Recycling Organic Fraction of Municipal Solid Waste: Systematic Literature Review and Bibliometric Analysis of Research Trends

The organic fraction is usually the predominant fraction in municipal solid waste, so its recycling is a potential alternative to disposal in landfill sites, as well as helping to reach targets included in the European Circular Economy Package. The existing body of knowledge in this research field is very large, so a comprehensive review of the existing scientific literature has been considered of interest to provide researchers and professionals with a detailed understanding of the status quo and predict the dynamic directions of this field. A systematic literature review and bibliometric analysis have been performed to provide objective criteria for evaluating the work carried out by researchers and a macroscopic overview of the existing body of knowledge in this field. The analysis of 452 scientific articles published from 1980 to 2019 has shown that the application of composting technologies is relevant, especially since 2014, when policies aimed at reducing emissions to the atmosphere were increased and focused on the use of this waste fraction to produce biogas. Nevertheless, the scientific field is still evolving to impose a model of a circular economy; in fact, emerging studies are being conducted on the production of biomethane, contributing to the decarbonised energy system.


Introduction
Municipal solid waste (MSW) management is an important challenge of the urban environment in most cities worldwide today. In fact, a big problem concerns the planning of treatment plants that can face the quantity and composition of municipal waste [1]; as a consequence, sustainable management solutions should be designed [2]. In this respect, several studies have reported that the composition of MSW varies significantly from one country, region or municipality to another, including food waste, metals, plastics, glass, textiles or inert materials, among others [3]. The composition of MSW depends significantly on factors such as lifestyle, economic level or legal framework, and knowledge of it is critical to determining the appropriate handling and management of these wastes [4,5]. However, the organic fraction (OFMSW) is usually the predominant fraction, in the case of its selective collection not being implemented; in fact, the percentage of the OFMSW worldwide is 46%, varying between 64% in developing countries and 28% in the case of higher-income ones [6]. Although the data indicate a slight reduction in the proportion of organic waste in 2025, it is forecast that solid waste The objective of this study was thus to develop a bibliometric analysis of the organic fraction of municipal solid waste recycling treatments, through the performance of (i) a systematic review for a quantitative analysis, (ii) a qualitative review using a science mapping study and (iii) the analysis of results.
This study will establish research themes, mapping researcher networks and recommendations for future studies in the research field of recycling OFMSW, contributing to the existing body of knowledge by assessing and highlighting the patterns and trends in the research field. Figure 1 shows the double integrated analysis performed to achieve the objectives of this study, including: (i) a systematic literature review (SLR) of the bibliographic records on recycling the OFMSW, and (ii) a bibliometric analysis of the identified documents. The sections below describe each of these procedures. quantitative analysis, (ii) a qualitative review using a science mapping study and (iii) the analysis of results.

Materials and Methods
This study will establish research themes, mapping researcher networks and recommendations for future studies in the research field of recycling OFMSW, contributing to the existing body of knowledge by assessing and highlighting the patterns and trends in the research field. Figure 1 shows the double integrated analysis performed to achieve the objectives of this study, including: (i) a systematic literature review (SLR) of the bibliographic records on recycling the OFMSW, and (ii) a bibliometric analysis of the identified documents. The sections below describe each of these procedures.

Systematic Literature Review
The SLR establishes a methodological approach that compiles all the empirical evidence that conforms to pre-specified eligibility criteria [30], in order to establish useful findings in the literature [31]. The SLR identifies any gap by minimising research bias and providing reliable results from which conclusions can be drawn and decisions made [32]. The review has been carried out following a search strategy and presents evidence on data sources, selection criteria and analysis [33] carried out according to the following four stages (Figure 1), according to Kitchenham's guidelines [34]:

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Planning and formulation of the problem. In this step, the SLR is planned, the problem is formulated and the scope of the review is set. To do that, it is necessary to have a clear definition of the proposed research questions, the exclusion criteria for the final selection of the significant documents and the definition of the expected results.

