Global trends of municipal solid waste research from 1997 to 2014 using bibliometric analysis

Abstract

This study uses a bibliometric approach to identify global trends related to the municipal solid waste (MSW). It applies related literature in the Science Citation Index Expanded (SCI-EXPANDED), Social Sciences Citation Index (SSCI), Conference Proceedings Citation Index - Science (CPCI-S) and Conference Proceedings Citation Index - Social Science & Humanities (CPCI-SSH), retrieved from the ISI Web of Science. The data used covers the period from 1997 to 2014. Analyzed aspects included document type, and publication output as well as distribution of journals, subject category, countries, institutions, title-words, author keywords, and keywords plus. An evaluating indicator, citation score, was applied to characterize the MSW publications. The research outputs of MSW had steadily increased in the field of energy fuels, engineering chemical and biotechnology applied microbiology, especially environmental sciences and engineering environmental. The predominance of Chinese institutions in terms of article count and a predominance of industrialized countries’ institutions in terms of citation score were compared. Finally, author keywords, words in title, and keywords plus were analyzed to provide research emphasis, with the developing trends and recent hotspots provided.

Implications: A systematic overview of scientific literature dealing with municipal solid waste (MSW) is provided by a bibliometric analysis. The analysis of author keywords helps in drawing the research trends in a special perspective. Research studies on food waste, life cycle assessment (LCA), and renewable energy tend to be a new research focus in the area of MSW. The conclusions could provide a reference to the decision making and policy of MSW management for the government to some extent.

Introduction

Municipal solid waste (MSW) is the abridgment of the waste generated from domestic activities by persons that is collected and treated by municipalities. Because of the exponential growth of population and urbanization, coupled with the improvement of living standard and the development of social economy, the amount of MSW throughout the world has increased a lot, which poses a potential threat of environmental dilapidation (Karak et al., 2012). Therefore, MSW has been one of the key topics for environmental protection and resource utilization nowadays (Renou et al., 2008; Murphy and McKeogh, 2004). The last decade was characterized by great progress in development of MSW disposal, energy conversion methods, and management. The techniques and management system of the MSW had changed dramatically, shifting from oversimplified procedures, such as collecting unsorted wastes first and then disposing them in landfills, to integrated and sustainable methods that incorporate waste reduction practices, classification, material recycling techniques, biological and thermal processes for energy recovery, and landfill disposal (Liamsanguan and Gheewala, 2008; Lombardi et al., 2015; Grimberg et al., 2015; Chen et al., 2014). Numerous studies have been carried out by researchers from different countries all over the world in various aspects of MSW, such as food waste (Yasin et al., 2013), technology innovation (Malkow, 2004; Tang et al., 2013), and life cycle assessment (LCA) (Chen et al., 2014). In addition, several scientometrics studies have been made to analyze the trends of solid waste (Yang et al., 2013a). The analysis of author keywords, words in title, and KeyWords Plus showed that MSW ranked first among the hot spots in the solid waste research. However, no systematic analysis of the scientific research on global MSW has been carried out to date.

Bibliometric techniques involve statistical methods of bibliography counting to evaluate and quantify the growth of literature for a particular subject. Meanwhile, they have already been widely applied in studies on scientific production and research trends in many science and engineering disciplines (Xie et al., 2008; Wang et al., 2013). In recent decades, more information, closer to the research itself, such as the distribution of different words in the paper title, author keywords, and KeyWords Plus, have been presented in the study of research trends (Wang et al., 2010; Zhang et al., 2010; Mao et al., 2010). In addition, the total local citation score (TLCS), the total global citation score (TGCS) (Ma et al., 2014), and the average citation frequency of an article in the local collection (ALCS) were also calculated in this paper. Moreover, our study also focused on the contrastive analysis between the highly productive and highly cited institutions through the rank of ALCS.

The present study aims at mapping the trends of global MSW research from 1997 to 2014 and summarizing the predominant research achievements by using the bibliometric method, including scrutiny of the country and subject categories and the innovative methods, considering author keywords, KeyWords Plus, and words in title. To this end, a comprehensive bibliometric analysis focusing on all relevant peer-reviewed articles on MSW was performed. Specific efforts are made in this paper to (1) summarize the significant publication performances in MSW research with basic statistics, such as the chronological distribution of articles, and the most relevant subject categories, scientific journals, and institutions, (2) identify the research focuses and the hot area of MSW research, and (3) track the shift of MSW research over the last 18 years.

