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Article

Research Trends in the Recovery of By-Products from Organic Waste Treated by Anaerobic Digestion: A 30-Year Bibliometric Analysis

by
Pablo Castillo García
1,
María José Fernández-Rodríguez
1,
Rafael Borja
2,
Juan Manuel Mancilla-Leytón
1,* and
David de la Lama-Calvente
2
1
Departamento de Biología Vegetal y Ecología, Facultad de Biología, Universidad de Sevilla, 41080 Sevilla, Spain
2
Spanish National Research Council (CSIC), Instituto de la Grasa (IG), Department of Food Biotechnology, Campus Universidad Pablo de Olavide, Edificio 46, Ctra. de Utrera, km 1, 41013 Sevilla, Spain
*
Author to whom correspondence should be addressed.
Fermentation 2024, 10(9), 446; https://doi.org/10.3390/fermentation10090446
Submission received: 5 August 2024 / Revised: 22 August 2024 / Accepted: 23 August 2024 / Published: 27 August 2024
(This article belongs to the Special Issue Anaerobic Digestion Technology for the Transformation and Utilization of Organic Wastes)
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Abstract

:
The prevailing extractive economic model is unsustainable due to the finite nature of resources, thereby necessitating the development of alternative models and policies. The anaerobic digestion (AD) process is key to achieving this objective, as it facilitates the conversion of organic waste into biogas and nutrient-rich digestate. This approach is aligned with the principles of a circular economy and contributes to a reduction in carbon emissions. This study aims to conduct a comprehensive bibliometric analysis of the literature published over the past three decades (1993–2023). The analysis will be based on data drawn from the Scopus database and then analysed using the VOSviewer software, which allows for the interconnection of the revised bibliography through a series of selected keywords. The results demonstrated the existence of four clusters: (i) the beneficial valorisation of waste; (ii) volatile fatty acids and biohydrogen as added value by-products resulting from AD; (iii) lignocellulosic substrates and their by-products; and iv) the main products of AD, biogas and digestate. The bibliometric analysis demonstrates a growing interest in AD within the biorefinery concept in recent years, showcasing its potential for effective waste management and integration into the production chain through the principles of the circular economy.

