Introduction

The water resource system is an indispensable part of the ecological civilization system. In the last few decades, the existence of emerging contaminants in the aquatic environment has become a worldwide issue (Luo et al. 2014; Li et al. 2020). An efficient sewage treatment system is one of the important ways to protect water resources.

Recent advances in microalgae wastewater treatment have offered ample opportunities to develop next-generation water treatment processes. A variety of microalgae, including Chlorella, Chlamydomonas, and Spirulina, have a good removal effect on organic matters (Mujtaba and Lee 2017; Mujtaba et al. 2017; Zhu et al., 2020), heavy metal ions (Wang et al. 2010; del Rosario Martinez-Macias et al. 2019), phenolic compound (Surkatti and Al-Zuhair 2018), and possible medical waste (Xiong et al. 2016) in sewage. On the other hand, microalgae after sewage treatment have strong potential economic value. Lipid in microalgae is easily converted into biofuels by biological/thermochemical methods (Zhu et al. 2018; Zhu et al. 2019), expected to alleviate the energy crisis to a certain extent. Certain food additives can also be produced by microalgae such as carotene, astaxanthin, EPA, and so on (Polishchuk et al. 2015). All of these are the unparalleled competitiveness of other conventional methods.

In brief, research in the field of microalgae wastewater treatment is expanding from depth to breadth. Here, we present a systematic review and bibliometric analysis on the application of microalgae for wastewater treatment.

Bibliometric is a branch of information science and philology, which refers to the quantitative analysis of all knowledge carriers using mathematical and statistical methods (Zupic and Cater 2015). It analyzes the number of papers, the frequency of citations, the country of publication, and other literature information, to compare the research differences between countries and regions at various time periods, and reveal the development law, research status, and future trends of the field (Bornmann and Mutz 2015; Du et al. 2014; Wang et al. 2018).

This study comprehensively examines the current status and the future of this research field through detailed review and large-scale bibliometric analysis, providing a certain basis for the optimization of future research directions. The contribution of this study lies in the provision of valuable insights for the future development of microalgae-based wastewater treatment through the bibliometric analysis of publications on the topic during the recent 20 years.

Material and methods

This search was carried out through the Web of Science Core Collection using the “ISI Web of Knowledge” database of Web of Science, in April 2020.

The search terms used were [TS = (microalga* OR alga*) AND TI = (effluent OR wastewater OR waste*water OR wasted water OR sewage OR biogas slurry)] in the Advanced Search. Until 2020 April 10, 2954 records were retrieved. Three hundred thirty-three invalid records focusing on toxicological analysis were eliminated after manual screening. The remaining 2621 documents are valid samples for the analysis, with an effective rate of 88.7%.

These search terms were determined by the expectation of locating the most relevant papers for the special issue on wastewater treatment by microalgae. These terms have a limit that the title must include one or more possible pending sewage, while microalgae were kept broad to give a complete picture of the research in this area as much as possible.

The scientometric analysis was firstly carried out by using the “analyze the results” tool of the Web of Science database. The basic information was downloaded as “document type,” “author,” “publication year,” “countries/regions,” “organizations-enhanced,” “source title,” and “subject area.”

As a second step, supplementary information was manually retrieved for the articles used like the total citation numbers pear year, the numbers and countries of multinational author papers, h index of authors, publishing dates and citation numbers of certain papers, and so on. Some data had undergone basic calculations such as average citation numbers pear year. If an article had multiple authors or countries, the article would be considered as the literary work of all authors and countries.

Finally, the most cited papers were analyzed, and the relevant information was summarily recorded to measure the impact of the original research found in the former steps. This article discussed the most relevant parts of these results, rather than posting full information in tables.

Results and discussion

Document types

Through keyword search and manual selection, a total of 2621 articles were obtained. Table 1 lists the types, numbers, and proportions of these documents. It is notable that the sum of all types of documents is greater than 2621, since some documents occupy multiple document types in the database. A representative example is that the conference paper “Progress in the biological and chemical treatment technologies for emerging contaminant removal from wastewater: A critical review” at the 14th International Conference on Environmental Science and Technology can be counted as both an article and a proceeding paper. After excluding all duplicate documents, the total number is still 2621.

