Bibliometric Analysis of Municipal Solid Waste Management Research: Global and South African Trends

: Municipal solid waste management has become one of the most important environmental management issues around the world. In this study, a bibliometric analysis of the literature related to municipal solid waste management from a global and South African perspective was performed using the software, VOSviewer. Relevant scientiﬁc literature was sourced from the Scopus database. Results showed that, globally, articles based on this topic started during the 1968–1969 period, whereas in South Africa such articles only appeared in 1996. The keyword analysis showed that most studies were related to waste-to-energy technologies, waste treatment, and other management aspects. Globally, emerging keywords representing new research areas were COVID 19-related waste streams, life-cycle assessments, and the role of municipal solid waste management in the circular economy. In contrast, South Africa’s prominent keywords were municipal solid waste, developing country, landﬁlls, waste treatment, waste-to-energy technologies, pollution, greenhouse gas emissions, and others. Based on these results, and possible solutions to reduce the amounts of MSW generation rates, recommendations are made to bring South African research on par with international trends.


Introduction
Globally, the generation of municipal solid waste (MSW) is on the rise as a result of rapid population growth, increased economic development and industrialiasation, urbanization, and growing consumerism in society [1][2][3][4][5]. The total MSW generated in Central Asia and Europe amounted to 392 million tons in 2016, and this amount is projected to increase to 440 million tons in 2030, thus representing an increase of approximately 11% [6]. North America generated about 289 million tons of MSW in 2016, and this is expected to increase by 21% in 2030. However, these amounts are relatively lesser in some of the developing regions. For example, the Sub-Saharan African (SSA) region produced only 174 million tons of MSW in 2016, with a projected growth of 269 million tons in 2030 [6].
MSW has a complex composition and usually emanates from households and commercial enterprises. MSW comprises different types of paper, bottles, plastics, as well as garden wastes, discarded foods, furniture, clothes, appliances, and even batteries [2,4,[7][8][9]. To reduce the adverse and harmful nature of this waste stream, it is imperative to manage it effectively from its origin to the point of its final disposal or treatment or recycling. MSW is an intricate procedure and involves many steps, such as waste collection transport routes, temporary storage points, waste treatment, disposal, as well as energy recovery strategies, and recycling, the prime purpose being to enhance environmental protection, public health, and the need to comply with local government regulatory requirements [1,10,11]. Effective solid waste management is also one of the issues that are relevant in reaching the targets for the Sustainable Development Goal 11 of the United Nations, which involves working towards the attainment of sustainable cities and communities [12]. Similarly, Sustainable landscapes and research priorities. Moreover, there is an increase in bibliometric analyses across many sectors such as the circular economy, energy supply and conservation research, construction, and the environment [20][21][22][23][24][25][26][27][28][29][30][31]. In this paper, a bibliometric analysis was employed on selected keywords with the objective of determining trends, patterns, and gaps in the knowledge about MSWM.

Data Collection
The bibliometric analysis in the present study was derived from data in the scopus database. The MSWM keyword was used to extract data from this database. The search was done by considering article titles, abstracts, and keywords. However, some filters were applied to only include articles in English and from the Environmental Sciences discipline. However, no filter was applied on the year of publication, thus enabling a longitudinal trajectory in the study of this topic. In addition, only articles (including articles in press) were included. This process was repeated in a second search, but this time the search was limited to South Africa.

Data Processsing
The data were then exported in the comma-separated values (CSV) format and further cleaned to remove duplicate entries, irrelevant articles, or articles with incomplete references. Subsequently, it was then viewed by VOSviewer as described by Sarquah [20].

Data Analysis
The number of articles, authors with the most publications, countries that are leading in scientific production, and the co-occurrence of keywords were amongst some of the parameters that were analysed using VOSviewer. The VOSviewer software is known for its high-quality visualization characteristics and end-user-friendliness. It also provides homogenous mapping and clustering of data ( Figure 1) [32].

