1. Introduction
Automatic waste collection (AWC) technology plays a big role in the waste management systems shaping and maintaining smart and sustainable cities, leading the way to create smarter cities, improve quality of life today and secure a greener planet for future generations. The system makes urban environments cleaner and healthier, and reducing waste helps them become greener and more sustainable [
1].
AWC is a modern and efficient waste collection system that improves the urban image, optimizes the selective collection at source, reduces the environmental impact, decreases the cost per ton collected compared to conventional systems, and offers a smart service 24 h a day, 365 days a year, and achieves citizen satisfaction. This solution has more impact in densely populated areas, especially urban areas and cities [
2,
3].
Today pneumatic waste collection systems are increasing their popularity, even if they have been around for decades, mostly due to the increasing environmental awareness of cities and their managers and policymakers. The benefits of these systems for the city are reducing personnel costs, truck and fuel costs, reducing CO2 emissions, traffic, and, of course, achieving a more pleasant and safe environment for people living in the area where the system is in use.
Benefits of the pneumatic system for the European and global objectives in terms of recycling levels, circular economy and the sustainable development goals should also be highlighted [
4]. Other benefits of this waste collection system are the use of a pipeline instead of manual or truck operation, creating high-level sanitary conditions, realizing completely closed garbage collection and transportation, and eliminating cross-pollution [
5].
An LCA life cycle assessment is considered an effective methodology to assess any product or systems environmental impact. This approach allows quantifying and evaluating the environmental impact generated by a product during the entire production and operation stage, and even by the activity that takes place during the entire life cycle of the product, from the extraction of the material premium to the final disposal [
6]. Moreover, applying a life cycle perspective allows comparative assessments of processes and identification of benefits [
7]. LCA studies also allow to carry out sensitivity analyses when technological pathways are involved in helping in the identification of key points where future optimization and innovation efforts must focus [
8].
Different authors have performed different LCA in different locations with different systems, as summarized below. Punkkinen et al. [
9] evaluated the life cycle inventory (LCI) in terms of atmospheric emissions of a hypothetical stationary pneumatic waste collection system within an existing, densely populated city infrastructure in Helsinki, Finland, compared with a conventional door-to-door alternative. According to these authors, a pneumatic waste system would generate more air emissions due to its high electricity consumption and the manufacture of system components. On the other hand, at the local level, in the waste collection area, emissions would decrease as collection traffic would be reduced. If the case area is increased, the total NOx emissions would be 24% lower, whereas SO
2 emissions would be 17 times higher.
Uson et al. [
10] carried out a comparative LCA between a pneumatic waste collection system with a truck collection system in a neighborhood of Zaragoza, Spain. Results showed that, when operating at loads close to 100%, the pneumatic collection system had better environmental performance compared to the conventional system.
Perez et al. [
11] compared the environmental performance of different municipal waste collection and transport systems using the LCA methodology. They concluded that the environmental impact from pneumatic systems is higher than from conventional systems. Furthermore, within the conventional systems, underground installations have a higher impact than surface containerization systems.
Chàfer et al. [
2] studied the influence of the electricity consumption when evaluating six different waste collection systems (trucks—electric, gas, diesel, diesel–electric, gas–electric—and stationary pneumatic waste collection) in terms of LCA in the city of Barcelona, Spain. Their results showed that the energy source might trigger variations up to 80%.
This study presents, for the first time, a comparative life cycle analysis of three different urban waste collection systems: a traditional pneumatic collection with a conventional central, a pneumatic system with an AutoWaste Collect Compact System, and a truck collection. This LCA includes the collection of four waste fractions (organic, rest, paper and cardboard, and packaging) in an AWC Airport. This study is based on the international standards ISO 14040 and 14044. A cradle-to-grave life cycle analysis has been carried out, which includes the production, use and end of life of the systems. A lifetime of 30 years has been considered, although the results can be extrapolated over time.
The environmental impact was calculated following the life cycle analysis methodology with the ReCiPe indicators, giving impact points by categories of damage, and IPCC 2013 GWP100a, giving equivalent kilograms of CO2 emitted into the atmosphere. In addition to comparing different systems and scenarios, the results also provide valuable information to know which equipment or element has a more significant impact on the environment and, thus, to redesign, modify, and optimize the system towards a more sustainable system.
3. Results and Discussion
Considering the infrastructure of each system and what they consume during its 30 years of operation, together with the hypotheses detailed above, the results of the LCA of three urban waste collection systems were analyzed: two of them are pneumatic collection systems, one with a conventional collection center and the other with an AutoWasteCollect compact collection system, and the third is the traditional collection system with trucks.