Table 1 The distribution of the references by the type of the document
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According to the distribution of documents listed in Table 1, 80.7% of the references were articles, followed by proceeding papers and reviews, accordingly accounting for 10.4% and 5.9%. The proportion of all the other literature types of references was notably small (2%). In this article, a comprehensive analysis of all the abovementioned literature types was carried out.

Historical development of the field

It is an important clue on the underlying supporting structures and incentives by analyzing the data on the historical development of a research field. In this part of investigation, the following items were discussed: (1) the number of published papers according to the length of time, which reflected the development of research in this field to a certain extent; (2) the number of citations for published papers (Cn1), which reflected the degree of attention paid to the field; and (3) the number of citations of the papers published each year (Cn2), which reflected the quality of the papers of the year. The distribution of the papers by the publication year is shown in Table 2. The analysis of publication year showed that the number of papers on microalgae-based wastewater treatment exponentially increased during the last 20 years.

Table 2 The distribution of the papers by the publication year
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Although there had been some research on the treatment of wastewater by microalgae before 2010, both the number of studies and their impact were stabilized in a low level. However, as from 2010, research in this field emerged in an endless stream, and the number of publications surged. The literature in the past 5 years (2015–2019) accounted for 63.1% of the total. At the same time, the number of citations for published papers also surged in around 2010, which showed that the application of microalgae in sewage treatment attracted more attention. Table 2 describes that there was still a strong development space for the research on microalgae-based wastewater treatment because the number of papers and citations has not reached the highest point around 2019.

It is also notable that papers published between 2010 and 2014 held the highest number of citations. Especially in 2011, the total number of citations of published papers reached the highest number of 6593. The average citations of the literature in that year were also the highest in the past 20 years, reaching 75.8 times. Table 3 lists the most cited papers in 2011. Through a brief analysis of the article, all of the five papers were related to bioenergy without exception, which reflected that the high attention was paid to the coupled wastewater treatment with biofuel production by microalgae in that period.

Table 3 The most cited papers in 2011
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However, the citation frequency and average citation frequency of papers published since 2015 were not as high as 2010–2014. On one hand, the number of citations of the papers was a long-term accumulation process, and excellent papers also needed time accumulation to receive a higher frequency of citations. On the other hand, the quality of a paper is determined by not only the number of citations but also its specific research value and direction.

Most active countries

An analysis of the number of papers and citations on a national scale was carried out to determine the most active countries. Due to the existence of exchanges and cooperation between countries, some papers will be calculated multiple times.

Table 4 shows that the most publishing single country was the China with 581 papers, comprising 22.2% of the sample, followed by the USA (386, 14.7%), India (286, 10.9%), Spain (187, 7.1%), and Korea (144, 5.5%). It also reflects that the USA held the highest citation number up to 12,667. The next four countries were China (11110), India (8066), Spain (5485), and Mexico (3636). It is notable that although South Korea ranked fifth in the number of published papers, its total citations were lower than Mexico during the investigation time.

Table 4 The most publishing countries
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Table 4 also shows sufficient research investment and incentive policies in this field of China, the USA, and India, without which it would be impossible for researchers to actively publish high-quality papers in related fields. However, it seemed that these three countries still have a long way to improve the overall level of articles.

The published papers and citations are only one reference when discussing active countries. Adequate cooperation not only has a positive effect on the promotion of research capabilities, but also reflects international influence in a certain country. Table 5 lists the top 20 number of papers completed by scholars from multiple countries. It shows that China was the country with the largest number of cooperation with other countries in the world, reaching 151 times.

Table 5 Multinational author papers top 20
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Among these multinational author papers, China and the USA had the most frequent exchanges, with 73 times, accounting for 24.3%, and tripled the second place (the USA and Korea). Based on the fact that these two countries held the highest number of published documents and the highest frequency of citations in the world, these close exchanges between China and the USA not only promoted the development of microalgae-based wastewater treatment research but also enhanced the international reputation of both parties. It also reflected the lack of international exchanges in India, although its number of published papers ranked third.

Distribution of research institutions

One certain aspect of research may be carried out simultaneously by multiple research institutions in one country, and thus, it is necessary to evaluate the papers published by research institutions. A total of 2157 research institutions had been screened from 2621 documents. The top 20 research institutions with the number of published articles and the citations of their published papers are listed one by one in Table 6.