1. Introduction

The management of all generated organic waste represents a significant challenge, with an estimated 14% of all food produced globally going to waste [1]. According to the United Nations Environmental Programme (UNEP), by 2022, food-derived organic waste accounted for 1.05 billion tonnes of all organic waste, of which, 60% was classified as household waste [2]. In order to mitigate the adverse effects of climate change and the economic impact of the uncontrolled disposal of these wastes, the European Parliament adopted the General Programme for the Environment of the European Union up to 2030, which sets out six priority objectives. In short, they advocate for: (i) accelerating the transition to a non-toxic circular economy; (ii) pursuing zero pollution of air, water, and soil, as well as light and noise pollution, and protecting the health and well-being of people, animals and ecosystems; and (iii) protecting, preserving, and restoring marine and terrestrial biodiversity [3]. In this context, the European Circular Economy Action Plan [4], adopted in 2020, as part of the European Green Deal [5], is presented as the new agenda for sustainable growth. Its objective is to reduce pressure on natural resources and to create sustainable jobs for people, regions, and cities.
Both local and international policies reflect the alterations that must be incorporated into the production system, and the socio-economic problems associated with the depletion of resources and the energy crisis. In fact, the National Integrated Energy and Climate Plan (2021–2030) for Spain advocates: “Renewable electricity generation will represent 74% of the total in 2030, in line with a path towards a 100% renewable electricity sector by 2050” [6]. The current extractive economic model, which is predicated on the pursuit of continuous growth, has been demonstrated to be unsustainable due to the finite nature of the resources in question and the inherent limitations of the model itself [7]. Therefore, it is imperative to develop new models and policies with the aim of mitigating the current situation. Following this scenario, the United Nations Framework Convention on Climate Change (UNFCCC) presented the Sustainable Development Goals (SDGs) of the 2030 Agenda [8]. Among the 17 goals presented, which aim to “free the human race from the tyranny of poverty”, at least six of them (i.e., 6. Clean water and sanitation; 7. Affordable and clean energy; 8. Decent work and economic growth; 9. Industry, innovation and infrastructure; 11. Sustainable cities and communities; 13. Climate action) are directly linked to the promotion of technologies that can be embedded in a circular economy model.
One of the most significant of these technologies is anaerobic digestion (AD). AD refers to a biological process whereby organic matter is transformed, by the coordinated and interrelated activity of diverse microorganisms in the absence of oxygen, into a biogas (a mixture of methane (CH4) and carbon dioxide (CO2), primarily) and a nutrient-rich digestate that can be used as a fertiliser or organic amendment for crops, thereby facilitating nutrient recycling [9,10]. Furthermore, AD systems are regarded as a key component of a biorefinery. In 2004, the United States Department of Energy (DOE) defined a biorefinery as “an overall concept of a processing plant where biomass feedstocks are converted and extracted into a spectrum of valuable products”. The German Biorefineries Roadmap, published in 2012, provided a more comprehensive definition: “A biorefinery is characterised by an explicitly integrative, multifunctional overall concept that uses biomass as a diverse source of raw materials for the sustainable generation of a spectrum of different intermediates and products (chemicals, materials, bioenergy/biofuels) allowing the fullest possible use of all raw material components”. These objectives necessitate integrating various methods and technologies [11].
The application of AD within a biorefinery offers a sustainable approach for the processing of otherwise industrial organic wastes. This process allows for a reduction in the carbon footprint and the controlled conversion of nitrogenous and sulphur compounds. Moreover, the multi-stage transformation of complex organic matter could be interrupted, and the intermediates used as raw materials in the chemical industry [11]. Therefore, the effluents produced during the AD process (biogas and digestate) serve as precursors in the generation of a range of useful bioenergy/biofuel elements, such as methane, molecular hydrogen, ethanol, and butanol, which can be further converted to organic acids such as succinic acid and biopolymers such as bioplastics.
In order to promote the circular economy and the imperative of closing production cycles, a systematic review of the specific literature is required, with a particular focus on the treatment of the organic matter through AD, within the broad concept of biorefinery. This will enable the identification of current knowledge gaps and facilitate the decision-making process for future research. In this context, bibliometric analysis enables the examination of a large volume of scientific literature, facilitating the identification of interconnections between documents that may be linked by a common theme, such as the number of citations and publications, or the recurrence of keywords and research topics. This type of analysis is useful for mapping scientific knowledge and its evolution over time. It allows the reader to gain an overview, identify areas of research where further study is needed, find new research ideas, and learn about previous contributions in a field [12]. This technique has gained considerable popularity thanks to the advances, availability, and accessibility of bibliometric software such as Gephi, Leximancer, and VOSviewer, as well as scientific databases such as Scopus and Web of Science. Furthermore, the development of interdisciplinary bibliometric methodologies in the sciences has also contributed to its growing use [12].
The present study aims to carry out a comprehensive bibliometric analysis of the literature on AD and biorefinery published over the past three decades (1993–2023). This study will allow for the identification of the topics that have drawn the attention of the scientific community. This systematic approach was conducted using the internationally recognised Scopus database. The data extracted from this database were then analysed using the VOSviewer software version 1.6.20, which allows for the interconnection of the revised bibliography through a series of selected keywords. Finally, the most recurrent terms, represented as the largest clusters, are discussed.

2. Materials and Methods

Using the Scopus database, the terms “anaerobic digestion” and “biorefinery” were combined and the search was carried out within the “article title, abstract, keywords” option. Then the “analyze results” available option was selected. Information was collected for the period 1993–2023. Documents published in the current year 2024 were excluded so that only full years were considered. A total of 802 documents were then analysed. This provided sufficient information for the bibliometric analysis: authorship, geographical distribution of publications, areas of knowledge, funding, etc. The choice of Scopus for this analysis was due to its extensive coverage and comprehensive multidisciplinary content, which is crucial for ensuring a robust and detailed review. Scopus is widely recognized for its advanced search capabilities and citation analysis tools, making it a suitable choice for our study compared to other databases such as Web of Science or PubMed.
Additionally, the most significant keywords from the documents were exported and processed with VOSviewer software to determine the thematic relationships between the selected articles. This tool made it possible to create diagrams in which each keyword is represented by a node. These nodes are then interconnected when the represented terms are related in the assessed documents [13]. To investigate these connections, a keyword co-occurrence analysis was performed with a minimum of 30 iterations. The analysis was further refined and synonymous words or words that imply a very similar meaning to another keyword were combined using the Thesaurus tool. This allowed, for example, to group “Biorefineries” and “Biorefinery concept” as a single node on the map, improving the results and grouping of terms. Additionally, any term that did not provide information about the field of study, such as “Article” or “Controlled study” was removed. Thus, the number of nodes was reduced without altering the significance of the findings.
Finally, the selected documents were subjected to a detailed review based on the clusters obtained to evaluate the current status of the use of anaerobic digestion and the broad concept of biorefinery in energy recovery from waste worldwide, as well as its evolution over the years. This allows the identification of trends and future projections in this emerging field, which can be a key factor in initiating new research and can greatly help identify employment opportunities in the short and medium term.

