Enhancing Sustainable Energy Through Cutting-Edge Waste Biorefinery Technologies

Abstract

Biorefineries play a critical role in addressing current global sustainability challenges by converting renewable biomass into valuable products such as biofuels, biochemicals, and bioenergy. The necessity of biorefineries has increased due to the growing demand for eco-friendly alternatives to fossil fuels, mitigating climate change, and reducing environmental impact. Utilizing agricultural byproducts, forest residues, and municipal waste, biorefineries harness feedstock through techniques such as pretreatment, anaerobic digestion, and thermochemical transformation, alongside advanced methods including electrochemical processes, hydrogenation of bio-derived substances, and the implementation of complex biocatalysts within biomass-oriented biorefineries. These processes enhance the efficiency of bioenergy production by breaking down complex biomass into simpler components. The current capacity of biorefineries is expanding globally, with increasing interest in advanced technologies such as gasification, fermentation, and enzymatic conversion. While biorefineries offer significant potential for reducing waste and promoting a circular economy, challenges such as scalability and cost efficiency remain. Future trends include integrating new biotechnological advances and optimizing resource recovery systems to improve yield, profitability, and sustainability in the bioeconomy.

1. Introduction

The international inclination for sustainable energy is experiencing an upward trajectory as the urgency surrounding climate change, diminishing fossil fuel reserves, and ecological degradation intensifies [1,2]. This transition towards environmentally friendly energy alternatives demands innovative methodologies for energy generation that surpass traditional practices. A particularly promising frontier that is gaining momentum is the advancement of waste biorefinery technologies [3]. Biomass waste constitutes an essential resource for biorefineries, facilitating the synthesis of biofuels and other high-value commodities while advancing sustainability initiatives. The incorporation of diverse biomass categories, including agricultural residues, food waste, and forestry by-products, enhances the efficacy of bioconversion methodologies. Agricultural residues, such as straw, husks, and shells, can be proficiently transformed into biofuels via thermochemical and biochemical techniques [4]. Likewise, the substantial generation of food waste offers considerable prospects for bioconversion, significantly mitigating the environmental effects associated with waste disposal [5]. Moreover, forestry and urban waste can be processed to produce biofuels and catalytic agents, thereby enhancing the economic feasibility of biorefineries [6].

Together, these strategies enable biorefineries to use a wide range of technologies, including biochemical, thermochemical, and advanced electrocatalytic processes, to convert organic waste into usable energy forms. These pioneering technologies are consistent with the principles of circular economy and sustainability, as they facilitate the closure of material loops, decrease waste generation, and enhance resource utilization [7,8]. In contrast to conventional energy sources that rely on limited fossil fuels, biorefineries draw upon waste materials that are not only abundantly available but frequently under-exploited. This approach not only alleviates the pressure on landfills but also reduces the greenhouse gas emissions linked to waste decomposition and the combustion of fossil fuels [9]. Furthermore, biorefineries have the potential to strengthen local economies by generating employment opportunities within waste management and energy production sectors, thereby augmenting their societal value [10].

One of the most advantageous characteristics of waste biorefinery technologies is their adaptability. These facilities are capable of processing a wide range of feedstocks, including lignocellulosic biomass, food waste, animal manure, and sewage sludge, each of which poses distinct challenges and prospects for energy recovery [11]. For instance, lignocellulosic biomass, sourced from agricultural residues such as straw and wood chips, can be transformed into biofuels via methodologies including pyrolysis, gasification, and fermentation [12]. Likewise, food waste, which significantly contributes to methane emissions when disposed of in landfills, can be converted into biogas through anaerobic digestion. The resultant biogas can subsequently be purified and utilized for electricity generation or as transportation fuel. Additionally, electrocatalytic methodologies, exemplified by microbial fuel cells, are being innovated to convert organic matter within wastewater directly into electricity, emphasizing the diverse array of technological strategies being invested [13].

Biorefineries represent a transformative methodology for the conversion of biomass into an array of valuable commodities, encompassing biofuels, chemicals, and materials. Although numerous proposed biorefinery supply chains remain in theoretical frameworks, several tangible instances exemplify the effective realization of integrated biorefinery approaches. A particularly noteworthy instance is the Biomass Innovation Cluster situated in the Netherlands, which emphasizes the conversion of agricultural waste into bio-based products [14]. This cluster encompasses a diverse array of stakeholders, including agricultural producers, technology innovators, and manufacturers, collaboratively striving to establish a circular economy. Among the successful biorefineries within this cluster is the BioMCN facility, which facilitates the transformation of non-food biomass into methanol [15]. This facility employs agricultural residues, such as straw and wood chips, thereby demonstrating a model wherein waste biomass is adeptly converted into a valuable chemical. Another significant illustration is NexSteppe, a corporation dedicated to the cultivation of high-yield biomass crops specifically engineered for bioenergy applications [16]. Through collaboration with farmers to propagate dedicated energy crops, NexSteppe has established a sustainable supply chain that feeds into biorefineries for the production of advanced biofuels. Their partnership with existing refineries exemplifies the effective integration of biomass into conventional energy systems, thus minimizing waste and augmenting profitability. The Drax Power Station in the United Kingdom further exemplifies a compelling case of a biorefinery concept in operational practice. Originally a coal-fired power generation facility, Drax has undergone a transition towards renewable energy by incorporating biomass. It utilizes sustainably sourced wood pellets, converting waste wood into electricity [17]. This transformation underscores the capacity to adapt pre-existing infrastructure for biorefinery purposes, integrating biomass as a principal feedstock.

These instances signify an escalating trend towards the establishment of integrated biorefineries that not only convert waste biomass into multiple products but also achieve this in an economically viable and environmentally sustainable manner. By capitalizing on industrial clusters, innovative technological advancements, and collaborative partnerships, these facilities illustrate the potential for biorefineries to assume a pivotal role within a circular bioeconomy, advancing from conceptualization to fully operational systems. As this field continues to evolve, it is anticipated that an increasing number of projects will emerge, thereby further strengthening the viability of biorefineries and their capability to contribute to sustainable development objectives. The widespread implementation of waste biorefinery technologies presents numerous challenges, notwithstanding their inherent potential. Economic feasibility remains a significant issue, given that the initial capital outlay and ongoing operational expenditures for biorefinery facilities can be substantial [18]. The development of efficient and scalable methodologies capable of processing heterogeneous waste streams while optimizing energy recovery is essential for the sustained success of this sector. Moreover, the establishment of policy frameworks and incentives is imperative to facilitate the incorporation of waste biorefineries into pre-existing energy systems and waste management infrastructures. Additionally, international collaboration and the exchange of knowledge will be vital in addressing these obstacles and advancing innovation within the industry [19]. The assimilation of waste biorefineries into the global energy paradigm signifies a substantial advancement in the shift towards a more sustainable and circular economy. By converting organic waste into renewable energy, these technologies are positioned to play a crucial role in diminishing dependence on fossil fuels and mitigating environmental degradation. As technological innovations persist in enhancing the efficiency and scalability of waste biorefineries, they possess the potential to emerge as a fundamental component of sustainable energy infrastructures globally, thereby contributing to a cleaner and more resilient future ignited within the waste biorefinery sphere [9].

1.1. Feedstock Diversification in Waste Biorefinery Technologies

Feedstock diversification in waste biorefinery technologies is critical for enhancing sustainability, optimizing resource utilization, and improving the economic feasibility of biorefineries. By incorporating a wide range of organic waste materials, biorefineries can generate a broader array of bio-based products while simultaneously reducing environmental impacts. The principal facets of this diversification encompass the employment of heterogeneous feedstocks, including agricultural byproducts and food waste, which contribute to the diminution of landfill waste and the alleviation of methane emissions. The conversion of low-value compounds, like lignin, into valuable coproducts contributes to environmental sustainability [20]. A diverse feedstock supply can stabilize supply chains and minimize reliance on single sources, thus improving biorefinery economics [21]. The integration of various feedstocks facilitates the production of high-value derivatives, including biofuels and bioplastics, which can stimulate market growth [22]. The inherent variability in feedstock quality necessitates the adoption of advanced conversion technologies to optimize process yields [21]. Overcoming the recalcitrance of specific biomass types, such as fruit and vegetable waste, requires innovative pretreatment strategies to enhance their applicability [23]. While feedstock diversification provides several advantages, it also presents challenges in terms of processing efficiency and technological adaptation. Ongoing research and technological advancements are essential to overcoming these barriers and fully realizing the potential of diverse feedstocks within biorefinery systems. This diversification includes the use of agricultural by-products, food discards, municipal solid waste (MSW), effluent sludge, and non-traditional feedstocks such as algae and industrial residues. Through this approach, feedstock diversification not only addresses waste management issues but also enhances the efficiency and resilience of biorefinery systems.

1.1.1. Agricultural Residues and Food Waste

Agricultural residues, including straw, husks, and corn stover, represent many biomass resources that can serve as feedstocks within biorefinery operations [24]. Historically, these residues were mostly burned or left to decay, releasing large amounts of carbon dioxide and methane into the air. However, the conversion of agricultural residues in biofuels, biochar, or other bioproducts not only serves to alleviate greenhouse gas emissions but also generates supplementary revenue streams for agricultural producers. Lignocellulosic biomass, present in agricultural residues, is characterized by a high content of cellulose, hemicellulose, and lignin, rendering it a viable feedstock for bioethanol production via fermentation after pretreatment and enzymatic hydrolysis [4]. Food waste, which constitutes a considerable global concern, also represents an underexploited resource for biorefinery applications. Over one-third of food produced worldwide is discarded, resulting in methane emissions when such waste is deposited in landfills. Food waste, which is abundant in carbohydrates, lipids, and proteins, can be transformed into biofuels (including biogas through anaerobic digestion) and biochemicals. Innovative processing technologies, such as hydrothermal liquefaction and pyrolysis, can also be employed to process food waste, converting it into bio-crude oil, which may be refined into biodiesel or bio-jet fuel [4,25].

Additionally, their susceptibility to biological digestion is viewed as a key benefit for enhancing the biodegradability and strength of composite materials. Figure 1 illustrates the sources of agricultural fillers used in producing bio-composites and biofuels. Using agricultural by-products reduces dependence on landfills and lowers greenhouse gas emissions, promoting environmental sustainability [4]. Combining biofuel production with soil nutrient recycling improves farming practices and helps reduce the effects of climate change. Notwithstanding its potential, several challenges including feedstock logistics, contamination, and seasonal availability must be systematically addressed to facilitate the scaling of production. Moreover, biorefineries integrate a wide range of processes to convert biomass integrate multiple stages such as chemical, biological, and thermal transformations, thereby facilitating the optimal exploitation of biomass, reducing by-products, and producing revenue across various product streams. Agricultural residues encompass substantial quantities of cellulose, hemicellulose, and lignin, crucial constituents that can be converted into a variety of outputs. The pretreatment phase is imperative for deconstructing biomass and rendering these constituents available for subsequent conversion processes, such as enzymatic hydrolysis, fermentation, or thermochemical methods.

Table 1 elucidates biorefinery technologies related to the conversion of 10 tons of agricultural waste into various products, including bioethanol, biogas, biochar, and lignin-derived chemicals, emphasizing the aspects of Technology Readiness Levels (TRLs), developmental maturity, and revenue potential. TRLs serve as a standardized evaluative framework that assesses the developmental stage of technologies, extending from TRL 1 (fundamental research) to TRL 9 (commercial implementation). Technologies are categorized as Emerging (TRL 4–6), Mature (TRL 7–8), or Commercial (TRL 9), based on their preparedness for industrial-scale application. For example, pretreatment technologies, such as steam explosion, are classified as mature (TRL 7–8) and proficiently generate lignocellulosic biomass for subsequent processing, albeit yielding low revenue due to a scarcity of by-products. Enzymatic hydrolysis and fermentation are regarded as commercial technologies (TRL 9), effectively transforming biomass into fermentable sugars and bioethanol, respectively. Bioethanol, recognized as the most economically valuable product, presents substantial revenue potential. Anaerobic digestion is characterized as both mature and commercial, facilitating the production of biogas and digestate, which generates moderate revenue. Emerging technologies, exemplified by pyrolysis (TRL 6–8), are capable of converting digestate into biochar and bio-oil, whereas lignin valorization, situated at an initial stage (TRL 4–5), possesses the capability for producing high-value lignin-based products. Moreover, the production of biochar is presently confronted with various obstacles that impede its market viability, including elevated costs and insufficient financial backing [27]. Recent technological innovations have contributed to a decrease in production expenses, thereby enhancing its accessibility, with market prices fluctuating between 200 and 1000 per ton [28]. The utilization of biochar remains relatively restricted, predominantly in developed nations, suggesting that it constitutes an incipient practice rather than a fully established commercial endeavor [29]. The capacity of biochar to enhance agricultural productivity and alleviate climate change impacts indicates a potentially advantageous trajectory, provided that existing barriers are effectively mitigated [30]. Collectively, these technologies enhance the utilization of agro-waste, thereby advancing economic viability and environmental sustainability. As biorefinery technologies progress, the efficiency of biomass conversion is poised to enhance, resulting in reduced waste, improved product recovery, and increased economic viability. Integrated systems within biorefineries exhibit the potential to enhance the utilization of biomass, thus promoting a circular economy distinguished by reduced environmental impacts and optimized economic advantages.

1.1.2. Municipal Solid Waste (MSW) and Wastewater Sludge

Municipal solid waste (MSW), comprising a heterogeneous mixture of organic and inorganic materials, represents a significant feedstock for biorefineries [45]. Contemporary waste separation and preprocessing methodologies facilitate the extraction of organic components from MSW, permitting their transformation into bioenergy. Techniques such as gasification and anaerobic digestion are employed to convert these organic constituents into biogas, which is then utilized for the generation of electricity, thermal energy, or upgrading to renewable natural gas [46]. Moreover, the by-products of these conversion processes, including biochar derived from pyrolysis, may serve as soil amendments, thereby completing the cycle of waste utilization [47]. Similarly, wastewater sludge, which is an output of sewage treatment facilities, is abundant in organic matter and can be developed for energy generation. Anaerobic digestion stands as the predominant technique for the conversion of sludge into biogas. The solid residuals from this procedure can undergo additional treatments aimed at nutrient recovery, resulting in the production of fertilizers and other valuable commodities. In addition, pellet production from solid waste is an innovative method of waste management that converts municipal solid waste and other organic materials into dense, energy-rich pellets. These pellets can be used as a renewable fuel source, reducing landfill waste and minimizing environmental impacts. The process involves treating and transforming various solid wastes, such as agricultural residues, food waste, and paper products, into compact, uniform pellets through a series of steps, including drying, crushing, mixing, and densification. This not only reduces waste volume but also creates a valuable alternative energy source. The production of pellets involves several key steps, including separation, collection, drying, crushing, mixing, and densification processes. Figure 2 illustrates the collection and processing of raw materials at the site following their separation from municipal solid waste. This approach not only mitigates the environmental repercussions associated with wastewater treatment but also enhances energy recovery efforts within the framework of a circular economy. Recently, Xing et al. [48] reported key findings on the co-treatment of municipal solid waste incineration fly ash (MSWI FA) and municipal sludge, emphasizing enhanced sludge dewatering and fly ash dechlorination. The study shows that adding MSWI FA significantly improves sludge dewatering due to the physical and chemical interactions between the two materials. Co-treatment also effectively reduces chlorinated compounds in the fly ash, lowering its toxicity. Optimal ratios of MSWI FA and sludge were identified to maximize these benefits. This innovative method not only improves sludge management but also reduces environmental hazards and treatment costs, offering a promising waste management solution.