Table 6 The most publishing research institutions
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It was found that the Chinese Academy of Sciences was the most publishing institution (2.3%), followed by India Institute of Technology (2.2%), Council of Scientific & Industrial Research (2.1%), Harbin Institute of Technology (1.8%), and The National Institute of Water and Atmospheric Research with University of Minnesota (1.6%).

These results suggest that the research in this area concentrated in main national research centers and institutions of higher education. As the highest academic institution of natural sciences in China, the Chinese Academy of Sciences had a leading role in the development of this field in China, while the other 7 Chinese universities on the Table 6 had made indelible contributions. It is notable that a total of 15 institutions (75%) in Table 6 are affiliated to China, the USA, and India, which proves that these three countries have made irreplaceable contributions to the development of microalgae-based wastewater treatment.

Author distribution

Research institutions are composed of a certain number of researchers. Based on the sample of the papers, Table 7 shows that the top 20 publishing authors.

Table 7 The most publishing authors
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The most publishing authors were Ruan R (35 papers) publishing mostly in the area of energy fuels and the application in agriculture of microalgae, followed by Munoz R (29 papers), Craggs RJ (28 papers), Chen P (27 papers), Zhang YH, and Zhou WG (24 papers). It is notable that Ruan R was the author with the most number of citations, while Min M possessed the highest average citations per paper. What they had in common was that both their research concentrated in the area of biodiesel production from microalgae dealing with sewage. However, it turned out to be that not all high-level research must be related to bioenergy. For example, the research of Craggs RJ mostly focused on water resources and engineering.

It is admitted that there is still certain possibility that some authors working in the fields related to microalgae-based wastewater treatment were not included for some reasons. So it should be cautious in interpreting findings of this table.

Most influential journals

The data from journals where researches on microalgae-based wastewater treatment were published are also important. From 2000 to 2019, 578 journals published papers related to microalgae wastewater treatment. The titles of the top 20 most publishing journals and their 5-year impact factors were listed, as shown in Table 8.

Table 8 The most publishing journals on the topic
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“Bioresource Technology” was the most publishing journal with 365 published papers, followed by “Water Science and Technology” (138 papers), “Algal Research-Biomass Biofuels and Bioproducts” (119 papers), “Water Research” (82 papers), and “Journal of Applied Phycology” (73 papers). The subject heading of Biotechnology lists the first and the third journals of the table, suggesting that the field of bio-energy finds a firm place in researches related to microalgae-based wastewater treatment. Table 8 also shows that the impact factors of “Bioresource Technology” and “Water Research” in the past 5 years were accordingly 6.589 and 8.424, which indicated that the research of microalgae-based wastewater treatment had been well spread in these two journals.

In addition, the journals in Table 8 covered a variety of basic sciences, such as chemistry, biology, management, and applied sciences including biotechnology, chemical engineering, and energy, demonstrating that this field was an interdisciplinary combination research. At the same time, areas frequently appeared like engineering and energy also implied that researchers had an increasing interest in the application of the field.

Most published subject areas

The Web of Science database assigns one or more research fields to each journal in order to use a more systematic subject analysis and highlight the differences between disciplines. The literature retrieved in this article was divided into 55 fields. Table 9 lists the top 20 research fields assigned to the largest number of journals.

Table 9 The subject area of the papers
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Table 9 shows that “Environmental Sciences Ecology” was the area where 36.2% of the sample was published, followed by “Engineering” (34.2%), “Biotechnology Applied Microbiology” (32.0%), “Energy Fuels” (22.6%), and “Agriculture” (16.1%). Although it had not entered the top five, “Water Resources” was still the last field with a proportion of more than 10% (15.8%), only 0.3% lower than “Agriculture.”

Ecology, engineering and biotechnology ranked top three in Table 9, fully demonstrating that microalgae-based sewage treatment could not only be deeply explored by a theoretical basis, but also be close to practical applications. And Energy and agriculture ranked the fourth and fifth, which pointed out the close aforementioned connection between the microalgae wastewater treatment and bio-energy with aquaculture.

Twenty most-cited papers

The data on the most cited papers provide important information on the development of the field. By reading such articles, the depth of research in this field and the hot issues of peer concern could be quickly obtained. The most-cited 20 papers were determined, and the citation details for these papers were given in Table 10 with the references of these papers listed in refs.