Material and Methods

The ISI Web of Science (WoS) published by Thomson Reuters is considered to be the most important source of data for bibliometric analysis in the sciences (van Leeuwen, 2006). Compared with other databases like Scopus, its records are more consistent and standardized (Bettencourt and Kaur, 2011), allowing us to extract title text and author names. More importantly, they cited references for our bibliometric research. Moreover, WoS has a broad scope of the MSW relevant journals and covers multiple types of literature. Therefore, we chose WoS as the data source for our research in this paper, as well as regarding many international conferences on MSW as important platforms for MSW discussion. In view of this, the subfield databases Science Citation Index Expanded (SCI-EXPANDED) and Social Sciences Citation Index (SSCI), Conference Proceedings Citation Index—Science (CPCI-S) and Conference Proceedings Citation Index—Social Science & Humanities (CPCI-SSH) were also used as the data sources in this paper. “Municipal* solid* waste*” OR “urban* solid* waste*” were used as keywords to search titles, abstracts, and author keywords from 1997 to 2014. For the research on MSW, author keyword and word cluster analyses were made using Microsoft Excel 2007, and the Global Citation Scores (GCS) and Local Citation Scores (LCS) were acquired by using HistCite. The impact factor values from Journal Citation Reports (JCR) 2013 were also added for the identified journal titles.

Results and Discussion

Document types, publication output

The distribution of document types identified by the WoS was analyzed, from which eight document types were found. The article was the most frequently used document type, accounting for 81% of total production, followed distantly by proceedings papers (18%). Others include reviews (4.4%), editorial materials (0.72%), meeting abstracts (0.47%), corrections (0.18%), book chapters (0.06%), and letters (0.05%), which showed lower significance than articles and proceedings papers. As the dominant document types, 6,800 articles and 1,550 proceedings papers were then analyzed in the subsequent study.

MSW research developed gradually before 2002 and rapidly in the following years. As shown in Figure 1, the number of articles increased from 145 in 1997 to 219 in 2002, but rapidly increased from 219 in 2002 to 800 in 2013. Possible reasons for the dramatic increase might be the emergence of the serious global ecological and environmental crisis, the general improvement of the environmental consciousness and the particular attention to MSW, especially in developing countries, and so on. The continuous increase observed until today had a small negative peak in 2011. One possible reason for this drop of publications could be the economic crisis of 2009, which led to national economic difficulties and consequently to a possible shift in research emphasis from MSW to other areas like economic development, biotechnology, and energy fuel in 2010, which affected the research investment and the number of publications related to MSW in 2011. To some extent, this also showed good agreement with the following analysis of subject categories. However, the publications were still showing a faster growth trend on the whole. An exponential model described the relationship between the annual cumulative number of all publication and the year published for the periods from 1997 to 2014 (Figure 1). A significant correlation between the number of articles and the year was observed with a high coefficient of determination (R² = 0.9401). The exponential curve fitting indicated that there was a high growth rate for annual articles. Based on the exponential model during the period of 1997–2014, the number of scientific articles on MSW was predicted to continue growing at a high rate.

Figure 1. Trends of publications referring to MSW production.

Publication patterns: source titles and subject categories

Articles (7,876) were published in 1464 journals in 139 subject categories in WoS. Table 1 lists the 10 journals with the greatest number of published articles referring to MSW research from 1997 to 2014. Waste Management published the most articles, the count of which was 848 (10.8%), followed by Waste Management Research (445) and Bioresource Technology (348). For comparison, the trends in the six journals with the greatest number of articles are shown in Figure 2. The numbers of articles in Waste Management and Journal of Hazardous Materials increased significantly from 2005 to 2009, while the number in Journal of Hazardous Materials decreased in recent years. The numbers of articles in Chemosphere and Resources Conservation and Recycling fluctuated from year to year, while that in Bioresource Technology steadily increased from 2006. These 6 most productive journals consisted of 2,329 articles in total, accounting for 30% of all MSW publications.

Figure 2. The growth trends of the top six journals.

Besides such well-known academic journals, conferences and related meetings are also an important platform for discussions on MSW in the course of its development. Three important conferences were Annual North American Waste to Energy Conference, International Conference on Waste Management and Technology, and International Conference on Waste Management and the Environment. Combined, they collected a total of 170 (79, 50, and 41, respectively) conference papers published in WoS.

For subject categories analysis, 7,876 articles (including 24 articles without subject category information) were analyzed statistically. As illustrated in Figure 3, based on the continuous increase in the number of articles per category, MSW research had increased in the categories of energy fuels, engineering chemical, and biotechnology applied microbiology, especially environmental sciences and engineering environmental, while fluctuated slightly in engineering civil. Our analysis also showed that after 2010 there was a small fall in the top 6 categories, except for energy fuel and biotechnology applied microbiology, which showed increasing public concerns with energy fuel and biotechnology. These results were in coherence with the analysis of journals to some extent. Subject categories comprising at least 500 articles were those that fell under the categories of environmental sciences (4,254; 54%), engineering environmental (3,104; 39%), energy fuels (1,047; 13%), engineering chemical (636; 8%), and biotechnology applied microbiology (632; 8%). These 5 most productive subject categories, all of which are branches of chemical and environmental science, consisted of 5,979 articles in total, accounting for 76% of all MSW publications.