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Selection of the database (s), keywords and the search string. The second step defines the most appropriate bibliographic databases, search string and keywords for the document search. The main problem in carrying out searches in the databases is to determine the keywords and search chains that allow scientific documents relevant to the objectives of the research to be identified. In consequence, it should be necessary to have a large enough number of keywords to not restrict the number of studies, but it should be also specific enough to include only studies related to the research field under study. The first set of pre-selected records could be obtained thanks to the application of a search string.
• Selection of the literature. This is a key step to guarantee the selection of a significant number of relevant documents. The most relevant documents are those enclosing the data necessary to address the research questions of the SLR. They will be selected following the guidelines of the PRISMA flow diagram [35]. Flow diagram is a collective term for a diagram representing a flow or set of dynamic relationships in a system.

Systematic Literature Review
The SLR establishes a methodological approach that compiles all the empirical evidence that conforms to pre-specified eligibility criteria [30], in order to establish useful findings in the literature [31]. The SLR identifies any gap by minimising research bias and providing reliable results from which conclusions can be drawn and decisions made [32]. The review has been carried out following a search strategy and presents evidence on data sources, selection criteria and analysis [33] carried out according to the following four stages (Figure 1), according to Kitchenham's guidelines [34]:

•
Planning and formulation of the problem. In this step, the SLR is planned, the problem is formulated and the scope of the review is set. To do that, it is necessary to have a clear definition of the proposed research questions, the exclusion criteria for the final selection of the significant documents and the definition of the expected results.

•
Selection of the database (s), keywords and the search string. The second step defines the most appropriate bibliographic databases, search string and keywords for the document search. The main problem in carrying out searches in the databases is to determine the keywords and search chains that allow scientific documents relevant to the objectives of the research to be identified. In consequence, it should be necessary to have a large enough number of keywords to not restrict the number of studies, but it should be also specific enough to include only studies related to the research field under study. The first set of pre-selected records could be obtained thanks to the application of a search string.
• Selection of the literature. This is a key step to guarantee the selection of a significant number of relevant documents. The most relevant documents are those enclosing the data necessary to address the research questions of the SLR. They will be selected following the guidelines of the PRISMA flow diagram [35]. Flow diagram is a collective term for a diagram representing a flow or set of dynamic relationships in a system. • Identification of time horizon; selection of the database(s). Finally, and before science mapping, it is essential to establish different periods based on the number of relevant documents identified, as well as the main elements and inflection points of the research field.

Bibliometric Analysis: Performance Analysis and Science Mapping
Bibliometric analysis was carried out by performance analysis and science mapping; its objective was the obtainment of a spatial illustration of the connection between disciplines, specialities, individual documents and authors [28]. The performance analysis quantifies the impact of the citation of scientific production; on the other hand, the mapping of science shows the social analysis, and the intellectual and conceptual evolution in field research, as well as its evolution and dynamic characteristics. To do that, the free scientific mapping tool SciMAT (Science Mapping Analysis Software Tool) [28] was used. This software is based on the analysis of co-words and the h-Index, incorporating methods, algorithms and measurements in the workflow of the general science mapping, from preprocessing to the visualisation of results [28].
The application of SciMAT enables the detection of the research themes, where an equivalence index [28] is generated, followed by the clustering of thematic keywords using the simple centres algorithm [36] in order to identify the most relevant themes. It continues with the creation of two-dimensional strategic diagrams based on the degree of interaction of different research topics (centrality) and the internal strength value of the research topic object of study (density). In these diagrams, the following four different research topics are reflected by periods ( Figure 2a):

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Motor themes. These include important and developed topics in the research field.

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Highly developed and isolated themes. These are well developed topics, but, unlike motor themes, they are not important for the research field object of study.