Municipal Solid Waste Management in the World
Based on the results obtained from the literature search and analyses, 7374 articles linked to MSWM were generated from the scopus database. Globally, in the following years-1968, 1969, and 1974-only one paper was published annually, respectively (Figure 2). However, from the year 1981 onwards, the number of published papers increased exponentially ( Figure 2). This trend may be attributed to the rapidly growing importance of MSWM challenges that are facing many countries. Some European countries had to deal with challenges such as limited landfill space and the threat of environmental pollution on water bodies, climate change risks, and public health concerns [8]. Landfilling was

Municipal Solid Waste Management in the World
Based on the results obtained from the literature search and analyses, 7374 articles linked to MSWM were generated from the scopus database. Globally, in the following years-1968, 1969, and 1974-only one paper was published annually, respectively ( Figure 2). However, from the year 1981 onwards, the number of published papers increased exponentially ( Figure 2). This trend may be attributed to the rapidly growing importance of MSWM challenges that are facing many countries. Some European countries had to deal with challenges such as limited landfill space and the threat of environmental pollution on water bodies, climate change risks, and public health concerns [8]. Landfilling was no longer regarded as a viable strategy to deal with increasing volumes of MSW; therefore, alternative solutions had to be investigated, including waste minimization in the form of charging landfill fees, economic incentives, waste treatment, reuse, recovery, and recycling [14]. In a way, such a change in the direction of waste management was leading to the introduction of a circular economy (CE). According to Pires and Martinho [33], a CE includes reuse, refurbishment, and remanufacturing, or reducing, reusing and recycling of waste materials [34]. In North America, working towards a CE has prioritized waste reduction over the traditional waste disposal by landfilling [35]. Similarly, there is also great impetus for the creation of the CE in countries such as China and Japan [11]. In some countries, the carbon footprint is used as a tool for evaluating the CE [36]. The recycling of mixed paper, plastics, and organic wastes in the industrial production of materials can reduce CO 2 emissions that are negatively implicated in global warming [37]. The analysis of academic articles on resource retention options showed some variations in the CE operationalization principle [38].
no longer regarded as a viable strategy to deal with increasing volumes of MSW; therefore, alternative solutions had to be investigated, including waste minimization in the form of charging landfill fees, economic incentives, waste treatment, reuse, recovery, and recycling [14]. In a way, such a change in the direction of waste management was leading to the introduction of a circular economy (CE). According to Pires and Martinho [33], a CE includes reuse, refurbishment, and remanufacturing, or reducing, reusing and recycling of waste materials [34]. In North America, working towards a CE has prioritized waste reduction over the traditional waste disposal by landfilling [35]. Similarly, there is also great impetus for the creation of the CE in countries such as China and Japan [11]. In some countries, the carbon footprint is used as a tool for evaluating the CE [36]. The recycling of mixed paper, plastics, and organic wastes in the industrial production of materials can reduce CO2 emissions that are negatively implicated in global warming [37]. The analysis of academic articles on resource retention options showed some variations in the CE operationalization principle [38].  2022  2020  2018  2016  2014  2012  2010  2008  2006  2004  2002  2000  1998  1996  1994  1992  1990  1988  1986  1984  1982  1980  1978  1976  1974  1968 Number of articles Year       The occurrence of keywords from the year 2002 to 2022 is shown in Figure 6. There are few emerging topics, and they are shown by the yellow color ( Figure 6). Some of the newly emerging research involved COVID-19-related wastes, CE, and attendant constraints. In fact, COVID19 research appeared 47 times and it examined the health-care waste associated with the COVID-19 pandemic [56]. Some of the key research led to the detection of the SARS-CoV-2 RNA in MSW leachate [57]. Detecting the COVID-19 viral RNA in MSW leachate is useful when there is no adequate treatment system and sanitation infrastructure [57]. The general key finding in some of the COVID-19 studies was that health care waste and its management increased during the pandemic [56].
Words that appeared many times during the year 2015 (shown in light green) included China and waste disposal ( Figure 6). The reason why the word China had high incidence could be attributed to the increased research conducted in that country since 2015 to the present. Important research topics included the following ones: (1) sustainable waste management and waste-to-energy projects [58]; (2) changing city-level greenhouse gas emissions from MSW treatment and driving factors [59]; (3) comprehensive emission inventory of hazardous air pollutants from MSW incineration [60]; and (4) how can waste management contribute toward the development of a greenhouse gas sink [61]. Other researchers examined the identification of suitable, future waste disposal sites using the  The size of the circle or the circumference of the spheres in Figure 4 is dependent on the number of the occurrence of certain keywords [39]. In Cluster I, Italy has the biggest circle. In Cluster II, Japan has the biggest circle. The biggest circle in Cluster III, IV, V, VI, VII, VIII, IX, X, and XI belong to Brazil, Spain, Switzerland, Turkey, Iran, China, India, Israel, and the USA, respectively. Saudi Arabia and Malaysia had the biggest circles in Cluster XII and XIII ( Figure 4).