1.1.3. Algae and Non-Conventional Feedstocks

Algal biorefineries represent a highly promising technological advancement for the generation of sustainable energy, particularly in geographical areas characterized by controlled agricultural resources [50]. Algal biomass, including microalgae and cyanobacteria, offers a sustainable source for biofuel production as they grow rapidly without competing for arable land or food sources [51]. Recently, there has been increasing interest in marine microalgae for bioethanol production due to their high osmotic tolerance and ability to thrive in marine environments [52]. The biochemical composition of these species, such as polysaccharides, proteins, and lipids, supports the production of bioethanol and other value-added compounds [53]. However, the carbohydrate extraction process is essential for making microalgal polysaccharides more accessible for fermentation, as direct absorption by fermentative microorganisms is hindered by their complexity [54]. Methods like acid and enzymatic treatments are commonly used, although challenges in efficiency and cost remain [55]. The utilization of household waste and algal biomass for biofuel production presents a sustainable framework for waste management and renewable energy production. Figure 3 illustrates the conversion of fruit and vegetable waste (FVW) into value-added products. Large quantities of FVW are produced daily by food and wholesale markets. Despite having a shorter shelf life than other waste types, organic waste still poses environmental risks due to its interactions with the surrounding ecosystem. Organic waste generated in households, including food remnants and used oils, can be transformed into biofuels such as biodiesel or bioethanol via methodologies like anaerobic digestion and fermentation [56]. Algae, especially microalgae, possess a high concentration of lipids and carbohydrates, rendering them an exemplary feedstock for biofuel synthesis. Moreover, algal cultivation can be conducted utilizing wastewater, thereby alleviating competition with agricultural crops and addressing concerns related to water consumption. The integration of algal cultivation with the valorization of household waste stimulates a synergistic framework, promoting enhanced recycling of waste materials while simultaneously yielding valuable biofuels.

Recently, Maity et al. [57] elucidated several essential findings concerning the viability of marine microalgae and cyanobacteria in the field of bioethanol production. The research underscores a notable diversity of marine microalgae and cyanobacteria that may be utilized as feedstocks for bioethanol synthesis. Such diversity is essential, as it presents a range of biochemical pathways and metabolic capabilities that can be exploited for the generation of biofuels. Their research demonstrates that the biochemical composition of selected microalgae and cyanobacteria is conducive to bioethanol production. A high carbohydrate concentration, particularly in specific species, is critical, given that carbohydrates can be fermented into ethanol. Their results deliberate on the fermentation potential of carbohydrates extracted from these organisms. It was observed that certain strains exhibited substantial fermentation efficiency, which is essential for the economic feasibility of bioethanol production. Their research highlights the ecological benefits associated with the utilization of marine microalgae and cyanobacteria. These organisms can be cultivated on nonagricultural land and utilize seawater, thereby not competing with food crops for freshwater resources. Notwithstanding the encouraging findings, the manuscript also delineates challenges to the commercial deployment of these feedstocks. Challenges such as the costs associated with cultivation, harvesting, and processing must be resolved to render bioethanol production from marine microalgae and cyanobacteria economically viable. Many research groups promote further research aimed at optimizing cultivation methodologies and enhancing fermentation processes. This includes genetic engineering and optimizing metabolic pathways to improve yield and efficiency. Moreover, the potential of marine microalgae and cyanobacteria as alternative feedstocks for bioethanol highlights their advantages and the challenges that need to be overcome for practical implications.

2. Anaerobic Digestion in Waste Biorefinery Technologies

Anaerobic digestion (AD) constitutes a thoroughly well-known biochemical process that facilitates the conversion of organic waste into biogas and digestate via microbial activity under anaerobic conditions [58]. The process is fundamentally characterized by four sequential stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Hydrolysis entails the breakdown of intrinsic organic molecules, including proteins, fats, and carbohydrates, into more elementary monomers such as sugars, fatty acids, and amino acids [59]. Subsequently, these simpler compounds undergo fermentation during the acidogenesis phase, resulting in the production of volatile fatty acids, alcohols, hydrogen, and carbon dioxide. The acetogenesis stage follows, wherein acetogenic bacteria transform these intermediate products into acetic acid (CH₃COOH), hydrogen (H₂), and carbon dioxide (CO₂). The concluding phase, methanogenesis, a crucial biological process that engages methanogenic archaea in converting CH₃COOH and H₂ into biogas, is predominantly composed of methane (CH₄) and CO₂ [60]. The resultant biogas serves as a renewable energy source suitable for electricity generation and heating applications or as a substitute for conventional natural gas. Furthermore, the remaining material, referred to as digestate, is abundant in essential nutrients such as nitrogen, phosphorus, and potassium, rendering it advantageous as a soil conditioner or fertilizer in agricultural practices. A notable advantage of AD resides in its capacity to convert a diverse array of organic wastes—including food waste, agricultural residues, manure, and wastewater sludge—into renewable energy while concurrently diminishing greenhouse gas emissions. By capturing methane that would otherwise be emitted into the atmosphere from the decomposition of organic waste, AD plays a crucial role in mitigating one of the most potent greenhouse gasses. This dual function of waste minimization and energy generation positions AD as a pivotal technology for promoting circular economies and sustainability objectives. Moreover, the utilization of digestate facilitates the recycling of nutrients back into the soil, thereby lessening dependence on synthetic fertilizers, which are energy-intensive to produce and can exacerbate environmental degradation through phenomena such as eutrophication. Nonetheless, despite its numerous benefits, AD encounters several challenges that impede its broader implementation and operational efficiency. A primary challenge pertains to the heterogeneity of feedstocks, as various organic materials exhibit significant differences in their biochemical composition and degradability. For instance, a lignocellulosic biomass demonstrates resistance to microbial degradation and necessitates pre-treatment methods, such as thermal or chemical hydrolysis, to enhance biogas yields [58,59,60].

Saha et al. [61] elucidate the merits of anaerobic co-digestion (AcoD) in enhancing biogas yield and its process efficacy while concurrently acknowledging the obstacles such as logistical expenditures and the necessity for pretreatment infrastructure. This underscores the significance of ascertaining the optimal mixing ratio of diverse substrates for the efficacious application of AcoD. Experiential evidence suggests that the utilization of agricultural residues in place of energy crops can mitigate environmental repercussions while simultaneously supplying nutrient-dense co-substrates. Their results indicate that a consistent supply of agricultural residues is imperative for the successful execution of AcoD methodologies. Furthermore, the presence of contaminants in biogas, including hydrogen sulfide (H₂S) and water vapor, necessitates the application of upgrading technologies to purify biogas into biomethane, making it suitable for grid injection or utilization as fuel for vehicles. Research innovations in co-digestion, which involves the simultaneous processing of multiple feedstocks, have shown promise in optimizing biogas production by achieving a balance in nutrient content and enhanced microbial synergy. Additionally, advancements in microbial engineering, digester design, and process control are persistently enhancing the overall efficiency and scalability of AD systems. Specifically, AD emerges as a highly promising approach to addressing both waste management and renewable energy production challenges. It contributes to the mitigation of greenhouse gas emissions, facilitates nutrient recycling, and provides a renewable energy source, thus establishing itself as a cornerstone of contemporary sustainability initiatives.

Recently, Alharbi et al. [62] demonstrated that co-digestion of microalgae with cow manure substantially enhanced biogas production, particularly at a 50% microalgae concentration. Methane yields were significantly higher for Anabaena sp. (345 mL CH₄/g VS) compared to Chlorella sp. (297.96 mL CH₄/g VS) and the control (138.32 mL CH₄/g VS). Additionally, the nutrient-rich slurry generated from this process improved seed germination rates, especially for Sorghum bicolor, which achieved a 94.2% germination rate. The study also reported a threefold increase in biogas production when microalgae were co-digested with cow dung compared to mono-digestion. Their research was illustrated by utilizing pilot-scale anaerobic digesters, specifically plastic vessels with a cumulative capacity of roughly 20 L, as shown in Figure 4a. The digesters were purged of oxygen and sealed with butyl rubber caps, further reinforced with M-seal to ensure anaerobic conditions. Three different microalgae concentrations (25%, 50%, and 75% v/v) were used to assess biogas production, which was measured daily via the water displacement method. The experiment was conducted under mesophilic conditions at 36.85 °C, with the containers shaken twice daily for 1–2 min prior to recording biogas levels (Figure 4b). The flame test was employed to assess the quality of the produced biogas (Figure 4c). This test was carefully performed in a darkened room, with a Bunsen burner connected to one end of a small plastic pipe and the bio-digester linked to the other end to complete the setup. A seed germination assay was conducted to evaluate the quality of the biogas slurry. The slurry, derived from the biogas production using Anabaena sp. at 50% concentration, showed the highest seed germination rates compared to the control (cow manure). The study found that Sorghum bicolor achieved the highest germination rate (94.2%) with the microalgae-associated biogas slurry (Figure 4d), while the lowest germination rate (5.2%) was observed in the control group of Vigna unguiculata. Biogas treatments with microalgae were significantly different (p < 0.05) from control treatments. Seed germination was notably enhanced by the biogas slurry, which contained essential enzymes and chemical factors. According to Zhao et al. [63], 75% of biogas leachate can promote the germination of Vicia faba L. seeds.

Similarly, Miyuki et al. [64] reported that a 50% increase in the germination index is a strong indicator of the absence of harmful substances in the substrate materials. Despite methodological challenges, ongoing innovations in biogas upgrading, feedstock pre-treatment, and co-digestion are enhancing the efficiency and versatility of AD. These advancements are reinforcing AD’s pivotal role in promoting a sustainable, circular economy, making it a key technology in the transition towards cleaner energy solutions. Moreover, AD constitutes a fundamental component in enhancing the operational efficiency of biorefineries, significantly influencing their economic sustainability, scalability, technical viability, and ecological impact. Within the framework of biorefineries, AD adeptly transforms organic waste into biogas, thereby maximizing energy recovery and minimizing waste generation. The economic sustainability of this process is contingent upon various factors, including the availability of feedstock, operational expenditures, and the market prices of biogas and the resultant digestate byproducts.

3. Thermal Conversion in Waste Biorefinery Technologies

Thermochemical conversion represents an essential mechanism in the reconfiguration of biomass and organic waste into energy, fuels, and valuable chemicals, thereby playing a significant role in both sustainable waste management and the generation of renewable energy [65,66]. The two predominant methodologies of thermochemical conversion are pyrolysis and gasification, which employ heat and chemical interactions under varying conditions to decompose organic materials. Pyrolysis transpires at temperatures ranging from 400 °C to 700 °C in an oxygen-deficient environment, resulting in the thermal decomposition of biomass into bio-oil, biochar, and syngas [66]. The liquid fraction known as bio-oil can be refined into transportation fuels including diesel and gasoline; however, its comparatively low energy density concerning fossil fuels presents a challenge that ongoing advancements in upgrading technologies seek to address. Biochar, a solid residue rich in carbon, is increasingly recognized not only for its role as a soil variation that enhances soil fertility and moisture retention but also for its potential to sequester carbon within soils, thereby contributing to efforts aimed at mitigating climate change. Syngas, a gaseous incorporation of hydrogen, carbon monoxide, methane, and other trace gasses, is applicable for heat and electricity generation or can be further processed into chemicals and fuels such as methanol or hydrogen, signifying pyrolysis as a multifaceted process yielding various end products [66,67,68,69].

As an additional method of thermochemical conversion, gasification functions at elevated temperatures, generally between 800 °C and 1200 °C, and in the presence of a regulated quantity of oxygen or steam [70]. This partial oxidation mechanism transforms organic materials into syngas, which can be utilized in an array of energy applications, including combined heat and power (CHP) systems, or converted into liquid fuels and chemicals. The elevated temperatures associated with gasification facilitate the breakdown of complex molecules and diminish the production of deleterious emissions, such as dioxins and furans, thus rendering gasification a more environmentally benign alternative to traditional combustion techniques for waste-to-energy conversion. Syngas generated from gasification can undergo refinement and enhancement via catalytic processes such as Fischer–Tropsch synthesis, which enables the generation of high-value fuels like synthetic diesel and jet fuel. This adaptability in feedstock and the capability to produce a diverse range of energy products position gasification as a promising strategy for addressing the challenges inherent in waste management and renewable energy production, particularly for materials that are challenging to recycle, such as plastics and municipal solid waste (MSW) [71].

Thermochemical conversion presents numerous advantages over biological methodologies such as anaerobic digestion, particularly with respect to feedstock versatility and energy efficiency [72]. The capacity to process an extensive array of biomass sources, encompassing lignocellulosic materials like wood, agricultural residues, and non-recyclable wastes, significantly enhances the adaptability of these technologies. Furthermore, thermochemical processes generally yield superior energy outputs and can function with increased efficiency, especially when integrated with combined heat and power systems [73,74]. Recently, Jiang et al. [70] conducted a study that produced biochar from coconut shell (BC-CS), rice husk (BC-RH), and cow manure (BC-CM) at 800 °C, examining their properties under varying thermal treatment conditions. BC-CM demonstrated the highest BET surface area (263.3 m²/g) and total pore volume (0.164 cm³/g), indicating its potential for efficient adsorption. In methylene blue (MB) adsorption kinetics, BC-CM showed the highest equilibrium adsorption capacity of 2.18 mg/g at 10 ppm and a 0.5 g dose, outperforming BC-RH and BC-CS. The adsorption process followed a pseudo-second-order kinetic model, suggesting chemisorption as the main mechanism for MB adsorption.