Table 10 The most cited papers
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Table 10 shows that de-Bashan’s review (2004) of the status of phosphorus removal in wastewater has been most frequently cited, up to 846 times. This paper was followed by Ahluwalia (2007, 827 times) on microbial removal of heavy metals in wastewater and the collection of biomass. It is worth noting that three of the five articles are related to biodiesel. Most of the highest-cited 5 papers maintained a high average annual citation number (greater than 60); however, there were still two articles that reached a number higher than 60. One is Cai (2013, 68 times per year) on status about nutrient recovery from wastewater by microalgae, while the other one is Holkar (2016, 69 times per year) on possible approaches on textile wastewater treatments.

In addition, Table 10 also shows that the most cited papers are concentrated between 2010 and 2013, accounting for 65%. Only one highly cited document was published in 2016, and the research content of this article was the method of textile wastewater treatment, only mentioning the use of algae in biological methods to treat wastewater. This phenomenon reflects that there is a possibility that no widely recognized work in this field was found recently, due to factors such as technical bottlenecks, research enthusiasm, and social evaluation.

Comparative discussion

Until April 2020, there has been no published scientometric study of research on the algae and wastewater treatment in the scientific community, but only one publication about the bacterial algae symbiosis system on the sewage treatment (Qi et al. 2019). It investigated the research activities and tendencies of algae-bacteria symbiotic wastewater treatment technology by bibliometric method from 1998 to 2017 based on SCI-EXPANDED database, and concluded that China and the USA had the largest amount of publications, and the cooperation between them is the closest in the world, which is consistent with the conclusion of this paper. It also utilized VOSviewer as manifestations instead of a diagram in some cases to illustrate its opinions. Meanwhile, a critical review and bibliometric analysis about microalga-derived biodiesel was also analyzed by Ma et al. (2018).

In addition, there was still a part of bibliometric literature related to sewage treatment although it had nothing to do with microalgae. Some documents were related to what kind of wastewater was treated. Zheng et al. evaluated industrial wastewater treatment research from 1991 to 2014, based on the Science Citation Index Expanded (SCIE) database and applied a method named “word cluster analysis” to trace the research hotspots (Zheng et al. 2015). Qian et al. carried out a bibliometric analysis based on the science citation index expanded from Web of Science to assess the research pattern and tendencies of pharmaceutical wastewater treatment from 1994 to 2013 (Qian et al. 2015). Other documents focused on the method of wastewater treatment including nanomaterials (Zhao et al. 2018), electrochemical technology (Zheng et al. 2017), biosorption technology (Ho 2008), etc.

Among all the few bibliometric analysis literature, the discussed topics covered annual output, mainstream journals, WOS categories, major countries, institutions, and a small number of paper discussed title analysis, author keyword analysis, and keywords plus analysis. In terms of presentation form, tables were the most common form to visually display data and some articles had graphs to enhance the contrast effect at the same time.

In general, although this article was slightly single in form of expression (only tables), the content of the discussion was comprehensive and detailed, and comprehensively showed the development and possible future research directions of the field of microalgae-based wastewater treatment from 2000 to 2019.

Conclusion

Microalgae wastewater treatment is one of the most common and highly potential wastewater treatment methods. A metrological analysis of the literature on microalgae wastewater treatment in the past 20 years provided helpful insights into the research in this field. The results showed that the research on microalgae wastewater treatment had grown exponentially during the last two decades especially with the development of the research on algae and bio-energy. China and the USA played an essential role in the directions, depths, and contents of this field. The results also provided valuable information on the citations of the research on microalgae wastewater treatment, which had significant impacts and further incentives for the researchers, their institutions, and their countries to do more influential research in this area. In addition, the literature on microalgae wastewater treatment concentrated in ecology, engineering, and biotechnology, which was also proved by the most cited papers. The development of interdisciplinary fields has a side promotion effect on the development of other basic sciences. It is also important to note that bibliometric research was a supplement to the other types of qualitative research such as content analysis. It intuitively provides a more complete research picture in the form of data within a certain time frame with large number of references. Finally, it should be admitted that due to the single source of the literature and the continuous updating of the database, the conclusions of this article will have certain limitations.