3.4.1. Phase evolution analysis of research topics
1990 to 2019 was divided into three periods, and the top 50 keywords in each year were selected for analysis. Each node in Fig. 4 represents a keyword; larger nodes represent higher frequency of 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 the period of 2000–2009, research on sanitation and children's health in developing countries remained important. Meanwhile, as shown in Fig. 4 (b), “water” became the most-studied subject and “health” was also important. In September 2000, global heads of state voted to pass the United Nations Millennium Declaration and set a series of goals to be met by 2015, known as the "Millennium Development Goals" (MDG). The United Nations Millennium Development Goals have called issues of water and sanitation to the forefront of international development efforts. Safe drinking water is essential for human health and sustainable development and managing water is an essential component of the UN's MDG. While most countries are committed to increasing access to safe water, there is little consensus on how to actually improve water services (Galiani et al., 2005). During this period, researchers discussed ways to increase accessibility and use of water and sanitation, as well as the roles of private providers and civil society institutions in providing water and sanitation services (Al-Hmoud and Edwards, 2005; Sansom, 2006). In addition, they focused on the effects of sanitation reforms and policies on a variety of health outcomes, including public health, environmental health, rural health, health of schoolchildren, and health for poor people (Agoramoorthy and Hsu, 2009; Fry et al., 2008; McKee et al., 2006; Ulukanligil and Seyrek, 2003; Clark and Gundry, 2004; Anwar, 2003).
The breadth and depth of research on sanitation increased significantly during the period of 2010–2019. As shown in Fig. 4 (c), “sanitation”, “water” and “health” remain the most popular subjects, and “hygiene” as well as “impact” became more prominent. The United Nations 2030 Agenda for Sustainable Development proposes 17 sustainable development goals, of which Sustainable Development Goal (SDG) 6 aims to ensure availability and sustainable management of sanitation and water for all people, in all settings. In particular, SDG 6.2 is to achieve access to adequate and equitable sanitation and hygiene for all by 2030 (UNICEF/WHO, 2017). The work in hygiene is aimed at nurturing good hygiene practices and there is growing recognition that comprehensive hygiene behavior improvements should be integral to prevention efforts (Alexander et al., 2019; Bartram and Platt, 2010; Andrade et al., 2019). During this period, a growing number of researchers conducted surveys and assessments of different hygiene conditions, including community-based or school-based hygiene facilities, water container hygiene, personal hygiene (residents, children, parents, etc.) and food hygiene (Andrade et al., 2019; Appiah-Brempong et al., 2018; Al-Khatib et al., 2015; Biran et al., 2012;). Some researchers estimated the impact of drinking water, sanitation, and hygiene improvements on children’ growth and development (Pickering et al., 2019; Johri et al., 2019; Ercumen et al., 2018; Dearden et al., 2017; Garn et al., 2016).
3.4.2. Keyword clustering analysis
We also conducted further keyword clustering analysis using CiteSpace. On the basis of keyword co-occurrence 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.
Table 4
The clusters and main keywords within the clusters
No. | Size | | Cluster name | Main keywords |
1 | 72 | 0.777 | drinking water | sanitation; health; diarrhea; hygiene; disease; developing country; children |
2 | 67 | 0.731 | sustainability | water; management; system; waste water |
3 | 65 | 0.795 | biofilm | contamination; escherichia coli; public health; bacteria |
4 | 63 | 0.755 | epidemiology | prevalence; transmission; outbreak; Africa |
5 | 48 | 0.829 | WaSH | infection; growth; mortality; malnutrition |
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 (Dzwairo et al., 2006; Cotton et al., 2013; Hoque et al., 2006).
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) sustainability and decision making in water and sanitation management systems (Iribarnegaray and Seghezzo, 2012), (2) sustainable services in water supply and sanitation (Behailu et al., 2016) as well as (3) the selection of sustainable sanitation arrangements (Mara et al., 2007). They also explored the connections between governance and sustainability and discussed their possible roles in water and sanitation management systems (WSMS) (Iribarnegaray and Seghezzo, 2012). The researchers are committed to developing and evaluating the application of several methodological tools to sustainability of water supply and sanitation services (such as analytic hierarchy process, Sustainable Water Governance Index (SWGI), Life-Cycle Analysis), focusing on relevance, effectiveness and sustainability (Boukhari et al., 2018; Iribarnegaray and Seghezzo, 2012; Jones and Silva, 2009; Cotton et al., 2013). In addition, upgrading the coverage of wastewater management and services and increasing their efficiency is necessary to improve the environmental in cities (Joseph, 2006). Many researchers in this cluster have also studied the problems of providing and financing provision for wastewater management as well as the influence of wastewater treatment on improved public health (Garcia, 2006; Naik and Stenstrom, 2012). Their findings indicate that wastewater management problems must be addressed not by the State directly but through a greater degree of participation and involvement by society (Garcia, 2006). They also concluded that an increase in wastewater treatment availability reduces mortality caused by disease (Naik and Stenstrom, 2012).
The third cluster investigates biofilm, with keywords including contamination, escherichia coli, public health, and bacteria. There are still many populations worldwide that do not have access to safe drinking water (Whitley et al., 2019;). The bacteriological quality of some water sources is in doubt because of their proximity to both point and diffuse sources of pollution and can thus present public health risks (Dorice et al., 2010). Therefore, many researchers in this cluster investigated bacterial contamination, bacterial quality, bacterial morbidity, and sources of contamination of various water sources (such as domestic Wells, springs, rivers, surface and groundwater in rural settlements, and community drinking water), as well as sediments in sanitary sewers (Ngasala et al., 2019; Dorice et al., 2010; Oluyege et al., 2009; Baig et al., 2012; Schmitt, 1992). Their studies have shown (1) the importance of greater attention to household contamination, environmental sanitation control and water contamination (Abdelrahman and Eltahir, 2011); (2) the need to develop appropriate sanitation strategies (Dorice et al., 2010); (3) the need for new regulatory mandates for the distance of domestic wells from sanitation systems to protect human health (Ngasala et al., 2019); (4) the importance of water quality monitoring and healthy domestic hygiene practices (Baig et al., 2012).
The next cluster focuses on epidemiology and includes prevalence, transmission, outbreak, Africa and other keywords. In this cluster, researchers analyzed the prevalence of various epidemic diseases (such as diarrhea, hepatitis A, trachoma, Helicobacter pylori, Ebola and cholera) and the potential risk factors (Brown, 2000; Oswald et al., 2017; Nurgalieva et al., 2002; Mallow et al., 2018; Ali et al., 2017). Many scholars focused on the role of hygienic and sanitation improvements in reducing the prevalence of epidemic diseases (Imada et al., 2016; Nurgalieva et al., 2002). In addition, some scholars have explored the differences in epidemic prevalence across different geographical regions and ethnic groups (Goh et al., 2011; Choi, 2003).
Cluster 5 focuses on Water, Sanitation and Hygiene (WaSH), and includes keywords such as infection, growth, mortality and malnutrition. Stunted growth among children is associated with poor WaSH, due to the effect of infection on intestinal nutrient absorption (Budge et al., 2019). Researchers in this cluster focused on assessing WaSH practices and their association with adolescent nutrition and exploring the linkages between stunted growth and WaSH (Chattopadhyay et al., 2019; Kwami et al., 2019). They also investigated the effectiveness of WaSH intervention in reducing the prevalence of childhood diseases like diarrhea and anemia. Most of these studies use the Randomized Controlled Trial method (Dey et al., 2019; Fancony et al., 2019). In addition, some researchers examine WaSH practices and associated risk factors among different groups (Hall, 2019; Hsan et al., 2019; Whitley et al., 2019). Researchers have found that efforts to improve nutrition and WaSH behaviors are most effective in promoting long-term health outcomes for children when implemented early on (Kwami et al., 2019). The implementation of an effective WaSH awareness program is useful along with improved water supply and sanitation to improve WaSH practices (Hsan et al., 2019).