The results are presented grouped by indicators, ReCiPe (impact points) and the IPCC2013 GWP (kg CO2 equivalent), in addition to carrying out the study using two different energy mixes: the 2014 energy mix, chosen from the Ecoinvent database, and an energy mix that includes the incorporation of renewable energies in the production of electricity in our country (20% hydroelectric production, 30% solar production, and 50% wind production), which we will call hypothetical since it is not found in the Ecoinvent database.
3.1. Results Obtained Using the ReCiPe Indicator
Figure 5 shows that the traditional collection system with trucks is the collection system that has the fewest impact points, with 484 points per ton of waste per year. On the other hand, of the pneumatic collection systems, the compact AutoWaste system has fewer impact points than the conventional collection center (629 and 748 impact points, respectively). As far as pneumatic collection systems are concerned, the compact AutoWaste system unit reduces the environmental impact by 19% compared to a system with a conventional collection unit.
Regarding the contribution of each of the life cycle phases to the environmental impact, the contribution is different if the system is a pneumatic collection or if it is a conventional system with trucks. In the case of a traditional collection with trucks, the construction phase is the one that provides the highest contribution to the environmental impact, while in the pneumatic collection, the environmental impact is mainly due to the operational phase.
For the collection with trucks, the construction phase considers both the construction of the truck station and the construction of all trucks, hence a greater impact than that generated by the operational phase that only includes the few kilometers between the airport and the final destination of the waste.
In the case of pneumatic systems, the construction phase, with low impact, corresponds to the construction of the collection station and the pipes and manholes, which have carbon steel as their main material. In the operation phase, the greatest contribution to the impact is due to the operation of the fans. Of the two pneumatic collection systems, the operational phase with the lowest environmental impact is that of the AutoWaste compact system, with 454 impact points, compared to 566 impact points for the conventional pneumatic central. Therefore, the system with a compact central allows a 25% reduction in the environmental impacts of the operational phase compared to the conventional pneumatic central.
For the traditional collection system with trucks, the environmental impacts in the construction phase are three times higher than those in the operation phase. In the case of the compact pneumatic system, the environmental impacts in the operation phase are 2.6 times higher than in the construction phase. In contrast, the environmental impact of the pneumatic system with a conventional collection center in its operation phase is three times higher than in its construction phase.
The impact on the ecosystem of the three urban waste collection systems is shown in
Figure 6. In the case of the two pneumatic systems, this impact is due to the electrical consumption of the fans that drive the waste through the pipes and the diesel consumption of the truck that collects the container from the power plant to the waste treatment plant. In the case of the traditional truck collection system, it is due to the diesel consumption of the trucks that manage the collected waste.
Results are different when the analysis is carried out considering an energy mix that includes renewable energies in the production of electricity (
Figure 7). The collection station that has fewer impact points is the compact AutoWaste system with 293 impact points per ton of waste per year, although slightly lower than the conventional type collection station that has 303 impact points. The traditional collection system with trucks is the system that generates the most environmental impact, with 438 impact points.
Therefore, pneumatic systems reduce 45% of the environmental impact generated by traditional truck collection. Regarding pneumatic systems, the compact AutoWaste plant reduces the environmental impact by 3.5% compared to the system with a conventional collection plant.
Regarding the contribution of each of the life cycle phases to the environmental impact, in the three systems, the construction phase is the one that provides the highest contribution to the environmental impact. In the case of the two pneumatic systems, the contribution of the construction phase is 1.5% higher than the impacts generated during the entire operation phase. However, in the case of the traditional truck collection system, the construction phase is 5% higher than the impacts generated during the operational phase. In the comparison between systems, the same conclusions can be withdrawn as when the other energy mix was considered (
Figure 8).
3.2. IPCC 2013 GWP Indicator Results
The results of the life cycle analysis of the different urban waste collection systems with the environmental impact assessment method IPCC2013 100a show, according to
Figure 9a, that the traditional truck collection system is the one with the lowest impact, with 1293 kg CO
2 equiv./ton emitted to the atmosphere. In the case of the pneumatic systems, the compact AutoWaste central collection system, with 2322 kg CO
2 equiv./ton emitted to the atmosphere is the one with the lowest impact compared to the 2913 kg CO
2 equiv./ton emitted in the case of a conventional pneumatic collection. This is the quantitative value expected at 100 years, while at 20 years, as shown in
Figure 9b, the traditional collection system with trucks emits 1446 kg CO
2 equiv./ton, followed by the compact AutoWaste pneumatic collection system, with 2584 kg and, finally, the conventional pneumatic system emits 3237 kg CO
2 equiv./ton.
In both perspectives, the same trend of environmental impact is fulfilled. The system with the least impact is that of the traditional collection with trucks and of the pneumatic systems. The system with a compact central allows a 25% reduction in environmental impacts compared to the conventional pneumatic central.