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Emerging and declining themes. These include little-developed and non-important topics in the research field. • Basic and transversal themes. These are important topics in the research field object of study, but they are not well developed.
Subsequently, conceptual links between research topics, in different periods, as well as the strength of association between the themes through the inclusion index [37] are detected. The following two types of graphics are used for their representation:

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Overlay graph (Figure 2b). The number of words shared by both periods is represented on the horizontal arrow. The upper incoming arrow shows the number of new words in period 2, and the upper outgoing one shows the words that disappear in period 2. Additionally, the contribution of research topics to the whole field of research is quantitatively and qualitatively measured by bibliometric measurements. Among others, the number of documents published, number of citations of the documents, most cited authors, most cited publications and different variants of the h-index [38] are detected. The application of SciMAT enables the detection of the research themes, where an equivalence index [28] is generated, followed by the clustering of thematic keywords using the simple centres algorithm [36] in order to identify the most relevant themes. It continues with the creation of twodimensional strategic diagrams based on the degree of interaction of different research topics (centrality) and the internal strength value of the research topic object of study (density). In these diagrams, the following four different research topics are reflected by periods ( Figure 2a):

•
Motor themes. These include important and developed topics in the research field.

•
Highly developed and isolated themes. These are well developed topics, but, unlike motor themes, they are not important for the research field object of study.

•
Emerging and declining themes. These include little-developed and non-important topics in the research field.

•
Basic and transversal themes. These are important topics in the research field object of study, but they are not well developed.
Subsequently, conceptual links between research topics, in different periods, as well as the strength of association between the themes through the inclusion index [37] are detected. The following two types of graphics are used for their representation:

Results and Discussion
The SLR method and the science mapping study of the relevant documents were applied to carry out an exhaustive analysis of the OFMSW recycling treatment research field. The results obtained are summarised in Figures 3-6 and Tables 1-4, and they are described in detail in the following sections. Additionally, the contribution of research topics to the whole field of research is quantitatively and qualitatively measured by bibliometric measurements. Among others, the number of documents published, number of citations of the documents, most cited authors, most cited publications and different variants of the h-index [38] are detected.

Results and Discussion
The SLR method and the science mapping study of the relevant documents were applied to carry out an exhaustive analysis of the OFMSW recycling treatment research field. The results obtained are summarised in Figures 3-6 and Tables 1-4, and they are described in detail in the following sections.

Systematic Literature Review
The results obtained by applying the methodology of the SLR are presented below, including the definition of the research questions, the search process, the PRISMA flow diagram ( Figure 3) and the data and search results collection.  • Third period (2014-2019). Finally, in the last period (Figure 4c), there is a greater number of documents (245), so it is possible to differentiate four research topics: compost, natural gas, greenhouse gases and solid waste. Three of these research topics are considered motor themes (compost, natural gas and greenhouse gases), and only one of them is classified as declined (solid waste). The performance measures highlight two research topics: compost and natural gas; they show an important impact rate and achieve higher h-indices compared to the remaining topics.
Although the compost theme appears as the main one, research focused on the use of biomethane in natural gas networks emerges as a way to reduce Greenhouse Gas (GHG). This is a clear example of the important role of waste management in the decarbonisation of the energy system.

Performance Analysis
After the SLR was performed, a total of 452 documents published within the time horizon (1980-2019) were obtained. Finally, the following configuration in SciMAT for the bibliometric analysis was established: (i) the word as the unit of analysis, (ii) the analysis of co-occurrence to build the networks, (iii) the index of equivalence to measure the similarity to standardize the networks, and (iv) the kmeans clustering algorithm to detect the themes; finally, documents were analysed taking into account the year of publication, journals cited, authors and number of citations. The results obtained are summarised below: • Documents per year. Figure 6 shows the distribution by year of the 452 publications selected. It is worth stressing that, in general terms, the number of studies was not high, except in the year 2017, when it exceeded 40. Before 2007, no more than eight publications related to the research field analysed were observed per year; however, since 2007, a continuing increase in the number of articles can be seen. This result highlights the obligation to apply Article 4 of Directive 2006/12/EC of the European Parliament and of the Council of 5 April 2006 on waste, which established the obligation to apply the waste management hierarchy; it indicates an order of preference for management hierarchy that reduce the production of waste and capture the progression of a material or product through consecutive phases of waste management, and it includes, in this order: prevention; preparation for reuse; recycling; another type of valorisation, for example, energy recovery; and elimination. • Most relevant journals. A total of 192 journals were identified in this analysis. Table 3 shows the journals of 28.5% of the documents analysed, classified in descending order according to the number of citations of the document. Most of these research journals focus on the application of processed organic solid waste, biogas utilisation, energy policy and soil science, among others. Table 3 also includes the most cited publications in each journal; it should be noted that the numbers of publications and citations are closely related, except for the BioCycle journal; that is, the most prolific sources are those with the greatest impact in the research field.

