Hydrogen Production from Biowaste: A Systematic Review of Conversion Technologies, Environmental Impacts, and Future Perspectives

Abstract

The escalating climate crisis and unsustainable waste management practices necessitate integrated approaches that simultaneously address energy security and environmental degradation. Hydrogen, with its high energy density and zero-carbon combustion, is a key vector for decarbonization; however, conventional production methods are fossil-dependent and carbon-intensive. This systematic review explores biowaste-to-hydrogen (WtH) technologies as dual-purpose solutions, converting organic waste to clean hydrogen while reducing greenhouse gas emissions and landfill reliance. A comprehensive analysis of different conversion pathways, including thermochemical (gasification, pyrolysis, hydrothermal, and partial oxidation (POX)), biochemical (dark fermentation, photofermentation, and sequential fermentation), and electrochemical methods (MECs), is presented, assessing their hydrogen yields, feedstock compatibilities, environmental impacts, and technological readiness. Systematic literature review methods were employed using databases, such as Scopus and Web of Science, with strict inclusion criteria focused on recent peer-reviewed studies. This review highlights hydrothermal gasification and dark fermentation as particularly promising for wet biowaste streams, like food waste. Comparative environmental analyses reveal that bio-based hydrogen pathways offer significantly lower greenhouse gas emissions, energy use, and pollutant outputs than conventional methods. Future research directions emphasize process integration, catalyst development, and lifecycle assessment. The findings aim to inform technology selection, policymaking, and strategic investment in circular, low-carbon hydrogen production.

1. Introduction

Since the early 21st century, global greenhouse gas (GHG) emissions, primarily CO₂, CH₄, and N₂O, have surged to approximately 50 gigatons of CO₂-equivalent annually, driving unprecedented climate disruptions, including wildfires, heatwaves, and ocean acidification [1,2,3,4]. Despite a temporary decline during the COVID-19 pandemic, emissions rebounded to 53.8 Gt CO₂eq in 2022, exceeding pre-pandemic levels by 2.3% [5,6]. Fossil fuel combustion, which satisfies over 70% of the global energy demand, remains the dominant contributor (68% of emissions), with the BRICS and G7 nations collectively responsible for 60% of the global GHG [7,8].

Concurrently, conventional waste management practices, such as landfilling and incineration, exacerbate environmental degradation, emitting 1.6 billion tonnes of CO₂eq annually from organic waste decomposition alone [3]. This dual crisis, driven by energy emissions and unsustainable waste disposal, demands integrated solutions to decouple economic growth from ecological harm. The global waste crisis is intensifying, with municipal solid waste (MSW) volumes projected to rise 70% by 2050, driven by urbanization and population growth [9]. Biowaste encompasses a wide spectrum of organic residues generated across multiple sectors, including agricultural waste (e.g., crop residues, straw, and husks), forestry waste (e.g., sawdust, bark, and wood chips), industrial organic waste (e.g., food-processing sludge and glycerol), municipal solid waste (organic fraction), and food waste from households, restaurants, and supply chains. These streams vary significantly in composition, biodegradability, and moisture content, which, in turn, influence their suitability for different hydrogen production technologies. Agricultural residues and forestry biomass are typically lignocellulosic in nature, rich in cellulose, hemicellulose, and lignin, making them suitable for thermochemical and pretreated biochemical conversion routes. Industrial organic waste streams, on the other hand, often have high chemical oxygen demands (CODs) and are suitable for anerobic processes.

Among these, food waste stands out due to its sheer volume, high biodegradability, and compatibility with low-energy biological conversion processes. According to global estimates, food waste accounts for approximately 1.3 billion tonnes annually, constituting roughly 44% of the municipal solid waste, with a moisture content exceeding 70%. These characteristics make it particularly well-suited for biological hydrogen production methods, such as dark fermentation, photofermentation, and hydrothermal gasification, which can process wet substrates without requiring extensive drying or pretreatment. Furthermore, addressing food waste aligns with broader sustainability goals, including emission reduction, landfill diversion, and the development of a circular bioeconomy.

For these reasons, while this review acknowledges the diversity of biowaste feedstocks, it gives specific attention to food waste as a representative and strategically important substrate for waste-to-hydrogen (WtH) technologies.

Organic waste, constituting 44% of the MSW, includes food waste, agricultural residues, and garden debris, yet only 30% is valorized in regions with advanced biowaste policies [10].

Figure 1 illustrates projected trends in MSW generation across various global regions for the years 2016, 2030, and 2050 [11]. It highlights a significant and consistent increase in waste production across all regions over time. East Asia and the Pacific is the largest contributor, with waste generation rising from 468 million tonnes in 2016 to an anticipated 714 million tonnes by 2050. South Asia and Sub-Saharan Africa also exhibit sharp increases, particularly South Asia, which is projected to have output. Regions such as Europe and Central Asia and North America show more moderate growth. The data underscore the pressing need for sustainable waste management strategies, especially in rapidly urbanizing and economically developing regions [12].

Landfilling, which accounts for 70% of the global waste disposal, emits methane, 25 times more potent than CO₂ over a century, and contaminates soil and groundwater through leachate seepage. For instance, food waste, representing 1.3 billion tonnes of annual losses, generates 8–10% of the anthropogenic GHGs when landfilled, despite its high organic content (40–60% carbohydrates and 10–30% lipids), offering untapped potential for bioenergy recovery [13]. This paradox underscores the urgency of reimagining waste as a resource rather than a liability.

Table 1. Food waste hydrogen production potential by dark fermentation.

Scenario/Study	Reference Year	Food Waste Volume (Million Tonnes/Year)	H2 Yield (m3 H2/Tonne)	Total H2 Potential (Million Tonnes H2/Year)	Source/Notes
Baseline	2019	~931	80 (~7.2 kg)	~6.7	[15,18,19]
Projection—moderate growth	2030	~1200	80	~8.6	[16,19,20]
Business as usual (BAU)	2050	~2100	80	~15.1	[16,19]
2050 (25% Waste-to-H2 recovery)	2050	~525	80	~3.7	Assumes partial diversion to hydrogen recovery [16,18,19]
2050 (50% Waste-to-H2 recovery	2050	~1050	80	~7.5	Feasible with aggressive circular bioeconomy interventions [19]
2050—Optimized yield scenario	2050	~2100	100 (optimized)	~18.9	Assumes optimized process conditions for higher yield [15,19]

Note: The actual hydrogen yield may vary depending on the feedstock composition, moisture content, pretreatment method, and reactor configuration. The assumptions used here represent conservative modeling estimates, not maximum theoretical outputs.

shows the projected hydrogen production potential from food waste in various scenarios (2019–2050).

A baseline hydrogen yield of 80 m³ H₂/tonne is applied across most scenarios, based on typical values reported in the literature for dark fermentation under standard conditions [15,18,19,20]. An optimized yield scenario of 100 m³ H₂/tonne is included to reflect future technological advancements, such as improved microbial strains, enhanced catalysts, and integrated system designs.

