatures.