Trends in global research on Sanitation: A 30-year perspective from 1990 to 2019

Background began there of on this and the body of research continues to rapidly. Methods Based on the Web of Science Core Collection databases from 1990–2019, this paper applies the bibliometric method to analyze the research characteristics and developments of scientific publications about sanitation. Visualized knowledge mapping is employed to investigate development status, scientific collaboration, involved disciplines, research hotspots and emerging trends of this field. significantly the number of related publications increases The strength of countries is greater than that countries; the comprehensive strength the States ranks first field, followed by England Switzerland. This field is a highly interdisciplinary field that covers a wide range of interests, and institutional collaboration is increasing in this field. Keyword clustering analysis shows that the main research topics in the domain of sanitation could be summarized as: (a) drinking water; (b) sustainability; (c) biofilm; (d) epidemiology; and (e) WaSH. Meanwhile, keyword bursts analysis showed that the new research hotspots and research frontiers mainly concentrated on: (a) Sustainable development of sanitation services adapting to climate change; (b) Main determinants affecting child malnutrition; (c) Multinational research and model innovation; and (d) Evaluations on various aspects of performance. paper can serve reference for future research. (b) (c) Multinational research and (d) Evaluations

The purpose of this study is to comprehensively and systematically provide a bibliometric analysis of research on sanitation. More specifically, CiteSpace was used to conduct a detailed analysis and offer an in-depth and structured review of global expansibility. The data was evaluated for the period of 1990-2019. The overall research status and trends were investigated in addition to scientific collaboration and the involved disciplines. Then, the hotspots and evolution of this field were explored across three periods. The analyses of hotspots and emerging trends in this paper are not limited to results retrieved from CiteSpace; instead, we apply an expansive review that included additional valuable literature on the basis of the database, aiming to provide a comprehensive analysis. Our findings could assist researchers around the world to better understand the current state and latest research in this field and may serve as a potential guide for future study.

Materials And Methodology 2.1. Data collection
The Web of Science Core Collection from Thomson Reuters contains more than 12,000 influential academic journals that have been widely recognized by the international academic community. This paper takes the Science Citation Index Expanded (SCI-E) and Social Science Citation Index (SSCI) databases of the "Web of Science core collection" as the object database and sets TS= (sanitation or "sanitary equipment*" or "sanitary facilit*") as the retrieval condition, with a time span of 1990-2019, to search for related results. The search scope was "topic", including title, abstract and keywords. The 9,559 retrieved records were downloaded and saved as a plain text file in the format of "Full Record and Cited References", which was used as the sample in the paper.

Methodology 2.2.1. CiteSpace Software Parameters Setting
The tool used in this study, CiteSpace, is one of the most influential pieces of software in literature information analysis. The 5.5.R2 version of CiteSpace for 32-bit Windows with Java 8 was used. During execution, the parameters (e.g., time slice, node type, and pruning) in CiteSpace should be properly selected in accordance with the research objectives (Song et al. 2016). The parameters in CiteSpace were set as: (1) Time slicing from 1990 to 2019, years per slice = 1. (2) Node type = institution, category, keyword; (3) In the collaboration network and co-occurring network analysis, we selected the top 50 most-cited or most frequently occurring items from each slice for institutions, categories and keywords.

Comprehensive strength
The academic ability of one country is reflected in academic scale, influence and competence. Just as in other bibliometric studies (Yu et al., 2016), we selected eight indicators to evaluate the national comprehensive research strength of a single country: total number of articles, total number of citations, number of hot articles, number of hot articles citations, number of highly-cited articles (TOP100), number of highly-cited articles citations (TOP100), number of productive authors, and number of productive institutions. Standard scores are calculated using a standard method as shown in Formula (1), and the total score for each country was obtained through summing the eight standard scores as seen in Formula (2): 3. Results And Discussion 3.1. Temporal development and global geographic distribution analysis 3.1.1. Quantity of articles and citations In total, 9,559 valid records were obtained in the field of sanitation between 1990 to 2019. The trends in the quantity of articles over the last 30 years are shown in Fig. 1. From the perspective of quantity, in the first decade (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999), the average annual output was 89 articles, accounting for 9% of the total, while in the last decade (2010-2019), the average annual output was 666 articles, accounting for 70% of the total. In 2013, the United Nations designated November 19 as "World Toilet Day" to promote safe drinking water and basic sanitation facilities and a clean, comfortable and hygienic environment for all. As shown in Fig. 1, the number of publications in this field has risen sharply since 2013, with 1,078 in 2019. The increasing quantity of research indicates that the research on sanitation is in its "growth stage" and has great potential for development.
The total number of citations was 158,563 (1990-2019) and the average number of citations per publication was 16.6. The trends in quantity of citations in the last 30 years are shown in Fig. 1. The citation volume of articles on sanitation research before the 21st century grew slowly, with an average annual total citation volume of only 276.5. After 2000, the total citation volume increased steadily, and increased rapidly after 2010, with an average annual total citation volume of 2269.3

Global geographic distribution
In the past 30 years (1990-2019), 179 countries/regions have contributed to investigating sanitation, which means 91 percent of countries have scholars publishing in this field. Figure 1 shows the geographical distribution of published articles in this field. Different colors represent different numbers of documents in different geographic regions. Among the countries with more than 100 publications are three North American countries, ten European countries, seven Asian nations, seven African nations, plus Brazil in South America and Australia in Oceania. As shown in the diagram, articles about sanitation are mainly concentrated in the USA, England, Brazil, India, and Switzerland.    Control Prevention USA. The Swiss Tropical Public Health Institute (betweenness centrality, 0.14) and Harvard University (0.12) were relatively more influential than other institutions during this period.
From 2010 and 2019, the number of articles published increased rapidly. The most productive institutions during this period include University of London, Emory University and Stanford University.
From the perspective of influence on network, the London School of Hygiene and Tropical Medicine (betweenness centrality, 0.14) and Harvard University (0.10) were more influential than other institutions during this period. Overall, cooperation between research institutions increased during this period (See Fig. 2(c)).

Involved disciplines analysis
All the articles covered one of 219 ISI (Identified Subject Categories) in the WoS. The top 15 subject categories are shown in Table 3. Sanitation research is a multidisciplinary and interdisciplinary field that involves various subjects, such as environmental science, public health, water resources,  Co-occurring categories analysis is used to study interdisciplinary links and can reveal the intrinsic connections between various subject categories by constructing a subject-related network. Figure 3a shows that during the first period (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999), the top three categories in sanitation research were public environmental occupational health, environmental sciences and water resources. Public environmental occupational health (betweenness centrality, 0.37) in this period emerged as an intermediary that connected many different subject categories. In the second and third periods, in addition to public environmental occupational health, the betweenness centrality of environmental sciences is also greater than 0.1, which indicates that both are more important than other categories.
Moreover, as time goes by, the links between various subject categories become closer (See Fig. 3).
This result reflects the profound effects of sanitation because it shows that joint efforts are required to carry out global research.
3.4 Research topic analysis 3.4.1. Phase evolution analysis of research topics keyword occurrence. The links between nodes indicate that two relevant keywords appeared together in an article; a thicker link represents more frequent occurrences between the two keywords. As shown in Fig. 4 (a, b, c), the research nodes increased gradually across the three periods, and the cross fusion of each cluster was significantly enhanced. These results demonstrate that this field was constantly expanding and developing. During

Keyword clustering analysis
We also conducted further keyword clustering analysis using CiteSpace. On the basis of keyword cooccurrence analysis, keyword clustering analysis takes the frequency of co-occurrence as the object of analysis and uses the clustering statistical method to simplify the complex co-occurrence network relationship into the relationship between a relatively small number of groups (Zhong et al., 2008). Table 4 listed the top five clusters based on their sizes. However, since it is difficult to find specific information in the keywords from the cluster name alone, it is necessary to combine the main keywords in the cluster to analyze the specific research content contained in each cluster in a deeper way. The first cluster is related to drinking water. The main keywords contained in this largest cluster are the current top ten sanitation research keywords, including sanitation, health, diarrhea, hygiene, disease, developing country, and children. In this cluster, the researchers focus on the relationship between sanitation conditions and children's health and risk of disease (especially diarrhea) in developing countries. Safe drinking water is essential for human health and sustainable development (Forget et al., 2000). Therefore, this cluster also focuses on the studies of quality and management of drinking water at the supply and household levels ( The second cluster is about sustainability, including water, management, system, and wastewater. Access to improved sanitation facilities is key to the socioeconomic wellbeing and sustainable development of any society (Abubakar, 2017). In this area, researchers explored (1)

Keywords in different countries
According to Fig. 1, sanitation has been studied extensively on a global scale. Due to differences in geographical features, history and economy, the subjects studied vary between regions. Figure 5 provides the 5 most frequently used keywords in the 15 most influential countries. We found that, "sanitation" and "water" appear in the top 5 keywords for each country. However, there are differences among other keywords in different countries. For example, from 1990 to 2019, the most frequently used keywords in the USA were "sanitation" (849 times), "water" (361 times), "health" (357 times) "diarrhea" (282 times), and "hygiene" (236 times), while in Brazil, the most frequently used keywords were "Brazil" (87 times), "sanitation" (86 times), "water" (53 times), "children" (48 times), and "prevalence" (38 times). In addition, most countries (11/15) include "health" in highfrequency keywords and one third of countries pay more attention to research on "diarrhea" and sanitation-related "impact."

Research frontier analysis
Keywords with bursts refer to those keywords which increase sharply in frequency. Burst detection is a useful analytic method to find the keywords that receive particular attention from the scientific community during a certain period of time (Zhou et al., 2018). Therefore, bursting keywords can be used as indicators to predict research trends (Li and Chen, 2017). Table 5   . As shown in Table 5, we found that "infrastructure", "country", "undernutrition", "performance", "climate change", "determinant", "model" and "sustainability" were the emerging active topics in recent years. Combined with a keyword clustering analysis, the emerging sanitation research frontiers are described as follows: (1) Sanitation services adapting to climate change. Climate change threatens WaSH facilities and services (Alhassan and Hadwen, 2017). The effect of water on infectious disease deserves attention, particularly in low-and middle-income countries. Climate change effects will exacerbate water-borne disease challenges for the public health sector (Cisse, 2019). Therefore, research on the threats to current sanitation services from extreme weather events and the capacity of sanitation services to respond to climate change has increased rapidly in recent years. Some scholars have pointed out that climate change impacts, as well as increased water demand, pose serious risks to the provision of sustainable urban water services, e.g., drinking water, sanitation, and safe drainage (Johannessen and Wamsler, 2017). Moreover, a megacity may pose a greater risk due to its scale and complexity (Kim et al., 2018). Therefore, some studies have discussed Integrated Urban Water Management, which is the holistic management of urban water, sanitation, stormwater, and wastewater in order to achieve sustainable objectives of sanitation services (Kirshen et al., 2018). Meanwhile, because WaSH facilities tend to be unique in their vulnerability, they need region-based data to reveal local vulnerabilities that can inform strategic national or global planning. This requires more effort from researchers in different fields. In addition, rural fresh water and sanitation is a vulnerable sector in the context of climate change, so more research in this area is required.   (1) and (2) (Fig. 6). The indicators and the total standard score (TSS) of the USA are much higher than those of other countries, with a total standard score of 33.65. For England, the scores of the three indices TH, PA and THC are relatively high, and the total standard score is 16.36, putting it in second place. Although Switzerland has not published many particles, it has many citations and prolific authors, placing it in third. In terms of total publications, Brazil and India are the top two developing countries. As shown in Fig. 6, India's TSS scores are higher than Brazil's, which is mainly due to Brazil's low TMCC value.

Conclusions
This paper, using bibliometric analysis, provides a unique snapshot of sanitation research over the past 30 years. Publication data indicates increases in researcher output, especially after 2013.
Scholars from 91% of the countries in the world publish in this field. These publications are mainly concentrated in the USA, England, Brazil, India, and Switzerland. The USA ranks first in this field, followed by England and Switzerland, as reflected in its total number of articles, productive institutions and highly-cited papers. This domain is a highly interdisciplinary field that covers a wide range of interests, including environmental science public health, water resources, medicine, engineering, food science and microbiology.

Ethical Approval and Consent to participate
Not applicable.

Consent for publication
Not applicable.

Availability of data and material
All the data used in this paper are publicly available.

Competing interests
The authors declare that they have no competing interests.

Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Preparation of this manuscript was supported by Grant Nos. 71573179,             TSS: total standard score.