Figure 3. The growth trends of the top six subject categories.

Publication distribution of countries and institutions

The analysis of the contributions of different publications by country/territories was based on journal articles in which the address and affiliation of at least one author was provided. Articles originating from England, Scotland, Northern Ireland, and Wales were recategorized as United Kingdom, whereas articles from Hong Kong, Macao, and Taiwan were not included under the China category. There were 24 articles without any author address information on the WoS. The total number of articles for the distribution analysis of countries and institutions publications was 7,852.

For the 7,852 articles from 4395 institutions in 117 countries, the temporal analysis for the top 6 most productive countries from 1997 to 2014 is shown in Figure 4. During the study period, China showed significant research strengths, followed by the United States, Italy, Spain, Japan, and Canada. While the United States was clearly leading in terms of publications output in MSW before 2006, we found a big gap between China and rest of the selected countries after 2008. China had the highest growth rate from 2 in 1997 to 185 in 2013, and took first place in the years from 2006 to 2014. The results indicated that China had significantly increased its publication output in MSW in recent years. The significant focus of the Chinese researchers in the areas of MSW might indicate the outcomes of Chinese government policies and initiatives, considering MSW as one of China’s priority areas of environmental management (Chen et al., 2010; Yang et al., 2013b). Thus, there is also a strong correlation between the number of articles and the focus of national policies. It is also worth to mentioning the concurrently growing interest in scientific research, which can be easily observed in China in other scientific fields. Similarly, in the United States, Italy, and Spain the increasing trend could be connected to the increased national interest to upgrade MSW recovery and management.

Figure 4. The growth trends of the six most productive countries.

Of these six productive countries, four are from the major industrialized countries (G7: the United States, the United Kingdom, Germany, Canada, Italy, Japan, and France) and one from the developing countries. Thus, it was suggested that economic development and scientific investment have contributed much to the distribution. Overall, the number of WoS publications from all countries just listed had shown a faster growth since 2005. A likely reason for this growth may be the worldwide concern about MSW (Zhang et al., 2010; Pires et al., 2011). For example, the 3R (reduce, reuse, and recycle) Initiative was agreed upon at the G8 Sea Island Summit in June 2004 as a new G8 initiative, which promoted the process of research on MSW and advanced relative research to a certain extent.

The top 10 most productive institutions are listed in Table 2. The Chinese Academy of Sciences (137), Tongji University (136), and Zhejiang University (132) are the top three research institutions. It is worth noting that the top three research institutions are all from China. Furthermore, of the top 10 most productive institutions, there are 4 institutions from China, 2 from Spain, and 1 each from Denmark, the United States, Canada, and Taiwan. It is notable that no institution from Italy or Japan can be found in the top 10 most productive institutions. Yet it is also worth mentioning that when compared by LCS and GCS, the Technical University of Denmark showed dominance in article quality. It also got the highest ALCS with a frequency of 11.3, followed by the Universidad Autonoma de Barcelona with 7.0 and the University of Regina with 5.7, which indicated the high average quality of their articles. However, Chinese institutions ranked 7th, 8th, 9th, and 10th, respectively, in the citation frequency of articles, which might indicate that there had been a considerable problem with the quality of Chinese articles issued. It also can be drawn from the results in Figure 4 and Table 2 that there is also a strong correlation between the most relevant institutions and the most productive countries.

Table 1. Top 10 most productive journals, 1997–2014.

Journal	TP	R (%)	TLCS	TGCS	IF
Waste Management	848	1(10.8)	5299	11791	3.157
Waste Management Research	445	2(5.7)	1644	3765	1.114
Bioresource Technology	348	3(4.4)	2096	7838	5.039
Journal of Hazardous Materials	275	4(3.5)	2351	5923	4.331
Chemosphere	228	5(2.9)	1415	4997	3.499
Resources Conservation and Recycling	185	6(2.3)	1039	3087	2.692
Environmental Science Technology	157	7(2.0)	1229	3926	5.481
Water Science and Technology	120	8(1.5)	704	2024	1.212
Compost Science Utilization	101	9(1.3)	436	1161	0.662
Science of the Total Environment	97	10(1.2)	582	1682	3.163

Notes: TP, total number of publications; R (%), rank and percentage of total publication for a certain journal; TLCS, total local citation score, which is the number of times cited by other papers in the local collection, provided by HistCite; TGCS, total global citation score, which is the citation frequency based on the full WoS count at the time the data was download, provided by HistCite; IF impact factor, indexed in the Journal Citation Reports 2013.

