From pollutant removal to resource recovery: A bibliometric analysis of municipal wastewater research in Europe

Current trends and future directions of global research on wastewater to energy: a bibliometric analysis and review

This paper presents a structured bibliometric analysis and review of the research publications recorded in the Web of Science database from 2000 to 2023 to methodically examine the landscape and development of the 'wastewater to energy' research field in relation to global trends, potential hotspots, and future research directions. The study highlights three main research themes in 'wastewater to energy', which are biogas production through anaerobic digestion of sewage sludge, methane generation from microbial wastewater treatment, and hydrogen production from biomass. The analysis reveals activated sludge, biochar, biomethane, biogas upgrading, hydrogen, and circular economy as key topics increasingly gaining momentum in recent research publications as well as representing potential future research directions. The findings also signify transformation to SDGs and circular economy practices, through the integration of on-site renewables and biogas upgrading for energy self-sufficiency, optimising energy recovery from wastewater treatment systems, and fostering research and innovation in 'wastewater to energy' supported by policy incentives. By shedding light on emerging trends, cross-cutting themes, and potential policy implications, this study contributes to informing both knowledge and practices of the 'wastewater to energy' research community.

A bibliometric analysis of emerging contaminants (ECs) (2001-2021): Evolution of hotspots and research trends

Emerging contaminants (ECs) have attracted increasing attention in the past two decades because of their ubiquitous existence and high environmental risk. Understanding the progress of research and the evolution of hot topics is critical. This study provides a bibliometric review, along with a quantitative trend analysis of approximately 8000 publication records dated from 2001 to 2021. Wider distribution in various subjects was discovered in terms of publication numbers, indicating a strong tendency for EC research to become an interdisciplinary topic. Visualization of term co-occurrence analysis revealed that the ECs study went through three stages over time: identification and detection, traceability and risk, and process and control. Quantitative trend analysis revealed that antibiotics, microplastics, endocrine disrupting chemicals (EDCs), per/poly-fluoroalkyl substances (PFAS), pesticides, heavy metals, and nanoparticles are attracting increasing attention, whereas conventional pharmaceuticals, persistent organic pollutants, and materials such as benzotriazole, diclofenac, bisphenol A, carbamazepine, triclosan, and titanium dioxide exhibit a downward trend. PFAS and EDCs are considered potential future core hotspots for the hysteretic rise in research attention compared with conventional ECs. Furthermore, analysis of research linkage and the developing stages of ECs could be possible approach to determine the evolution of hotspots in ECs study. This study provides objective and comprehensive insights into the research landscape of ECs, which may shed light on future developmental directions for researchers interested in this field.

A bibliometric analysis of published literature on membrane photobioreactors for wastewater treatment from 2000 to 2022

With the focus on limiting greenhouse gas emissions, microalgae-based technology is a promising approach for wastewater treatment, combining cost-effective operation, nutrient recovery, and assimilation of CO2. In addition, membrane technology supports process intensification and wastewater reclamation. Based on a bibliometric analysis, this paper evaluated the literature on membrane photobioreactors to highlight promising areas for future research. Specifically, efforts should be made on advancing knowledge of interactions between algae and bacteria, analysing different strategies for membrane fouling control and determining the conditions for the most cost-effective operation. The Scopus® database was used to select documents from 2000 to 2022. A set of 126 documents were found. China is the country with the highest number of publications, whereas the most productive researchers belong to the Universitat Politècnica de València (Spain). The analysis of 50 selected articles provides a summary of the main parameters investigated, that focus in increasing the biomass productivity and nutrient removal. In addition, microalgal-bacterial membrane photobioreactor seems to have the greatest commercialisation potential. S-curve fitting confirms that this technology is still in its growth stage.

Bibliometric analysis and literature review of ultrasound-assisted degradation of organic pollutants

Ultrasound as a clean, efficient, and cheap technique gains special attention in wastewater treatment. Ultrasound alone or coupled with hybrid processes have been widely studied for the treatment of pollutants in wastewater. Thus, it is essential to conduct a review about the research development and trends on this emerging technique. This work presents a bibliometric analysis of the topic associated with multiple tools such as Bibliometrix package, CiteSpace, and VOSviewer. The literature sources from 2000 to 2021 were collected from Web of Science database, and the data of 1781 documents were selected for bibliometric analysis in respect to publication trends, subject categories, journals, authors, institutions, as well as countries. Detailed analysis of keywords in respect to co-occurrence network, keyword clusters, and citation bursts was conducted to reveal the research hotspot and future directions. The development of the topic can be divided into three stages, and the rapid development begins from 2014. The leading subject category is Chemistry Multidisciplinary, followed by Environmental Sciences, Engineering Chemical, Engineering Environmental, Chemistry Physical, and Acoustics, and there exists difference in the publications of different categories. Ultrasonics Sonochemistry is the most productive journal (14.75 %). China is the leading country (30.26 %), followed by Iran (15.67 %) and India (12.35 %). The top 3 authors are Parag Gogate, Oualid Hamdaoui, and Masoud Salavati-Niasari. There exists close cooperation between countries and researchers. Analysis of highly cited papers and keywords gives a better understanding of the topic. Ultrasound can be employed to assist various processes such as Fenton-like process, electrochemical process, and photocatalysis for degradation of emerging organic pollutants for wastewater treatment. Research topics in this field evolve from typical studies on ultrasonic assisted degradation to latest studies on hybrid processes including photocatalysis for pollutants degradation. Additionally, ultrasound-assisted synthesis of nanocomposite photocatalysts receives increasing attention. The potential research directions include sonochemistry in pollutant removal, hydrodynamic cavitation, ultrasound-assisted Fenton or persulfate processes, electrochemical oxidation, and photocatalytic process.

Innovative Membrane Technologies for the Treatment of Wastewater Polluted with Heavy Metals: Perspective of the Potential of Electrodialysis, Membrane Distillation, and Forward Osmosis from a Bibliometric Analysis

A bibliometric analysis, using the Scopus database as a source, was carried out in order to study the scientific documents published up to 2021 regarding the use of electrodialysis, membrane distillation, and forward osmosis for the removal of heavy metals from wastewater. A total of 362 documents that fulfilled the search criteria were found, and the results from the corresponding analysis revealed that the number of documents greatly increased after the year 2010, although the first document was published in 1956. The exponential evolution of the scientific production related to these innovative membrane technologies confirmed an increasing interest from the scientific community. The most prolific country was Denmark, which contributed 19.3% of the published documents, followed by the two main current scientific superpowers: China and the USA (with 17.4% and 7.5% contributions, respectively). Environmental Science was the most common subject (55.0% of contributions), followed by Chemical Engineering (37.3% of contributions) and Chemistry (36.5% of contribution). The prevalence of electrodialysis over the other two technologies was clear in terms of relative frequency of the keywords. An analysis of the main hot topics identified the main advantages and drawbacks of each technology, and revealed that examples of their successful implementation beyond the lab scale are still scarce. Therefore, complete techno-economic evaluation of the treatment of wastewater polluted with heavy metals via these innovative membrane technologies must be encouraged.

Bibliometric analysis of chitosan research for wastewater treatment: a review

Wastewater has negative impacts on the environment, such as destroying aquatic ecosystems and creating a shortage of clean water sources for consumption. In this paper, we provide a comprehensive bibliometric analysis of chitosan to better understand the evolution in degrading various pollutants as a wastewater treatment research limited only by photocatalyst system published in 2001-2021. The number of publications analyzed a total of 456 documents, which was conducted from the Scopus database. All data in this paper was visualized by using open-source software, VOSviewer and Tableau, to perform bibliometric analysis and scientific mapping. The reason for choosing chitosan is its ability to degrade various pollutants with high adsorption performance (from various sources: degradation Congo red 98.4%, methylene blue 99.36%, rhodamine B 95%, and Cd(II) 94%), non-toxicity, biodegradability, and abundantly available sources in nature. The analysis results show that the highest number of publications in 2016 was 66, and the highest number of citations was 2258. The network of keywords and innovations for wastewater treatment is USA and China as the most productive countries with many cooperative relations. This paper helps scholars understand the evolution of composite chitosan-based photocatalyst systems as research on wastewater treatment from a bibliometric point of view and inspires them to develop new efficient methods in synthesizing chitosan from fish by-products (waste) with high adsorption efficiency for various type of waste.

Mapping the research on desulfurization wastewater: Insights from a bibliometric review (1991-2021)

Desulfurization wastewater in coal-fired power plants (CFPPs) is a great environmental challenge. This study aimed at the current status and future research trends of desulfurization wastewater by bibliometric analysis. The desulfurization wastewater featured with high sulfate (8000 mg/L), chlorite (8505 mg/L), magnesium (2882 mg/L) and calcium (969 mg/L) but low sodium (801.82 mg/L), and the concentrations of the main contaminants were critically summarized. There was an increasing trend in the annual publications of desulfurization wastewater in the period from 1991 to 2021, with an average growth rate of 15%. Water Science and Technology, Desalination and Water Treatment, Energy & Fuels, Chemosphere, and Journal of Hazardous Materials are the top 5 journals in this field. China was the most productive country (58.3% of global output) and the core country in the international cooperation network. Wordcloud analysis and keyword topic trend demonstrated that removal/treatment of pollutants dominated the global research in the field of desulfurization wastewater. The primary technologies for desulfurization wastewater treatment were systematically evaluated. The physicochemical treatment technologies occupied half of the total treatment methods, while membrane-based integrated processes showed potential applications for beneficial reuse. The challenges and outlook on desulfurization wastewater treatment for achieving zero liquid discharge are summarized.