Hydrogen has emerged as a key player in decarbonization strategies due to its high energy density (120 MJ/kg) and zero direct CO₂ emissions during use [21]. However, 95% of the global hydrogen production relies on steam methane reforming (SMR), emitting 9–12 kg of CO₂ per kilogram of hydrogen and perpetuating fossil fuel dependence [22,23,24,25]. Transitioning to renewable hydrogen is critical for achieving net-zero targets, but scalability is hindered by feedstock scarcity and costs [26].

Waste-to-hydrogen (WtH) pathways, particularly those leveraging food waste, offer a dual advantage: diverting organic waste from landfills while producing low-carbon hydrogen at a 60–80% lower GHG intensity than SMR [9,27]. For example, gasification of 1 tonne of food waste can yield 40–50 kg of hydrogen, offsetting 0.5 tonnes of coal-equivalent emissions [28].

Different feedstocks used for biohydrogen (bio-H₂) production include lignocellulosic biomass (agricultural residues, forestry waste, and dedicated energy crops), food waste, municipal solid waste, industrial organic waste, and wastewater sludge. Lignocellulosic materials are abundant and rich in cellulose and hemicellulose, making them suitable for biochemical conversion routes, like fermentation and microbial electrolysis cells, after appropriate pretreatment processes [29]. Food waste, characterized by high carbohydrate and moisture contents, is compatible with dark fermentation and hydrothermal gasification [30,31].

Organic fractions of municipal solid waste and industrial effluents serve as economical and sustainable substrates due to their continuous availability and high organic loadings, especially in anerobic and photofermentation processes [32]. Algal biomass, although still under research, offers rapid growth rates and does not compete with food crops, making it a promising future feedstock [33]. Each feedstock type presents distinct advantages and limitations depending on the moisture content, biodegradability, and the need for pretreatment, influencing the selection of the appropriate hydrogen production technology [34].

The primary objectives of this review are to (i) present technical details and summarize the latest research directions and trends of thirteen prominent WtH technologies, (ii) compare the environmental impacts of these pathways based on four critical environmental indicators (GHGs, energy consumption, water use, and air pollution), and (iii) identify knowledge gaps and provide recommendations for future WtH research. It is expected that the findings of this review will facilitate informed decision making and guide future research and development efforts toward the advancement and implementation of sustainable waste-to-energy solutions.

2. Review Methodology

This review employs a systematic and structured methodology to comprehensively identify, evaluate, and synthesize relevant literature on biowaste-to-hydrogen technologies. The literature search was conducted across established databases, including Scopus, Web of Science, ScienceDirect, SpringerLink, IEA, and PubMed, covering publications from 1995 to 2025. Google Scholar was used only as a supplementary source to capture gray literature and early-access publications; however, all references ultimately included were verified as originating from peer-reviewed journals or official institutional reports. To ensure rigor, the search employed strategic combinations of keywords and Boolean operators, including terms related to waste-to-energy and biowaste utilization; hydrogen production pathways (thermochemical processes, gasification, pyrolysis); biological processes (dark fermentation, photofermentation); and lifecycle assessment. The selection process was governed by strict inclusion criteria: Additional filters considered technology maturity (TRL ≥ 3), study type (experimental or validated modeling), geographic relevance to regions with active hydrogen policy or deployment, environmental assessments, and the integration of such technologies within circular economy models. Only English-language studies with sufficient methodological detail were included.

Studies concentrating on fossil-fuel source content were included for comparison purposes. The collected literature was then categorized based on the hydrogen production pathway: thermochemical routes, biochemical routes, electrochemical routes, and physicochemical routes. Studies were critically appraised for their relevance to the field, data validity, and scientific quality, with priority given to those presenting validated comparative assessments, experimental data, and pilot-scale implementations. This rigorous methodological framework ensures that the review provides an up-to-date, reliable, and balanced synthesis of the current state of biowaste-to-hydrogen research.

To determine the eligibility of studies for each synthesis, we first categorized all the included studies based on the type of hydrogen production technology they investigatednamely, thermochemical, biochemical, physicochemical, or hybrid systems. Key characteristics of each study, including the feedstock type, technology applied, and reported outcomes, were tabulated and compared against the predefined groups outlined in the eligibility criteria. Studies that reported relevant quantitative or qualitative data for a specific technology group, such as hydrogen yield or environmental impact metrics, were included in that group’s synthesis. Studies without sufficient data for particular outcomes were excluded from that synthesis but were retained for qualitative discussion or inclusion in other relevant syntheses. This method ensured that each synthesis comprised studies with comparable and homogeneous data, facilitating a more robust and meaningful analysis.

Data were standardized to common units (e.g., hydrogen yield per kilogram of feedstock) for synthesis. Missing summary statistics were estimated from figures or approximated when necessary. Environmental impacts were normalized to a consistent functional unit. Studies lacking sufficient data were excluded from quantitative synthesis but included in qualitative discussion.

Figure 2 illustrates the adopted methodology, which is detailed in the following subsections.

The diagram illustrates the information flow across the four stages of the review process: Identification, Screening, Eligibility, and Inclusion, showing the number of records initially identified, those screened and excluded, and the number ultimately included in the final analysis. The study selection process followed the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, as illustrated in the accompanying flow diagram. A total of three thousand three hundred twenty-three records were initially identified through six academic databases: Scopus (n = 1253), Web of Science (n = 536), ScienceDirect (n = 261), PubMed (n = 694), SpringerLink (n = 523), and IEA (n = 56). Before screening, 1468 records were removed—582 as duplicates, 635 marked as ineligible by automation tools, and 251 removed for other reasons. The remaining 1855 records were screened based on titles and abstracts, resulting in the exclusion of 1389 records. Of the 466 reports sought for retrieval, 57 could not be accessed, leaving 409 reports for full-text eligibility assessment. During this stage, 239 reports were excluded due to lack of relevance (n = 137), reliance on modeling approaches (n = 56), insufficient technology maturity (n = 21), or inadequate experimental data (n = 23). Ultimately, 172 studies met all the inclusion criteria and were included in the final review.

3. Current Biowaste-to-Hydrogen Conversion Technologies

3.1. Thermochemical Conversion

Thermochemical conversion technologies present promising avenues for sustainable hydrogen production from organic waste streams. They offer high efficiency and versatility. Recent research efforts by various scholars have contributed to advancing these technologies [35]. Figure 3 shows the general framework of thermochemical pathways for producing hydrogen from organic biomass.

3.1.1. Gasification

Gasification is the thermochemical conversion of carbon-based feedstock used in the waste-to-energy (WTE) process to produce combustible gaseous fuels [36,37] from a wide range of waste materials, including organic components of municipal solid waste (MSW), the paper industry, and residues from the pulp industry [26,38]. Forestry lignocellulosic biomass and agricultural residue can be gasified to produce syngas. Variations in the properties of the feedstock may affect both hydrogen (H₂) yields and gasification efficiency [39]. Equations (1) and (2) illustrate the primary chemical reactions that occur in the gasifier [34].