Table 2. Top 10 most productive institutions, 1997–2014.

Institution	TP	LCS	GCS	R(ALCS)
Chinese Academy of Sciences	137	438	1491	8 (3.2)
Tongji University, China	136	477	1031	7 (3.5)
Zhejiang University, China	132	387	916	9 (2.9)
Technical University of Denmark	111	1253	3168	1 (11.3)
Consejo Superior de Investigaciones Cientificas, Spain	111	461	2591	6 (4.2)
Tsinghua University, China	100	276	644	10 (2.8)
University of Florida, United States	93	468	1865	5 (5.0)
University of Regina, Canada	86	487	1425	3 (5.7)
Universidad Autonoma de Barcelona, Spain	68	479	1205	2 (7.0)
National Taiwan University	63	325	836	4 (5.2)

Notes: TP, total number of publications; LCS, local citation score, which is the number of times cited by other papers in the local collection, provided by HistCite; GCS, global citation score, which is the citation frequency based on the full WoS count at the time the data was download, provided by HistCite; R(ALCS), rank of average citation frequency of an article in the local collection.

Research emphasis: Author keywords, words in title, and KeyWords Plus

The examination of author keywords in the present study reveals that 12,135 author keywords were used from 1997 to 2014: 9,019 (74%) keywords were used only once, 1,394 (11%) were used twice, and 544 (4.4%) were used three times. The large number of once-only author keywords probably indicated a lack of continuity in research and a wide disparity in research aims (Chuang et al., 2007). Only 1,179 (9.7%) keywords were used more than three times, indicating that mainstream research on MSW focused on a small area. These keywords were calculated and ranked using 6-year intervals to minimize year-to-year fluctuations.

Table 3 shows the top 30 most frequently used author keywords along with their rankings and percentages. Through the analysis of the top 30 most frequently appearing author keywords, we could roughly draw the research trends from 1997 to 2014. Except for “municipal solid waste,” “solid waste,” “waste” (the searching words studied in the present work), and “MSW” (the meaning is the same as “municipal solid waste”), most of the top 30 author keywords were related to the following aspects: treatment and disposal methods (landfill, composting, anaerobic digestion, incineration, leaching, biodegradation, pyrolysis, gasification, combustion; 1,935, 33%), pollutants (heavy metal, leachate, fly ash, bottom ash, landfill leachate, landfill gas; 1090, 18%), renewable energy (biogas, methane, biomass; 437, 7.4%), management (waste management, solid waste management; 282, 4.8%), types of waste (sewage sludge, food waste; 220, 3.7%), recycle (recycling; 203, 3.4%), life cycle assessment (160, 2.7%), and environment (environment, soil; 137, 2.3%). This indicates that the research on the study of treatment and disposal methods and the pollutants attracted more attention.

Table 3. Top 30 most frequency of author keywords, 1997–2014.

	TP	R (%)
Author keywords	1997–2014	1997–2014	1997–2002	2003–2008	2009–2014
Municipal solid waste	1239	1 (21)	1 (23.1)	1 (21.1)	1 (20.4)
Landfill	468	2 (7.9)	3 (9.9)	4 (7.6)	2 (7.6)
Compost	445	3 (7.5)	2 (11.3)	3 (8.0)	5 (6.4)
Heavy metal	429	4 (7.3)	4 (7.8)	2 (8.5)	4 (6.5)
Anaerobic digestion	389	5 (6.6)	6 (5.3)	5 (6.1)	3 (7.1)
Leachate	227	6 (3.8)	7 (5.0)	7 (4.3)	9 (3.3)
Biogas	220	7 (3.7)	16 (2.4)	13 (3.1)	6 (4.3)
Incineration	217	8 (3.7)	5 (5.5)	6 (4.5)	12 (2.8)
Recycling	203	9 (3.4)	8 (4.4)	10 (3.5)	10 (3.2)
Waste management	200	10 (3.4)	18 (2.3)	11 (3.2)	7 (3.7)
Solid waste	191	11 (3.2)	9 (3.9)	9 (3.5)	11 (2.9)
Fly ash	178	12 (3.0)	11 (3.3)	8 (3.9)	13 (2.5)
Life cycle assessment	160	13 (2.7)	52 (1.0)	17 (2.1)	8 (3.4)
MSW	151	14 (2.6)	13 (3.0)	12 (3.2)	14 (2.2)
Sewage sludge	137	15 (2.3)	10 (3.3)	15 (2.3)	15 (2.1)
Methane	111	16 (1.9)	14 (2.4)	20 (1.7)	16 (1.8)
Leaching	109	17 (1.8)	12 (3.1)	14 (2.6)	28 (1.2)
Biomass	106	18 (1.8)	51 (1.0)	18 (2.1)	17 (1.8)
Bottom ash	99	19 (1.7)	23 (1.9)	16 (2.1)	23 (1.4)
Landfill leachate	93	20 (1.6)	62 (0.9)	19 (2.1)	21 (1.5)
Food waste	83	21 (1.4)	139 (0.5)	28 (1.2)	18 (1.7)
Solid waste management	82	22 (1.4)	24 (1.8)	48 (0.9)	19 (1.5)
Waste	82	23 (1.4)	31 (1.3)	26 (1.3)	22 (1.5)
Biodegradation	82	24 (1.4)	26 (1.6)	23 (1.5)	24 (1.3)
Pyrolysis	76	25 (1.3)	15 (2.4)	33 (1.1)	30 (1.1)
Gasification	75	26 (1.3)	49 (1.0)	51 (0.9)	20 (1.5)
Combustion	74	27 (1.3)	20 (2.1)	21 (1.7)	44 (0.8)
Environment	71	28 (1.2)	29 (1.4)	34 (1.0)	26 (1.2)
Soil	66	29 (1.1)	41 (1.0)	24 (1.5)	36 (1.0)
Landfill gas	64	30 (1.1)	37 (1.1)	31 (1.2)	35 (1.0)