In separate research, Suratman et al. [75] explored the effects of various drying methodologies on the physico-chemical characteristics of biomass, particularly focusing on biomass okara, which exhibits notable susceptibility to both oxidation and thermal degradation. Thermogravimetric analysis (TGA) revealed that biomass undergoes decomposition in three distinct stages—the evaporation of moisture, the breakdown of cellulose and hemicellulose, and the degradation of lignin—with the drying technique exerting no substantial influence on the organic constituents. Their results imply that the choice of drying method, along with pyrolysis temperature and activation agents, plays a critical role in customizing biochar properties for its utilization in environmental remediation and energy applications. Nonetheless, despite these benefits, there remain substantial challenges to overcome, including the elevated capital expenditures linked with the establishment and maintenance of thermochemical conversion facilities, as well as the variability in feedstock composition, which can influence process efficiency. For gasification, cleaning syngas and removing tar remain major challenges. Continued innovation is needed to prevent equipment damage and produce high-quality fuels and chemicals.

Moreover, the economic feasibility of this process is contingent upon various factors, including feedstock prices, energy efficiency, and the prevailing market demand for biofuels and biochar products. The adaptability of thermochemical conversion represents a significant advantage, as these processes can be tailored for implementation in both small- and large-scale biorefineries, thereby ensuring extensive applicability across diverse industries. Technical feasibility is strengthened by innovations in reactor design, process optimization, and the synergistic integration of thermochemical conversion with other biorefinery processes to enhance resource efficiency. The environmental significances of thermochemical conversion are generally favorable, particularly when evaluating its capacity to diminish greenhouse gas emissions, convert waste biomass into renewable energy, and endorse practices associated with the circular economy. Nevertheless, challenges persist in the field of emission reduction, byproduct management, and the enhancement of overall energy efficiency to position thermochemical conversion as a sustainable alternative for future applications.

4. Electrocatalytic Upgrading in Biorefineries

Electrocatalytic upgrading constitutes an advanced approach within the field of waste biorefineries, utilizing catalysts activated by electrical energy to promote the conversion of low-value waste materials into higher-value fuels and chemicals [76]. This strategy harnesses renewable electrical energy such as that derived from solar or wind sources to induce chemical reactions that would typically necessitate considerable thermal or pressure inputs. The potential of electrocatalytic upgrading is substantial within the context of sustainable energy, as it not only increases the economic value of products sourced from waste but also provides a viable avenue for the decarbonization of chemical processes that have historically relied on fossil fuel resources. The efficacy of electrocatalytic upgrading in waste biorefineries is determined by several critical factors. The electrode architecture plays a pivotal role in enhancing the performance of electrocatalytic processes by optimizing the surface area and reactivity of the materials utilized. Additionally, improving energy efficiency in these processes is crucial for reducing operational costs and mitigating environmental impacts, particularly through the integration of advanced materials like intermetallics, which offer superior performance compared to conventional cathodes. In addition, the capacity to generate hydrogen on demand represents another critical factor, as it mitigates dependence on external hydrogen sources and infrastructure, thereby facilitating the upgrading process. The electrochemical reactions associated with deoxygenation are also critical, as they enhance the characteristics of bio-oils, rendering them more acquiescent to subsequent processing. Finally, the overall efficiency of electrocatalytic upgrading is improved by the incorporation of these following factors, which collectively strengthen the stability and energy content of the upgraded bio-oils. Therefore, a comprehensive approach that encompasses electrode design, energy efficiency, hydrogen generation, and deoxygenation mechanisms is imperative for the optimization of electrocatalytic upgrading in waste biorefineries. Furthermore, this approach enhances economic sustainability by enhancing the value derived from waste streams and optimizing energy consumption, with prospective decreases in operational expenditures as renewable electricity becomes increasingly available. The potential for scalability is considerable, as electrocatalytic systems can be modified for a diverse range of biorefinery scales, encompassing laboratory settings to large-scale industrial applications. The technical feasibility relies on advancements in catalyst development, electrode design, and process integration, which are critical for enabling the efficient and selective conversion of complex biomass molecules. The environmental implications of electrocatalytic upgrading are favorable, as it diminishes dependence on fossil fuels, reduces greenhouse gas emissions, and enhances the utilization of renewable energy sources for the valorization of biomass. Nevertheless, obstacles persist in the upscaling of cost-effective technologies and in further enhancing the efficiency of electrocatalysts to facilitate broader adoption of this process in sustainable biorefineries.

4.1. Electrocatalytic Upgrading of Bio-Oil

The electrocatalytic upgrading of bio-oil represents an innovative approach designed to enhance its economic value and functional utility. Bio-oil, derived from biomass through thermochemical processes such as pyrolysis or hydrothermal liquefaction, benefits significantly from this advanced methodology [77]. Bio-oil, characterized by its high oxygen content, corrosiveness, instability, and low energy density, requires significant refinement before it can be effectively utilized as a transportation fuel or chemical feedstock. Traditional upgrading methods like hydrotreating are energy-intensive and involve high temperatures, pressures, and expensive hydrogen sources [78]. By contrast, electrocatalytic upgrading offers a sustainable alternative, operating under milder conditions, often at ambient temperature and atmospheric pressure, and utilizing renewable electricity to drive chemical transformations. The primary goal is the selective deoxygenation of oxygenated compounds, thereby increasing the energy density and stability of bio-oil. Key deoxygenation mechanisms include hydrodeoxygenation (HDO), which reduces oxygen-containing functional groups by forming water, as well as decarboxylation and decarbonylation, which remove oxygen in the form of carbon dioxide or carbon monoxide, respectively. Hydrogenation and hydrogenolysis are chemical reactions that help improve bio-oil by making it less reactive and increasing its energy content [79]. Hydrogenation adds hydrogen to unsaturated carbon bonds, while hydrogenolysis breaks carbon–oxygen bonds. To make these reactions work well, they require effective electrocatalysts—substances that speed up the reactions.

Transition metals like nickel, cobalt, and molybdenum are promising because they are cheaper than noble metals like platinum and palladium. High-entropy alloys (HEAs) are also interesting because they can be customized and resist fouling. Electrocatalytic upgrading has some advantages over traditional methods. It can be more energy-efficient, work at lower temperatures, and produce specific products more effectively. However, there are challenges to overcome, such as the complex nature of bio-oil, issues with catalyst performance, and slow reaction rates. To address these challenges, researchers continue improving catalyst designs, using new materials like nanostructures and applying computer modeling. Despite these difficulties, electrocatalytic upgrading shows great promise for turning bio-oil into valuable fuels and chemicals, supporting the use of renewable biomass.

Du et al. [80] conducted a study that revealed several important findings regarding the electrochemical generation of hydrogen alongside the upgrading of biomass-derived isobutanol. The electrodeposited Ni(OH)₂ electrocatalyst demonstrated excellent electrocatalytic activity for isobutanol electro-oxidation, achieving a Faradaic efficiency of up to 92.4% for the production of isobutyric acid at a potential of 1.45 V versus the reversible hydrogen electrode (RHE). This high efficiency highlights the process’s effectiveness in converting isobutanol into valuable chemical compounds while simultaneously generating hydrogen. Notably, the study showed a significant reduction in the electrolysis voltage required for hydrogen production. Specifically, the voltage was reduced by approximately 180 mV at a current density of 40 mA cm⁻² when isobutanol was used as the feedstock. This reduction is significant, as it suggests lower energy consumption for hydrogen production, improving the process’s economic feasibility. The research also sheds light on the mechanisms of the electro-oxidation pathway. It was found that side reactions primarily result from the oxidative cleavage of carbon–carbon bonds in isobutyraldehyde, an intermediate in the reaction, leading to by-products such as formic acid and acetone. This highlights potential areas for optimizing the process to reduce unwanted by-products. The approach not only focuses on hydrogen generation but also emphasizes the sustainable conversion of biomass into valuable chemicals. The synthesis of isobutyric acid from isobutanol under mild conditions marks a significant advancement in utilizing biomass feedstocks for both energy and chemical production. Overall, these findings suggest that the proposed electrocatalytic method for hydrogen generation, coupled with biomass upgrading, is a promising strategy for improving the efficiency and sustainability of electrochemical processes.

Recently, nanoporous metal catalysts have been recognized for their exceptional electrocatalytic activity due to their unique pore structure and large specific surface area, which enhances the efficient utilization of catalytic sites, making them highly suitable for furfural (FFR) upgrading. In the study by Belgsir et al. [81], a Cu catalyst demonstrated a remarkable 90% conversion of FFR, 80% selectivity towards FFA, and 72% Faradaic efficiency. To further improve product rates and Faradaic efficiency in the electrochemical hydrogenation of FFR, Fu et al. [82] demonstrated microcrystalline (4 μm-sized) and nanocrystalline (400 nm-sized) Cu electrodes via electrodeposition. Compared to the bare Cu electrode, the nanocrystalline Cu catalyst more than doubled the productivity of FFA with 2-MF, emphasizing the critical role of surface roughness and specific surface area in enhancing electrocatalytic performance (Figure 5a).

Moreover, yang et al. [83] demonstrated a series of nanoporous Cu electrocatalysts (NP-Cu) through the chemical dealloying of a CuAl alloy for the electrocatalytic conversion of FFR to FFA. By adjusting the concentration of NaOH, they were able to optimize the pore size and active surface area of the catalysts. The NP-Cu₂ catalyst exhibited exceptional electrocatalytic performance, achieving 96% selectivity for FFA and 95% Faradaic efficiency, largely attributed to its optimal pore size, which facilitated H_ads formation and enhanced mass transfer (Figure 5b). The authors further advanced this field by synthesizing homogeneous Cu species anchored on 3D hierarchically porous carbon (Cu/NC900), characterized by a high specific surface area and abundant ultrathin N-doped carbon nanosheets. This catalyst achieved 99% FFR conversion, 100% FFA selectivity, and 95% Faradaic efficiency (Figure 5c). Mechanistic studies confirmed that FFA selectivity is highly dependent on the applied potential, as it impacts the concentration of active hydrogen (H_ads) and FFA radicals on the Cu surface (Figure 5d). Additionally, DFT calculations revealed that the coexistence of Cu⁺ and Cu⁰ significantly contributes to both the kinetics and thermodynamics of the electrocatalytic hydrogenation process, synergistically promoting the reaction (Figure 5e). By integrating these nanoporous structures and optimizing catalyst properties, these studies highlight the potential of Cu-based electrocatalysts for efficient and selective FFR upgrading.

4.2. Sustainable Chemicals Synthesis via Electrocatalytic CO₂ Reduction

Electrocatalytic carbon dioxide reduction (CO₂RR) constitutes a highly promising methodology for mitigating carbon emissions while facilitating the conversion of CO₂ into valuable chemical substances and fuels [84]. This mechanism employs an electrocatalyst to facilitate the reduction in CO₂ into products such as carbon monoxide (CO), formic acid (HCOOH), methane (CH₄), ethylene (C₂H₄), and various alcohols within an electrochemical cell framework. The reduction in CO₂ occurs at the cathode, supported by an externally applied voltage, whereas oxidation reactions transpire at the anode. A major challenge in CO₂RR is overcoming the stability of the CO₂ molecule to achieve efficient conversion into desired products, which requires multiple electron and proton transfer steps. The specific reaction pathways and product selectivity depend on the catalyst material, applied potential, electrolyte composition, and the prevailing reaction conditions. For example, reactions involving 2-electron reductions typically yield CO or HCOOH, whereas 6- or 8-electron reductions culminate in the production of CH₃OH, CH₄, or C₂H₄ [85].

Figure 5

(a) Nanocrystalline Cu catalyzing FFR to FFA and 2-MF. (b) NP-Cu electrocatalysts catalyzing FFR to FFA. (c) Cu catalyst on N-doped porous carbon transforming FFR to FFA. Elsevier. (d) Mechanism for FFR conversion to FFA. (e) Reaction mechanism on Cu⁺ and Cu⁰ active sites. Reproduced with permission from [82] published by Elsevier, 2024.

[Figure omitted. See PDF]

The design of catalysts plays a crucial role in determining the efficiency and selectivity of CO₂RR. Copper (Cu) is particularly distinguished for its capacity to generate hydrocarbons, despite variability in its selectivity. Silver (Ag) and gold (Au) exhibit high selectivity for CO production; however, their high cost poses limitations on their practical applicability. Transition metals such as nickel (Ni), cobalt (Co), and iron (Fe) are also under investigation for their catalytic potential in converting CO₂ to CO or formate. Moreover, metal–nitrogen–carbon (M-N-C) catalysts present a more economically viable alternative, with adjustable properties aimed at enhancing reaction performance. The reaction condition, encompassing electrolyte composition, and pH, further affects the CO₂RR mechanism. Alkaline conditions are generally conducive to formate production, while neutral or acidic environments may promote the generation of CO or hydrocarbons.

Recently, Zhang et al. [84] have introduced various strategies and materials to improve the efficiency and selectivity of CO₂RR process. One key approach is mixed gas reduction, where cyclic voltammetry is employed for simultaneous reduction in CO and CO₂, achieving product selectivities ranging from 25% to 40% for formic acid (HCOOH) and ethylene (C₂H₄) production. Mukhopadhyay et al. [86] demonstrated another strategy that involves enhancing local CO₂ concentration through a nitrile-modified metal–organic framework (MOF), which increases CO₂ concentration by 27-fold and achieves over 90% selectivity for HCOOH at specific potentials [86]. Additionally, bimetallic catalysts, such as CuAg/CeO₂, demonstrate a high Faradaic efficiency (FECO) of 84% for CO production, attributed to electron delocalization and optimized electronic structures [87]. Moreover, porous copper frameworks show mechanical stability and a large surface area, enabling the efficient production of hydrocarbons like methane and ethylene [88]. While these advancements are promising, challenges related to catalyst stability and cost continue to limit practical industrial implementation. Future research must address these issues to improve the viability of CO₂RR technologies further. Xia et al. [89] reveal that the CO₂ CO₂RR efficiency of the catalysts was assessed utilizing a three-electrode flow cell configuration. The implementation of linear sweep voltammetry (LSV) demonstrated that both Cu₅Ce₉₅Ox and Cu₉₀Ce₁₀Ox attained current densities exceeding 420 and 850 mA cm⁻² at a potential of −1.6 V relative to the reversible hydrogen electrode (RHE), markedly exceeding the performance metrics of unmodified CuO and CeO₂ compounds (see Figure 6a). The product selectivity for CO₂RR was indeed examined across a potential spectrum ranging from −0.6 to −1.6 V versus RHE. Both catalysts yielded a diverse array of products, encompassing hydrogen (H₂), carbon monoxide (CO), methane (CH₄), and ethylene (C₂H₄), in addition to trace quantities of formate, acetate, and ethanol (illustrated in Figure 6b,c).