C + H₂O ⟶ CO + H₂

(1)

C_xH_y + xH₂O ⟶ xCO + (x + y/2)H₂

(2)

Key thermodynamic assumptions include equilibrium-based modeling, with minimization of Gibbs free energy, and the presence of complete drying and devolatilization phases. The primary product is syngas, consisting mainly of CO, H₂, and CH_4, and minor quantities of carbon dioxide, CO₂ [40]. To enhance the purity of the hydrogen, the water–gas shift reaction (WGSR) can be employed, as depicted in Equation (3) [41].

CO + H₂O ⟶ CO₂ + H₂

(3)

Before proceeding with the WGSR, it is commonly necessary to eliminate sulfur and tar from syngas using catalysts [42]. The last step involves the removal of CO₂ through pressure-swing adsorption (PSA), thereby achieving an H₂ stream with purity levels as high as 99.9% [43]. Figure 4 shows a schematic diagram of the gasification process. Gasification takes place within a low-oxygen environment, typically characterized by pressures between 1 and 33 atm and at temperatures from 800 to 1400 °C, depending on the reactor design (e.g., fixed-bed, fluidized-bed, or entrained flow) [44]. Recent experiments have indicated that elevated gasification temperatures typically result in higher yields of hydrogen (H₂) [26,45].

Gasification is a technologically mature and high-yield thermochemical route for hydrogen production, capable of processing diverse lignocellulosic and carbon-rich feedstocks. Its scalability and syngas flexibility make it suitable for industrial-scale applications. However, its operational requirements, such as high temperatures, low-moisture feedstock, and sophisticated gas-cleaning systems, pose significant technical and economic barriers. Tar formation and catalyst deactivation remain persistent challenges that must be addressed to optimize system efficiency and reliability [36,44,45]. Pilot and commercial examples include the GoBiGas project in Sweden and NREL’s pilot-scale fluidized-bed gasifier, which provides empirical performance data supporting hydrogen-rich syngas production from biomass and waste feedstocks [46,47].

3.1.2. Pyrolysis

Pyrolysis is one of the thermochemical conversion methods that involves heating biomass without oxygen to produce syngas, bio-oil (in liquid form, it can also generate hydrogen independently), and charcoal (in solid form) [48]. Hydrogen generation via pyrolysis occurs through secondary reactions, governed by the reactor’s operational parameters [49]. In addition to hydrogen, the gaseous stream of the pyrolysis process contains significant amounts of light hydrocarbons and CH₄, which can be converted back to H₂ by the water–gas shift (WGS) and steam reforming (SR) processes [50]. According to Foong et al., 2021 [51], pyrolysis typically occurs in the absence of oxygen, at a temperature ranging from 350 to 900 °C and pressures between 0.1 and 0.5 MPa. Meanwhile, steam reforming, which converts methane to syngas, occurs at 700 °C.

Fast pyrolysis, conducted at ~500 °C with high heating rates (>100 °C/s) [51]. Thermodynamically, the hydrogen yield increases with increasing temperature due to enhanced cracking and secondary reforming reactions, but excessive temperatures can reduce the solid carbon content. Assumptions for modeling include a biomass moisture content of <10% and inert gas (N₂) atmospheres to avoid partial oxidation. Several pilot-scale systems, such as the Fraunhofer pyrolysis plant in Germany and projects in Canada (e.g., Enerkem), have demonstrated the scalability of pyrolysis for biohydrogen production [52,53].

Figure 5 illustrates the pyrolysis process of biowaste for hydrogen production. Various types of biomass waste, including agricultural residues, like corn straw [54], tea waste [55], rice straw [56], teff husk [57], and wheat straw, as well as forest waste, like beech wood [7] and pinewood sawdust [58], are suitable feedstocks for pyrolysis. Some previous studies have shown contrasting findings, where the gaseous product yield rises in CO₂ environments, the H₂ yield decreases due to a higher amount of CO produced from the presence of CO₂ [59]. Plasma pyrolysis presents numerous advantages over traditional pyrolysis methods, such as improved gas yields, increased energy content, and higher inherent temperatures with low tar formation levels [9,60].

Pyrolysis enables the thermal decomposition of biowaste in an oxygen-limited environment, producing hydrogen-rich gases, with typical yields ranging from 2 to 6 mol H₂/mol of biomass, especially when coupled with reforming [51,53]. The process operates across 350–900 °C and is effective for dry, lignocellulosic substrates, such as agricultural residues and forest biomass [54,55,56]. However, limitations include low hydrogen selectivity, tar generation, and reduced efficiency under high-moisture conditions. Plasma-assisted or catalytic pyrolysis has shown promise in enhancing hydrogen yield and gas quality [52].

Recent advances in pyrolysis-based hydrogen production highlight its potential as a sustainable energy pathway, particularly for biomass and waste feedstocks. Key challenges remain in feedstock variability, catalyst deactivation, and reactor integration. Ni-based catalysts are widely used but prone to coking, prompting interest in more robust alternatives, such as nanostructured and perovskite-based materials. Two-stage systems combining pyrolysis with catalytic reforming have shown improved hydrogen yields, though scalability issues persist.

Environmental assessments affirm pyrolysis as a low-emission route, especially when co-producing biochar. However, energy demands and catalyst-related emissions require further optimization. Economic feasibility hinges on feedstock cost, catalyst durability, and supportive policy. Emerging innovations, like microwave-assisted and plasma pyrolysis, offer promising avenues for improving efficiency and scalability.

3.1.3. Partial Oxidation

Partial oxidation (POX) is a type of thermochemical reaction characterized by partial combustion of a sub-stoichiometric component when a fuel–air mixture is insufficient in quantity within a reformer. This outcome in syngas production is enhanced with hydrogen, which can be further utilized, for instance, in fuel cells. The process is further categorized into thermal partial oxidation (TPOX) and catalytic partial oxidation (CPOX) (see Figure 6). The feedstock employed in this method is biogas produced from biowaste; however, it primarily comprises heavy-oil fractions, presenting difficulties in processing and utilization [61]. POX is a non-catalytic thermochemical process that gasifies raw material in the presence of oxygen (Reactions (4) and (5)) at temperatures typically ranging from 1300 to 1500 °C, as well as at pressures ranging from 3 to 8 MPa [62]. When compared to steam reforming (H₂:CO = 3:1), it generated more carbon monoxide (H₂:CO = 2:1 or 1:1). Therefore, the process is completed by transforming carbon monoxide with steam to hydrogen and carbon dioxide (see Equations (4)–(6)).