In terms of research linkages or collaborations with other countries, the USA and the United Kingdom (UK) had most linkages with 64 and 63 other countries, respectively. They were followed by China (59), Germany (55), India (50), Italy (47), France (42), Spain (39), Canada (37), and Japan (36). The 64 links observed for the USA were between countries such as France, UK, Japan, Netherlands, Germany, India, Turkey, Canada, and South Africa. Interestingly, among these countries, the UK had the highest links (63) ( Figure 5). Linkages were observed between USA, Australia, China, and India. Notably, some links were also found between Bangladesh and Thailand. Spain was linked to countries such as Argentina, Poland, Czech Republic, Portugal, and Mexico ( Figure 4).
Furthermore, Bangladesh, Thailand, Saudi Arabia, and Malaysia collaborated on COVID-19-related personal protective equipment (PPE) waste research [40]. They have suggested incineration to reduce the increase in PPE wastes. Researchers from Canada, North Korea, and the USA investigated the potential of converting cardboard waste into lactic acid. Thus, cardboard waste can be an important source for producing lactic acid and has associated economic benefits based on technoeconomic analysis [41]. Since cardboard forms part of MSW, this study is of particular interest because waste conversion practices and resource recovery are very important for a sustainable bioeconomy.
Researchers from Sri Lanka and India developed a user interface tool that uses Google Earth to establish spatial information relevant for the siting of alternative dumping sites [42]. The Chinese-North Korean collaboration has created a model for urban biowaste management. The model performed optimally, and important policy recommendations were suggested [43]. Poorly planned urbanization is linked to illegal solid waste disposal and water pollution. This promoted a rapid impact assessment matrix for urban water shed management by researchers from Brazil and Canada [44]. Their results showed that by using rapid impact assessment matrix, the negative impacts on urban water streams as a result of urbanization can be reduced. The Japanese-Thailand collaboration has investigated the effects of compost bin design, design preference, waste collection performance, and their data clearly showed that appropriate bin design increased the effectiveness of waste recovery rates [45].
A total of 32,726 keywords were generated during the literature review. The threshold used was a minimum of five occurrences per keyword; the most dominant keyword was "MSW" which appeared 5842 times, followed by "waste management" and "solid waste management" which occurred 4199 and 2634 times, respectively ( Figure 5).
The keyword analysis revealed three distinct Clusters. The three keywords mentioned above fall under Cluster I ( Figure 5). In Cluster I, MSWM is closely linked to efforts towards the achievement of sustainable development goals. Some important research on MSW in Cluster I focused on life cycle assessment (LCA) of MSW [46], promotion of MSW recycling in the developing countries [47], and the contribution of MSW towards greenhouse gas emissions [48]. Some of the key findings from such studies were that a balance between landfill and incineration was the optimal waste management option [46].
Cluster II keywords were largely made up of words such as "chemical elements", "incineration", and "pollution abatement". In Cluster II, the transformation of hazardous elements in ash from waste incineration plants was investigated [49], and using integrated strategies for MSWM may lead to a reduction in greenhouse gas emissions by 2050 [48].
Cluster III was largely occupied by "anaerobiosis", "anerobic digestion", "bioreactors", and the "management of sewages" (Figure 5). Some of the key research in Cluster III involved the environmental impact assessment of the organic fraction from MSW treatment by anaerobic digestion [50]. Anaerobic digestion transforms organic matter to biogas and sludge under anaerobiosis. The process is effective in reducing greenhouse gas emissions, and the resultant biogas can be used as a renewable energy source. Moreover, the resulting digestate can also be used as fertilizer to improve soil quality [50]. In Cluster III, "pollutant removal" and "wastewater treatment" were closely linked. The elimination of toxic chemicals from wastewater is one of the strategies to help reduce the environmental impacts of such waste materials [51]. Leachate contamination of water also poses a significant public health risk [52].
