Research progress, trends, and updates on anaerobic digestion technology: A bibliometric analysis
Graphical abstract
Introduction
There is a global concern about the possibilities to overcome the impacts of climate change and support sustainable development (Klenert et al., 2020). In general, the impacts of climate change are mainly associated with the consumption of non-renewable fuels and the resulting emissions of greenhouse gases (GHG), which cause negative environmental consequences and affect human life (Brauer and Khan, 2021). Otherwise, the incorrect disposal of solid and liquid residues from various industries contributes to GHG emissions (Chen, 2018). Considering the GHG emissions by sector, the disposal of wastes is responsible for 3.2% of the GHG emissions, including wastewater (1.3%) and landfills (1.9%), whereas, considering the global GHG emissions, the food waste is responsible for 6% of the total (Ritchie and Roser, 2020a). Moreover, considering the emissions per type of fuel in 2020, the utilization of fossil fuels represented 93.22% of CO2 emissions around the world (the respective percentages are 40.15% for coal, 31.81% for oil, and 21.26% for gas) (Ritchie and Roser, 2020b). Therefore, the correct management of organic wastes has grown into a challenge for different industrial sectors, which aimed to meet the Sustainable Development Goals (SDGs) established by the United Nations Framework Convention on Climate Change (UNFCCC) (United Nations, 2020). Among the 17 SDGs established, waste valorization into more sustainable paths support the achievement of the following goals:
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SDG 6 (clean water and sanitation), since AD is worldwide used for effluent treatment, which contributes to diminishing the pollution of water bodies
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SDG 7 (affordable and clean energy), since the waste could be used for electricity and thermal energy generation through methane upgrading from biogas
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SDG 13 (climate action), by avoiding environment pollution, GHG emission, and by encouraging other sectors to contribute in achieving the goals.
Innovative technologies enabling appropriate waste management and energy recovery pathways are crucial to uphold sustainable development based on bioeconomy (Yaashikaa et al., 2020). The recovery of bioenergy from waste (waste-to-energy) can support the achievement of profitable processes with environmental conservation while mitigating losses, environmental side-effects, and avoiding GHG emissions (Van et al., 2020). Therefore, the design of innovative processes for bioenergy recovery can avoid organic waste disposal by landfilling or incineration, and consequently contribute in gradually establishing a circular economy because the wastes could be then used as feedstock to obtain value-added products at the end of the production cycle.
One of the most employed biological processes for organic waste management and energy recovery is anaerobic digestion (AD), whereby organic waste is treated without oxygen (Zamri et al., 2021). The AD reactions are carried out by a consortium of microorganisms that transform complex organic matter into simple soluble compounds and eventually produce biogas and many other valuable compounds (Nguyen et al., 2021). AD can be classified by considering the feed and maintenance of organic matter in wet AD (solids < 15%) and dry AD (solid > 15%) (Jimenez-Castro et al., 2020). Both processes present many advantages and disadvantages, which have been extensively discussed in the literature (Atelge et al., 2020; Kothari et al., 2014; Li et al., 2019; Nguyen et al., 2021; Xu et al., 2018b; Zamri et al., 2021).
The AD occurs in four phases: i) hydrolysis; ii) acidogenesis; iii) acetogenesis; and iv) methanogenesis (Kothari et al., 2014). Hydrolysis is characterized by transforming complex compounds with high molecular weight (i.e. proteins, carbohydrates, and lipids) into simple soluble molecules (i.e. sugars, amino acids, and short-chain fatty acids). In the acidogenic phase, bacteria use the low molecular weight material introduced in the metabolic cycle (Atelge et al., 2020). The acidogenic bacteria are the most abundant and active microorganisms present in the AD process (Xu et al., 2018b). Syntrophic acetogenesis converts glucose into volatile fatty acids consisting mainly of acetate, and then homoacetogenesis converts carbon dioxide (CO2) and hydrogen (H2) to acetate (Nie et al., 2007). In the methanogenesis phase, the microorganisms transform the acetic acid, hydrogen, and carbon dioxide into methane, which is the main product of the biogas (Cremonez et al., 2021; Neumann et al., 2016). AD is a versatile process, resulting in many products, such as methane, hydrogen, volatile fatty acids, and fertilizer. From an environmental perspective, methane-rich biogas can be considered a renewable energy carrier that can be transformed into heat and electricity, decreasing in this way the environmental impacts caused by fossil fuels (Hakawati et al., 2017; Li et al., 2019; Mao et al., 2015).