The results of the life cycle analysis of the different urban waste collection systems with the IPCC2013 20a environmental impact assessment method show, according to
Figure 10b, that the system with the lowest impact is the central wastewater station system. The AutoWaste compact type collection with 613 kg CO
2 equiv./ton emitted to the atmosphere, followed by the other pneumatic collection system, the conventional one, with 620 kg CO
2 equiv./ton emitted to the atmosphere. Therefore, the system that emits the most CO
2 kg is the traditional system with trucks, with 1174 kg CO
2 equiv./ton. This is the quantitative value that is expected in 20 years, while at 100 years, it is 533 kg CO
2 equiv./ton emitted into the atmosphere for the AutoWaste compact pneumatic system, 537 kg CO
2 equiv./ton emitted into the atmosphere for the conventional pneumatic system, and the one that generates the most emissions is the traditional system with trucks with 1046 kg CO
2 equiv./ton emitted into the atmosphere. In both perspectives, the same trend of environmental impact is fulfilled. The system with the least impact is the pneumatic system with an AutoWaste compact control unit.
Looking at the results of the 100-year perspective (
Figure 10a), which is the most recommended for studying the impact on global warming of a system, the two pneumatic systems considered in this study allow a 95% reduction in the environmental impact generated compared to the traditional truck system. In the case of the two pneumatic systems, the AutoWaste compact collection centre system reduces the impact by almost 1% compared to the conventional collection centre.
3.3. Environmental Payback
The environmental payback in time provides very useful information since it allows obtaining the impact of each system from year to year. At time zero, the impact corresponds to the value of the construction phase of each system and year after year, and it is linearly increased by the impact of the operation phase. From there, information is derived, on the one hand, of the total impact at the desired time and, on the other hand, when the systems will have similar impacts. It is worth mentioning that the results shown correspond to an operating time set at a lifetime of 30 years.
According to the 2014 energy mix, the life cycle analysis based on the ReCiPe indicator (
Figure 11a) and IPCC2013 (
Figure 11b) show that for the traditional truck collection system, although initially, the impacts are higher than pneumatic systems, over the years of operation, a growing trend of impacts is generated, but with a much lower slope than in the case of pneumatic systems. In the case of pneumatic systems, the AutoWaste system presents a smaller slope than the conventional pneumatic plant.
According to the hypothetical energy mix that includes renewable energies, the life cycle analysis based on the ReCiPe indicator (
Figure 11c) and IPCC2013 (
Figure 11d) shows that the compact-type plant and the conventional plant show an increasing trend of impacts, but the compact type system has a lower slope than the conventional type plant. In the case of the traditional collection system with trucks, at the beginning, when only the construction phase is considered, its value is considerably higher than in the case of pneumatic systems. Over the years, when the operation phase is present, the trend increases, although its slope is slightly lower than in the case of pneumatic systems. Even so, after 100 years of analysis, the environmental impact continues to be higher than in the case of pneumatic systems.
Although the environmental payback period shows the same trend when evaluated based on ReCiPe (
Figure 11c) and based on IPCC 2013 (
Figure 11d), a bigger difference is observed between the traditional system with trucks and pneumatic systems when the environmental depreciation is based on the IPCC 2013. For this case, it is observed that, at 100 years, the environmental impacts generated by the traditional system with trucks would be 17% higher than that generated by pneumatic systems.
4. Conclusions
This LCA study shows that the energy mix used is decisive in the results obtained. For an LCA obtained using the 2014 energy mix in Spain that is included in the Ecoinvent database, the traditional collection system with trucks is the system that generates the lowest environmental impact. In relation to the two pneumatic collection systems analyzed, the pneumatic collection system with the AutoWaste compact central unit, the annual flow of greenhouse gases into the atmosphere (kilograms of carbon dioxide equivalent for 30 years and per ton) can be reduced up to 25% compared to a pneumatic collection system with a conventional central.
For an LCA obtained using an energy mix that allows a better representation of today’s reality and its trend, which includes the use of renewable energies in the production of electricity (i.e., 20% hydroelectric production, 30% solar production and 50% wind energy production), the AutoWaste compact central pneumatic collection system is the system that generates the lowest environmental impact, and that of trucks is the one that generates the highest impact. Among the two pneumatic collection systems analyzed, the compact AutoWaste plant allows the annual flow of greenhouse gases to be reduced by 1% compared to a pneumatic collection system with a conventional plant.
These evaluated and quantified values show that of the two analyzed pneumatic systems, the gases emitted by the AutoWaste compact central pneumatic collection system are clearly reduced compared to a conventional pneumatic collection system with a conventional central unit for a lifetime of 30 years. Therefore, it is a system that is recommended to be implemented in cities where optimal air quality and mitigation of climate change are key objectives.