The Clusters also show the different research streams under MSWM ( Figure 5). Some of the research streams involved refuse-derived-fuel (RDF). RDF appeared 38 times and some research looked at policy options for sustainable RDF production [53]. RDF production is regarded as a substitute for using fossil fuels along with its associated environmental benefits [53]. Computer modelling and machine learning appeared 28 and 19 times, respectively. Another research strand involved developing multi-city models for MSW generation predictions by Lu et al. [54] and Zhang et al. [55]. The results by Zhang et al. [55] showed that population and GDP are two of the most important indicators in MSW prediction. Similarly, key factors influencing MSW generation are annual precipitation, population density, and mean annual temperature [54]. This is because precipitation increases the moisture content of MSW, thus increasing the total weight. In addition, high precipitation and temperature also influences human behavior, hence the consumption of goods and services. High precipitation may also contribute to increased crop production, thereby leading to more organic waste generation, whereas high population numbers are linked to more waste items being produced per person.
The occurrence of keywords from the year 2002 to 2022 is shown in Figure 6. There are few emerging topics, and they are shown by the yellow color ( Figure 6). Some of the newly emerging research involved COVID-19-related wastes, CE, and attendant constraints. In fact, COVID19 research appeared 47 times and it examined the health-care waste associated with the COVID-19 pandemic [56]. Some of the key research led to the detection of the SARS-CoV-2 RNA in MSW leachate [57]. Detecting the COVID-19 viral RNA in MSW leachate is useful when there is no adequate treatment system and sanitation infrastructure [57]. The general key finding in some of the COVID-19 studies was that health care waste and its management increased during the pandemic [56].
Words that appeared many times during the year 2015 (shown in light green) included China and waste disposal ( Figure 6). The reason why the word China had high incidence could be attributed to the increased research conducted in that country since 2015 to the present. Important research topics included the following ones: (1) sustainable waste management and waste-to-energy projects [58]; (2) changing city-level greenhouse gas emissions from MSW treatment and driving factors [59]; (3) comprehensive emission inventory of hazardous air pollutants from MSW incineration [60]; and (4) how can waste management contribute toward the development of a greenhouse gas sink [61]. Other researchers examined the identification of suitable, future waste disposal sites using the analytic hierarchy process (AHP) and geographic information systems (GISs) [62,63], and a quantitative analysis of MSW disposal charges in China [64].

Municipal Solid Waste Management in South Africa
Generally, in South Africa, the articles published annually during the 1996-2021 period have never exceeded ten (Figure 7). Although the highest number of articles (8) were published in 2020, only one article was published in the following years, 1996, 1997, and 2004, respectively (Figure 7). From then, the number of articles increased, but dropped dramatically in 2008 probably due to the after-effects of the global financial crisis, which could have affected research funding and research priorities in a negative manner. Therefore, in 2008, only one paper was published compared to two from the previous year. Later, there was an increase in articles as from the year 2010 to 2013 and during the 2015-2019 period, largely due to the increasing attention being paid to MSWM and how it is constrained by limited resources and illegal dumping of waste in South Africa [17]. For the current year (2022), no paper has been published yet (Figure 7). analytic hierarchy process (AHP) and geographic information systems (GISs) [62,63], and a quantitative analysis of MSW disposal charges in China [64].

Municipal Solid Waste Management in South Africa
Generally, in South Africa, the articles published annually during the 1996-2021 period have never exceeded ten (Figure 7). Although the highest number of articles (8) were published in 2020, only one article was published in the following years, 1996, 1997, and 2004, respectively (Figure 7). From then, the number of articles increased, but dropped dramatically in 2008 probably due to the after-effects of the global financial crisis, which could have affected research funding and research priorities in a negative manner. Therefore, in 2008, only one paper was published compared to two from the previous year. Later, there was an increase in articles as from the year 2010 to 2013 and during the 2015-2019 period, largely due to the increasing attention being paid to MSWM and how it is constrained by limited resources and illegal dumping of waste in South Africa [17]. For the current year (2022), no paper has been published yet (Figure 7).
It has been reported that South Africa is behind developed countries when it comes to waste management research [17,18]. Moreover, some of the policies and legislation in South Africa were derived from Europe and that is why there are so many implementation pitfalls because the geographical contexts are different. Nevertheless, the South African government has committed itself to an increase in expenditure on waste management It has been reported that South Africa is behind developed countries when it comes to waste management research [17,18]. Moreover, some of the policies and legislation in South Africa were derived from Europe and that is why there are so many implementation pitfalls because the geographical contexts are different. Nevertheless, the South African government has committed itself to an increase in expenditure on waste management from 0.5% to 1.5% of the GDP in order to create conditions conducive to the growth of the CE. However, the question of how the CE will be implemented effectively in a developing country with so many policy failures remains unanswered [18].
A total of 54 institutions have contributed to the existing body of knowledge of MSWM in South Africa, of which some of them are shown in Table 2. The University of KwaZulu-Natal contributed the most articles, followed by the University of the Witwatersrand and the University of Johannesburg in descending order (Table 2). On the other hand, other institutions contributed relatively fewer publications (Table 2). CE. However, the question of how the CE will be implemented effectively in a developing country with so many policy failures remains unanswered [18].  Table 2. The University of KwaZulu-Natal contributed the most articles, followed by the University of the Witwatersrand and the University of Johannesburg in descending order ( Table 2). On the other hand, other institutions contributed relatively fewer publications ( Table 2). In the SADC region, South Africa was central in terms of research collaborations as it exhibited seven research links with other countries in the world (Figure 8). All the other countries had one link or collaboration, except for Ethiopia and the UK which had two links. This is because the search was limited to South Africa. Based on Figure 8, there were a total of five different Clusters. In Cluster I, the most important countries are Canada, China, and South Africa. The UK and Ethiopia are featuring prominently in Cluster II. In  In the SADC region, South Africa was central in terms of research collaborations as it exhibited seven research links with other countries in the world (Figure 8). All the other countries had one link or collaboration, except for Ethiopia and the UK which had two links. This is because the search was limited to South Africa. Based on Figure 8, there were a total of five different Clusters. In Cluster I, the most important countries are Canada, China, and South Africa. The UK and Ethiopia are featuring prominently in Cluster II. In Cluster III, IV and V, Australia, Botswana, and France came out to be the most important countries (Figure 8). Although a total of seven countries have collaborated with South Africa, the biggest collaborator is the UK. Researchers from both countries modeled greenhouse gas emissions from MSW disposal across Africa [65], assessed air quality management in South Africa [66], and evaluated leachate recirculation with cellulase addition to enhance waste biostabilisation and landfill gas production [67].
Although most countries that collaborated with South Africa had nearly an equal number of research outputs, the citations varied greatly. Australia and Botswana had three documents each with 63 and 54 citations, respectively. China, France, Ethiopia, and Canada had two documents with 51, 22, 21, and 18 citations, respectively. In addition, the UK and South Africa had 60 and 6 articles, respectively, with 995 and 118 citations, respectively (Table 3).
Cluster III, IV and V, Australia, Botswana, and France came out to be the most important countries (Figure 8). Although a total of seven countries have collaborated with South Africa, the biggest collaborator is the UK. Researchers from both countries modeled greenhouse gas emissions from MSW disposal across Africa [65], assessed air quality management in South Africa [66], and evaluated leachate recirculation with cellulase addition to enhance waste biostabilisation and landfill gas production [67]. Although most countries that collaborated with South Africa had nearly an equal number of research outputs, the citations varied greatly. Australia and Botswana had three documents each with 63 and 54 citations, respectively. China, France, Ethiopia, and Canada had two documents with 51, 22, 21, and 18 citations, respectively. In addition, the UK and South Africa had 60 and 6 articles, respectively, with 995 and 118 citations, respectively (Table 3). The network visualization in Figure 9 shows three main Clusters associated with MSWM. MSWM is closely associated with landfill sites (Figure 9). The aforementioned words are found in Cluster I. The word "MSWM" appeared 45 times, followed by "waste disposal" and "solid waste management" which occurred 44 and 26 times, respectively. Furthermore, the words leachate treatment, leaching, and sewage also appeared in Cluster  The network visualization in Figure 9 shows three main Clusters associated with MSWM. MSWM is closely associated with landfill sites (Figure 9). The aforementioned words are found in Cluster I. The word "MSWM" appeared 45 times, followed by "waste disposal" and "solid waste management" which occurred 44 and 26 times, respectively. Furthermore, the words leachate treatment, leaching, and sewage also appeared in Cluster I. The effect of cellulase to enhance waste biostabilisation and landfill gas production has been investigated. Cellulase augmentation to leachate was found to be economically viable [67]. The use of organic wastes for the denitrification of landfill leachate showed the suitability of organic MSW to sustain denitrification, thus offering a low-cost method for alternative landfill treatment [68]. Determining pollutant concentrations after long-term leaching showed that water flow needs to be taken into account in order to improve leachate quality and quantity [69].