A comparative analysis of Faradaic efficiency (FE) for CH₄ and C₂H₄ at varying potentials is presented in Figure 6d,e. At a potential of −1.2 V relative to RHE, Cu₅Ce₉₅O_x demonstrated a current density of 230 mA cm⁻², with an FE for CH₄ exceeding 60% (as depicted in Figure 6f). In a similar vein, Cu₉₀Ce₁₀O_x exhibited remarkable stability for over 500 h at −1.4 V versus RHE (See Figure 6g), sustaining a high current density of 620 mA cm⁻² and an FE of 60% for C₂H₄. An elevation in Ce(III) concentration, as evidenced in the Ce 3d spectra, contributed positively to the enhanced material stability during the CO₂RR process. In the domains of current density, selectivity, and stability, both Cu₅Ce₉₅O_x and Cu₉₀Ce₁₀O_x showcased superior performance in comparison to other materials reported in the literature, thereby emphasizing their potential for practical applications characterized by enhanced selectivity and stability (See Figure 6h,i). Figure 7 emphasizes the critical role of surface protons (*H) and the *CHO intermediate in determining CO₂RR selectivity. The *CO−*CHO pathway has a lower reaction free energy of −0.36 eV compared to the *CO−*CO coupling reaction, which has a free energy of 1.42 eV. This suggests that the *CHO intermediate is more favorable and plays a key role in influencing the formation of final products, such as CH₄ or C₂H₄. Overall, emphasizing the need to tailor surface chemistry of catalysts is crucial for achieving precise control over catalytic performance. This approach provides key insights for attaining desired selectivity and ensuring long-term stability in CO₂RR technology. Nevertheless, substantial challenges persist that must be addressed prior to the widespread implementation of CO₂RR. Enhancing energy efficiency while concurrently maintaining high selectivity for desired products, particularly multi-carbon compounds, remains a formidable task due to the presence of competing reaction pathways. Additionally, improving the longevity of electrocatalysts to avert deactivation, as well as ensuring the scalability of the process through the utilization of cost-effective materials and optimized reactor configurations, are essential for large-scale applications. Despite these challenges, electrocatalytic CO₂ reduction holds significant potential for sustainable carbon management. It offers a feasible method for converting CO₂ into valuable chemical products, contributing to climate change mitigation and the progression towards a carbon-neutral economy.

Furthermore, the economic feasibility of CO₂ reduction technologies are increasingly becoming prominent, particularly in light of escalating carbon pricing mechanisms, incentives for carbon capture initiatives, and the surging demand for renewable fuels and chemicals. Nevertheless, the initial investment costs and energy prerequisites associated with CO₂ conversion systems continue to present considerable obstacles. Scalability represents a significant advantage, as CO₂ reduction systems can be seamlessly integrated into pre-existing biorefineries and expanded in conjunction with innovations in reactor and catalyst technologies. The technical viability is conditional on the advancement of efficient catalysts, the optimization of reaction conditions, and the dependable integration with both upstream and downstream processes within biorefineries. From an environmental perspective, CO₂ reduction markedly diminishes greenhouse gas emissions, promotes circular economy practices through the utilization of waste CO₂, and enhances the operational cleanliness of biorefineries. Prospective innovations in renewable energy-driven systems will further enhance the sustainability and economic efficiency of CO₂ reduction in waste biorefineries.

4.3. Hydrogenation of Bio-Based Molecules

Another important application of electrocatalysis in the domain of waste biorefineries is the hydrogenation of bio-derived molecules. The hydrogenation of bio-derived molecules is of foremost significance in the valorization of renewable feedstocks, providing a sustainable framework for the synthesis of high-value chemicals and fuels [90]. This procedure encompasses the incorporation of hydrogen into unsaturated organic compounds sourced from biomass, including furfural, levulinic acid, and various platform molecules, thereby transforming them into more reduced, stable, and economically advantageous products. Catalytic hydrogenation, frequently performed utilizing metal-based catalysts such as nickel, palladium, or copper, facilitates selective transformations that are crucial for maximizing product yields and characteristics. Notably, electrocatalytic hydrogenation has surfaced as a promising research methodology owing to its capacity to function under milder conditions and harness renewable electricity, thereby expanding the overall sustainability of the process. This technique not only aids in the generation of biofuels and chemicals but also assumes a critical role in mitigating the carbon emissions associated with conventional chemical hydrogenation methods. Developments in catalyst design, especially those emphasizing nanoporous architectures and bimetallic systems, have exhibited superior activity and selectivity, providing valuable perspectives for the efficient hydrogenation of bio-derived molecules.

Recently, Machineni et al. [91] conducted a thorough examination of diverse methodologies for the generation of biohydrogen (bioH₂) from lignocellulosic substrates as shown in Figure 8, concentrating on both biotic and abiotic strategies. BioH₂ is characterized as a carbon-neutral, renewable energy source, possessing considerable ecological advantages, which is vital for the transition from conventional fossil fuels to sustainable energy alternatives. The authors elucidate two fundamental modalities for bioH₂ generation namely biotic and abiotic processes. Biotic methodologies encompass dark fermentation, which transpires in the absence of illumination, and photo fermentation, which employs light to enhance hydrogen synthesis. These biological mechanisms are critical for enhancing bioH₂ yields derived from lignocellulosic biomass. Although less prominently featured, abiotic techniques are examined within the framework of thermochemical processes that may serve to complement biological approaches in bioH₂ generation. A significant element underlined is the criticality of reactor configuration in influencing the efficiency of both biotic and abiotic techniques. The optimization of reactor design is imperative for the maximization of hydrogen yields. Furthermore, the significance of biocatalysts in the pretreatment of biomass is emphasized, as these catalysts enhance the accessibility of lignocellulosic substrates for microbial activity, thereby improving fermentation efficiency. In addition, the research also investigates the potential of both natural and genetically modified microalgae in the production of bioH₂, indicating that such organisms can generate hydrogen under specific conditions, thereby presenting a promising pathway for sustainable energy generation. Ultimately, the research groups discuss the industrial viability of bioH₂ production, highlighting the necessity for scalable and sustainable systems that can adequately fulfill regional energy requirements. The synergy of innovative reactor architectures, enhanced biomass pretreatment methodologies, and the integration of both biotic and abiotic processes is essential for the advancement of bioH₂ production. Overall, the authors provide a comprehensive overview of current techniques and potential advancements for biohydrogen (bioH₂) production from lignocellulosic feedstock [91].

In other research conducted by He et al. [92], the viability of biomass-derived platform compounds for the sustainable synthesis of renewable nitrogen-containing chemicals through reductive amination is comprehensively examined. The study emphasizes the essential function of biomass as a renewable feedstock for the generation of valuable platform chemicals, which can subsequently be converted into nitrogen-containing compounds essential for various applications within sectors such as agriculture and pharmaceuticals. The reductive amination methodology is emphasized for its efficiency and sustainability, capitalizing on renewable resources while reducing waste, and positioning it as a promising strategy for the synthesis of nitrogen-containing chemicals. The research presents data concerning yield and selectivity, illustrating the process’s capacity to attain elevated product yields, which is crucial for the economic viability of the approach. Furthermore, the selectivity towards particular nitrogen-containing chemicals is analyzed, demonstrating the potential for customized production tailored to specific applications. The authors also discuss the environmental benefits of using chemicals from biomass instead of traditional fossil fuels. They emphasize the significant reduction in carbon emissions and reduced dependence on non-renewable resources. This transition signifies a critical advancement toward a more sustainable and ecologically responsible chemical industry. Their study highlights future research directions, including optimizing reaction parameters, exploring alternative biomass feedstocks, and scaling up for commercial viability. It underscores the potential of biomass-derived compounds for producing renewable nitrogen-based chemicals via reductive amination, offering technical and environmental benefits for sustainable chemical production [92].

For sustainable chemical synthesis methodologies, the deoxydehydration (DODH) reaction represents a critical catalytic mechanism employed for the selective removal of oxygen atoms from vicinal diols, resulting in the formation of unsaturated compounds as shown in Figure 9. This reaction is particularly advantageous for transforming biomass-derived substrates, including sugars and polyols, into commercially significant chemicals and fuels. Within the DODH framework, the removal of two adjacent hydroxyl groups (OH) generates the establishment of a carbon–carbon double bond, thereby producing an unsaturated compound. Due to their superior activity and selectivity, Rhenium (Re)-based catalysts are frequently employed in DODH reactions. These catalysts promote the conversion of bio-sourced feedstocks, such as mucic acid, into unsaturated derivatives like alkenes and adipates, which serve as crucial intermediates in the manufacture of plastics, lubricants, and other industrial chemicals. The efficacy of the DODH reaction is contingent upon several parameters, including catalyst composition, support material, reaction conditions, and the selection of reducing agent. Through meticulous optimization, the DODH reaction presents a sustainable pathway for the synthesis of unsaturated products from renewable biomass resources.

Recently, Harth et al. [93] investigated the catalytic deoxydehydration (DODH) reaction using rhenium (Re)-based catalysts, emphasizing their high activity in the selective dehydroxylation of vicinal diols to produce unsaturated products. This process is particularly relevant for upgrading biomass-derived mucic acid into renewable adipates. The study identified that the choice of support material plays a critical role in the catalyst’s effectiveness, with carbon (C) and titanium dioxide (TiO₂) emerging as the most suitable supports for Re catalysts, yielding significant DODH activity.

Furthermore, pretreatment conditions were found to greatly influence the catalyst’s performance. A reductive pretreatment procedure was essential for achieving high yields of fully dehydroxylated products, with up to 88% yield at a relatively low reaction temperature of 120 °C, using methanol as the reducing agent. The research indicated that the activity of DODH depends on having both metallic and high-valent rhenium (Re) sites. The study suggested that particle growth during reductive thermal treatment enhances the formation of bifunctional sites, crucial for the reaction mechanism. The researchers proposed a bifunctional reaction mechanism for DODH on monometallic Re catalysts, which aligns with the experimental observations, providing an explanation for the catalysts’ high performance under the tested conditions. Overall, their study underscores the importance of catalyst support materials, pretreatment conditions, and a deep understanding of the underlying reaction mechanisms to optimize the DODH process for converting bio-based aldaric acids into valuable adipic acid derivatives.

The approaches connected to sustainable chemical production encounter a variety of obstacles, including the high expenses linked to renewable feedstocks, processes that require considerable energy input, and challenges associated with scalability. Improvements in catalyst efficiency, selectivity, and longevity are also imperative to optimize performance in extensive operational contexts. Moreover, the management of waste and the reduction of carbon emissions persist as pivotal issues in the transition from fossil fuel dependence. Prospective innovations are contingent upon the development of economically viable and resilient catalytic systems, the enhancement of energy efficiency via green technologies, and the incorporation of bio-based materials. An increased emphasis on research pertaining to circular economy frameworks, the integration of renewable energy sources, and the intensification of processes will be crucial in overcoming these challenges and attaining methods of chemical production that are both environmentally sustainable and economically feasible.

5. Advanced Biocatalysis in Waste Biorefinery Technologies

Advanced biocatalysis represents a forefront domain within the field of waste biorefinery technologies, which encompasses the utilization of biological catalysts, predominantly enzymes and microorganisms, to facilitate the transformation of waste substrates into valuable energy products and biochemicals [6]. The principal benefit of biocatalysis lies in its ability to operate under mild conditions, such as low temperatures and pressures, while maintaining high specificity and efficiency, thereby reducing energy consumption and environmental impact. Progressions in enzyme engineering, microbial metabolic pathways, and integrated biocatalytic systems are advancing the discipline, thereby enhancing the economic viability and environmental sustainability of biorefineries [94]. Recently, Silva et al. [95] delineated numerous significant discoveries regarding the application of dye-decolorizing peroxidases (DyPs) within biorefineries. Their research studies underscore the remarkable enzymatic efficacy of DyPs in the degradation of diverse dyes, a factor that is dominant for the treatment of wastewater in such contexts. This enzymatic activity stems from the ability of DyPs to oxidize a wide range of aromatic compounds, making them highly effective in breaking down complex pollutants. The research elucidates that DyPs exhibit broad substrate specificity, which enables them to effectively degrade various categories of dyes, including those that present considerable challenges in terms of degradation. This adaptability is indispensable for addressing the heterogeneous industrial effluents encountered in biorefineries. The authors suggest that using DyPs in biorefineries could significantly improve how efficiently dyes are removed from wastewater. This could make the biorefinery processes more sustainable. Their research highlights the environmental advantages of DyPs, as these enzymes help reduce the toxicity of dye-contaminated water, lowering the negative impact of industrial activities on the environment. This approach aligns with the principles of green chemistry and sustainable development. Additionally, the authors emphasized the need for future research to focus on optimizing conditions that enhance DyP activity and exploring genetic engineering strategies to improve their efficiency. Such initiatives could facilitate the advancement of more proficient biocatalysts for industrial applications.

Moreover, the study presents compelling evidence that dye-decolorizing peroxidases may assume a crucial role in strengthening the sustainability and operational efficiency of biorefineries by effectively degrading dyes and alleviating environmental degradation. These findings open new opportunities for future research and practical applications in the field of biocatalysis. Furthermore, the economic feasibility of advanced biocatalysis is significantly increased by the utilization of waste feedstocks, which not only diminishes raw material expenditures but also generates valuable byproducts. Nonetheless, the optimization of enzyme stability, reaction kinetics, and the reduction in enzyme costs continue to present formidable challenges in enhancing overall economic efficiency. The potential for scalability is encouraging, as biocatalysis can be implemented across various scales, ranging from small-scale applications to industrial-scale biorefineries, particularly with the progression of bioreactor innovations and continuous processing methodologies. The technical viability is liable upon the creation of robust and highly specific biocatalysts capable of enduring industrial conditions and converting a variety of waste streams. With regard to environmental implications, biocatalysis, in terms of its impact on the environment, is considered eco-friendly because it works under gentler conditions than traditional chemical methods. This lowers energy use and helps reduce greenhouse gas emissions, making it an important part of sustainable waste biorefinery processes.

5.1. Enzymatic Hydrolysis for Biomass Conversion

One of the most efficient processes in the field of advanced biocatalysis is the enzymatic hydrolysis of lignocellulosic biomass—plant-derived materials that encompass cellulose, hemicellulose, and lignin. Lignocellulosic biomass, sourced from agricultural residues, forestry by-products, and even municipal solid waste, serves as a fundamental feedstock for biofuel production, particularly in the field of bioethanol [96]. Nonetheless, the inherently recalcitrant characteristics of lignin, which functions as a protective encasement around cellulose fibers, render the biomass resistant to degradation. Advanced biocatalysis is dedicated to the innovation of highly proficient enzyme systems capable of decomposing lignocellulosic substrates into fermentable sugars with minimal requirement for pretreatment. Enzymes such as cellulases, hemicellulases, and ligninases have been meticulously engineered to extend their activity, stability, and resilience against inhibitors frequently present in biomass [6]. Through the optimization of these enzyme systems, it becomes feasible to attain superior yields of biofuels while concurrently diminishing the costs and complexities associated with pretreatment methodologies. For instance, recent advancements in enzyme blends—combinations of diverse enzymes—have significantly enhanced the degradation of cellulose and hemicellulose across various biomass sources. These enzyme mixtures can be specifically tailored to accommodate particular feedstocks, thereby maximizing sugar liberation for subsequent fermentation into ethanol or alternative biofuels [96,97,98].

Recently, Zarina et al. [99] elucidated that the enzymatic hydrolysis of effluent streams originating from wastewater treatment facilities (WWTPs) yielded several notable outcomes. The enzyme mixtures synthesized via Irpex lacteus (IL) facilitated the liberation of 31.2 mg of sugars per gram of desiccated wastewater screenings. This finding underscores the efficiency of the IL enzyme in deconstructing the carbohydrate constituents present within these effluent streams. By contrast, a commercially available enzyme formulation attained a substantially elevated sugar release of 101 mg per gram of dried screenings, corresponding to a 90% degree of saccharification. This implies that commercial enzymes may exhibit greater efficiency in sugar extraction from such substrates. Their research further examined the extraction of lipids and proteins from sewage sludge prior to undergoing hydrolysis. Nevertheless, the findings revealed that this recovery methodology was not beneficial in expanding sugar production. This observation underscores the complexities associated with the incorporation of resource recovery and saccharification methodologies. The results illustrated that the IL enzyme possesses potential applications that extend beyond conventional lignocellulosic biomass, thereby demonstrating its adaptability in processing diverse effluent streams from WWTPs. The outcomes reinforce the notion that wastewater screenings are particularly amenable to valorization via enzymatic hydrolysis, representing a feasible approach to diverting waste from landfills. This is congruent with the objectives of achieving waste treatment and renewable energy benchmarks established by the European Union. Overall, their study compellingly demonstrated the feasibility of employing enzymatic hydrolysis for sugar production derived from WWTP waste streams, while concurrently identifying the limitations associated with lipid and protein recovery in enhancing sugar yields [99].

Moreover, the economic feasibility of enzymatic processes is affected by the declining costs associated with enzymes, attributed to innovations in enzyme engineering and production methodologies. This process also mitigates waste generation by utilizing low-cost feedstocks such as agricultural byproducts, forestry residues, and municipal solid waste, thereby enhancing operational cost-effectiveness. The scalability of enzymatic hydrolysis represents a significant advantage, as biorefineries are increasingly adopting this technology for extensive applications, owing to advancements in enzyme efficiency and bioreactor configurations. The technical viability of this approach is boosted by continuous research focused on enzyme optimization, pretreatment strategies, and process integration aimed at improving yield and overall productivity. From an environmental perspective, enzymatic hydrolysis presents considerable benefits by functioning under mild operational conditions, thereby decreasing energy requirements and minimizing the generation of toxic byproducts, thus establishing it as a sustainable and environmentally benign method for biomass conversion in waste biorefineries.

5.2. Microbial Electrosynthesis

Microbial electrosynthesis represents a growing biocatalytic methodology that combines the competencies of microorganisms with electrochemical mechanisms to transmute waste materials into bioenergy [100]. This methodology leverages on the intrinsic metabolic functions of particular microorganisms that possess the ability to utilize electrons sourced from an external electrical supply to reduce carbon dioxide or organic substrates into biofuels and biochemicals [101]. It exemplifies an exceptionally effective mechanism for the capture and storage of energy within chemical bonds, thus providing a sustainable avenue for the valorization of waste materials. In a conventional microbial electrosynthesis configuration, microorganisms are immobilized on an electrode situated within an electrochemical cell. These microorganisms can directly acquire electrons from the electrode and employ them to facilitate reduction in substrates, such as CO₂ or volatile fatty acids, into commercially valuable products like acetate, ethanol, or methane [102]. Progressive research endeavors concentrate on enhancing the efficiency of electron transfer between the electrode and microorganisms, refining reactor designs, and genetically engineering microbial strains with optimized metabolic pathways. The incorporation of renewable electricity, sourced from solar or wind energy, into microbial electrosynthesis significantly enhances the sustainability of this research methodology, thereby offering a direct mechanism for the conversion of surplus renewable energy into storable biofuels or biochemicals. In the field of biochemical process synthesis, the microbial electrosynthesis of ammonia (MES of ammonia) signifies a groundbreaking biotechnological method that harnesses microorganisms, typically inclusive of electroactive bacteria, to effectuate the conversion of nitrogen (N₂) into ammonia (NH₃) via the use of an electric current [103]. This procedure transpires within a bioelectrochemical system wherein microbial organisms at the cathode promote nitrogen reduction under mild conditions, thereby providing a more sustainable and energy-efficient alternative to conventional chemical methodologies such as the Haber–Bosch process.

Recently, the study conducted by Li et al. [104] unveiled several significant findings concerning the utilization of biologically self-assembled transmembrane electron pathways for the biosynthesis of ammonia within the framework of microbial electrosynthesis. Figure 10 depicts the research substantiating the findings that iron sulfide (FeS) nanoparticles could establish an artificial electron transfer pathway, thereby facilitating the translocation of electrons from the extracellular milieu into the periplasmic compartment, independent of the involvement of CymA, which is conventionally regarded as critical for electron transport in Shewanella oneidensis MR-1. This revelation posits a novel paradigm for electron transfer that emulates the characteristics of type IV pili. The research accomplished a remarkable average ammonia production rate of 391.8 μg·h⁻¹·L reactor⁻¹. This rate signifies a considerable enhancement in the efficiency of ammonia synthesis attributable to the activation of a greater number of electron transfer conduits by the self-assembled FeS nanoparticles. The selectivity for ammonia production was documented at 98.0%, indicating that the process is exceptionally specific for ammonia synthesis as opposed to the formation of alternative byproducts. Furthermore, the cathode efficiency was assessed at 65.4%, which underscores the efficacy of the electron transfer mechanism in transforming electrical energy into chemical energy in the form of ammonia. Their research also discerned that the amide group present in protein-like entities situated in the outer membrane could facilitate the transference of electrons from the extracellular environment into the intracellular compartment, functioning synergistically with c-type cytochromes. This finding contributes a novel perspective to the comprehension of the interactions between bacterial entities and semiconductor materials. Collectively, these results underscore the potential applicability of biologically self-assembled structures in enhancing microbial electrosynthesis processes, particularly in the field of ammonia production, while also paving the way for future inquiries into electron transfer mechanisms within microbial systems.

5.3. Metabolic Engineering of Microorganisms

Metabolic engineering constitutes the genetic alteration of microorganisms to enhance their capacity for the conversion of waste substrates into desired end products. Through the manipulation of metabolic pathways in bacteria, yeast, or fungi, researchers can augment the synthesis of biofuels, chemicals, and other economically significant compounds [105]. This approach holds particular relevance in biorefineries, where specific by-products derived from waste biomass can be converted into high-value chemicals [106]. A notable instance of the successful implementation of metabolic engineering is the production of bioethanol utilizing genetically modified strains of Saccharomyces cerevisiae, which was commonly referred to as baker’s yeast. Conventional yeast strains are capable of fermenting simple sugars, such as glucose, into ethanol; however, they exhibit limitations when dealing with more complex sugars like xylose, which is abundant in hemicellulose [107].

By genetically engineering these yeast strains to metabolize a wider array of sugars, biorefineries can enhance ethanol yields from lignocellulosic feedstocks. Furthermore, metabolic engineering is being utilized to synthesize alternative biofuels, including biobutanol, biohydrogen, and biodiesel, in addition to platform chemicals such as succinic acid, lactic acid, and polyhydroxyalkanoates (PHAs) [108]. These products can function as precursors for biodegradable plastics, green solvents, and various environmentally sustainable materials. Chen et al. [109] examine the progress and obstacles within the discipline of microbial metabolic engineering pertaining to the biosynthesis of fatty acids. Usually, fatty acids are derived from both animal and plant sources, as depicted in Figure 11. TGA, which denotes triacylglycerol; FAs, representing fatty acids; PUFAs, referring to polyunsaturated fatty acids; and OCFAs, specifically odd-chain fatty acids, particularly ω-3 fatty acids such as eicosapentaenoic acid and docosahexaenoic acid, are vital to human health. Nonetheless, conventional sources like fish and marine organisms are inadequate to satisfy the escalating demand arising from population growth. Traditional methodologies for fatty acid synthesis, encompassing chemical synthesis and extraction processes from flora and fauna, encounter substantial challenges. These challenges encompass sustainability concerns and elevated expenses, which hinder their capability to satisfy human nutritional requirements. Their results underscore metabolic engineering as a viable remedy. This methodology entails genetic alteration to establish novel metabolic pathways, thereby facilitating the efficient and economically feasible production of fatty acids. This technique extends advantages in terms of sustainability and economic feasibility when compared with conventional methods. The authors encapsulate the most recent research endeavors aimed at leveraging metabolic engineering for fatty acid synthesis [109]. This encompasses the creation of innovative microbial strains and metabolic pathways that enhance fatty acid yield while diminishing production costs. Their results also deliver a succinct evaluation of the contemporary challenges encountered in the domain such as the imperative for more efficient microbial strains and the optimization of metabolic pathways to elevate fatty acid production. Ultimately, the prospective advancements in the field highlight the potential for further innovations in metabolic engineering approaches that could be ample in more sustainable and economically viable methods for fatty acid production. This type of research provides a comprehensive examination of the current state of microbial metabolic engineering focused on fatty acid synthesis, highlighting its importance, challenges, and potential future directions.

Moreover, metabolic engineering of microorganisms enhances the economic feasibility of waste biorefineries by facilitating the effective utilization of inexpensive waste substrates, enhancing product yields, and broadening the spectrum of valuable metabolites that can be synthesized. Through the application of genetic engineering techniques, microorganisms can be specifically designed to convert targeted waste feedstocks, thereby diminishing the costs associated with raw materials and processing [107]. The potential for scalability is optimistic, as modified microbes can be adapted for implementation in industrial scale biorefineries, contingent upon the optimization of bioprocessing systems to promote high levels of productivity and stability. The technical viability of this approach is intricately associated with advancements in the fields of synthetic biology and systems biology, which facilitate precise alterations in microbial metabolic pathways to attain desired product configurations and improve resilience to challenging industrial environments [110]. From an environmental perspective, metabolic engineering supports the sustainability of waste biorefineries by helping to use waste more efficiently, cutting down emissions, and lowering energy use. This makes it an important technology for eco-friendly bio-based production.

5.4. Synthetic Biology for Custom Enzymes and Pathways

Synthetic biology is propelling the forthcoming era of advancement in biocatalysis by facilitating the design and fabrication of custom enzymes and metabolic pathways. This process encompasses the synthesis of biological components, including enzymes, genes, and regulatory elements, which can be incorporated into artificial systems specifically configured for designated functions [111]. In the context of waste biorefineries, synthetic biology can be employed to engineer innovative enzymes with superior capabilities for the degradation of intricate waste materials or to modify microorganisms to possess entirely novel metabolic pathways. For example, methodologies in synthetic biology can be applied to engineer enzymes capable of hydrolyzing plastic waste, such as polyethylene terephthalate (PET), into its constituent monomers, which can subsequently be repurposed into new materials or converted into bioenergy. Furthermore, synthetic biology facilitates the construction of biosynthetic pathways that transform low-value waste streams into high-value biochemicals, thereby enhancing the profitability and efficiency of the biorefinery process [112]. Moreover, designing custom enzymes and metabolic pathways using synthetic biology faces several significant limitations that hinder progress. These challenges arise from the complexity of enzyme functionality, the stability of enzyme structures, and the efficiency of metabolic pathways [113,114]. One key limitation in enzyme design is structural flexibility.

Current approaches often assume a static view of protein structures, ignoring the dynamic nature of enzymes that can shift between multiple conformations, reducing the effectiveness of synthetic enzymes [115]. Another issue is the low expression levels and thermal instability of engineered enzymes, which can limit their practical application in industrial settings [116]. Furthermore, iron–sulfur enzymes pose unique challenges due to limited cluster supply and instability, which can create bottlenecks in engineered pathways [117]. In mammalian systems, the accumulation of misfolded proteins can trigger stress responses, complicating the development of engineered cell factories for biopharmaceutical production [115]. Despite these obstacles, advances in computational design and deep learning offer promising solutions for more efficient enzyme engineering, indicating a bright future for synthetic biology [116].

Furthermore, the field of synthetic biology aimed at the engineering of custom enzymes and metabolic pathways within microorganisms is revolutionizing waste biorefinery technologies by facilitating the development of custom metabolic processes that enhance the efficiency of biomass conversion. By employing genetic engineering techniques, microorganisms are adeptly modified to synthesize specific enzymes that significantly augment the degradation of waste materials into high-value products such as biofuels, biochemicals, and various bioproducts. This level of customization serves to enhance the economic feasibility of biorefineries by optimizing the utilization of low-cost feedstocks and maximizing product yields, while concurrently diminishing the necessity for expensive external enzymes. The scalability potential is highly feasible, as synthetic biology can be efficiently adapted for large-scale industrial applications, utilizing engineered microorganisms designed for high productivity and robust performance in industrial environments. The advancement of technical feasibility is occurring at a rapid pace, propelled by innovations in metabolic engineering, enzyme design, and pathway optimization, which collectively enhance efficiency and product specificity. The environmental implications of employing synthetic biology within waste biorefineries are profound, as it facilitates the transformation of waste into renewable products, reduces dependence on fossil fuels, mitigates greenhouse gas emissions, and fosters sustainable resource utilization. Table 2 provides an overview of biomass feedstocks together with their relevant biofuel and chemical conversions conversion methods, in addition to the pertinent yields.