CH₄ + O₂ ⟶ CO + 2H₂

(4)

CH₄ + 2O₂ ⟶ CO₂ + 2H₂O

(5)

CH₄ + H₂O ⟶ CO + 3H₂

(6)

The results of POX differ depending on the reaction conditions and the hydrocarbon being oxidized. For instance, when methane undergoes POX, it produces a combination of water and carbon dioxide, whereas propane (C₃H₈) POX results in a mixture of water, carbon dioxide, and hydrogen [64]. Partial oxidation (POX) gasification is a commonly adopted method for hydrogen production because it uses a wide range of feedstocks and offers various advantages compared to other methods, like electrolysis or steam reforming [64,65]. Partial oxidation yields hydrogen through sub-stoichiometric combustion of hydrocarbon-rich feedstocks, achieving H₂ outputs typically in the range from 2 to 6 mol H₂/mol of carbon [61]. It operates at elevated temperatures (1300–1500 °C) and pressures (3–8 MPa) [62]. POX is particularly adaptable to heavy oils or syngas precursors, offering high conversion rates and moderate scalability. However, it often results in CO-rich syngas (H₂:CO ≈ 1:1 to 2:1), necessitating downstream water–gas shift and purification steps. The process is further hindered by the oxygen demand, capital cost, and lower H₂ selectivity relative to those of steam reforming [61]. Partial oxidation of biowaste presents a promising thermochemical route for hydrogen production, combining waste valorization with clean energy generation. Enhancements, such as supercritical water gasification, improve the conversion efficiency, with optimal oxidant ratios (e.g., 0.2 for glucose), maximizing hydrogen yields. While the process benefits from the abundant availability of biowaste and potential integration with other thermochemical pathways, challenges remain in catalyst stability, process optimization, and scalability. Continued research into advanced catalysts, effective pretreatment strategies, and hybrid systems is essential to fully realize the potential of biowaste partial oxidation in a circular hydrogen economy.

3.1.4. Hydrothermal Gasification

A recent research investigation has shown that using catalysts for the gasification of biowastes in supercritical and subcritical water improves the efficiency of hydrogen production. Salimi et al. [31] presented that wet biomass, such as straw, sawdust, and organic waste (lignin and sewage sludge), was gasified under hydrothermal conditions. Furthermore, the investigation of the hydrothermal gasification of these feedstocks assessed the impacts of pressure, temperature, residence duration, and the inclusion of alkali metals (like K₂CO₃ or KOH). In the presence of K₂CO₃ and KOH, aromatic compounds, carbohydrates, glycine, and biowaste underwent complete gasification, resulting in a hydrogen-rich product, with CO₂ emerging as the primary carbon compound [66].

Okajima et al. [30] examined the impacts of different factors, including the pressure, reaction temperature, molar ratio of water to carbon, reaction time, and catalysts, that affect the decomposition efficiency of wet biowaste, such as garbage, pig sludge, and paper sludge. The catalysts employed were KOH, nickel (Ni-5132P), NaOH, KCl, K₂CO₃, and Na₂CO₃. Low pressures, high temperatures, and high molar ratios of water to carbon are associated with higher yields of hydrogen gas. The optimal conditions were found to be 10 MPa, 700 °C, and 20 min, with a water-to-carbon molar ratio of around 20, resulting in hydrogen gas generation between 1500 and 2300 milliliters per gram of dry waste biomass.

HTG is highly versatile, capable of processing a broad spectrum of feedstocks, including agricultural residues, microalgae, and even synthetic polymers. Its integration with downstream systems, such as solid oxide fuel cells, enhances the overall energy efficiency and environmental performance. However, key challenges remain, including catalyst stability, material corrosion, and the economic feasibility of high-pressure reactor systems. Future research will focus on optimizing catalyst formulations, improving process integration, and evaluating lifecycle performance to facilitate commercial-scale deployment. HTG offers a compelling pathway for renewable hydrogen production, particularly from wet biowaste streams that are unsuitable for conventional thermal processes.

3.1.5. Biogas Reforming

Biogas reforming is an emerging thermochemical pathway for hydrogen production that leverages methane-rich biogas typically produced via anerobic digestion of organic waste, such as food waste, agricultural residues, and wastewater sludge. The primary component of biogas, methane (CH₄), can be converted to hydrogen through steam reforming, autothermal reforming (ATR), or dry reforming processes, using catalysts under high-temperature conditions (700–900 °C). Steam reforming of biogas follows a similar mechanism to that of natural gas reforming but requires additional gas-cleaning steps to remove CO₂, H₂S, and siloxanes for catalytic conversion. Biogas reforming offers several advantages, including the reuse of waste-derived methane, relatively mature catalytic systems, and compatibility with existing natural gas infrastructure.

Recent studies have shown that co-digestion of multiple organic substrates, such as food waste with lignocellulosic biomass or wastewater sludge, can enhance biogas yield and stability, increasing the hydrogen potential of this route. For instance, Jaafar (2010) [67] demonstrated effective biogas generation from date palm pulp waste, while Radeef et al. (2016, 2020) [68] explored the impacts of date seed size and oil content on biogas production in co-digested systems. Yong et al. (2015) [69] highlighted the benefits of co-digesting food waste and straw, reporting synergistic effects on methane output. These biogas streams, when coupled with reforming technologies, represent a promising hybrid approach to hydrogen production, particularly in decentralized settings or regions with established biogas infrastructure. However, technical challenges, such as the sulfur poisoning of catalysts, CO₂ dilution, and low methane concentrations, necessitate further innovation in gas upgrading and process integration. As such, biogas reforming stands as a valuable intermediary route, bridging traditional waste-to-energy systems with emerging hydrogen economies (see Figure 7).

3.2. Biological Fermentation

The exploration of biological pathways for hydrogen (H₂) production is advancing due to their potential benefits, such as lower energy requirements, reduced CO₂ emissions, and feasibility under ambient pressure and temperature conditions [70]. However, most biological methods are still in the early stages of laboratory and pilot-scale testing. Substantial endeavors are required to transition these technologies to industrial-scale implementation [71].

3.2.1. Dark Fermentation

Dark fermentation has emerged as a promising technology for hydrogen production from biowaste due to its high hydrogen yield potential and compatibility with a wide range of organic substrates [72]. Dark fermentation involves the anerobic conversion of organic compounds to hydrogen and volatile fatty acids by mixed microbial consortia [73]. Hydrogen-producing fermentative microorganisms have extensive metabolic versatility. Microorganisms perform various fermentations depending on the biotic and abiotic conditions they encounter within the same species or between diverse species of archaea and bacteria [17,74]. These microorganisms can produce H₂ through both butyrate- (Equation (7)) and acetate-type fermentation pathways (Equation (8)) [75,76,77,78].