Cluster II is characterized by studies on the greenhouse effect, carbon dioxide (CO 2 ), and other gas emissions. The ways in which greenhouse gases are accounted and reported in the waste sector in South Africa were studied and different accounting methodologies were identified [70]. Greenhouse gas emissions from the management of MSW in the eThekwini Municipality were investigated by Friedrich and Trois [71]. The results showed that MSW recycling can help to prevent the emission of approximately 113,275 tons CO 2 by replacing virgin materials with recycled materials. Furthermore, the modelling of greenhouse gas emissions from MSW disposal has indicated that such releases will increase in Africa and will further contribute to climate change [65]. I. The effect of cellulase to enhance waste biostabilisation and landfill gas production has been investigated. Cellulase augmentation to leachate was found to be economically viable [67]. The use of organic wastes for the denitrification of landfill leachate showed the suitability of organic MSW to sustain denitrification, thus offering a low-cost method for alternative landfill treatment [68]. Determining pollutant concentrations after long-term leaching showed that water flow needs to be taken into account in order to improve leachate quality and quantity [69]. Cluster II is characterized by studies on the greenhouse effect, carbon dioxide (CO2), and other gas emissions. The ways in which greenhouse gases are accounted and reported in the waste sector in South Africa were studied and different accounting methodologies were identified [70]. Greenhouse gas emissions from the management of MSW in the eThekwini Municipality were investigated by Friedrich and Trois [71]. The results showed that MSW recycling can help to prevent the emission of approximately 113,275 tons CO2 by replacing virgin materials with recycled materials. Furthermore, the modelling of greenhouse gas emissions from MSW disposal has indicated that such releases will increase in Africa and will further contribute to climate change [65].
Cluster III is characterized by research themes covering the Gauteng province, despite the literature being open to the whole of South Africa. Key studies have focused on sustainable solid waste management in developing countries with emphasis on Cluster III is characterized by research themes covering the Gauteng province, despite the literature being open to the whole of South Africa. Key studies have focused on sustainable solid waste management in developing countries with emphasis on Johannesburg [17]. For example, two research areas which received attention included: waste-to-energy recovery in Johannesburg [72], and spatial and temporal variations in the microbiological pollution occurring in the Vaal River of South Africa [73]. Other findings indicated that landfill airspace in Johannesburg will be depleted by the year 2023 [72]. The least commonly appearing key words were "urban area" and "sewage", which featured 25 and 20 times each. The articles on sewage management do not seem to be emerging topics because some of the topics were published as early as 2010 and 2017 [74,75].
Based on Figure 10, there are no new emerging topics since the year 2020, and this is deduced from the absence of keywords in yellow. However, some words which had a high incidence around the year 2015 included "MSW" and "developing country" (Figure 10). The studies on those keywords included: co-operatives as a development mechanism to support job creation and sustainable waste management in South Africa [76], sustainable solid waste management in developing countries, and studies of the institutional factors involved in the management of solid wastes in Johannesburg [17], status of MSWM policy implementation in developing countries [77], sustainability of composting as an alternative waste management option for developing countries [78], and the characterization, recovery, and recycling potential of solid waste amongst some of the local universities [79]. The results showed that co-operatives may play an important role in the formalization of the informal waste sector in the developing countries. However, the informal waste sector remains fragmented and still faces numerous challenges. The sector is currently operating through verbal contracts, thus leading to the emergence of opportunistic and unsustainable enterprises [76]. The research conducted has also shown that ineffective MSWM emanates from institutional failures to implement and enforce existing policies and regulations [17]. Similarly, other researchers such as Mmereki et al. [77] have reported inadequacies in the implementation of policy related to MSWM and weak institutional support in Botswana. The study by Snyman and Vorster [78] indicated that most of the MSW generated in Pretoria is being landfilled, thus identifying the application of unsustainable waste management methods. By contrast, composting the organic fraction of MSW would save airspace and reduce the MSW by nearly 43%, thereby increasing the lifespan of existing landfills.