3. Results

3.1. Topic’s Publication Growth

Based on the cadence of publications, two clear periods were observed (Figure 1). The initial period, spanning 14 years from 1993 to 2007, is represented by only three publications. However, in the second period, from 2008 onwards, the topic of study began to gain popularity and the number of documents per year increased pseudo-exponentially up to 2022 (i.e., from two documents in 2008 to 165 in 2022) (Figure 1). Therefore, the second period accounts for over 99.5% (799 out of 802) of the total scientific production. Despite the low number of publications during the first period, the article published in 2007 by Yazdani and Gonzalez [14] has the highest number of references (812). This high citation count underscores the significant impact and relevance of the information contained in the article. The article addresses a crucial issue in the biofuels sector: the economic viability of biofuels such as biodiesel and bioethanol. It highlights the potential of biorefineries to co-produce higher-value products alongside biofuels, focusing specifically on the use of glycerol-rich by-products generated during biodiesel production. The numerous citations of this manuscript reflect its influence on the advancement of the biorefinery field and its ongoing relevance in discussions on optimizing biofuel production processes.
The 802 documents identified through the search were distributed as follows according to their respective categories: 65.71% articles, 20.07% reviews, 8.48% book chapters, and 4.36% conference papers. The remaining 1.39% were classified under the remaining categories (editorials, books, erratum, conference proceedings, and notes) (Figure 2). These documents were also grouped by subject area, the three most common being “Environmental Science”, “Energy”, and “Chemical Engineering”, with 458, 415, and 254 publications, respectively (Figure 3).

3.2. Keyword Analysis

After eliminating words with no meaningful information and grouping terms with similar meanings into broader categories, the results of the keyword analysis show four clusters (Figure 4), each associated with keywords based on their degree of relationship (Table 1). Cluster 1 integrates the keywords “anaerobic digestion” and “biorefinery”, highlighting the importance of AD with terms such as “circular economy”, “recycling”, “sustainable development”, “waste treatment”, “valorisation”, “renewable energy”, “global warming”, and “greenhouse gases”. Cluster 2 focuses on the growth conditions of microorganisms in AD processes. Cluster 3 includes biotechnological processes such as alcoholic fermentation of lignocellulosic residues (“alcohol”, “bioethanol”, “hydrolysis”, etc.). Finally, Cluster 4 groups together the main products of AD, with “biogas” at the core, “digestate” and its use as fertiliser at one end, and the use of effluent for microalgal cultivation at the other (Figure 4 and Table 1).

4. Discussion

Due to the wide range of possible applications of the biorefinery concept to the AD process, this section has been divided into the clusters identified in the bibliometric analysis for the sake of clarity.
This section discusses the content of each keyword cluster obtained from the co-occurrence analysis obtained in VOSviewer. During the development of the article, the literature review, to know the current status of AD as a process integrated into the biorefinery, was crucial to deepen the knowledge.

4.1. General and Beneficial Aspects of Waste Valorisation

The traditional economic model based on a linear process treated the by-products generated with no commercial value as waste, which has been either stored, partially treated, and/or disposed of by incineration or landfill. The growing awareness of the environmental impact of anthropogenic activities has led to the strengthening of the idea of a greener perspective, such as the circular economy model [15]. Composting and AD are processes that allow the direct reuse of these by-products to be transformed into new materials and energy. With regard to the AD, it has two essential roles in the circular economy, namely, the conversion of organic biomass and the production of renewable energy. AD is then presented as a key factor in reducing greenhouse gas (GHG) emissions, but its impact falls beyond this. In fact, its implementation tackles either directly or indirectly all the 17 SDGs: it reduces poverty by creating sustainable jobs, boosts economic growth, improves agricultural productivity, reduces dependence on fossil fuels, promotes inclusive and gender-equal education, combats climate change by reducing pollution, and transforms cities into sustainable spaces [16]. Thereby, AD is a very promising bioconversion technology for the socio-economic advancement of society, but methane production alone is not sufficient to justify the investment required to create the necessary infrastructure [10]. Therefore, AD should be embedded in a biorefinery scenario and be a part of a more complex and interrelated system with other technologies and processes.
In brief, a comprehensive biorefinery system can be split into three main stages: (i) extraction of compounds of commercial interest; (ii) conversion of biomass into a valuable material; and (iii) energetic valorisation. The bibliography contains a wide range of extraction processes. The type of solvent, acidity, time, and temperature are the most common parameters that need to be assessed for an optimal extraction process [17]. In addition, according to these authors, technologies such as microwaves, ultrasounds, or autoclaves are also available. These processes permit the production of a variety of products, including polyphenols, reducing sugars, terpenoids, proteins, fibre, and so forth [18]. Nevertheless, the optimisation of the extraction process requires further research due to the specificity of the target compound and the nature of the substrate [17].
Concerning conversion techniques, these include hydrothermal combustion for the generation of biochar, with several applications (energy, soil amendment, biogas yield enhancement, etc.); composting for the production of fertilisers; saccharification for the production of oligosaccharides and sugars, with several applications (food, energy, etc.); dark fermentation for the generation of VFA; etc. [19]. Nevertheless, as with the aforementioned extraction processes, optimisation of the process necessitates further investigation and the evaluation of the principal control parameters. However, these have been extensively studied and are less dependent on the substrate matrix. For example, biochar produced by pyrolysis occurs at temperatures between 550 and 850 °C, with minimal variation in output when biochar is used for soil amendment [19]. Consequently, conversion into valuable materials is a more prevalent strategy for valorisation.
Although extraction and conversion techniques are available, these are contingent on the substrate and the available resources. Thus, these stages may or may not be included in a biorefinery scenario. However, the energetic valorisation of the untreated biomass or the exhausted residue following the extraction and/or conversion steps is consistently a key consideration. Energy valorisation of biomass through biochemical conversion includes the production of bioethanol, biobutanol, hydrogen, and methane [20]. Additionally, according to these authors, biomass can be thermochemically converted into syngas, bio-coal, or biochar. The efficiency of these processes depends on several factors, including the nature of the feedstocks and the infrastructure employed [21]. This forces a return to lab-scale experimentation when a substantial change in the system is proposed or required. Nevertheless, the integration of these techniques within a biorefinery scenario makes it possible to comprehensively manage wastes and helps to mitigate the impact of physico-chemical changes in feedstocks. For instance, the sequential extraction of total phenolic compounds (TPC) and reducing sugars (RS) from the macroalga Rugulopteryx okamurae revealed no significant differences in methane yield between different batches, while the non-extracted batches showed differences of up to 80% [22]. Moreover, recent developments in machine learning tools and algorithms suggest a promising outlook for the future of these systems, which require comprehensive optimisation of a multitude of parameters simultaneously [23].
The present bibliometric study is primarily concerned with the integration of AD within a biorefinery framework. It also delineates the specific research areas that have attracted the greatest interest from the scientific community. The bibliometric analysis revealed that the most investigated topics are those related to the production of VFA and hydrogen, the study of lignocellulosic biomasses and their derived by-products, and the generation of methane and digestate.