Simultaneous nitrification, denitrification and phosphorus removal: What have we done so far and how do we need to do in the future?

Nitrogen and phosphorus contamination in wastewater is a serious environmental concern and poses a global threat to sustainable development. In this paper, a comprehensive review of the studies on simultaneous nitrogen and phosphorus removal (SNPR) during 1986-2022 (538 publications) was conducted using bibliometrics, which showed that simultaneous nitrification, denitrification, and phosphorus removal (SNDPR) is the most promising process. To better understand SNDPR, the dissolved oxygen, carbon to nitrogen ratio, carbon source type, sludge retention time, Cu²⁺ and Fe³⁺, pH, salinity, electron acceptor type of denitrifying phosphorus-accumulating organisms (DPAOs), temperature, and other influencing factors were analyzed. Currently, SNDPR has been successfully implemented in activated sludge systems, aerobic granular sludge systems, biofilm systems, and constructed wetlands; sequential batch mode of operation is a common means to achieve this process. SNDPR exhibits a significant potential for phosphorus recovery. Future research needs to focus on: (1) balancing the competitiveness between denitrifying glycogen-accumulating organisms (DGAOs) and DPAOs, and countermeasures to deal with the effects of adverse conditions on SNDPR performance; (2) achieving SNDPR in continuous flow operation; and (3) maximizing the recovery of P during SNDPR to achieve resource sustainability. Overall, this study provides systematic and valuable information for deeper insights into SNDPR, which can help in further research.

Sources of copper into the European aquatic environment

Chemical contamination from point source discharges in developed (resource-rich) countries has been widely regulated and studied for decades; however, diffuse sources are largely unregulated and widespread. In the European Union (EU), large dischargers report releases of some chemicals, yet little is known of total emissions (point and diffuse) and their relative significance. We estimated copper loadings from all significant sources including industry, sewage treatment plants, surface runoff (from traffic, architecture, and atmospheric deposition), septic tanks, agriculture, mariculture, marine transport (antifoulant leaching), and natural processes. A combination of European datasets, literature, and industry data were used to generate export coefficients. These were then multiplied by activity rates to derive loads. A total of approximately 8 kt of copper per annum (ktpa) is estimated to enter freshwaters in the EU, and another 3.5 ktpa enters transitional and coastal waters. The main inputs to freshwater are natural processes (3.7 ktpa), agriculture (1.8 ktpa), and runoff (1.8 ktpa). Agricultural emissions are dominated by copper-based plant protection products and farmyard manure. Urban runoff is influenced by copper use in architecture and by vehicle brake linings. Antifoulant leaching from boats (3.2 ktpa) dominates saline water loads of copper. It is noteworthy that most of the emissions originate in a limited number of copper uses where environmental exposure and pathways exist, compared with the bulk of copper use within electrical and electronic equipment and infrastructure that has no environmental pathway during its use. A sensitivity analysis indicated significant uncertainty in data from abandoned mines and urban runoff load estimates. This study provided for the first time a methodology and comprehensive metal load apportionment to European aquatic systems, identifying data gaps and uncertainties, which may be refined over time. Source apportionments using this methodology can inform more cost-effective environmental risk assessment and management. Integr Environ Assess Manag 2022;00:1-17. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

A comprehensive analysis of evolution and underlying connections of water research themes in the 21st century

This work aimed to reflect the advancements in water-related science, technology, and policy and shed light on future research opportunities related to water through a systematic overview of Water Research articles published in the first 21.5 years of the 21st century. Specific bibliometric analyses were performed to i) reveal the temporal and spatial trends of water-related research themes and ii) identify the underlying connections between research topics. The results showed that while top topics including wastewater (treatment), drinking water, adsorption, model, biofilm, and bioremediation remained constantly researched, there were clear shifts in topics over the years, leading to the identification of trending-up and emerging research topics. Compared to the first decade of the 21st century, the second decade not only experienced significant uptrends of disinfection by-products, anaerobic digestion, membrane bioreactor, advanced oxidation processes, and pharmaceuticals but also witnessed the emerging popularity of PFAS, anammox, micropollutants, emerging contaminants, desalination, waste activated sludge, microbial community, forward osmosis, antibiotic resistance genes, resource recovery, and transformation products. On top of the temporal evolution, distinct spatial evolution existed in water-related research topics. Microplastics and Covid-19 causing global concerns were hot topics detected, while metagenomics and machine learning were two technical approaches emerging in recent years. These consistently popular, trending-up and emerging research topics would most likely attract continuous/increasing research input and therefore constitute a major part of the prospective water-related research publications.