5.5. Integrated Biocatalytic Systems

The combination of diverse biocatalytic methodologies into a cohesive and efficient system represents a notable progression within the discipline. Through the integration of enzymatic, microbial, and electrochemical catalysis, biorefineries are allowed to enhance the transformation of waste feedstocks into an expanded array of products [140]. Such integrated frameworks facilitate continuous processing while concurrently diminishing the necessity for energy-demanding procedures such as distillation or separation, thereby mitigating operational expenditures and environmental repercussions [141]. A pertinent illustration of this concept is the synthesis of enzymatic hydrolysis and microbial fermentation within a consolidated bioprocessing (CBP) framework. In the context of CBP, enzymes, and microorganisms operate in concert to decompose biomass and convert the resultant sugars into biofuels within a singular step, obviating the requirement for distinct hydrolysis and fermentation phases [142]. This streamlining of the process contributes to cost reduction and enhances overall efficiency.

Recently, Kumar et al. [143] elucidated substantial discoveries concerning the amalgamation of fermentation and microbial fuel cell (MFC) technologies aimed at the conversion of waste into energy. Their research delineates municipal solid waste (MSW) and agricultural residues (AR) as principal sources for the generation of bioenergy, which can be effectively transformed into biofuels and bioelectricity through sophisticated fermentation and MFC methodologies. The integrated system is anticipated to yield approximately 7.17 trillion liters of bioethanol annually from MSW and AR, alongside 6.12 × 10⁻⁴ MW/m² of bioelectricity, thereby highlighting the considerable energy potential inherent in organic waste. From an economic perspective, this system could yield an excess of USD 465 billion to the global energy market, thereby illustrating the financial feasibility of waste-to-energy conversion. Furthermore, the environmental advantages of MFCs are emphasized by their capacity to diminish chemical oxygen demand (COD) levels in wastewater, consequently enhancing water quality while simultaneously producing energy. Their research further highlights the significance of this integrated technology in advancing a circular economy, fostering waste reduction and enhanced waste management practices, thereby contributing to sustainability within both energy and environmental domains. Overall, their study presents a promising approach for the transformation of organic waste into precious energy resources while addressing environmental challenges, thereby supporting sustainable development objectives.

6. Integration with the Circular Economy in Waste Biorefineries

The integration of waste biorefinery technologies with the principles of the circular economy signifies a crucial progression towards the achievement of sustainable industrial frameworks. The circular economy emphasizes the minimization of waste, the continued utilization of materials, and the restoration of natural systems through the elimination of waste and pollution at the design phase [144]. Within the framework of waste biorefineries, this objective is realized via the proficient transformation of waste materials into valuable commodities, including biofuels, biochemicals, and bioenergy. These facilities not only address environmental issues related to waste but also promote resource recovery, close material loops, and reduce reliance on primary raw materials [145].

6.1. Resource Recovery and Waste Valorization

The fundamental principle of the circular economy is to reduce the extraction of natural resources by recovering and reusing materials derived from waste streams. Waste biorefineries exemplify this concept by converting diverse types of organic waste including agricultural residues, food waste, and industrial by-products into renewable energy and products with added value [22]. This approach is consistent with the objective of minimizing the environmental impact of industries while enhancing resource efficiency. Waste valorization represents a pivotal strategy in the integration of biorefineries within the framework of the circular economy. By converting waste into valuable products, biorefineries can reduce the amount of waste sent to landfills or incineration, thereby decreasing pollution and lowering greenhouse gas emissions. For instance, food waste can be transformed into biogas via anaerobic digestion, which is applicable for heating or electricity production. The by-products generated from biogas production, such as digestate, can serve as a nutrient-dense fertilizer, further facilitating circular material flows. The incorporation of circular economy principles into biorefineries transcends broad energy production. The biochemicals and biomaterials produced can find applications in manufacturing, agriculture, and pharmaceuticals, thus reducing dependence on petrochemical-derived products [146]. This concept enables industries to dissociate economic growth from resource consumption, culminating in a more sustainable industrial framework.

6.2. Industrial Cooperation in Biorefinery Development

Another key aspect of integrating biorefineries within the circular economy is the concept of industrial symbiosis. This approach advances collaboration among various industries to repurpose by-products from one sector as raw materials for another, creating a network that enhances resource utilization. Waste biorefineries can serve as vital hubs in these ecosystems by processing waste from multiple industries and converting it into reusable products [147]. For example, a food processing facility may supply organic waste to a biorefinery, which then transforms it into biofuels or biochemicals. These products can either be returned to the original facility or used in other sectors, such as transportation or packaging. Additionally, nutrient-rich by-products from bioenergy production can be redirected to agriculture as fertilizers, completing the nutrient cycle. This symbiotic approach improves resource efficiency, reduces operational costs, and minimizes waste across industries while fostering collaboration and innovation for more resilient industrial systems that can effectively tackle environmental and economic challenges [50].

To propose feasible biorefinery frameworks, several critical factors must be considered beyond fundamental technologies, such as material accessibility, transportation logistics, production costs, and governmental support. Key considerations include locating biorefineries strategically near abundant and reliable feedstock sources, such as agricultural residues, forestry byproducts, or municipal solid waste. For instance, rural areas with high agricultural output or regions adjacent to major urban centers could serve as optimal locations due to their proximity to biomass and waste streams, thereby reducing transportation distances and associated costs [50]. Efficient logistics are essential for transporting bulky, low-energy-density biomass, a challenge that can be addressed by situating biorefineries close to feedstock sources and establishing regional collection systems. A decentralized network of smaller biorefineries feeding into a larger central processing facility could help lower transportation costs and reduce environmental impacts [148].

Biorefinery designs must also incorporate scalable, energy-efficient separation technologies, such as membrane filtration and gasification, that can handle heterogeneous waste streams. These processes should be adaptable to different biomass sources, ensuring operational flexibility and optimizing the production of high-value biofuels and chemicals. Site selection should balance proximity to feedstock, access to transportation networks (e.g., roads, railways, or ports), and potential markets for the biorefinery’s end products [149]. The development of industrial clusters or eco-industrial parks, where the byproducts of one process are utilized as inputs for another, can create synergies, minimize waste, and reduce costs. Furthermore, close integration with established chemical or energy industries can reduce operational costs through shared infrastructure. The availability of skilled labor is another essential consideration for biorefinery operations. Locating facilities near academic and research institutions can ensure a steady supply of trained personnel and advance innovation. Government initiatives can also incentivize workforce development within the renewable energy sector, addressing potential labor shortages. Furthermore, modular biorefinery designs that allow for gradual scaling based on demand and available feedstock can reduce the financial burdens associated with construction and operation. Hybrid models that integrate energy production with high-value products, such as bioplastics or specialty chemicals, can diversify revenue streams and enhance economic resilience. Government policies play a critical role in supporting the competitiveness of bio-based products, which often struggle to compete with petroleum-based alternatives [150]. Financial mechanisms, such as subsidies, tax incentives, carbon pricing, and renewable fuel mandates, can significantly enhance the viability of biorefineries. Public–private partnerships and access to funding for research and development are also crucial for accelerating the commercialization of advanced biorefinery technologies.

The broad adoption of waste biorefinery technologies encompassing chemical reduction, pyrolysis, gasification, fermentation, and electrochemical processes has transitioned from experimental, laboratory-scale applications to large-scale facilities focused on converting waste into valuable products and biofuels as alternatives to non-renewable energy sources. However, despite the promising potential of these technologies, numerous challenges remain [18]. Even though conceptual frameworks are indispensable, it is equally important to highlight real-world examples of successful, fully operational biorefineries, as illustrated in Figure 12. These systems must efficiently transform waste biomass into high-value products, underpinned by supply chains that integrate global biorefineries. The recent developments in their core technologies are outlined in Table 3. For instance, POET has increased bioethanol production through advanced bioprocessing, while LanzaTech utilizes biorecycling to capture carbon emissions and convert them into sustainable fuels. Enerkem converts residual biomass into circular methanol, and UPM Chemicals produces next-generation biochemicals from wood. St1 is engaged in biogas and wind energy initiatives, with ongoing biogas upgrading processes, while Avantium is developing plant-derived polyethylene furanoate (PEF) and PlantMEG. Fibenol focuses on biomaterials derived from forestry waste, and facilities such as Maabjerg BioEnergy and Omega Green are advancing carbon-neutral biodiesel production.

However, these enterprises face significant challenges, with market penetration being a primary obstacle. This involves not only securing sufficient consumer demand for bio-based products but also achieving market acceptance amid competition from fossil-based alternatives. Price competitiveness remains a persistent issue, necessitating either cost reductions or the creation of high-value products to gain market share. Additionally, effective biomass supply chain management is crucial, particularly in terms of transportation logistics for bulky feedstocks, infrastructure development, and mitigating seasonal and geographic variability in biomass supply. Technological challenges also exist, particularly in the separation and purification of biofuels, biochemicals, and bioplastics, where scalable, energy-efficient methods are essential for achieving high product purity while maintaining economic viability [151]. The successful implementation of biorefineries is further complicated by regulatory and legislative frameworks. Compliance with environmental regulations, securing policy support through subsidies or tax incentives, and meeting industry standards for sustainability certifications are all critical. Furthermore, sustainable practices, financial investment, and cross-sector collaboration are essential to ensuring the long-term success of the biorefinery sector. Addressing these interconnected factors is crucial for establishing a robust and commercially viable biorefinery industry.

6.3. Closed-Loop Systems and Circular Flows

Closed-loop systems constitute a foundational element of the circular economy, with biorefineries serving a pivotal function in the establishment of such frameworks. Within closed-loop systems, waste materials are perpetually reintegrated into production processes, thereby eliminating waste and diminishing the dependence on external resources. Waste biorefineries can be engineered to function as closed-loop systems by harnessing all by-products and residues produced during biorefinery operations [152]. For example, in the process of biofuel production, the solid residues remaining post-processing—such as lignin derived from lignocellulosic biomass—can be repurposed as feedstock for the synthesis of biochar or renewable chemicals. Biochar, subsequently, can be applied in agricultural settings as a soil amendment, thereby enhancing soil fertility and facilitating carbon sequestration. This generates a circular flow wherein every output is reintegrated into the system as an input, thus optimizing material utilization and mitigating environmental repercussions. Likewise, the water utilized in biorefinery processes can be subjected to recycling and treatment for reuse, which diminishes water consumption and strengthens the sustainability of the operation [153]. By closing the loops on materials, water, and energy, waste biorefineries play a crucial role in the development of regenerative systems that align with the principles of the circular economy.

6.4. Reducing Carbon Footprint and Promoting Decarbonization

The focus of the circular economy on optimizing resource utilization and minimizing waste generation plays a pivotal role in advancing decarbonization initiatives [154]. Through the conversion of waste materials into biofuels, biorefineries diminish the reliance on fossil fuels, consequently lowering greenhouse gas emissions and fostering the development of low-carbon energy sources. Biofuels derived from waste, including biodiesel, bioethanol, and biogas, exhibit a reduced carbon footprint in comparison to conventional fossil fuels, thereby establishing their significance within a circular energy framework [155]. The incorporation of renewable energy sources such as solar and wind power into biorefineries presents a promising avenue for sustainable energy production. This integration not only enhances energy efficiency but also supports the transition towards a low-carbon economy. The combination of biomass, solar PV, and wind turbines in microgrid systems can provide reliable energy in both grid-connected and off-grid scenarios, ensuring stability and efficiency [156]. Utilizing biocrude from biomass pyrolysis alongside solar PV allows for dispatchable electricity production, addressing the intermittency of solar energy and enhancing overall system reliability [157]. Integrating renewable energy systems can significantly benefit agricultural practices by powering essential operations such as irrigation and processing, thus promoting socio-economic development in rural areas. Farmers can achieve greater energy independence and reduce costs through the adoption of renewable technologies, which can also lower greenhouse gas emissions [158]. Despite the advantages, challenges such as the need for advanced energy storage technologies and the optimization of renewable energy systems remain. Addressing these issues is crucial for maximizing the potential of integrated renewable energy solutions in biorefineries.

6.5. Sustainable Product Design and Market Integration

The outputs produced by waste biorefineries such as bioplastics, biodegradable chemicals, and biofuels are similar to the principles of a circular economy, as they are engineered for longevity, reutilization, and recyclability [159]. These eco-friendly products possess the potential to supplant traditional materials that frequently exhibit single-use characteristics or pose challenges in terms of recyclability, thereby facilitating the transition towards a more circular economic model. For example, bioplastics are synthesized from renewable resources and are specifically formulated to decompose with greater ease than conventional plastics [160]. They can be assimilated into sectors such as packaging, agriculture, and consumer goods, wherein they contribute to diminishing the dependency on fossil-based plastics and mitigating environmental pollution [161]. Furthermore, the biodegradable characteristics of bioplastics ensure that they exert a lesser impact on landfills and marine pollution, thereby aligning with the objectives of the circular economy to minimize waste and rejuvenate natural systems. The market integration of biorefinery outputs also holds considerable importance in the establishment of sustainable supply chains [162]. By embedding biochemicals and biofuels into pre-existing industrial processes, enterprises can lessen their dependence on non-renewable materials and reduce their carbon footprints. This advancement underpins broader economic aspirations pertaining to sustainability, innovation, and resilience, propelling the adoption of circular economy methodologies across diverse sectors. The integration of waste biorefineries within the framework of circular economy principles establishes a sustainable approach to waste management and the generation of renewable energy and high-value products. By prioritizing resource recovery, implementing closed-loop systems, promoting industrial symbiosis, and encouraging sustainable product design, waste biorefineries play a pivotal role in minimizing waste, reducing carbon emissions, and dissociating economic growth from resource exploitation [162]. The alignment of waste biorefineries with the principles of the circular economy not only strengthens environmental sustainability but also boosts economic resilience and fosters innovation across multiple sectors.

7. Digitalization and AI Integration in Waste Biorefineries

The processes of digitalization and artificial intelligence (AI) are significantly reshaping waste biorefineries, thereby enhancing their operational efficiency, sustainability, and scalability [163]. Through the incorporation of sophisticated digital technologies, including AI, machine learning, the Internet of Things (IoT), and data analytics, biorefineries can enhance their operational optimization, improve the management of resources, and minimize energy consumption. Such advancements are essential for harmonizing waste biorefinery operations with the objectives of a circular economy, facilitating the production of biofuels, biochemicals, and bioenergy more intelligently and efficiently.