4.2. Volatile Fatty Acids and Biohydrogen as Anaerobic Digestion By-Products

VFAs (e.g., acetate, propionate, butyrate, valerate, isobutyrate, isovalerate) are short-chain fatty acids partially volatile at room temperature and pressure. They are also intermediates of AD during the acidogenesis phase [14]. VFAs are then essential for the efficient conversion of biomass to biogas. Moreover, acidogenesis is typically the fastest stage in AD, due to the rapid growth of the involved bacteria (usually 30–40 times faster than archaea) [21]. Consequently, if the AD system is not properly balanced, VFAs can accumulate, resulting in the acidification of the digester and the subsequent inhibition of the entire process. The avoidance of this phenomenon is possible by controlling and adjusting the pH to neutral values with chemicals, typically with sodium bicarbonate [21].
However, these VFAs also serve as precursors to a diverse range of biochemicals and biofuels [21]. For example, they can be converted into beneficial oils through microbial metabolisation or into polyhydroxyalkanoates (PHAs) bioplastics [24,25]. Hence, in a biorefinery scenario, the modification of the AD system with the objective of promoting VFAs production may increase the valorisation of the treated waste. Fundamental control parameters, such as pH, temperature, hydraulic retention time (HRT), and organic loading rate (OLR), can be adjusted to suppress archaea activity and prevent the conversion of these VFAs into methane [15].
A review of the literature reveals a multitude of bioreactor configurations for the production of VFAs. However, these configurations are presented as optimal for a given condition and feedstock. This makes it difficult to extrapolate results from one scenario to another [26]. For instance, while it has been demonstrated that alkaline pH values (9) enhanced the production of VFAs from strawberry extrudates, it has also been documented that for food waste, the highest VFA generation was achieved at low pH values (4–6) [27,28]. In any case, alkaline pH values are commonly more favourable for VFAs production [26]. Regarding the digester’s temperature, the mesophilic range (30–40 °C) has been proven to be the most effective for the production of VFA and the maintenance of system stability [29]. It is noteworthy that although thermophilic temperatures (50–60 °C) typically shorten the HRT and enhance the fermentation process, they also demand higher heat inputs and more rigorous operational management [30]. Fewer discrepancies have been found regarding OLRs. In general, OLRs higher than those optimal for methane production are beneficial for VFAs. In fact, values as high as 14 gVS L−1 d−1 have been reported to achieve higher VFA concentrations and yields [31]. However, excessively high OLR values may result in an imbalance of the acid-base system, potentially leading to the disruption of the fermentation process [26]. Finally, HRT must be assessed for the specific substrate and digester performance to ensure optimal physical contact between organic matter and microbial enzymes [26].
Given that high concentrations of VFAs are inhibitory to both hydrolysis acidification stages, the on-line separation of these compounds is key in a continuous system, as to achieve the highest yields [26]. According to these authors, several methodologies have been described in the literature, including membrane separation, electrodialysis, adsorption/ion exchange, and esterification. However, the development of an efficient general process remains a significant challenge, due to the relatively low concentration of VFA present in the digesters and the dependence on other factors, such as the nature of the fermentation broth and the intended subsequent use of the VFAs [26,32].
As with VFAs, biohydrogen is produced during the initial stages of the overall AD process (acidogenesis and acetogenesis). Additionally, it can be generated through the oxidation of acetate. Similarly, the suppression of methanogenic activity, typically by heating, can facilitate the formation of hydrogen. The primary variables that could be modified in order to increase the hydrogen production are temperature, pH, HRT, and OLR [33]. However, the use of facultative acidogenic–acetogenic microorganisms, the substrate pretreatment (for instance, to modify the C/N ratio), or the supplementation with essential nutrients (e.g., N, P, Fe) may represent alternative pathways to increase the biohydrogen yield of a specific substrate [33]. Both mesophilic and thermophilic ranges of temperature have been used successfully to produce biohydrogen through AD processes. Higher temperatures allow for better catalytic efficiency of the enzymes involved; however, the VFAs produced, such as acetic and butyric acid, inhibit production by lowering the pH [34]. Regarding the pH, the optimal value would be that which allows the maximum growth of the microorganism involved and the higher activity of the produced enzymes. Consequently, it strongly depends on the specific system. Nevertheless, acid-to-neutral values (5–7) are typically found in the literature [33,35]. The impact of HRT and OLR on biohydrogen performance are variables that need yet to be further studied. Nevertheless, preliminary studies suggest that HRT and OLR are strongly linked to substrate’ composition and digester set-up.
Biohydrogen formation through the acetate oxidation pathway is thermodynamically favourable at low hydrogen concentrations. It is, therefore, recommended to use mixing and partial pressure controls, through the removal of the produced biohydrogen, in order to achieve the desired results. This allows for higher liquid–gas mass transfer in the digester, which has the potential to increase biohydrogen production by 38%, while simultaneously reducing the potential formation of secondary metabolites, such as ethanol, butanol, and acetone [36,37]. The resulting gas mixture must undergo a purification process before utilisation as a source of energy. This process, also referred to as biohydrogen upgrading, can be achieved through a number of techniques, including absorption, membrane separation, cryogenesis, and pressure swing adsorption [15].
Despite the considerable potential of biohydrogen as an alternative source of clean and renewable energy, it is still considered a relatively fragile technology, given the limitations it currently faces in terms of scale-up, refining, processing, and storage of large volumes [38]. Besides, the low yield of hydrogen (1 to 3 mol H2 mol−1 glucose) can make its industrial production unfeasible. The search for improving biohydrogen productivity is still a prominent area of research, with proposals including the addition of nanoparticles, the use of genetic engineering, and cell immobilisation [33].