Search String * Records Scopus Records ISIWoS
"Organic waste" AND "municipal solid waste" AND "biogas" 250 235 "Organic waste" AND "municipal solid waste" AND "biomethane" 19 21 "Organic waste" AND "municipal solid waste" AND "natural gas" 18 15 "Organic waste" AND "municipal solid waste" AND "network" 20 9 "Organic waste" AND "municipal solid waste" AND "compost" 274 233   Table 4. Authors with more than ten published studies in the research field.

Systematic Literature Review
The results obtained by applying the methodology of the SLR are presented below, including the definition of the research questions, the search process, the PRISMA flow diagram (Figure 3) and the data and search results collection.

Planning and Formulation of the Problem
The research questions were determined before starting the search. The SLR of this study addressed the following research questions (RQ):

Selection of the Database, Keywords and Search Strings
Firstly, the Scopus and ISIWoS databases were selected because of their large numbers of international scientific publications and high impact techniques for any discipline. Secondly, the keywords related to OFMSW recycling were identified. Next, an advanced search was carried out in the field "Title/Abstract/Keyword" with the identified keywords and using the five search chains defined in Table 1. Finally, a total of 581 bibliographic records were identified for Scopus and 513 bibliographic records, for ISIWoS.

Selection of the Literature
Once these documents had been compiled, the guidelines of the PRISMA flow chart were applied to show the number of relevant documents (Figure 3). It can be seen that a total of 1094 bibliographic records were retrieved from the two selected databases. After removing 427 duplicates, 177 of the remaining 667 records were excluded based on their titles and abstracts; notes and errata were also excluded. The remaining 490 records were examined at the full-text level, which led to the exclusion of 86 additional records that did not cover the topics included in this study.

Identification of the Time Horizon
The recovery time interval of the literature was established from 1980 to 2019. To analyse trends in publication patterns, this time interval of the study was divided into three periods, considering both several relevant milestones and the number of documents selected. As a result, the following three periods were established: • First period   (2013), which ends in the year 2020. The Doha Amendment also applies, according to which the participating countries committed to reducing emissions by at least 18%, compared to 1990 levels. In the case of the EU, it undertook to reduce emissions by 20% below 1990 levels (United Nations, 1998).

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Third period (2014-2019). The end of this period corresponds to the date of the last articles included in the study, that is, the past year.

Science Mapping and Strategic Diagrams
From the analysis of the evolution of the strategic diagrams, the change in the development of the recycling of OFMSW can be seen, as well as the main milestones and inflection points. For the three periods considered (1980-2005, 2006-2013 and 2014-2019), and in order to analyse the temporal evolution, Figure 4 represents the strategic diagrams, showing the sizes of the spheres proportional to the numbers of documents published associated with each research topic. In addition, Table 2 shows the measures of performance obtained for each topic and period in terms of the number of documents, h-index, values of centrality and density. An analysis of these results, for each period, is discussed below.