Notes: TP, total number of publication; R (%), rank and its percentage in different period.

During the whole study period, rankings of these author keywords shifted. The author keywords of “gasification,” “biomass,” “landfill leachate,” “life cycle assessment,” and “food waste” exhibited higher growth rates than did any others, and were more frequently used in recent years— especially “food waste,” which ranked 139th in 1997–2002, and soared to 18th in 2009–2014. The research on food waste has posed a serious economic and environmental concern along with an awareness of food waste and has grown at the international level. Increasing focus has been given to food waste collection, treatment, and minimization (Bernstad et al., 2015; Halloran et al., 2014; Bernstad and la Cour Jansen, 2012). In addition, it appears from previous studies that the bioconversion of food waste to energy in terms of ethanol, hydrogen, methane, and biodiesel is economically viable (Uçkun Kiran et al., 2014). Management systems and bioconversion of food waste to energy have therefore attracted increasing interest recently. Another noteworthy point is that the author keywords of “life cycle assessment,” which ranked 52nd in 1997–2002, soared to 8th in 2009–2014. The popularity of life cycle assessments (LCAs) in analyzing MSW management systems has been illustrated by the numerous published studies of the life cycle emissions of these systems, as well as the substantial number of LCA computer models addressing MSW management. Over the past decades, many academics, as well as organizations such as the International Organization of Standardization (ISO) and the Society of Environmental Toxicology and Chemistry (SETAC), have contributed to the development of the methodology used to undertake LCAs (Cleary, 2009). LCA has been acknowledged as a tool enabling researchers to consider the full life cycle of MSW management system, and the holistic information provided by it has been used for decision support in MSW management planning (Zhao et al., 2009). Another shift is for the author keyword of “gasification,” which ranked 49th in 1997–2002, stayed around 51st in 2003–2008, and increased to 20th in 2009–2014 rapidly. Gasification has several advantages over traditional combustion of solid wastes, mainly due to the possibility of combining the type of starting waste, operating conditions, and features of the specific reactor (fluidized bed, moving grate furnace, direct melting furnace, rotary kiln) to obtain a syngas suitable for use in different applications as a chemical feedstock, or a fuel gas that can be burned in gas reciprocating engines or gas turbines to generate electricity (Arena, 2011). Moreover, recent policy to tackle climate change and resource conservation provided a renewed further impetus to the gasification technology development on MSW. Many relevant innovative technologies have been proposed, such as high-temperature gasification and gasification associated with a plasma process (Arena, 2012; Lombardi et al., 2012; Ruj and Ghosh, 2014). In recent years, the study on the technical reliability of this new technique has become a cause for concern (Arena et al., 2012; Consonni and Viganò, 2012). Another important shift is for the author keywords of “biomass,” which ranked 51st in 1997–2002, soared to 18th in 2003–2008, and reached 17th in 2009–2014. The development of renewable energy sources has clearly emerged as a promising policy toward enhancing the fragile global energy system with its limited fossil fuel resources, as well as for reducing the related environmental problems. Waste biomass utilization has been regarded as a viable alternative for energy production, encompassing a wide range of potential thermochemical, physicochemical, and biochemical processes (Iakovou et al., 2010). Municipal biomass waste (MBW) comprises kitchen waste (KW), vegetable/fruit residue (VFR), and waste activated sludge (WAS), which is a large class of biomass and a potential source of renewable energy. Therefore, the utilization of MBW accomplishes the dual goal of waste reduction and energy production. Consequently, research on an ideal way to treat MBW was of paramount importance for scholars (Liu and Wang et al., 2012; Liu and Gao et al., 2012; Wu et al., 2015). Use of the author keywords “landfill leachate,” which ranked 62nd between 1997 and 2002, suddenly increased to 19th in 2003–2008, and dropped to 21st in 2009–2014. The release of leachate to the environment is one of the major environmental impacts related to the disposal of waste. Landfill leachate contains four groups of pollutants: dissolved organic matter, inorganic macrocomponents, heavy metals, and xenobiotic organic compounds (Kjeldsen et al., 2002), resulting in a threat to the surrounding soil, groundwater, and surface water (Aktaş and Çeçen, 2001). As the burial of MSW in landfills is the most common disposal alternative in most countries, it has prompted a growing research interest in establishing a leading selective, reliable, and durable alternative for the treatment of heavily polluted leachates. Lately, widely varied scientific publications covering the collection, storage, and appropriate treatment of the highly contaminated landfill leachates or its manifestations have emerged (Renou et al., 2008). The frequently used author keywords of “landfill leachate” in recent years pointed to the great importance that has been attached to environmental protection. On the other hand, the author keywords “incineration,” “combustion” (the meaning is the same as “incineration”), and “pyrolysis” lost their research potential in the study period, since they gradually decreased in the list of frequently used author keywords. Their ranks, which were 5th, 20th, and 15th in 1997–2002, fell to 6th, 21st, and 33rd in 2003–2008, and then descended sharply to 12nd, 44th, and 30th in 2009–2014, respectively.

