## 5 Environmental Aspects  
[PERSON] et al. [34] quantified and compared, by means of a life cycle assessment (LCA), the potential environmental impacts of three selective collection systems (i.e., mobile pneumatic, multi-container, and door-to-door) modelled on densely populated urban areas. The results show, with a sensitivity analysis, that the mobile pneumatic system at an inter-city distance of 20 km shows the greatest environmental impacts and the greatest energy demand.  
A second LCA study on a hypothetical AWCS, located in Helsinki (Finland) was carried out by [PERSON] et al. [35]. Here, a conventional door-to-door waste collection system was compared with its hypothetical pneumatic alternative, also analysing if the number of waste fractions has an impact on the results. The results show that the AWCS has an overall higher impact due to the electricity consumed, but local CO2 emissions would decrease in the waste collection areas due to the lower truck traffic.  
[PERSON] et al. [36] carried out a LCA study where an AWCS was compared to a traditional truck collection system in Valdespartera (Zaragoza, Spain). The results show that to achieve the best performance, the AWCS needs to operate at loads close to 100%, otherwise, its impacts are higher than the traditional system.  
[PERSON] et al. [37] focused on the organic fraction of waste and carried out a LCA comparing door-to-door collection systems with pneumatic ones, also including the recycling process of such organic waste. The results show that despite the fact electricity production and consumption have a significant influence on the results, the energy savings from the recycling of the organic fraction are higher than the energy requirements.  
To evaluate the environmental impact of a real AWCS, [PERSON] et al. [38] evaluated different waste collection systems (trucks--electric, gas, diesel, diesel-electric, and gas-electric--and stationary pneumatic waste collection) using the LCA methodology. Given the high impact of the electricity used in AWCSs (as in the case above [35]), a sensitivity analysis of five energy sources (Spanish energy mix 2008, hydropower, photovoltaic, wind, and a renewable energy mix) was also carried out. This study was based on the system installed in 22@ (Barcelona, Spain) listed in Table 1. The results show that the energy source has a big impact on the results of the LCA, with variations of up to 80%. The environmental impact of each collection system depends strongly on the source of the energy used and, thus, decision-makers should consider the energy source and the expected evolution of the energy mix when considering the best waste collection systems from an environmental point of view. In a framework with mostly fossil-sourced energy, the truck collection system shows lesser environmental impacts due to its lower electricity use, whereas in a renewable energy environment, the stationary pneumatic waste collection system shows better performance.  
[PERSON] et al. [39] evaluated the impact of three different collection systems in an airport following the LCA methodology. The systems considered were a traditional AWCS, an innovative pneumatic AutoWaste compact collection system, and a truck system. The results show that the pneumatic collection system with the innovative AutoWaste compact central unit can reduce the annual flow of greenhouse gases into the atmosphere (kilograms of carbon dioxide equivalent for 30 years and per tonne) by up to 25% compared to a pneumatic collection system with a conventional central unit.