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First period . According to the strategic diagram of Figure 4a, the following four main research topics can be found in the 97 papers published in this period: compost, fertilisers, anaerobic treatment and renewable resources. Two of them are considered motor themes (compost and anaerobic treatment); one, transversal (renewable resource); and, finally, another, a basic one (fertilizers). The performance analysis for each topic (Table 2) complements the information provided by the strategic diagram; it may be observed that compost and fertilizer are the themes with a significant impact rate; they receive more than 100 citations and obtain higher h-indices compared to the remaining themes. These research topics show that the first treatments of the organic fraction of urban waste were aimed at producing compost to be used as fertiliser, applying simple technologies for the aerobic stabilisation of the biodegradable fraction. •  Second period (2006-2013). According to the strategic diagram of Figure 4b, in the 127 papers published in this period, an increase in research topics is seen, rising to five: compost, biogas, biofuel, biomethanization and anaerobic treatment. One of them is considered a motor theme (biogas); two, transversal (biofuel and biomethanization); and, finally, two of them are basic themes (anaerobic treatment and compost). The performance measures included in Table 2 reveal that compost and biogas are the most noted research topics. They obtained an important impact rate and achieved higher h-indices in comparison with the remaining topics. It can be seen that the application of the waste management hierarchy and the obligation to achieve the established objectives to reduce the percentage of biodegradable waste disposal in landfill have led to an evolution of the treatments towards anaerobic stabilisation or biomethanisation technologies in order to obtain a biogas that can be used to produce energy, as well as a digestate that, after a composting phase, can be used in agriculture.
• Third period (2014-2019). Finally, in the last period (Figure 4c), there is a greater number of documents (245), so it is possible to differentiate four research topics: compost, natural gas, greenhouse gases and solid waste. Three of these research topics are considered motor themes (compost, natural gas and greenhouse gases), and only one of them is classified as declined (solid waste). The performance measures highlight two research topics: compost and natural gas; they show an important impact rate and achieve higher h-indices compared to the remaining topics.
Although the compost theme appears as the main one, research focused on the use of biomethane in natural gas networks emerges as a way to reduce Greenhouse Gas (GHG). This is a clear example of the important role of waste management in the decarbonisation of the energy system.

Science Mapping, Overlay Graph and Thematic Evolution Map
The systematic analysis of the literature has shown the change in the development of the treatments for recycling OFMSW, as well as the main milestones and inflection points. Next, it was considered interesting to carry out a joint analysis of the evolution of the keywords and the thematic evolution of the field of the investigation. The results are shown in Figure 5 and discussed below.
The number of keywords per period and their evolution have been represented in Figure 5a, as well as the number of incoming and outgoing keywords, and the number and percentage of keywords that remain from one period to the next. It can be seen that the number of keywords increases over the periods, in parallel with the rise in the number of documents over the years. Thus, the number of keywords increased from 29 to 44 between the first and last periods, which meant a growth of 51.7%; this result indicates that the field of research is diversifying and continuing to increase, meaning that it is not yet a consolidated field. The increasing number of words and keywords shared between successive periods proves the growing thematic diversity of the field of research on the recycling of OFMSW. Figure 5b shows the thematic evolution of the field of research thanks to the analysis of its origins and its interrelations. The thickness of the lines represents the strength of the association measured by the inclusion index. The analysis of the graph from the point of view of the number of documents shows that the four thematic groups of the first period   Finally, it is noteworthy that the compost thematic group of the first period was kept with the same label in the second and third periods but with a greater number of central documents published during the last one (2006-2019). In the same way, the anaerobic treatment thematic group of the first period was maintained with the same label in the following period, although with a lower number of documents, evolving towards the solid waste thematic group during the last period.