4.3. Lignocellulosic Substrates and Their By-Products

Lignocellulosic biomasses are those composed significantly of cellulose, hemicellulose, and lignin and represent most of the agricultural wastes. Therefore, the scientific interest in lignocellulosic biomass has increased in recent years as a feedstock to produce bioenergy and derived products [39]. However, the conversion into valuable compounds is hampered by the complexity of its structure, mainly due to the presence of lignin, a complex phenolic polymer which is usually interlinked with hemicellulose, cellulose, polyphenols, and proteins [40]. The presence of these components gives these biomasses their hard-to-degrade properties. Consequently, their hydrolysis is often the limiting factor during the AD process [41]. Therefore, several pretreatments have been applied in order to break down its structure and increase their exposure to the enzymes released by hydrolytic bacteria [42]. However, the majority of the reported pretreatments, including hydrothermal or acidification processes, have the potential to collaterally generate or release toxic/inhibitory compounds such as acetyl groups, organic acids, furfurals, heavy metals, etc. [43]. Furthermore, the effectiveness of pretreatments applied to lignocellulosic materials faces other significant challenges, such as technology availability, cost, energy efficiency, and environmental impact, among others [43].
Applying the biorefinery principles, lignocellulosic biomass processing has the potential to facilitate valuable biomaterials. Regardless of the end use and the technology applied, the conversion of these biomasses into oligosaccharides and sugars has been identified as the most studied process in the reviewed bibliography. For instance, the cellulose and hemicellulose fractions, whose fermentation produces sugars and oligosaccharides are widely used for bioethanol production. This process is usually carried out by enzymatic digestion, followed by physico-chemical pretreatment, hydrolysate fermentation, and ethanol fermentation [44]. Advances in biotechnology have led to the development of more efficient enzymes and pretreatment methods, enhancing the overall yield and reducing costs [45]. In addition to its use as a source of bioethanol, glucose can act as a precursor for other products of commercial value, such as bioplastics, succinic acid, or fungal protein. These products have diverse applications: bioplastics are used in environmentally friendly packaging, succinic acid serves as a building block for various industrial chemicals, and fungal protein is a promising source of sustainable food and animal feed [46,47,48]. Moreover, the treatment of lignocellulosic biomass can produce organic acids, which can be used in the chemical industry as part of resins, pesticides, or fertilisers [49]. However, the residual lignin, which typically remains intact after the aforementioned treatments, has limited commercial value in today’s markets. Nevertheless, it could be considered as a potential source of heat and electricity production through combustion processes.
Indeed, lignocellulosic biomasses have been the subject of extensive assessment in relation to their potential as a source of energy. In addition to AD, other technologies, including pyrolysis, gasification, hydrothermal carbonisation, combustion, fermentation, saccharification, and transesterification have been employed to process these materials in order to produce bio-oils, alkenes, hydrogen, bioethanol, butanol, biodiesel, and biogas [50]. Additionally, research is ongoing to find new applications for lignin, such as in the production of high-performance materials, carbon fibres, and bio-based adhesives, which could potentially increase its economic value [51,52,53].

4.4. Main Products of Anaerobic Digestion: Biogas and Digestate

As exposed through the present bibliometric analysis, the primary products derived from a traditional AD process are biogas and digestate. Biogas has great energy potential and, after proper treatment, can be used in a variety of sectors, such as the automotive industry, and for injection into the natural gas grid, combined heat, and power generation [54]. Biogas is mainly composed of CH4 (50–75%) and CO2 (25–50%), although it also presents traces of H2, NH3, and H2S (>1%) [55]. The presence of methane gives biogas its energy potential. In fact, the calorific value of biogas can vary significantly, as described by Cellek et al. [56] and changes from 15.580 to 26.230 MJ kg−1 can be obtained when methane content varies from 55% to 75%. However, in order to increase its calorific value and reduce the corrosive potential of H2S, the methane fraction has to be separated and purified, a process that is commonly known as biogas upgrading [57]. Traditional upgrading processes include absorption (water, organic, or ammine scrubbing) or adsorption technologies or the use of physical membranes [56,58]. However, these processes do not efficiently remove the N2 and O2 content of biogas [57]. In order to tackle this technical issue, emerging technologies, such as cryogenic condensation or sublimation, and biological processes that take advantages of photosynthetic organisms have received great attention from the scientific community in the recent years [56]. Although, a recent study reported that membrane separation provides the most cost-effective solution [57].
Given the connection between CO2 and global warming, its release into the atmosphere should be minimised. Some alternatives currently exist, such as utilising CO2 in microalgae cultivation systems, where algae convert carbon dioxide into biomass through photosynthesis [59]. The resulting algal biomass can be used as a co-substrate in AD [60]. Furthermore, the conversion of CO2 into synthetic fuels has raised interest in the past decades; catalytic, plasma-chemical, photochemical, biochemical, solar, thermo-chemical, and electrochemical conversion technologies are viable options that address environmental regulations [61].
Finally, AD produces a semi-liquid effluent called anaerobic digestate, which is rich in essential nutrients such as phosphorus or nitrogenous organic compounds. This digestate can be used as a soil amendment or organic fertiliser [62]. This process reduces the environmental impact associated with mineral fertilisers by lowering GHG emissions and promotes the circular economy by returning the products of anaerobic digestion to the field. However, the characteristics of digestate vary significantly depending on the feedstock used and may present several limits for its use. For instance, digestates from animal manure may contain pathogens and heavy metals (copper, zinc, etc.) at phytotoxic concentrations [63]. Thereby, while the presence of pathogens can be addressed by simple thermal treatments, the removal of toxic compounds such as heavy metals or phenolic compounds may be challenging. For example, the most widely used methods for heavy metals removal in wastewater are bioadsorbents, although others are currently being studied, such as carbon-based methods or silicate binders, which are proving to be very efficient, safe, and low-cost methods for heavy metals removal [64]. On the other hand, nutrient availability may present another impediment factor to its direct use on soils. Several processes are available, including ammonia stripping, struvite precipitation, ion exchange, adsorption, evaporation, freeze concentration, membrane separation, and composting [65].
Other studies have focused on the thermochemical conversion of the anaerobic digestate in order to obtain energy, such as biochar, or added-value materials, such as syngas [66]. Among the available technologies, pyrolysis, hydrothermal carbonisation (HTC), gasification, and torrefaction are the most widely studied and applied [66].
Moreover, these processes need to comply with the local regulation, which, due to the significant lack of unanimity, impede the proposal of a general and efficient process [22]. Nevertheless, recent advances in machine learning and artificial learning have provided promising results on the application of fertiliser informatics as an efficient tool to design pathways to efficiently valorise anaerobic digestates [67].