Possible Solutions to Reduce MSW
As MSW generation continues to increase at an alarming rate globally, the environmental pollution caused by this waste stream has become a growing public health concern. There is, therefore, an urgent need to improve municipal waste management practices and the implementation of associated policies [82]. Existing literature shows that Lastly, another important research area entailed the composition of MSW, which is dependent on where the waste is coming from, thereby raising the potential for informing local waste management systems so that effective waste recovery is achieved [79,80]. At the University of Venda in the Limpopo province, it was found that the proportion of compostable wastes relative to other waste types was greatest in the student residences and kitchen facilities (40.6% and 40.5%), respectively [79]. This reveals an untapped potential for recycling such wastes at this university; an opportunity that can help in reducing the volume of prevailing waste streams. Furthermore, Nell et al. [81] have examined the composition of wastes from illegal dumpsites in a rural town in the Northern Cape province. Their study found 17 different waste fractions, of which some of their weight comprised garden waste (4%); glass (13%); recyclable plastics (15%); paper and cardboard (13%); and e-wastes and other household hazardous wastes (1%). In light of these results, it is imperative that municipal planning to minimize illegal dumping must take the needs and perceptions of local communities into consideration regarding their infrastructure problems [81].

Possible Solutions to Reduce MSW
As MSW generation continues to increase at an alarming rate globally, the environmental pollution caused by this waste stream has become a growing public health concern. There is, therefore, an urgent need to improve municipal waste management practices and the implementation of associated policies [82]. Existing literature shows that there are many ways to reduce MSW generation rates, and in this section, waste-to-energy generation and household waste segregation are put into perspective for reducing the management burden of MSW.
The utilisation of waste for the generation of energy may contribute towards their reduction and possible elimination. Waste-to-energy (WTE) generation refers to a waste treatment process that produces heat and energy from various waste sources, including MSW [83]. The process can be conducted by means of thermochemical processes such as incineration, pyrolysis, and gasification, or biological processes which entail biomethanation as well as composting [84]. The effective utilization of these technologies is contingent upon several factors, including the specific attributes of the waste feedstock, access to funding opportunities, and environmental aspects [85]. Across the globe, more than 1700 WTE plants have been established and the bulk of them are located in Asia Pacific (62%) and Europe (33%), while North America has few of them (4.5%) [85]. In South Africa, various WTE plants have been started in some of the metropolitan municipalities in provinces such as KwaZulu-Natal, Western Cape, and Gauteng [86]. For instance, in the Gauteng province, several landfill gas projects were commissioned in 2016 for enhancing both environmental and public health benefits. Despite their implementation challenges, especially the high operational costs, their potential energy output was estimated to be 8000 MWh yr −1 . When connected to the national electricity grid, such energy generation is expected to provide electricity to nearly 25,000 middle-income households [87]. There is, therefore, a need for appropriate policy adaptation in South Africa to curb existing challenges as the country has the highest theoretical potential of energy generation from MSW [88].
Another effective solution to reduce waste generation at the municipal level is by encouraging waste reuse and recycling. In Denmark, between the years 1993 and 2018, the recovery of municipal wastes has risen from 80% to 99% while composting increased from 9% to 17% [89]. On the other hand, landfilling declined from 20% to 1% [89]. These remarkable achievements can be ascribed to many years of investing resources, infrastructure, and intervention strategies into their waste management sector, thus providing useful lessons for other countries to learn. In addition, the recycling processes are not achievable without the participation of citizens in the segregation of wastes at home. Therefore, it is imperative to provide communities with relevant information, feedback, and incentives so that they can participate meaningfully in waste reclamation and recycling [90]. In Singapore, it was found that household knowledge of what wastes are recyclable played a very important role in their waste minimization strategy [91]. Similarly, in South Africa, residents in urban areas are constrained to participate effectively in household waste segregation largely because of inadequate space, lack of time and recycling knowledge, and inconvenient recycling facilities [92]. Hence, one of the recommendations suggested for increased household waste sorting in Johannesburg (South Africa) is to simplify household waste separation so that it is convenient to residents across all age groups [93].