Performance Analysis
After the SLR was performed, a total of 452 documents published within the time horizon (1980-2019) were obtained. Finally, the following configuration in SciMAT for the bibliometric analysis was established: (i) the word as the unit of analysis, (ii) the analysis of co-occurrence to build the networks, (iii) the index of equivalence to measure the similarity to standardize the networks, and (iv) the k-means clustering algorithm to detect the themes; finally, documents were analysed taking into account the year of publication, journals cited, authors and number of citations. The results obtained are summarised below: • Documents per year. Figure 6 shows the distribution by year of the 452 publications selected. It is worth stressing that, in general terms, the number of studies was not high, except in the year 2017, when it exceeded 40. Before 2007, no more than eight publications related to the research field analysed were observed per year; however, since 2007, a continuing increase in the number of articles can be seen. This result highlights the obligation to apply Article 4 of Directive 2006/12/EC of the European Parliament and of the Council of 5 April 2006 on waste, which established the obligation to apply the waste management hierarchy; it indicates an order of preference for management hierarchy that reduce the production of waste and capture the progression of a material or product through consecutive phases of waste management, and it includes, in this order: prevention; preparation for reuse; recycling; another type of valorisation, for example, energy recovery; and elimination. • Most relevant journals. A total of 192 journals were identified in this analysis. Table 3 shows the journals of 28.5% of the documents analysed, classified in descending order according to the number of citations of the document. Most of these research journals focus on the application of processed organic solid waste, biogas utilisation, energy policy and soil science, among others. Table 3 also includes the most cited publications in each journal; it should be noted that the numbers of publications and citations are closely related, except for the BioCycle journal; that is, the most prolific sources are those with the greatest impact in the research field. • Documents by author. A systematic literature review and performance analysis allowed the identification of a total of 1329 authors who have published on the theme addressed by the study's objective. Table 4 shows those authors with more than three published studies, as well as the total number of documents published and citations received; the h-index (Hirsch index), has been also included as a measure of the authors' professional quality, taking into account the number of times that their scientific articles have been cited [39]. According to the information analysis, Dubrovskis, V., Li, Y. and Kumar, A. have published the most articles on them, although Xi, B. has the highest h-index.

Conclusions
This document shows an analysis of the scientific literature that addresses the treatment of the biodegradable fraction of urban waste from 1980 to 2019. To do this, a transparent, rigorous and reproducible research procedure was applied to a collection of 452 articles published in indexed journals in the ISI Web of Science (ISIWoS) and Scopus database, with peer review before publication. Tendencies were analysed, considering an overview and a more specific analysis of three different time intervals during the period under review (1980-2005, 2006-2013 and 2014-2019).
The study has shown that the application of technologies that allow the use of the OFMSW as a biological nutrient in the framework of the circular economy is relevant, especially in recent years. In fact, the results show a regular rise in the number of studies published since 1980; this rise is more significant since 2014, when policies aimed at reducing emissions to the atmosphere through the development of renewable energies were increased. Although the treatments of this fraction have been applied for years, the scientific field is still evolving; this may be explained by the evolution of the legal framework, which imposes the need to continue working on the implementation of strategies to develop a model of a circular economy. One of the basic pillars of this new framework is the use of bioproducts, such as OFMSW, as a renewable energy resource.
The systematic analysis of the literature has shown the evolution of recycling treatment for OFMSW, as well as the main landmarks and inflection points. On the other hand, the strategic diagrams show the interest of researchers in different key issues, which have progressed from simple treatments, just applied to produce compost from the biodegradable fraction of municipal waste, up to its use to produce biofuel; this last application is within the framework of policies aimed at promoting the use of renewable energies that reduce the emissions of greenhouse gases.
This document has shown that emerging studies in this area are being conducted on the production of biomethane, called green gas, for application as fuel in vehicles or for injection into natural gas networks. Although these applications are currently in use, they are still not competitive, but their contribution to their circular economy as a sustainable alternative to waste disposal, as well as to reduce greenhouse gas emissions, is already clear. Biomethane is thus positioned as a fundamental element in the energy transition that will contribute to a decarbonised energy system in order to comply with global objectives.
The above findings provide researchers and professionals in the OFMSW recycling field with a detailed understanding of the status quo, predicting its dynamic directions; in consequence, this study is a valuable contribution to research concerning this field.