7.1. Process Optimization Through AI and Machine Learning

AI and machine learning (ML) technologies possess the potential to substantially enhance the operational efficacy of waste biorefineries through the optimization of intricate processes, including feedstock conversion, product yield maximization, and energy management [164]. ML algorithms exhibit the capability to scrutinize extensive volumes of process data, such as temperature, pressure, and chemical concentrations in real time, thereby discerning patterns and generating predictions aimed at performance optimization. Within a waste biorefinery, AI-enhanced systems can perpetually monitor and modify process parameters to enhance the efficiency of conversion methodologies such as fermentation, anaerobic digestion, and catalytic upgrading [165]. Recently Gupta et al. [166] demonstrated the increasing application of ML models in the design and optimization of organic waste management systems, aiming for enhanced resource circularity. ML-based models offer several advantages, including (1) reduced computation time, (2) elimination of the need for recalibration, (3) the potential for embedding physical laws into the data-driven framework, and (4) relatively straightforward integration within control system architectures. A range of ML techniques, such as neural networks (NNs), support vector machines (SVMs), logistic regression (LR), random forests (RFs), eXtreme gradient boosting (XGBoost), and k-nearest neighbors (KNNs), have been explored by the organic waste treatment research community. These models are employed to predict various outcomes, including the yield and composition of end products, process stability, and environmental impact.

For instance, gasification converts organic waste into syngas (a mixture of CO, H₂, CO₂, H₂O, CH₄, and higher-order hydrocarbons), ash, and biochar under oxygen-limited conditions at temperatures between 500 and 1200 °C. The yield and properties of the end products are influenced by factors such as feedstock characteristics, the gasifying agent, temperature and pressure within the reactor, reactor design, and the use of catalysts and sorbents. Gasification processes involve complex chemical reactions, as well as heat and mass transfer dynamics. To better understand the variation in output products based on input feedstock and operating conditions, multi-phase computational fluid dynamics (CFD) simulations have been developed [167]. However, these simulations are limited by their inability to account for all relevant factors and process conditions, thereby restricting their applicability in optimizing process design and control. By contrast, ML-based models for gasification have been utilized to predict the effects of a broader range of factors, such as reactor type, catalyst use, gasifying agent, temperature, and feedstock type, on the yields of syngas components and biochar. While these ML models have been successfully applied at the laboratory scale, they are still underdeveloped for scaling up to large-scale batch processes. The scalability of ML simulations to commercial-scale operations remains hindered by several challenges. These include the complexity of real-world processes, the need for extensive and high-quality datasets, difficulties in generalizing models to accommodate diverse operating conditions, and the substantial computational resources required for training and optimization. Furthermore, integrating ML models into existing industrial systems, ensuring their robustness under variable operational conditions, and addressing regulatory and interpretability concerns represent significant barriers to their widespread adoption in biorefineries.

7.2. Predictive Maintenance and Equipment Monitoring

Biorefineries face significant challenges in managing equipment maintenance due to the complexity and criticality of their assets. Traditional maintenance strategies are often reactive and inefficient, resulting in costly downtime and increased safety risks. However, the advent of artificial intelligence (AI) and predictive maintenance technologies provides a transformative solution to these issues. Digitalization facilitates the implementation of predictive maintenance protocols for biorefinery apparatus, thereby mitigating operational downtime and preventing costly repair expenditures [168]. Internet of Things (IoT) sensors, strategically deployed on essential machinery, perpetually monitor vital performance indicators such as vibration, temperature, and pressure, yielding real-time data that artificial intelligence (AI) algorithms employ to anticipate potential malfunctions prior to their manifestation. AI-driven predictive maintenance systems analyze historical equipment performance data and compare it with current operational parameters to identify signs of wear or malfunction [169]. Recently, Jambol et al. [170] highlighted the impact of AI-driven predictive maintenance in the oil and gas sector, where machine learning algorithms analyze equipment data to forecast maintenance needs before failures occur. By continuously monitoring equipment performance in real-time, AI can identify potential issues early, enabling proactive maintenance interventions. This approach reduces downtime, lowers maintenance costs, and enhances overall equipment reliability and safety. The successful implementation of AI-driven predictive maintenance requires a comprehensive strategy that includes robust data collection, analysis, and integration with existing maintenance practices. The adoption of such technologies in the oil and gas industry has led to significant improvements in equipment uptime, extended asset lifespans, and enhanced operational efficiency, outcomes that could similarly benefit biorefineries in optimizing equipment management and safety. However, several challenges hinder the widespread adoption of AI-driven predictive maintenance in biorefineries. The quality and availability of data are crucial for accurate AI models, but many industrial environments struggle with incomplete or noisy data, impacting prediction reliability. Integrating AI with inheritance systems often requires significant infrastructure upgrades. Real-time data processing, essential for early failure detection, demands considerable computational resources, adding complexity. Additionally, the interdependent nature of industrial systems makes predictive modeling difficult, while the “black box” nature of AI models undermines trust and decision making. AI scalability across diverse equipment and operational conditions, along with high costs for hardware, software, and specialized personnel, poses further challenges. Concerns about data security, regulatory compliance, and model maintenance complicate broader adoption, requiring a multidisciplinary approach for successful AI implementation.

7.3. Supply Chain Digitalization and Feedstock Management

Supply chain management constitutes an essential component of waste biorefinery operations, especially when addressing the complexities associated with heterogeneous and fluctuating feedstocks such as agricultural byproducts, municipal solid waste, or food discards [151]. The process of digitalization significantly improves supply chain efficacy by facilitating the real-time monitoring of feedstock availability, quality, and logistics, thereby guaranteeing that the biorefinery maintains a stable and dependable supply of resources. The integration of Internet of Things (IoT) sensors and blockchain technology is instrumental in the surveillance and documentation of feedstock from its origin to the biorefinery. Blockchain technology contributes to heightened transparency by offering an immutable record of feedstock provenance, quality parameters, and movement throughout the supply chain. This aspect is particularly crucial for verifying that feedstocks comply with sustainability benchmarks and that the outputs of the biorefinery are accredited as renewable or environmentally sustainable [171]. Additionally, artificial intelligence (AI)-driven systems can be employed to anticipate feedstock availability by analyzing meteorological patterns, agricultural yields, or trends in waste generation. Through the examination of both historical and current data, AI algorithms are capable of forecasting variations in feedstock supply, thereby assisting biorefineries in adjusting their operational strategies accordingly. This adaptability ensures that biorefineries can respond effectively to evolving market dynamics, mitigate feedstock scarcities, and enhance inventory management practices.

7.4. Enhanced Quality Control and Product Customization

AI and digitalization technologies are fundamentally transforming the quality control protocols within waste biorefineries by facilitating enhanced precision and real-time oversight of product quality [172]. AI systems possess the capability to scrutinize data derived from sensors and online analyzers to identify discrepancies in product composition, thereby ensuring that biofuels, biochemicals, and biomaterials adhere to rigorous quality standards. For instance, AI algorithms can identify contaminants or deviations from product specifications throughout the biofuel production process and initiate corrective measures to sustain consistency. This capability is especially critical when managing heterogeneous feedstocks that may introduce variability into the production continuum [173]. Automated quality control mechanisms diminish the necessity for manual intervention and permit expedited modifications to the production process, thereby augmenting overall efficiency and minimizing waste. Beyond the assurance of product quality, AI and digital technologies facilitate a heightened level of customization regarding biorefinery outputs. Through the analysis of market trends and consumer preferences, AI can assist biorefineries in recalibrating their production methodologies to create biofuels, biochemicals, or biomaterials specifically aligned with particular market needs. This adaptability enables biorefineries to expand their product portfolio and tap into profitable markets, thereby improving their economic viability and competitive advantage.

7.5. Sustainability Monitoring and Environmental Impact Assessment

Digitalization and AI possess the potential to assist waste biorefineries in monitoring and mitigating their ecological footprint, thereby advancing the objectives of sustainability inherent in the circular economy [174]. AI algorithms are capable of real-time tracking of resource utilization, energy consumption, and emissions, thereby yielding critical insights into the biorefinery’s environmental efficiency [175]. Environmental impact assessment methodologies, underpinned by AI, can scrutinize life cycle data encompassing feedstock procurement, processing, and product distribution to ascertain the biorefinery’s cumulative carbon emissions, water consumption, and waste production. Such insights empower biorefineries to pinpoint opportunities for enhancing resource efficiency, limiting emissions, and declining waste generation [176]. For instance, AI-improved systems can refine energy consumption by aligning renewable energy generation with the biorefinery’s energy requirements. Within an integrated framework, renewable electricity from solar or wind sources can be directed towards processes like electrocatalytic upgrading and other emerging technologies, ensuring that the biorefinery operates with minimal dependence on fossil fuels. Furthermore, AI can assist biorefineries in adhering to environmental regulations and sustainability benchmarks by perpetually monitoring emissions and waste outputs [177].

Most importantly, enhancing the quantitative association between biorefinery technologies and overarching objectives of sustainability and climate change mitigation necessitates the incorporation of life cycle assessment (LCA) to scrutinize their environmental and economic ramifications. LCA affords a thorough examination of resource utilization, energy expenditure, greenhouse gas emissions, and waste generation throughout the comprehensive process, ranging from feedstock acquisition to product delivery. By quantifying these effects, LCA can elucidate how biorefinery technologies aid in diminishing carbon footprints, reducing dependence on fossil fuels, and fostering practices aligned with the circular economy [178]. For instance, anaerobic digestion and microbial electrosynthesis present carbon-neutral avenues by transforming waste into energy whilst sequestering CO₂, whereas thermochemical and electrocatalytic methodologies can be fine-tuned for diminished emissions. Quantifying these reductions aligns biorefineries with international sustainability objectives, such as those delineated by the Paris Agreement, and establishes a transparent framework for the amplification of these technologies to fulfill climate change mitigation aspirations [179]. Reinforcing this connection ensures that biorefineries are recognized not merely as economic resources but as essential elements of global environmental initiatives.

8. Conclusions

Waste biorefineries are positioned at the leading edge of sustainable energy solutions and circular economy paradigms, presenting a transformative methodology for the management of organic waste. Through the conversion of waste into valuable biofuels, biochemicals, and bioenergy, these facilities substantially mitigate environmental pollution and greenhouse gas emissions, thereby advancing global sustainability objectives. The incorporation of cutting-edge technologies such as electrocatalytic upgrading, biocatalysis, and digitalization has significantly enhanced the efficiency and scalability of waste biorefineries, establishing them as essential elements of a circular economy. Electrocatalytic upgrading signifies a substantial technological innovation, employing renewable electricity to facilitate chemical reactions that transform waste-derived intermediates into higher-value products. This methodology corresponds with worldwide decarbonization initiatives by minimizing dependence on fossil fuels and curtailing carbon emissions. For instance, electrocatalytic processes can refine bio-oils derived from waste biomass into clean, superior-quality fuels, while CO₂ can be transformed into valuable chemicals and fuels, thereby closing the carbon loop and alleviating greenhouse gas impacts. The application of renewable electricity in these processes further amplifies the sustainability of biorefineries, diminishing their carbon footprint and facilitating their integration into a low-carbon energy framework. Advanced biocatalysis also assumes a vital function in optimizing waste biorefineries. Through the utilization of specialized enzymes or microorganisms, advanced biocatalysis enhances the efficiency of biochemical reactions that convert waste into biofuels and biochemicals. This technology improves reaction kinetics and product yields while decreasing the energy requirements for processing, thereby rendering biorefinery operations more economically viable and ecologically sustainable.

Digitalization and AI significantly transform waste biorefinery operations by facilitating real-time data analytics, optimizing processes, and enabling predictive maintenance functionalities. The deployment of Internet of Things (IoT) sensors in conjunction with data analytics instruments allows for the continuous observation of critical process parameters, including temperature, pressure, and chemical concentrations, thereby permitting dynamic modifications that enhance both operational efficiency and product quality. AI-driven algorithms systematically analyze both historical and real-time datasets to forecast maintenance requirements, minimize downtime, and optimize energy utilization. The implementation of digital twins, which replicate physical assets within a virtual framework, provides invaluable insights for process enhancement and risk mitigation, thereby empowering biorefineries to function more effectively and sustainably. The incorporation of waste biorefineries into larger industrial ecosystems through the mechanism of industrial symbiosis further reinforces the principles of a circular economy. By facilitating the exchange of waste and by-products with alternative industries, biorefineries can optimize resource utilization and diminish waste generation. For example, the by-products generated from biofuel production, such as biochar, can be repurposed as soil amendments within agricultural applications, while the waste heat produced by biorefineries can be harnessed for district heating or other industrial activities. This cyclical movement of materials not only alleviates environmental repercussions but also reinforces economic resilience and sustainability. Looking ahead, the ongoing evolution of waste biorefineries will be propelled by advancements in catalytic technologies, the integration of renewable energy sources, and innovations in digital technology. The proliferation of research focused on high-entropy alloys, metal–organic frameworks, and various advanced materials is expected to enhance the efficiency and selectivity of conversion processes. The increased incorporation of renewable energy resources alongside digital technologies will bolster the sustainability and operational efficacy of biorefineries, positioning them as pivotal contributors to the transition towards a circular bioeconomy. As the global appetite for sustainable energy solutions escalates, waste biorefineries are poised to assume a critical role in closing material loops, mitigating environmental impacts, and facilitating the creation of a more sustainable and resilient industrial framework.

9. Future Perspectives

Future research directions pertaining to biorefineries are primarily centered on the enhancement of operational efficiency, scalability, and sustainability, whilst simultaneously confronting both technical and economic obstacles. Prominent future perspectives encompass the following:

• (1). Integration of Renewable Energy: The assimilation of renewable energy modalities, including solar, wind, and biomass-derived electricity, within biorefineries, has the potential to diminish reliance on fossil fuels and mitigate greenhouse gas emissions. This methodology augments sustainability and energy efficiency, with technological advancements such as anaerobic digestion and microbial fuel cells facilitating energy loops that transmute waste into both bioenergy and valuable byproducts. Furthermore, the electrification of processes alongside energy storage solutions strengthens the transition towards carbon-neutral operational frameworks.

• (2). Development of Cost-Effective and Robust Catalysts: Research efforts are directed towards the formulation of economically viable, stable catalysts that are resilient to the demanding conditions prevalent in biorefineries. Innovative materials, such as metal–organic frameworks (MOFs), nanocatalysts, and bio-based catalysts, are currently under examination to enhance conversion efficiencies, selectivity, and scalability. The primary objective is to optimize process efficiency while simultaneously minimizing costs and ecological outcomes.

• (3). Optimization of Waste Utilization: The effective utilization of waste entails the transformation of various waste streams into valuable products, thereby maximizing the sustainability of biorefineries. Initiatives encompass the broadening of feedstock sources, the advancement of pretreatment technologies, and the integration of processes aimed at converting disparate waste fractions into biofuels, chemicals, and energy. Cutting-edge waste valorization techniques, including microbial electrosynthesis, also serve to strengthen resource recovery.

• (4). Advanced Process Integration: The incorporation of biochemical, thermochemical, and electrochemical processes within biorefineries serves to optimize resource utilization and enhance operational efficiency. This integration minimizes waste generation, amplifies energy recovery, and facilitates the production of multiple products from a singular feedstock, thereby extending both economic viability and sustainability.