Additionally, we also found that there existed a major difference between the topics of concern in the developed and developing countries. To illustrate the point, the comparison between the G7 (United States, United Kingdom, Germany, Canada, Italy, Japan, and France) and BRIC (Brazil, Russia, India, and China) countries in terms of the top 20 most frequency of author keywords is shown in Table 4. It indicates that developed countries, especially the G7, had undoubtedly played a dominant role in the current field of MSW along with a high ratio in both amount and percentage of corresponding keywords during the study period. Moreover, through the analysis and comparison, we found that research studies from the G7 were more concerned about the conversion of waste to energy, reflecting by higher rankings and percentages of energy-related author keywords, such as “anaerobic digestion,” “recycling,” “biogas,” and “waste-to-energy.” In addition, the G7 were more likely to pay attention to waste management than BRIC. Developing countries were focused more on waste disposal methods, especially pollutants control. “Heavy metals,” “leachate,” “fly ash,” “optimization,” and “bioreactor landfill” were more frequently used as author keywords in BRIC. It is also worth noting that “food waste” and “life cycle assessment” appeared in the list of the G7, which implied that the frontier and hot topic of MSW research gained wide attention among the developed countries.

Table 4. Comparison between G7 and BRIC countries on the top 20 most frequency of author keywords, 1997–2014.

Author keywords (G7)	TP	%	Author keywords (BRIC)	TP	%
Municipal solid waste	434	35.0	Municipal solid waste	424	34.2
Anaerobic digestion	167	42.9	Landfill	105	22.4
Landfill	211	45.1	Heavy metals	166	38.7
Heavy metals	100	23.3	Leachate	104	45.8
Compost	196	44.0	Fly ash	106	59.6
Incineration	97	44.7	Compost	73	16.4
Waste management	91	45.5	Anaerobic digestion	51	13.1
Recycling	87	42.9	Incineration	46	21.2
Biogas	87	39.5	Environment	34	47.9
Leachate	61	26.9	Biogas	32	14.5
Fly ash	60	33.7	Sewage sludge	29	21.2
Sewage sludge	52	38.0	Recycling	29	14.5
Leaching	49	45.0	Waste management	26	13.0
Environment	44	62.0	Sanitary landfill	24	14.3
Bottom ash	42	42.4	Optimization	24	50.0
Gasification	42	56.0	Biodegradation	24	29.3
Life cycle assessment	42	26.3	Methane	23	20.7
Waste-to-energy	40	65.6	Bioreactor landfill	23	57.5
Methane	39	35.1	Solid waste management	22	26.8
Food waste	36	43.4	Leaching	22	20.2

Notes: TP, total number of publications; %, its percentage in corresponding keywords.