4.5. Future Perspectives

This bibliometric analysis shows the necessity for a sustainable economy to transform by-products into high-value products, thereby moving away from the traditional concept of waste, which is typically incinerated or stored. In this context, the idea of bio-refineries emerges as a key element in both technical and scientific advancements. Bio-refineries have the potential to broaden our knowledge horizons and address the current barriers to the large-scale utilisation of certain residues. Among the existing technologies, AD is the most promising and significant process.
AD plays a crucial role in producing clean and renewable energy and contributes significantly to the achievement of the Sustainable Development Goals (SDG) set by the UN (e.g., SDG 7, affordable and clean energy). However, developing a comprehensive waste management system based on AD requires addressing potential issues related to the characteristics of substrates and the effluents generated. This necessitates evaluating the use of pretreatments, biogas upgrading, and digestate modification in an integrated manner. The progression from laboratory-scale experiments to industrial-scale plants is essential, as optimised parameters and strategies often do not yield the same results when scaled up. Addressing the challenges associated with increasing production volumes needs to be experienced and resolved. This is currently a major factor slowing down the commercialisation of AD from a biorefinery perspective.
In terms of monetising this integrated scenario, the specific literature provides limited insights into the economic aspects. This is mainly due to the strong correlation between profits and market volatility, which complicates the derivation of reliable conclusions independent of market fluctuations. For example, Delrue et al. [68] concluded that the price of a litre of biodiesel derived from AD by-products ranged between €1.94 and €3.35, which was significantly higher than the market prices at the time of the study. Furthermore, many economic assessments in the reviewed literature focus primarily on the valorisation of one specific co-product, neglecting the overall potential of the rich mixture of co-products generated by an efficient biorefinery. While much attention has been given to the commercialisation of methane, studies have highlighted that the most profitable output of an AD process is the valorisation of the anaerobic digestate as an organic fertiliser [67].
Despite the need for further research and investment in AD within the biorefinery context, this technology is viewed as essential for developing a sustainable future for upcoming generations. As the IEA Bioenergy Task Report 37 states: “We are still at the advent of the circular economy” [69]. The scientific, technological, and legislative challenges to making this technology a viable renewable energy source are numerous and varied. However, collective action and knowledge sharing are powerful tools that can help pave the way towards achieving the goals outlined in the Horizon Europe plan set for 2030.
Moreover, expanding research efforts to explore the co-benefits of AD, such as greenhouse gas reduction, nutrient recovery, and soil health improvement, can provide a more comprehensive understanding of its potential contributions to sustainability [16]. Interdisciplinary collaborations and public–private partnerships will be crucial in overcoming current limitations and accelerating the adoption of AD technologies on a commercial and industrial scale [70]. By promoting innovation and encouraging investment in AD and biorefinery projects, we can move closer to realising a circular economy that not only addresses waste management but also contributes to energy security and environmental protection [71].
An additional area of focus should be the development of more efficient and cost-effective pretreatment methods to enhance the biodegradability of various substrates [72]. Current pretreatment technologies, such as thermal, chemical, and mechanical methods, each have their advantages and limitations. Research into novel pretreatment techniques, potentially integrating multiple methods, could significantly improve the overall efficiency and economic viability of AD processes. This, in turn, would facilitate the conversion of a wider range of waste materials into valuable bioproducts [72].
Furthermore, policy and regulatory frameworks need to be developed to support the widespread implementation of AD and biorefineries [71]. Governments can play a crucial role by providing incentives for research and development, subsidising initial investments in AD infrastructure, and setting favourable tariffs for biogas and bio-based products. By creating a supportive policy environment, it will be possible to attract more private investment and foster a thriving market for AD technologies.

5. Conclusions

This bibliometric analysis highlights the critical need for a sustainable economy that transforms by-products into high-value products, shifting away from traditional waste disposal methods like incineration and storage. Biorefineries emerge as a key component in this transformation, offering solutions to the large-scale utilisation of residues. Among existing technologies, anaerobic digestion (AD) stands out as the most promising for producing clean and renewable energy, significantly contributing to the United Nations’ Sustainable Development Goals (SDGs), particularly affordable and clean energy (SDG 7).
Future research should focus on optimising pretreatment methods, biogas upgrading, and digestate utilisation to enhance AD efficiency and economic viability. Expanding studies on the co-benefits of AD, such as greenhouse gas reduction and nutrient recovery, alongside developing supportive policy frameworks, will be essential for advancing AD technologies. Interdisciplinary collaborations and public–private partnerships are crucial to overcoming current limitations, driving innovation, and accelerating the adoption of AD on a commercial scale, thereby contributing to a sustainable and circular economy.