Butyrate pathway:

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6 ⟶ 2{\mathrm{H}}_2 + 2{\mathrm{CO}}_2 + {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{COOH} \mathrm{\Delta }G = -206 \mathrm{kJ}/\mathrm{mol}$$

(7)

Acetate pathway:

$${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6 + 2{\mathrm{H}}_2\mathrm{O} ⟶ 4{\mathrm{H}}_2 + 2{\mathrm{CO}}_2 + 2{\mathrm{CH}}_3\mathrm{COOH} \mathrm{\Delta }G = -255 \mathrm{kJ}/\mathrm{mol}$$

(8)

The optimal pH range for dark fermentation (DF) is between 5.5 and 7.0; however, this should not be excessively low. It is essential to maintain hydraulic retention times (HRTs) at a level that allows for sufficient reaction time. Excessively low HRTs may lead to microorganism washout and reduced yields [79]. Dark fermentation experiments are frequently conducted at mesophilic temperatures (32–37 °C). However, recent research has indicated that higher yields of H₂ can be achieved under thermophilic conditions (55–70 °C) [73]. While dark fermentation can operate under both mesophilic (32–37 °C) and thermophilic (55–70 °C) conditions, the choice of the temperature regime significantly impacts both hydrogen yield and energy consumption. Thermophilic systems typically demonstrate higher hydrogen production rates, with reported yields reaching 2.8–3.8 mol H₂/mol glucose, an increase of 30–50% compared to those reported for mesophilic operation. However, this improvement comes at the cost of increased energy consumption, primarily due to the higher heating requirements needed to maintain thermophilic conditions. Estimated energy inputs for thermophilic dark fermentation range from 1.8 to 2.5 kilowatt hours per kilogram of hydrogen produced, compared to 1.0 to 1.5 kWh/kg H₂ for mesophilic systems. Whether the yield enhancement offsets the energy cost depends on the reactor insulation efficiency, local climate, and the source of the process energy (e.g., renewable vs. grid electricity). In contexts where waste heat or solar thermal energy is available, thermophilic fermentation may offer net environmental and energetic advantages [78,79,80]. Implementing efficient heat shock treatment in a serial bioreactor configuration and effluent recycling has been shown to reduce the lag phase in microbial growth and enhance productivity and yields [81]. However, the practical implementation of dark fermentation is significantly hindered by microbial instability. The hydrogen-producing consortia are highly sensitive to environmental fluctuations, such as pH shifts, feedstock variability, accumulation of VFAs, and the presence of hydrogen-consuming microorganisms, like methanogens. These factors can suppress hydrogen yield and lead to inconsistent reactor performance. Maintaining a stable microbial community over long operational periods requires frequent inoculum rejuvenation, pH buffering, and sometimes chemical or heat pretreatment to suppress non-hydrogenogenic bacteria.

Opportunities for dark fermentation also lie in the development of novel reactor configurations, such as membrane bioreactors or immobilized cell systems, to overcome these limitations and improve process efficiency [82]. In terms of reactor scalability, challenges arise due to biomass washout, inefficient mass transfer, and difficulties in ensuring uniform mixing at higher volumes. While batch reactors demonstrate feasibility at the lab scale, continuously stirred tank reactors (CSTRs) and upflow anerobic sludge blanket (UASB) systems used at larger scales often struggle with biomass retention and foaming. Additionally, heat management becomes more difficult at scale, particularly in thermophilic processes.

Downstream processing of the resulting hydrogen stream is also energy-intensive. Fermentative gas is typically diluted with CO₂ and water vapor, requiring further purification via pressure swing adsorption (PSA), membranes, or cryogenic separation, which diminishes the net energy efficiency. Moreover, VFAs and ammonia in the effluent require post-treatment if wastewater reuse is considered.

Furthermore, integration with other biotechnological processes, such as wastewater treatment or bioenergy production, offers synergistic benefits and enhances the sustainability of dark fermentation systems [83]. Figure 8 shows the schematic diagram of dark processes [9,84].

3.2.2. Photofermentation

Photofermentation is also one of the biological fermentation processes used to produce hydrogen from waste by photosynthetic bacteria under light exposure and anerobic conditions [85,86]. Purple non-sulfur bacteria (PNSBs), such as Rhodobacter, Rhodopseudomonas, and Rhodospirillum, are commonly employed microorganisms in photofermentation [87]. PNSBs are highly sensitive to light intensity and wavelength, nutrient ratios, and inhibitory compounds in the feed. Even minor variations in illumination or substrate concentration can result in drastic drops in H₂ productivity. Maintaining monocultures or functional consortia over long-term operations is difficult due to contamination by non-photosynthetic organisms and biofilm formation on reactor walls. Figure 9 shows the schematic diagram of the photofermenter bioreactor setup. The following equations (Equations (9) and (10)) illustrate that metabolic processes can proceed in two different pathways, depending on the availability of N₂ [84].

In the absence of N₂:

8H⁺ + 8e⁻ + 16ATP ⟶ 4H₂ + 16ADP + 16P_i

(9)

In the presence of N₂:

N₂ + 8H⁺ + 8e⁻ + 16ATP ⟶ 2NH₃ + H₂ + 16ADP + 16P_i

(10)

where ADP = adenosine diphosphate, and ATP = adenosine triphosphate. However, nitrogen remains crucial as a macronutrient for cell growth. The equations below (Equations (11)–(13)) illustrate H₂ production using different substrates, highlighting visible variances in H₂ outputs based on the carbon source.

Glucose: C₆H₁₂O₆ + 6H₂O + light ⟶ 12H₂ + 6CO₂

(11)

Butyrate: CH₃CH₂CH₂COOH + 6H₂O + light ⟶ 10H₂ + 4CO₂

(12)

Acetate: CH₃COOH + 2H₂O + light ⟶ 4H₂ + 2CO₂

(13)

The majority of recent studies have shown that the hydrogen production from biowaste using photofermentation was conducted in laboratory-scale settings, and an earlier examination applied a pilot-scale solar fermenter with acetate as the substrate [88]. The results of this investigation, conducted with a 90 L fed-batch photobioreactor, revealed relatively low conversion efficiency rates and H₂ yields, amounting to 12% and 0.35 mol/mol of acetate, respectively. Photofermentation leverages light-driven bacterial metabolism to convert organic acids to hydrogen, achieving theoretical yields of up to 12 mol H₂/mol of glucose, although practical yields are typically 0.3–4.5 mol H₂/mol of substrate, depending on the light intensity and bacterial strain [87,88]. Operating at 25–35 °C under anaerobic conditions, photofermentation benefits from utilizing dark fermentation byproducts, like acetate and butyrate. However, low photon conversion efficiency and oxygen sensitivity, and large-surface-area requirements currently restrict its scalability. Pilot studies have achieved yields of 0.35 mol H₂/mol of acetate in fed-batch photobioreactors [88].

Downstream processing is further complicated by the low partial pressure of hydrogen in the gas phase and the co-production of oxygen (under non-ideal conditions), which risks H₂-O₂ explosive mixtures. Gas stripping or vacuum-assisted collection techniques have been proposed but are still not economically feasible. Additionally, the liquid effluent often contains residual organics and dead biomass, demanding post-treatment before discharge or reuse.