In the title analysis, prepositions such as “of,” “in,” and other meaningless words were excluded. After eliminating these words, the top 20 most frequently used single substantives in the titles were analyzed, also in three 6-year periods, and are shown in Table 5. The ranks of “energy” and “assessment” had a significant increase. To some extent, the results mirrored the analysis of author keywords. The distribution of KeyWords Plus with its rank and percentage in different periods is shown in Table 6. KeyWords Plus is the result of Thomson Reuters editorial expertise in science, which supplied additional search terms extracted from the titles of articles cited by authors in their bibliographies and footnotes (Garfield and Sher, 1993). KeyWords Plus analysis as an independent supplement reveals the article contents with more details. There were some similar and dissimilar trends between the other two statistical results in the study period. As a whole, results of KeyWords Plus paralleled those of the author keywords, as did results of title words. “Biomass,” “life-cycle assessment,” and “leachate” were also the highlight of KeyWords Plus in the study period, which might identify current MSW research hotspots. However, KeyWords Plus gave special prominence to “management,” “behavior,” and “emissions,” and showed a rapid growth of “removal” and “manure,” which were all closely correlative to MSW. Meanwhile, it weakened the leading status of the treatment and disposal methods such as “landfill” and “compost,” which was different from the trends of author keywords and title words. KeyWords Plus with additional search terms may be more concerned about interdisciplinary and novel research direction than the mature direction in the field.

Table 5. Top 20 most used single words in title, 1997–2014.

	TP,	R (%)
Title words	1997–2014	1997–2014	1997–2002	2003–2008	2009–2014
Waste	3881	1 (49.3)	1 (51.1)	1 (48.3)	1 (49.3)
Solid	3052	2 (38.8)	2 (42.9)	2 (39.7)	2 (37.0)
Municipal	2854	3 (36.2)	3 (41.8)	3 (36.9)	3 (34.2)
Landfill	813	4 (10.3)	5 (9.2)	5 (10.7)	4 (10.4)
Compost	712	5 (9.0)	4 (10.5)	6 (9.5)	8 (8.3)
Organic	692	6 (8.8)	9 (7.4)	7 (8.4)	6 (9.4)
Ash	683	7 (8.7)	6 (9.0)	4 (11.5)	9 (7.0)
Management	635	8 (8.1)	13 (5.6)	9 (7.0)	5 (9.4)
Anaerobic	609	9 (7.7)	14 (5.0)	8 (7.5)	7 (8.7)
Incineration	471	10 (6.0)	8 (7.6)	10 (6.9)	17 (5.0)
Leachate	450	11 (5.7)	15 (4.9)	12 (6.4)	11 (5.6)
Fly	427	12 (5.4)	10 (6.0)	11 (6.5)	18 (4.6)
Treatment	402	13 (5.1)	24 (3.3)	16 (4.5)	10 (6.0)
Soil	395	14 (5.0)	11 (5.9)	19 (4.2)	16 (5.2)
Incinerator	385	15 (4.9)	7 (8.0)	13 (6.1)	28 (3.3)
MSW	369	16 (4.7)	12 (5.8)	14 (5.9)	23 (3.7)
Assessment	350	17 (4.4)	45 (2.1)	22 (3.9)	13 (5.4)
Energy	326	18 (4.1)	57 (1.8)	32 (3.3)	15 (5.3)
Heavy	315	19 (4.0)	17 (4.2)	17 (4.3)	20 (3.8)
Sludge	290	20 (3.7)	36 (2.6)	34 (3.1)	19 (4.3)

Notes: TP, total number of publications, R (%) rank and its percentage in different period.

Table 6. Top 30 most frequency of KeyWords Plus, 1997–2014.