Author Contributions

Conceptualization, P.C.G., J.M.M.-L., D.d.l.L.-C., R.B. and M.J.F.-R.; methodology, software, validation, formal analysis, investigation, resources, and data curation, P.C.G., J.M.M.-L. and M.J.F.-R.; writing—original draft preparation, writing—review and editing, and visualization, P.C.G., J.M.M.-L., D.d.l.L.-C., R.B. and M.J.F.-R. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in the article.

Acknowledgments

The authors thank the reviewers for their comments and suggestions, which have significantly improved the quality of this manuscript. Special thanks to M. M. Ballesteros Martín for her help in using the VOSviewer software.

Conflicts of Interest

The authors declare no conflicts of interest.

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Fermentation 10 00446 g001
Figure 1. Cumulative publications per year divided into two distinct periods, 1993–2007 and 2008–2023. Both periods are presented with their trend line and the corresponding fitting equations and determination coefficient (R2).
Figure 1. Cumulative publications per year divided into two distinct periods, 1993–2007 and 2008–2023. Both periods are presented with their trend line and the corresponding fitting equations and determination coefficient (R2).
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Figure 2. Distribution of the analysed data according to the document type. Numbers represent articles within the specific type.
Figure 2. Distribution of the analysed data according to the document type. Numbers represent articles within the specific type.
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Figure 3. Distribution of the data analysed according to the subject area. Numbers represent articles within a particular subject area. In some cases, documents have been added to more than one subject.
Figure 3. Distribution of the data analysed according to the subject area. Numbers represent articles within a particular subject area. In some cases, documents have been added to more than one subject.
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Figure 4. VOSviewer co-occurrence analysis results, based on the study’s keywords. Visually presenting 4 clusters: Cluster 1 in red, Cluster 2 in green, Cluster 3 in blue, and Cluster 4 in yellow.
Figure 4. VOSviewer co-occurrence analysis results, based on the study’s keywords. Visually presenting 4 clusters: Cluster 1 in red, Cluster 2 in green, Cluster 3 in blue, and Cluster 4 in yellow.
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Table 1. Words constituting each cluster were derived from the VOSviewer co-occurrence analysis and sorted alphabetically.
Table 1. Words constituting each cluster were derived from the VOSviewer co-occurrence analysis and sorted alphabetically.
Cluster 1Cluster 2Cluster 3Cluster 4
Anaerobic digestionAcetic acidAlcoholBiogas
BiorefineryAnaerobic growthBioethanolDigestate
Carbon dioxideBiodegradationBiofuelFertilisers
Circular economyBiomethaneBiotechnologyManures
Economic analysisBioreactorEnzymatic hydrolysisMicroalgae
Environmental impactBioremediationHydrolysisMicroorganisms
ExtractionButyric acidLignocellulose
Food wasteCarbonSubstrates
Global warmingChemical oxygen demand
Greenhouse gasesChemistry
Life cycleFermentation
PyrolysisHydrogen
RecyclingpH
Renewable energyVolatile fatty acids
Sustainable developmentWaste disposal
ValorisationWaste water
Waste treatment
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Castillo García, P.; Fernández-Rodríguez, M.J.; Borja, R.; Mancilla-Leytón, J.M.; de la Lama-Calvente, D. Research Trends in the Recovery of By-Products from Organic Waste Treated by Anaerobic Digestion: A 30-Year Bibliometric Analysis. Fermentation 2024, 10, 446. https://doi.org/10.3390/fermentation10090446

AMA Style

Castillo García P, Fernández-Rodríguez MJ, Borja R, Mancilla-Leytón JM, de la Lama-Calvente D. Research Trends in the Recovery of By-Products from Organic Waste Treated by Anaerobic Digestion: A 30-Year Bibliometric Analysis. Fermentation. 2024; 10(9):446. https://doi.org/10.3390/fermentation10090446

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Castillo García, Pablo, María José Fernández-Rodríguez, Rafael Borja, Juan Manuel Mancilla-Leytón, and David de la Lama-Calvente. 2024. "Research Trends in the Recovery of By-Products from Organic Waste Treated by Anaerobic Digestion: A 30-Year Bibliometric Analysis" Fermentation 10, no. 9: 446. https://doi.org/10.3390/fermentation10090446

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