3.2.3. Sequential Fermentation

The integration of dark fermentation and photofermentation, commonly referred to as sequential fermentation, has emerged as a promising approach for enhancing biohydrogen yields from organic waste feedstocks [89]. Dark fermentation, driven by anaerobic bacteria, such as Clostridium spp., primarily converts carbohydrate-rich substrates to hydrogen and organic byproducts, including volatile fatty acids (VFAs), under anaerobic, light-free conditions. Despite its simplicity and high substrate degradation rate, hydrogen production through dark fermentation alone is typically limited to 1.5–2.5 mol H₂/mol of glucose due to metabolic constraints and the accumulation of residual VFAs [90]. To overcome these limitations, a subsequent photofermentation step has been widely investigated. In this stage, purple non-sulfur photosynthetic bacteria (e.g., Rhodobacter sphaeroides) utilize the VFAs generated during dark fermentation, such as acetate, butyrate, and lactate, under light conditions to produce additional hydrogen. This integrated two-stage process improves the overall substrate conversion and can theoretically yield up to 8–9 mol H₂/mol of glucose, significantly surpassing the efficiency of either stage alone [91]. Figure 10 shows the sequential fermentation process.

Recent studies have focused on optimizing process parameters and reactor configurations to enhance system performance. For instance, Chen et al. [92] demonstrated that treating food waste hydrolysate through sequential dark fermentation and photofermentation increased the hydrogen yield by 60% compared to that of dark fermentation alone. Likewise, Rashid et al. [93] employed immobilized microbial consortia to extend the hydrogen production duration and stabilize the operation. Additional innovations, such as co-culturing and nutrient supplementation, have been shown to improve VFA utilization and light conversion efficiency.

Furthermore, hybrid systems that integrate sequential fermentation with photocatalytic purification or microbial electrolysis offer novel pathways for increasing hydrogen purity and system sustainability [94]. While scalability and light energy supply remain practical challenges, the integration of dark fermentation and photofermentation systems represents a highly adaptable and efficient strategy for sustainable hydrogen production from food and organic waste.

3.3. Microbial Electrolysis Cells (MECs)

Microbial electrolysis cells (MECs) are an emerging technology for hydrogen production from biowaste through electrochemical processes mediated by electroactive microorganisms [95]. It consists of an electrochemical device called a microbial electrolysis cell (MEC), which includes two electrodes installed in separate chambers [96]. Exoelectrogenic microorganisms form a biofilm on the surface of the anode, where the substrate undergoes oxidation to produce CO₂, electrons, and H⁺ ions (protons), while an external voltage source connects the electrodes, facilitating electron transfer to the cathode. In the cathodic chamber, protons flow through a membrane separating the two chambers, combining with electrons to produce H₂. The reactions depicted in both chambers involve acetic acid as the substrate at the anode (see Equations (14) and (15)) [97,98].

Cathode: 8H⁺ + 8e⁻ ⟶ 4H₂

(14)

Anode: CH₃COOH + 2H₂O ⟶ 2CO₂ + 8H⁺ + 8e⁻

(15)

Recent research has focused on improving the performance of MECs through the development of advanced bioelectrodes with enhanced catalytic activity and stability [99]. Additionally, optimization of reactor configurations, such as flow patterns and electrode spacing, has been explored to maximize hydrogen production rates and current densities [100,101]. Figure 11 shows a microbial electrolysis cell’s (MEC’s) schematic representation.

MECs utilize electroactive microorganisms under mild conditions (ambient temperature and pressure) to produce hydrogen at yields of 2–4 mol H₂/mol of acetate, with energy inputs as low as 0.2–0.8 V of external voltage and energy consumption of around 80–120 MJ/kg H₂ [98]. These systems are particularly suited for treating wastewater and fermentation effluents. Their low energy footprint and integration potential make them attractive for circular bioeconomy models. However, current limitations include membrane fouling, electrode degradation, and low current density, which constrain scalability and cost-effectiveness [101].

MECs can be configured in single-chamber or dual-chamber designs. Single-chamber MECs eliminate the need for a membrane separator but may suffer from gas crossover and lower hydrogen purity. Dual-chamber MECs separate the anode and cathode with a proton exchange membrane (PEM) or cation exchange membrane (CEM), which improves gas separation but increases internal resistance and cost.

Coulombic efficiency (CE), which represents the fraction of electrons recovered as hydrogen from the total available in the substrate, typically ranges between 60 and 90% under optimized lab conditions. However, energy recovery efficiencies drop in practice due to overpotentials, microbial competition, and losses in electrode kinetics. Furthermore, achieving high hydrogen yields requires careful control of microbial communities and substrate composition.

One of the key operational challenges in MECs is electrode fouling, particularly at the anode. Biofilm overgrowth, inorganic scaling, and clogging reduce electron transfer efficiency and require periodic cleaning or replacement of electrodes. Cathode materials, often composed of platinum or nickel, are prone to deactivation and add to system costs. Additionally, the use of ion exchange membranes introduces significant capital and maintenance expenses, especially when scaled up. Membrane fouling, ion leakage, and pH imbalances between chambers can severely impair long-term performance.

Despite these challenges, MECs offer an integrative approach to waste valorization and energy recovery. Several pilot-scale studies, such as those conducted by the University of Queensland and KAIST, have demonstrated practical feasibility with municipal wastewater and food waste substrates. However, cost-effective scaling of MECs remains constrained by materials, system complexity, and energy input requirements. Ongoing research is focused on low-cost electrode materials, membrane-free designs, and self-powered MEC configurations to improve economic feasibility.

4. Comparative Analysis of Technologies

Table 2. An overall comparative analysis of technologies [14,102,103,104,105,106,107,108].

Criteria	Gasification	Pyrolysis	POX	Hydrothermal Gasification	Dark Fermentation	Photofermentation	MECs
Feedstock Moisture Tolerance	Low–Moderate	Low	Moderate	High	High	Moderate	High
Operating Temperature (°C)	800–1400	40–900	1000–1500	250–700	30–70	25–40	Ambient
Hydrogen Yield (mol H2/mol)	5–10	2–6	2–6	8–12	2–4	4–12	2–4
Scalability/ Maturity	High	Medium	Medium	Low–Medium	Low–Medium	Low	Low
Energy Efficiency	Moderate–High	Moderate	Moderate	Moderate	Low–Moderate	Low	Moderate
Byproducts	CO, Tar, and CO2	Char and Bio-oil	CO, H2O, and CO2	CO2 and Salts	VFAs and CO2	CO2	Minimal Organics
Environmental Benefits	High (with CCS)	Moderate	Moderate	High	High	High (Solar-based)	High
Capital Cost	High	Medium	High	High	Low	Medium	Medium–High
Technology Complexity	High	Medium	High	Very High	Low	Medium	High
Suitability for Food Waste	Limited (Drying Needed)	Moderate	Moderate	Excellent	Excellent	Moderate	Good

Notes: Hydrogen Yield: High (>4 mol H₂/mol of substrate), Moderate (2–4 mol/mol), and Low (<2 mol/mol). Technology-Readiness Level (TRL): High (TRL = 8–9), Moderate (TRL = 5–7), and Low (TRL < 5). GHG Reduction Potential: High (>70% compared to that of SMR), Moderate (40–70%), and Low (<40%). Economic Competitiveness (LCOH): High (LCOH < USD 2/kg H₂), Moderate (USD 2–5/kg), and Low (>USD 5/kg). These classifications are derived from a synthesis of the recent literature and institutional reports, such as the IEA (2023) [109], U.S. D.O.E. Hydrogen Program (2022) [110], and Bridgwater (2012) [111].

summarizes key comparative criteria for seven leading biowaste-to-hydrogen technologies, based on recent peer-reviewed literature. The comparison considers the hydrogen yield, feedstock suitability, operational complexity, environmental benefits, and economic feasibility. This comparative analysis highlights the multifaceted tradeoffs between various biowaste-to-hydrogen technologies.