Lastly, since food waste is one of the biggest contributors of MSW and constitutes more than 50% of this waste stream, it is important to treat it accordingly. Therefore, to curb the increase in household kitchen waste, source separation followed by treatment is recommended [82]. Furthermore, anaerobic digestion can be applied, thus reducing its environmental impact by over 51% [93]. Fei et al. [82] also suggested the use of food waste disposers (FWDs) since they can result in significant waste reduction. Designing on-site biochemical food waste treatment plants was identified as another waste reduction option, provided there are large amounts of food wastes to be brought to such treatment plants. Furthermore, food waste can be reused as animal food and a source of compost to support agricultural activities [94]. Table 4   The emerging keywords at the world perspective during the 2020s were COVID-19 waste research and CE, electronic waste, research based on China, waste disposal facilities, organic wastes, sewage waste treatment, and associated health issues. In South Africa, MSWM research in terms of the circular economy and COVID-19 challenges is lacking. By contrast, the most prominent keywords for MSWM research in South Africa are urban area, developing country, MSW, methane emissions, and landfills. Documents relating to electronic waste in South Africa were lacking amongst the 60 papers mentioned in Table 4.

Summary of the Literature Trends
Globally, there was a total of 37,276 keywords related to MSWM research. Some of the most dominant keywords that occurred more than 1000 times in descending order were MSWM, waste management, solid waste management, waste disposal, refuse disposal, solid waste (solid wastes), landfills, incineration, recycling, waste treatment, and waste incineration ( Table 5). The least occurring keywords that appeared less than ten times included waste reuse, sustainability indicators, MSW generation rates, radiation waste, waste collectors, and food waste disposers.  [56,99] In South Africa, there was a total of 1194 keywords related to MSW research. Except for waste incineration and gas emissions, the words were almost identical. Some of the most dominant keywords in South Africa that occurred more than ten times were MSW, waste management, waste disposal, refuse, solid waste (solid wastes), landfill, recycling, waste treatment, and gas emissions ( Table 6). The examination of the 60 articles in South Africa showed that studies on waste incineration or incineration are missing. In contrast, the least occurring keywords did not exactly match up to the global trend. The words that appeared the least were: waste-to-energy technologies, bioenergy, biological water treatment, sustainable development goals, leachate recycling, waste companies, anaerobiosis, and food waste disposers.

Conclusions and Recommendations
This paper conducted a bibliometric analysis of the available scientific literature on MSWM at a global level and from a South African perspective. With such analyses, it was possible to determine existing patterns and trends in the research about MSWM while revealing similarities and dissimilarities at a global stage and within a South African context. Based on the results generated, the following conclusions and recommendations are summarized.
Although the MSWM research started as early as 1968 from a global perspective, in South Africa such research focus only started in 1996. This discrepancy shows that MSWM research at an international level is ahead of South African contributions. The pattern is partly driven by differences in situational contexts, whereby MSWM issues were given much attention quite early in the developed countries largely because of the inherent environmental and health risks associated with MSW landfilling. For example, in the USA, the MSWM research around 1968 was pre-occupied with the challenges presented by unrestrained municipal waste generation and the extent to which landfilling was constrained by a lack of new spaces, especially in the urban areas, and the potential for environmental pollution [108]. At that time, the incineration of municipal waste was seen as a viable solution to reduce the volume of waste generated in the urban areas but with rising health concerns due to gaseous emissions [108]. The same MSWM issues involving emissions received research attention in South Africa, but this occurred much later during the 1996-2004 period. At that time, much research effort went into the understanding of the different chemical pollutants that were in gaseous emissions and the leachate produced by waste decomposition [104,109,110].
At an international level, the total number of research outputs, keywords relevant to MSWM, and the number of collaborations were relatively more for developed countries than is the case in South Africa. Globally, the research clusters consisted of keywords such as WTE technologies, sustainable development, circular economy (CE), and life-cycle assessment (LCA). On the other hand, the research clusters in South Africa consisted of MSW, landfills, waste treatment, WTE research, microbiological pollution, waste management cooperatives, and greenhouse gas emissions. These discrepancies in MSWM research indicate differences in research priorities and capabilities. This shows that more resources should be allocated to South African MSWM research so that it can be on par with international trends, especially in comparison with developed countries. Therefore, more research prioritization and funding are recommended in South Africa for the following research topics: Furthermore, given the limited international collaborations that South Africa has with other countries, there is a need to initiate and reinforce existing research linkages with other countries, thus involving a greater sharing of resources, capabilities, and experiences than is currently the case. Similarly, to stay current with the latest MSWM research, trends require further literature re-assessments from time to time to document new trends and identify research gaps.

Limitations of Study
The current study focused on peer review articles and book chapters that have been included in the Scopus database. It is possible that some important literature from conference papers and technical papers may have been missed. This has also been reported by other authors that conducted bibliometric analysis.