	TP,	R (%)
Keywords Plus	1997–2014	1997–2014	1997–2002	2003–2008	2009–2014
Municipal solid-waste	1898	1 (29.4)	1 (16.0)	1 (27.2)	1 (33.3)
Sewage-sludge	472	2 (7.3)	5 (6.1)	3 (6.4)	2 (8.0)
Management	439	3 (6.8)	24 (2.3)	2 (6.6)	4 (7.9)
Heavy-metals	393	4 (5.8)	21 (2.7)	12 (3.7)	3 (8.0)
Water	372	5 (4.8)	3 (6.8)	6 (5.6)	5 (5.6)
Behavior	309	6 (4.7)	6 (5.3)	9 (4.3)	6 (4.9)
Soil	305	7 (4.7)	12 (4.1)	8 (5.2)	9 (4.6)
Emissions	304	8 (4.5)	4 (6.4)	5 (5.8)	12 (3.8)
Fly-ash	289	9 (4.4)	8 (4.8)	11 (3.8)	8 (4.7)
Combustion	287	10 (3.9)	2 (6.8)	4 (6.1)	20 (3.2)
Model	255	11 (3.8)	9 (4.4)	7 (5.4)	21 (3.1)
Degradation	248	12 (3.7)	17 (2.9)	15 (3.2)	10 (4.3)
Systems	242	13 (3.7)	14 (3.4)	10 (4.3)	16 (3.6)
Performance	234	14 (3.6)	80 (1.0)	25 (2.5)	7 (4.7)
Sludge	227	15 (3.5)	14 (3.4)	14 (3.3)	14 (3.6)
Anaerobic-digestion	224	16 (3.5)	31 (2.1)	21 (2.7)	11 (4.1)
Incineration	207	17 (3.2)	43 (1.7)	19 (2.9)	13 (3.7)
Biomass	190	18 (2.9)	138 (0.7)	26 (2.5)	15 (3.6)
Organic-matter	189	19 (2.9)	33 (1.8)	13 (3.4)	25 (2.9)
Carbon	187	20 (2.9)	13 (3.7)	16 (3.1)	30 (2.6)
Temperature	179	21 (2.8)	36 (1.8)	26 (2.5)	22 (3.1)
Organic fraction	173	22 (2.7)	29 (2.1)	31 (2.1)	23 (3.1)
Removal	173	23 (2.7)	123 (0.7)	37 (2.0)	18 (3.4)
Manure	167	24 (2.6)	75 (1.0)	38 (1.8)	19 (3.3)
Dibenzo-p-dioxins	167	25 (2.6)	7 (4.8)	17 (3.0)	49 (1.9)
Residues	157	26 (2.4)	75 (1.0)	24 (2.5)	27 (2.7)
Life-cycle assessment	151	27 (2.3)	— (0)	93 (1.0)	17 (3.5)
Waste	147	28 (2.3)	30 (2.1)	34 (2.0)	31 (2.4)
Leachate	145	29 (2.2)	141 (0.7)	35 (2.0)	28 (2.7)
Speciation	144	30 (2.2)	23 (2.5)	18 (2.9)	52 (1.8)

Notes: TP, total number of publications; R (%), rank and its percentage in different period.

Conclusion

Based on 7,876 MSW articles retrieved from the WoS, this bibliometric study provided an overview of the research in MSW and identified some significant points in the research throughout the investigation period. The following conclusions were drawn from this study:

1. MSW research studies have significantly increased in the past 18 years, especially from 2002 to 2013.

2. There were in total 1464 journals distributed in the 139 subject categories. MSW research studies had steadily increased, especially at the beginning of the 21st century, in the fields of energy fuels, engineering chemical, biotechnology applied microbiology, and especially environmental sciences and engineering environmental, while fluctuated slightly in engineering civil.

3. The industrialized countries, with a longer tradition in research in this field, especially the G7, of which four countries were in the top six productive countries, had generated more research studies. Developing countries like China, which had become the most productive country since 2006, represented the highest growth rate. Furthermore, the top 3 productive research institutions are all from China, whereas no institution from Italy or Japan can be found in the top 10 most productive institutions. An interesting observation that also needs to be pointed out is the predominance of Chinese institutions in terms of article count and a predominance of industrialized countries’ institutions in terms of citation score. It may be inferred from this difference that articles written by scholars affiliated to industrialized countries’ institutions are less in terms of quantity but more cited in terms of scientific impact than Chinese institutions.

4. According to the analysis of the frequency of title words, author keywords, and KeyWords Plus, research on the study of treatment and disposal methods and the pollutants attracted more attention. In addition, the author keywords of “gasification,” “biomass,” “landfill leachate,” “life cycle assessment,” and “food waste” exhibited higher growth rates than did any others, and were more frequently used in recent years. This indicates that research on these aspects has drawn increasing attention among researchers, especially the research on “food waste” and “life cycle assessment.” It also implies that the management innovation and renewable resources are the focal point in the area of MSW. Additionally, we also found that there exists a major difference between the topics of concern in the developed and developing countries. Research studies from the G7 were more concerned about the conversion of waste to energy, and those from BRIC were focused more on waste disposal methods, especially pollutants control. It is also worth noting that the frontier and hot topics of MSW research like food waste and life cycle assessment gained wide attention among the developed countries.

Limitations and future work

One limitation of the approach is that Web of Science database might have omitted other relevant research. Moreover, other means of communication like oral communication between scientists, internal reports between research institutions, or other informal channels are also important.

An interesting avenue for future work would be to conduct a more detailed analysis and bring a more in-depth discussion into the results that have been introduced in the preceding sections, which could be conducted with innovative bibliometric tools to discern. For instance, some significant questions include: What is the root cause of the citation pattern? How have the important publications shaped (or still are shaping) the field of MSW?

Additional information

Notes on contributors

Haibin Chen

Haibin Chen is a research professor at the School of Environmental Science & Engineering, Huazhong University of Science and Technology, in Wuhan, China.

Wei Jiang

Wei Jiang, Yu Yang, Yan Yang, and Xin Man are PhD candidates in environmental engineering with the School of Environmental Science & Engineering, Huazhong University of Science and Technology, in Wuhan, China.