• (5). Sustainable Water and Resource Management: The implementation of effective water management practices encompasses the recycling of processed water, the utilization of zero-liquid discharge systems, and the reclamation of resources from byproducts. Energy-efficient methodologies for water treatment and reuse contribute to the reduction in environmental impact and operational expenditures, ensuring that biorefineries uphold principles of sustainability.

• (6). Life Cycle Assessment and Circular Economy: Life cycle assessment (LCA) is an important tool for understanding the environmental impact of biorefinery processes, helping to find ways to improve them. The circular economy focuses on reusing resources by turning waste into useful products and keeping materials in use for longer, reducing the need for new alternative resources.

• (7). Microbial and Enzyme Engineering: The engineering of microbes and enzymes significantly strengthens biomass conversion efficiency. Microbial engineering focuses on the optimization of microorganisms to process a variety of feedstocks and endure industrial conditions, while enzyme engineering is directed towards the creation of robust enzymes that facilitate the breakdown of biomass more effectively, thereby enhancing both the scalability and sustainability of biorefineries. These research domains aspire to elevate the environmental and economic sustainability of biorefineries, thereby contributing to the global progression towards a circular, low-carbon economy.

Author Contributions

M.S.: Conceptualization, Methodology, Writing—review and editing, G.-P.C.-C.: Visualization and Investigation, K.K.: Writing—Original Draft, R.P.: Formal Analysis, N.A.-Q.: Resources supervision, R.B.: Conceptualization, Methodology, Writing—original draft, B.-W.H.: Investigation, I.-C.L. and Y.-Y.L.: Funding acquisition. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

No new data were created.

Acknowledgments

This research was funded by the Center for Environment Toxin and Emerging Contaminant Research, the Center for Conservation and Research, the Ministry of Education of Taiwan (R.O.C.), and Cheng Shiu University, Taiwan (R.O.C.).

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Various sources of agricultural-based fillers for the production of biocomposites and biofuels. Reproduced with permission from [26] published by Elsevier, 2024.

Figure 2. Flowchart of food waste and garden waste in pellet making. Reproduced with permission from [49] published by Elsevier, 2024.

Figure 3. Valorization of household wastage. Reproduced with permission from [26] published by Elsevier, 2024.

Figure 4. (a) Laboratory-scale anaerobic digesters; (b) gas collection methods; (c) flame test experiment; (d) seed germination rates for different agriculturally important seeds. Reproduced with permission from [62] published by Elsevier, 2024.

Figure 6. Electrocatalytic CO2RR performance: (a) LSV curves of Cu5Ce95Ox, Cu90Ce10Ox, CuO, and CeO2 with CO2 as the reaction gas. Gaseous product FE at various applied potentials for (b) Cu5Ce95Ox and (c) Cu90Ce10Ox. FE of (d) CH4 and (e) C2H4 for Cu5Ce95Ox, Cu90Ce10Ox, and other references. Long-term stability test for CO2RR with (f) Cu5Ce95Ox at −1.2 V vs. RHE and (g) Cu90Ce10Ox at −1.4 V vs. RHE Comparison of selectivity (h) and stability (i) for CH4 and C2H4 products. Reproduced with permission from [89] published by Elsevier, 2024.

Figure 7. Mechanism of electrocatalytic CO2 hydrogenation adsorption. Reproduced with permission from [89] published by Elsevier, 2024.

Figure 8. Key stages in BioH2 production from non-edible biomass waste. Reproduced with permission from [92] published by Royal Society of Chemistry, 2020.

Figure 9. (a) Interaction between metallic (orange) and high-valent (blue) Re sites on the Re/C catalyst facilitates the formation of activated hydrogen species and the deoxydehydration reaction. The Re site undergoes reduction (to a low-valent state, yellow) and re-oxidation during the cycle. (b) The following hydrogenation step requires metallic Re sites [93].

Figure 10. Schematic representation of the mechanism of microbial ammonia synthesis: OmcA and MtrC are surface decaheme cytochrome c components of the extracellular iron oxide respiratory system; MtrB is an outer membrane component; MtrA is a periplasmic decaheme cytochrome c component. CymA is a membrane-anchored tetraheme c protein; MQ is menaquinone, and MQH2 is its reduced form. NapB is the periplasmic cytochrome c subunit of nitrate reductase, while NapA is the molybdopterin-binding subunit. NrfA is the nitrite reductase that produces ammonia. Reproduced with permission from [104] published by ACS Publications, 2024.

Figure 11. Conventional approach for fatty acid production from animal and plant sources. TGA: triacylglycerol, FAs: fatty acids, PUFAs: polyunsaturated fatty acids, OCFAs: odd-chain fatty acids. Reproduced with permission from [109] published by Elsevier, 2024.

Figure 12. Mapping of biorefinery representation globally.

Table 1

Overview of biorefinery technologies, maturity levels, mass balance, and revenue potential for converting 10 tons of agro-waste into valuable products like bioethanol, biogas, and biochar.

Process or Technology Used	TRL Stage	Feed (x ton/y)	Recovered Products	Mass Balance (Recovered)	Revenue Potential	Refs.
Pretreatment (e.g., Steam Explosion, Dilute Acid Hydrolysis)	Mature	10 tons of agro-waste	Pre-treated biomass (lignocellulosic fractions), liquid fractions	8 tons of solid biomass, 2 tons of liquid fractions	Low revenue. Estimated: USD 50–100/ton	[31,32]
Enzymatic Hydrolysis	Commercial	8 tons of pre-treated biomass	Sugars (glucose, xylose)	6.5 tons of sugars, 1.5 tons of residual biomass	Moderate: Sugars can sell for USD 200–300/ton.	[33,34]
Fermentation (e.g., Bioethanol Production)	Commercial	6.5 tons of sugars	Bioethanol, CO2	3.5 tons bioethanol, 1.5 tons CO2	High: Bioethanol prices range from USD 500–700/ton, with msp USD 218/metric ton for direct and USD 264/metric ton for indirect pathways.	[35,36]
Anaerobic Digestion	Commercial/Mature	1.5 tons of residual biomass	Biogas (methane, CO2)	1.2 tons biogas (0.8 tons methane, 0.4 tons CO2), 0.3 tons digestate	Methane can generate power or sell for USD 300–400/ton. Digestate offers USD 50–100/ton, with potential revenue of USD 360–480.	[37,38]
Pyrolysis (for Biochar and Bio-oil)	Emerging	0.3 tons digestate (from AD process)	Biochar, bio-oil	0.2 tons biochar, 0.1 tons bio-oil	Moderate: Biochar (USD 500–800/ton) and bio-oil (USD 300–500/ton) revenues depend on demand.	[39,40,41]
Lignin Valorization	Early Stage	2.5 tons of lignin-rich biomass (from pretreatment)	Lignin-based chemicals, composites	2 tons of lignin-based products	Potentially high: Lignin derivatives (polymers, adhesives) can earn USD 1000–2000/ton, with potential revenue of USD 2000–4000.	[42,43,44]

Table 2

Different biomass feedstocks, their conversion technologies, and corresponding biofuel yields.

Feedstock Used for the Biorefinery	Conversion into Products	Specific Technology Used for Conversion	Yield	Scale of Operation	Ref.
Sugar beet	Biofuel	Enzymatic hydrolysis, fermentation, pyrolysis	350–400 (L/Ton)	Potentially continuous (high-input feedstock)	[118]
Rice husk	biofuel	Enzymatic hydrolysis, gasification, fermentation	200–250 (L/Ton)	Batch/continuous depending on biomass availability	[119]
Wood apple	biofuel	Gasification, fermentation	150–200 (L/Ton)	Continuous for large scale, batch for small scale	[120]
Municipal solid waste	H2 gas	Steam reforming reaction	0.569 kg/capital/day	Continuous (large scale)	[121]
Organic fraction of municipal solid waste	Formic acid pretreatment	Dark fermentation	H2 yield 31.6 mL/gVS with 5% formic acid pretreatment, biochemicals	Continuous (large scale, dependent on feedstock)	[122]
Pure cheese whey (CWP)	Biochar and Biogas	Anaerobic digestion	Biochar/CH4, (358 mL CH4/g VSadd, 2 g biochar/gVS)	Continuous (for oil extraction and conversion)	[123]
Rice straw	CH4/H2 gas	Hydrolytic acidification of AD	H2 yield (74–94 times)	Continuous, large-scale potential	[124]
Jackfruit peels and seeds waste	Bioenergy	Combustion	High heating value (~16.5 MJ/kg) with low emissions of CO, CO2, and SO2.	Batch (small scale, low feedstock input)	[125]
Winery solid waste: grape marc (GM)	Biofuel	Combustion, thermochemical methods, kinetic and thermodynamic analysis, ash analysis, etc.	GCV: 19.3 MJ/kg, ash: 5.74 wt.%, CaO: 75.5%, MgO: 6.8%.	Batch or continuous (depending on process size)	[126]
Forestry waste residues from breadfruit tree (BFT), Macaranga spp. (MCG), Acacia mangium (ACM), and fig tree (FGL) leaves.	Biogas	Thermochemical analyses	Bio-oils and bio-gasses, FGL for biochar production	Continuous (depending on feedstock and biomass type)	[127]
Paddy waste (rice straw)Part of a zero-waste biorefinery model.	Bioethanol	Alkaline pretreatment	Bioethanol and chemical production: 79% pure lignin, 96% silica, and nutrients. Energy efficiency: 0.529, surpassing the current process (0.449).	Continuous (part of zero-waste biorefinery model)	[128]
Rice oil	Biodiesel production	Transesterification	Vegetable oil from grain bran (as a catalyst)	Continuous (large scale for biodiesel production)	[129]
Pine needle forest biomass	Methane	Anaerobic digestion	Methane production increased by 21.4% with pretreatment compared to untreated biomass.	Continuous (for higher methane production	[130]
Lime fruit waste	Biogas	Digestion	Biogas production increased by 40% in pretreated waste, yielding 93.2 mL/g VS.	Continuous (large-scale digestion)	[131]
Acai seeds	Methane	Anaerobic Digestion	CH4 yield 156.65 mL/g TS	Continuous (large-scale)	[132]
Hemp, watermelon, and sugarcane wastes	Bioethanol	Anaerobic fermentation	Bioethanol production (optimum values: pH 4.5, and 35 °C)	Batch (small-scale fermentation)	[133]
Marine macroalgae waste	Bioethanol	Thermal acid hydrolysis	Enhanced bioethanol production, 2.4 g/L	Batch (small scale for enhanced bioethanol production)	[134]
Industrial processing residues of tomatoes, fennel, potato, and carrot as carbon sources	Bio-H2 gas	Thermoanaerobacterium thermostercoris	Significantly higher biohydrogen produced.	Batch (dependent on feedstock variability)	[135]
Waste oils: palm, olive, linseed, sea buckthorn, cottonseed oils	Biofuels	Hydrolysis	High quality biofuels	Continuous (suitable for biofuel production)	[136]
Amalfi-coast waste lemon-seed oil	Biofuel	Electrocatalytic	98.2% of biodiesel production among the total reaction production	Continuous (high-input waste oil)	[137]
Used cooking oil	Biofuel	Electrocatalytic	98.58% of biodiesel production among the total reaction production	Continuous (high-input waste oil)	[138]
Soybean oil	Biofuel	Electrocatalytic	98.03 of biodiesel production among the total reaction production	Continuous (large scale for biodiesel production)	[139]

Table 3

Global biorefineries and core technology advancements utilizing biowaste for the production of high-value products.

Country	Development Area	Description	Examples/Companies
USA	Bioethanol, Biochemicals	Large-scale production of bioethanol from corn and biochemicals from biomass.	POET Biorefining (Sioux Fallsm, SD, USA), Abengoa Bioenergy (Hugoton, KS, USA), LanzaTech (Skokie, IL, USA)
Canada	Advanced Biofuels	Focus on converting forest biomass into biofuels and bioproducts.	Enerkem (Montreal, QC, Canada), LanzaTech (Edmonton, AB, Canada)
Argentina	Biodiesel Production	Biodiesel production from soy and other agricultural feedstocks.	Biocombustible (Buenos Aires, Argentina)
Brazil	Bioethanol, Biochemicals	Ethanol from sugarcane; leadership in 2G ethanol technologies.	GranBio (São Paulo, Brazil), Raizen (Rio de Janeiro, Brazil)
Paraguay	Renewable Diesel	Developing renewable diesel production infrastructure.	Omega Green (Villeta, Paraguay)
South Africa	Gas-to-Liquid (GTL)	Biorefinery utilizing natural gas and biomass feedstock.	Mossel Bay Biorefinery (PetroSA) (Mossel Bay, South Africa)
Nigeria	Biofuels for Energy Security	Focus on reducing dependence on imported fuels with local biofuels production.	Nigerian National Petroleum Corporation (Abuja, Nigeria)
Germany	Bioplastics, Biochemicals	Advanced bioproducts and polymer research.	BASF (Ludwigshafen am Rhein, Germany), Clariant (München, Germany)
Netherlands	Biochemicals, Renewable Energy	Leadership in biobased chemicals and sustainable energy.	Avantium (Amsterdam, The Netherlands)
Sweden	Biofuels and Biochemicals	Production of biodiesel and biofuels using innovative feedstocks.	St1 Biofuels (Gothenburg, Sweden)
Finland	Advanced Biorefineries	Renewable fuels and biochemicals from forestry residues.	UPM Biorefining (Lappeenranta, Finland)
India	Bioethanol and Biogas	Producing ethanol from sugarcane molasses and exploring biogas solutions.	IOC Bio-Refinery (Panipat, India), Praj Industries (Pune, India)
Kenya	Bioenergy for Rural Areas	Small-scale biorefineries supporting local energy needs and agriculture.	Lake Victoria Bioenergy Plant (Kisumu, Kenya)
Thailand	Bioplastics and Biofuels	Development of bioplastics and bioethanol from cassava and sugarcane.	Bangchak Corporation (Bangkok, Thailand)
China	Biochemicals and Biofuels	Large-scale bioethanol and chemical production from agricultural residues.	COFCO Biorefineries (Beijing, China), Shandong Longlive Bio-Technology (Yucheng, China)
Australia	Enzymes and Biofuels	Focus on biofuels production and enzyme technologies.	Microbiogen (Macquarie Park, Australia), Agrifuels (Wagga Wagga, Australia)
New Zealand	Biochemicals	Development of sustainable biochemicals from local feedstocks.	LanzaTech (Auckland, New Zealand)

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