**4. Conclusions**

In this study, pneumatic collection is presented as an alternative to conventional trucking systems, reducing noise and odour effects, presenting potential space savings, increasing pedestrian areas, and apparently decreasing resource consumption. The energy assessment of door-to-door and pneumatic collection, focusing on the biodegradable fraction collection and its subsequent recycling by anaerobic digestion indicated that, when the organic fraction is collected separately, out of the bulky fraction, the pneumatic collection could be a suitable alternative because the energy requirements are balanced with the savings from the anaerobic digestion process. These results highlight the idea that energy recovery from MSW is a feasible alternative to face the challenge to move towards smart cities. Moreover, the combination of this technology with renewable energy supply, such as solar and wind (when high insolation and land availability are present), and new information and communications technologies, such as sensors to retrieve the fill level of containers, could be a good option to handle the cities transition along with the implementation of encouraging renewable technologies.

In the performed sensitivity analysis, one of the variables that potentially have a significant impact on the results is the effectiveness of the biodegradable fraction collection since when the efficiency varies from 100% to 40%, the energy savings are reduced to 57% for door-to-door collection and to 66% for the pneumatic system. This result highlights the importance of people awareness and information campaigns about the benefits of the biodegradable waste selective collection. On the other hand, other variable with potential influence on the results is the population density. An increase of 75% of the population density means 27% of PED savings for both collection systems due to the avoided impacts from the anaerobic digestion treatment.

This study aims to develop a general suitable model for Spain, based on average values, facilitating the decision-making process for the MSW collection and management. It is important to highlight the need of carrying out specific case studies with the aim of validating the model and to support the results of this work.