Only seven of these pathways were selected for the overall biowaste-to-hydrogen conversion comparative analysis in Table 2, based on the following criteria: (i) their relevance to current commercial or near-commercial applications, (ii) the availability of consistent and comparable data across multiple performance indicators (e.g., hydrogen yield, scalability, and environmental impact), and (iii) their suitability for biowaste feedstocks, particularly food waste. Due to the heterogeneity of the study designs, feedstocks, measurement units, and outcome reporting formats, no statistical meta-analysis was performed. Instead, a structured qualitative synthesis was conducted, and key performance indicators (hydrogen yield, energy use, and GHG emissions) were extracted and compared narratively and graphically across technology categories.

Thermochemical methods, such as gasification and pyrolysis, exhibit high hydrogen yields and commercial readiness, particularly in regions with robust infrastructure. However, these approaches require low-moisture feedstocks and complex operational systems, making them less suitable for high-moisture biowaste, like food waste. Gasification, while efficient, demands high capital investment and stringent tar removal processes. Pyrolysis, especially when coupled with reforming or plasma techniques, has gained traction but is still constrained by reactor scalability and tar management.

Partial oxidation presents moderate yield potential and fuel flexibility but is less favored due to CO contamination and reliance on oxygen. Conversely, biological and electrochemical technologies align more with decentralized and low-energy systems. Dark fermentation and microbial electrolysis cells (MECs) are particularly promising for food waste due to their compatibility with wet organic matter and lower energy requirements.

Although these processes are still under development, the recent literature demonstrates meaningful improvements in microbial efficiency and reactor configurations. Hydrothermal gasification is the most favorable technology for food waste, bypassing the drying stage and achieving high hydrogen yields under supercritical conditions. Despite its technical complexity, its environmental performance and feedstock compatibility position it as a frontrunner in future waste-to-hydrogen systems.

Among the various technologies evaluated, hydrothermal gasification (HTG) stands out as the most suitable for hydrogen production from food waste. This recommendation is based on the high moisture content of food waste, which often exceeds 80%, and renders it inefficient for conventional thermal conversion methods, such as pyrolysis and dry gasification, both of which require substantial drying. HTG, however, operates under high-temperature and high-pressure conditions, allowing it to process wet feedstocks directly and convert them to hydrogen and carbon dioxide without the need for energy-intensive drying.

Empirical findings underscore the effectiveness of HTG in this application. Studies such as those by Okajima et al. (2007) [30] and Salimi et al. (2016) [31] have reported hydrogen yields ranging from 1.5 to 2.3 L per gram of dry biomass, indicating one of the highest conversion efficiencies among the available technologies. Furthermore, the incorporation of alkali catalysts, like potassium carbonate (K₂CO₃) or potassium hydroxide (KOH), has been shown to significantly enhance gasification efficiency and hydrogen selectivity. HTG also offers environmental advantages, particularly when integrated with renewable energy sources or industrial waste heat, contributing to its feasibility as a sustainable solution.

In addition to HTG, dark fermentation presents a compelling alternative for food waste valorization. This biological process is particularly advantageous due to its low energy requirements, operation under ambient conditions, and high compatibility with food-derived substrates. Beyond hydrogen production, dark fermentation also generates valuable volatile fatty acids (VFAs), which can be further upgraded through integrated downstream processes, increasing the overall energy and resource recovery from the waste stream.

Moreover, microbial electrolysis cells (MECs) serve as a promising complementary technology to dark fermentation. MECs can be effectively employed to recover additional hydrogen from fermentation effluents and wastewater, thereby enhancing the overall hydrogen yields and system sustainability. This combination of biological and electrochemical processes represents a holistic approach to maximizing hydrogen recovery.

No formal sensitivity analysis was conducted due to the qualitative nature of this systematic review and the heterogeneity in the study methodologies and outcome reporting. However, several robustness strategies were embedded in the study design:

(i)

Only peer-reviewed studies or institutional reports were included;

(ii)

Studies without experimental data or with technology-readiness levels of below 3 were excluded;

(iii)

Model-only studies were analyzed separately and not included in the quantitative comparisons. Future meta-analytical studies may benefit from subgroup-based or scenario-based sensitivity assessments to better evaluate the robustness of synthesis findings.

5. Environmental Impact Assessment of Hydrogen Production Technologies

As hydrogen emerges as a cornerstone of global decarbonization strategies, its production pathways must be scrutinized through a comprehensive environmental lens [112]. This section compares thirteen hydrogen production technologies based on four critical environmental indicators: greenhouse gases (GHGs), energy consumption, water use, and air pollution (see Figure 10, Figure 11, Figure 12 and Figure 13, respectively). The analysis integrates data from the recent literature [113,114,115,116,117] and a graphical synthesis of key metrics, offering a holistic evaluation that can inform policy, investment, and technological prioritization.

5.1. Greenhouse Gas Emissions

Figure 12 illustrates the comparative GHG emissions, measured in kilograms of CO₂-equivalent per kilogram of hydrogen (kg CO₂-eq/kg H₂), across a range of hydrogen production pathways utilizing different feedstocks. Conventional non-renewable technologies, such as coal gasification and SMR, exhibit the highest GHG emissions, at approximately 19 kg CO₂-eq/kg H₂ and 10.5 kg CO₂-eq/kg H₂, respectively [4,118].

Electrolysis using grid electricity shows similarly high emissions (~14 kg CO₂-eq/kg H₂), largely due to the current carbon intensity of national electricity grids that still rely significantly on fossil-fuel-based generation [119]. This underlines the critical importance of the electricity mix used in electrolysis systems, which can either amplify or mitigate environmental impact.