Technical advances on current research trends and explore the future scope on nutrient recovery from waste-streams: a review and bibliometric analysis from 2000 to 2020

An exponentially growing global population has led to an increase in nutrient pollution in different aqueous bodies. Although different processes have successfully removed nutrients from wastewater on a large scale, a limited number of studies have been reported on efficiency, cost-effectiveness, and future potential of physical, chemical, and biological nutrient recovery methods to overcome the depletion of natural resources. Therefore, researchers need to understand current research trends by applying different approaches to investigate higher efficient nutrient recovery technologies. In this article, the research patterns and in-depth review of various nutrient recovery processes have been circumscribed with the application of bibliometric and attractive index (AAI) vs. activity index (AI) analysis. The performance, advantages, limitations, and future prospects of different nutrient recovery methods have also been addressed. More than 70% of study publications were published in the last decade in chemical and biological processes, which might be related to more rigorous effluent quality rules and increasing water pollution. The future prediction in the field of nutrient recovery has been predicted using S-curve analysis, and it was found that the number of publications in the saturated state in chemical methods was highest. However, the growth rate of the biological-based nutrient recovery methods is greater, which may be because of their huge research scope, cost-effectiveness, and easy operation methods. This study can assist researchers in understanding the current research scenario in nutrient recovery techniques and provide the research scope in nutrient recovery from wastewater in the future.

Wastewater treatment and emerging contaminants: Bibliometric analysis

In recent years, emerging contaminants have been found in the wastewater, surface water, and even drinking water, which should be treated to ensure the safety of our living environment. In this study, we provide a comprehensive summary of wastewater treatment and emerging contaminants research from 1998 to 2021 by using the bibliometric analysis. This study is conducted based on the Web of Science Core Collection Database. The bibliometix R-package, VOSviewer and CiteSpace software are used for bibliometric analysis and science mapping. A dataset of 10, 605 publications has been retrieved. The analysis results show that China has produced the most publications. China and the United States have the closest cooperation. Analysis of the most cited papers reveals that the purification or removal techniques such as ozonation or membrane filtration can effectively remove pharmaceutical compounds from the water environment. We also found that the efficient detection of emerging contaminants and the optimization of removal methods are current challenges. Finally, future research directions are discussed.

Ecological Risk Evaluation and Removal of Emerging Pollutants in Urban Wastewater by a Hollow Fiber Forward Osmosis Membrane

Forward osmosis (FO) is a promising technology for the treatment of urban wastewater. FO can produce high-quality effluents and preconcentrate urban wastewater for subsequent anaerobic treatment. This membrane technology makes it possible to eliminate the pollutants present in urban wastewater, which can cause adverse effects in the ecosystem even at low concentrations. In this study, a 0.6 m² hollow fiber aquaporin forward osmosis membrane was used for the treatment of urban wastewater from the Valladolid wastewater treatment plant (WWTP). A total of 51 Contaminants of Emerging Concern (CECs) were investigated, of which 18 were found in the target urban wastewater. They were quantified, and their ecotoxicological risk impact was evaluated. Different salts with different concentrations were tested as draw solutions to evaluate the membrane performances when working with pretreated urban wastewater. NaCl was found to be the most appropriate salt since it leads to higher permeate fluxes and lower reverse saline fluxes. The membrane can eliminate or significantly reduce the pollutants present in the studied urban wastewater, producing water without ecotoxicological risk or essentially free of pollutants. In all cases, good recovery was achieved, which increased with molecular weight, although chemical and electrostatic interactions also played a role.

A critical review of advanced nanotechnology and hybrid membrane based water recycling, reuse, and wastewater treatment processes

One of the modern challenges is to provide clean and affordable drinking water. Water scarcity is caused by the growing population in the world and pollutants contaminate all remaining water sources. Innovative water treatment solutions have been provided by nanotechnology. Microorganisms, organic suspensions, and inorganic heavy metal ions, among other things, are common water contaminants. Since antiquity, a wide range of water clean-up methods have been employed to address this issue. Breakthroughs in water purification procedures have occurred during the previous four decades, with the most significant one being the use of nanomaterials and nanomembranes. Nanoparticles and nanomembranes (polymeric membranes) have recently been used in engineered materials (TiO₂, ZnO, CuO, Ag, CNT's and mixed oxide nanoparticles, for example). Engineered nanomembranes, nanocomposites and nanoparticles have been used in this review article's discussion of water purification technologies. The review also discusses the risk and solutions of using nanoparticles and nanocomposites in the future.