The use of scientific information from published documents can determine the research trends and gaps in a field. Recently, bibliometric studies have been widely used to disseminate the prospects in diverse research fields, such as trends on biomass and bioenergy (Ferrari et al., 2020), bioenergy from bio-waste (Obileke et al., 2020), and the status of studies in bioenergy under climate change, offering a fresh perspective and suggestions to researchers and policymakers for future research and policy formulation (Zhang et al., 2021b).
Therefore, to elucidate research progress and trends in AD technology, this study presents a bibliometric analysis. Firstly, the search was conducted in the timespan from 1980 to 2020 to observe the global research trends. Then, the timespan was focused over the last five years of publications (2015–2020), aiming to summarize the technological advancement and current development trends for the AD process from a bibliometric viewpoint. Different from previous review articles, which use literature search to identify and explain important concepts and advances in a certain field, the present study focused on a broad analysis of AD to identify the main gaps that still need to be studied in the field. Finally, a deep discussion on the challenges and prospects have been provided to support future research and decision-making in this field of science and technology.
Section snippets
Material and methods
The bibliometric study used scientific output data from the core collection of Science Citation Index Expanded (SCI-E) – Clarivate Analytics' ISI – Web of Science© (WoS) and was performed as presented in Fig. 1. The type of search applied was “advanced search” applying the following logic operation: TS= (“anaerobic digestion”) NOT KP= (“anaerobic digestion”), to obtain reliable and accurate details based in selected words included in the title, abstract, author's keywords, and not in the
Research tendencies analysis by bibliometrics
The results of the bibliometric analysis on AD are presented in the next sections, indicating the most prominent research areas, author keywords, authors, journals, among others. Each part of the results was discussed to provide insightful data on research progress, trends, and updates on AD technology.
Industrial perspectives, research progress and directions
AD dynamics are strongly affected by the microbial community applied to the digestion since it is a biochemical route conducted by a great variety of microbial groups at different stages (Gannoun et al., 2016; Wang et al., 2018). Some AD applications include the production of biofuels (like hydrogen and methane), bulk chemicals, bioplastics, and allows chemicals’ removal from wastewater (Wainaina et al., 2019). Depending on the characteristics of the inoculum, it is possible to engineer the
Conclusion
This bibliometric study explored the research trends and gaps on anaerobic digestion technology over the last five years of publications. It provided an update in this field of science and technology. AD has been used as one of the most effective processes to treat organic wastes. The number of publications from 2015 to 2020 corresponds to almost 46% of all studies published regarding AD since 1980. The bibliometric research revealed a lack of information regarding organic waste in industries
Author contributions statement
Larissa Castro Ampese; William Gustavo Sganzerla and Henrique Di Domenico Ziero: Conceptualization; Data curation; Writing – Original draft; review & editing; revision.
Ackmez Mudhoo; Gilberto Martins and Tânia Forster-Carneiro: Writing – Original draft; review & editing; revision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by the Brazilian Science and Research Foundation (CNPq) (productivity grant 302473/2019-0); Coordination for the Improvement of Higher Education Personnel (CAPES, Brazil) (Finance code 001); and São Paulo Research Foundation (FAPESP, Brazil) (grant numbers 2018/05999-0, 2018/14938-4, and 2019/26925-7). The contents of this article were crosschecked for similarity in the Turnitin platform.
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