## 2 Bibliometric Analysis  
The database Scopus was used as a reference in the bibliometric analysis carried out in this paper. Compared to other databases such as Web of Science, this one includes a large number of documents about technological topics [12]. The query used was based on key terms related to the topic, which were \"automated waste collection\" and \"vacuum waste collection\". A total of 19 documents were found. Moreover, R-tool and the bibliometrix library were selected as the software used to perform the bibliometric analysis [13]. Figure 1: AWCS scheme. (1) Indoor inlets (on each floor), (2) outdoor inlets (with domestic door), (3) outdoor inlets (with professional door), (4) indoor inlets (on ground floor), (5) pipeline network, (6) separation and compaction unit (terminal), (7) blowers—de-pressurisation system, and (8) air treatment system. Source: URD. Figure 2 shows the trend in the number of publications in the period analysed between 2009 and 2022. It highlights the small number of existing scientific publications. This small number of publications related to vacuum waste collection shows a stark contrast with the number of publications related to the main alternate collection method, that is, using surface vehicles to pick up waste either from citizen's houses or from aggregated collection points (containers, underground containers, etc.). A simple search on the Scopus database of vehicle routing research for \"waste collection\" and \"vehicle routing\" (a search that, if refined, would yield several more results), produced 320 research studies. These studies ranged from 1974 [14] to more recent studies from 2023 [15]. These studies cover and profit from many technological advances, from ant colony optimisation algorithms [16] to neural networks [17]. This vehicle-based collection method also benefits from the application of research not specifically oriented toward waste collection, but to other uses of vehicles (mainly package delivery), and is a research topic well-studied in the literature. In fact, vehicle routing is a recurrent research problem in optimisation, problem-solving, and artificial intelligence research, and a quick search for \"vehicle routing problem\" or \"VRP\" (as it is often referred to in the literature) yields thousands of research studies (VRP is a generalisation of TSP, the travelling salesman problem, a problem studied since the mid-19 th century). Another facet of this anomalous lack of publications which indicates that, albeit the number of published articles is scarce, there is a serious research effort behind vacuum waste collection, is the number of filled patent applications, such as in [18; 19; 20], with some of these patents dating back to the 1970s [21]. Figure 3 presents a three-field plot which relates authors, keywords, and affiliations. The maximum number of each field was set to 20. The figure shows the relationship between 20 authors, 19 keywords, and 18 affiliations. The keywords \"waste management\" and \"greenhouse gases\" were the most used keywords. Moreover, the two affiliations that published the most documents about the topic were \"University Transportation Research Center\" at City College (New York, NY, USA) and \"Universitat de Lleida\" (Spain). Figure 4 shows the most relevant sources on the topic during the period analysed. The source with the highest number of publications was \"Waste Management\" followed by \"Environmental Modelling and Software\", \"Journal of Cleaner Production\", and \"Tunneling and Underground Space Technology\". Figure 2: Annual scientific production between 2009 and 2022 using R-studio software and bibliometric library of the automated waste collection system (AWCS) scientific literature found in Scopus. Figure 5 presents the world's scientific production on the topic. Countries are marked in two colours. Those indicated in grey are where no research was completed related to the topic; blue-marked countries are those where scientific research and publications were carried out.