In contrast, renewable hydrogen production technologies demonstrate substantially lower emissions. Electrolysis powered by renewable energy sources, such as wind and solar, results in emissions as low as ~1 kg CO₂-eq/kg H₂, assuming the electricity is nearly carbon-free [113,114,120]. Other pathways (including photoelectrochemical (PEC) water splitting and biological hydrogen production via biophotolysis, dark fermentation, and microbial electrolysis cells (MECs)), show similarly low emissions (typically 0.7–2.5 kg CO₂-eq/kg H₂) when using organic or waste feedstocks [121].

Technologies like plasma gasification and hydrothermal gasification, which utilize municipal solid waste or wet biomass, display moderate GHG profiles (~3.5 kg CO₂-eq/kg H₂), making them attractive options for integrating waste valorization and climate mitigation [122].

Overall, these findings reinforce that hydrogen produced via renewable and bio-based methods provides the lowest carbon footprint. However, the analysis also reveals variation within renewable pathways, influenced by factors such as conversion efficiency, feedstock sourcing, and system boundaries considered in LCAs [123,124,125].

5.2. Energy Use

The comparative energy analysis of hydrogen production technologies, as illustrated in Figure 13, reveals substantial variability in energy demand per kilogram of hydrogen produced, ranging from less than 100 MJ/kg H₂ to over 200 MJ/kg H₂. This has direct implications for environmental sustainability, particularly in the context of carbon emissions and renewable energy integration.

Conventional fossil-based methods, namely, SMR and coal gasification, exhibit the lowest external energy requirements (~140–160 MJ/kg H₂ for SMR and ~190–200 MJ/kg H₂ for coal gasification). SMR is widely commercialized due to its high efficiency and relatively low cost; however, it is also associated with significant CO₂ emissions, averaging 9–12 kg CO₂ per kg H₂ without carbon capture and storage (CCS) [113]. Coal gasification, despite being slightly more energy-intensive, carries an even higher carbon footprint due to the carbon-intensive feedstock.

Electrolysis technologies present a cleaner alternative but with higher energy demands. Grid-powered electrolysis requires approximately 210 MJ/kg H₂, while renewable-powered electrolysis performs slightly better (~190 MJ/kg H₂). The environmental performance of electrolysis is highly sensitive to the carbon intensity of the electricity used. When powered by solar or wind, it can achieve near-zero lifecycle emissions [126], but if fossil-heavy grid electricity is used, its emissions may rival those of SMR.

Biomass gasification and methane pyrolysis represent intermediate cases, with energy uses of ~160 MJ/kg and ~120 MJ/kg H₂, respectively. Biomass gasification offers net-zero or even net-negative emissions when coupled with CCS, positioning it as a sustainable transitional method [30]. Methane pyrolysis avoids CO₂ emissions by producing solid carbon as a byproduct, although the energy requirements for reactor heating still pose a challenge [127].

Photoelectrochemical (PEC) and biological methods, such as dark fermentation, photofermentation, and biophotolysis, are less mature technologies but are conceptually appealing due to their use of solar energy and organic waste. PEC methods show the highest energy intensity among all the considered technologies (>220 MJ/kg H₂), primarily due to poor solar-to-hydrogen efficiency and high capital costs [128]. Biological pathways, although renewable, also suffer from low hydrogen yields and high energy losses. Dark fermentation and photofermentation lie between 130 and 160 MJ/kg H₂, limited by incomplete substrate utilization and low light conversion efficiencies [129]. Biophotolysis, which uses algae and sunlight, is particularly inefficient and often exceeds 160 MJ/kg H₂, primarily due to biological limitations in photon use and oxygen sensitivity [64].

Plasma gasification and hydrothermal gasification, which utilize waste or wet biomass feedstocks, demonstrate energy demands of ~180–190 MJ/kg H₂ [130]. These technologies offer the advantage of waste valorization and operate under supercritical or high-temperature conditions, enabling high conversion efficiencies and reduced waste [131]. However, their high operational energy requirements limit widespread application without external energy subsidies.

Finally, microbial electrolysis cells (MEC) present the lowest energy use among the biological techniques (~80 MJ/kg H₂), leveraging organic-rich wastewater as both the substrate and electrolyte. MECs achieve this using a low external voltage to stimulate microbial hydrogen production, making them particularly attractive for decentralized wastewater treatment facilities [132]. However, scalability and electrode material costs remain unresolved issues.

5.3. Water Use in Hydrogen Production Technologies

Water intensity is an often-overlooked but critical environmental parameter in hydrogen production, particularly as the world faces increasing water stress due to climate change and population growth. As illustrated in Figure 14, hydrogen production technologies vary substantially in their water use, with values ranging from approximately 4 L/kg H₂ (steam methane reforming) to over 15 L/kg H₂ (biophotolysis, MEC, and hydrothermal gasification). These differences are shaped not only by the process chemistry but also by the feedstock type and water integration in ancillary operations, such as cooling and feedstock pretreatment.

Fossil-based hydrogen production technologies, such as SMR and coal gasification, exhibit the lowest water intensities, at approximately 4–6 L/kg H₂ and 6–7 L/kg H₂, respectively. This relatively low water use is due to their dry feedstocks (natural gas and coal) and reliance on thermal energy rather than aqueous processing [123]. However, these methods contribute significantly to greenhouse gas emissions, and water use may increase substantially when considering carbon capture and storage (CCS), which typically introduces additional cooling and processing demands [133].

Electrolysis, both grid-based and powered by renewable energy, requires around 10–11 L/kg H₂. The stoichiometric water requirement for electrolysis is ~9 L/kg H₂, with additional volumes needed for system cooling and purification [126]. The key environmental tradeoff here lies in the origin of the electricity: Electrolysis powered by solar or wind has the potential for net-zero emissions, but in water-scarce regions, the availability of clean water may constrain deployment, especially at gigawatt-scale facilities [89,134].

Biomass gasification and methane pyrolysis represent intermediate water use technologies, requiring ~9 L/kg H₂ and ~5 L/kg H₂, respectively. While biomass gasification requires water for gasification reactions and sometimes for biomass pretreatment, methane pyrolysis avoids water-intensive steps by thermally cracking methane to hydrogen and solid carbon, making it relatively frugal in terms of water demand [135]. Still, methane pyrolysis remains dependent on natural gas infrastructure, and the management of solid carbon residues poses a logistical challenge.

Photoelectrochemical (PEC) water splitting, which integrates sunlight directly into water electrolysis, has a water use of ~13 L/kg H₂. This reflects not only the stoichiometric requirement but also additional process water needed to maintain photoactive surfaces and temperature control in experimental setups. Despite its promise as a solar-driven clean hydrogen source, PEC technology remains at the lab scale due to challenges in efficiency and material stability [136].

Biological pathways, such as dark fermentation, photofermentation, and biophotolysis, exhibit high water intensities (12–16 L/kg H₂). These processes often use aqueous substrates, like organic waste, acids, or algal suspensions, and require significant water for microbial cultivation, substrate handling, and system maintenance [129]. Although they can be integrated with wastewater treatment, their large water footprint poses scalability concerns in water-limited regions.

Waste-to-hydrogen technologies, including plasma gasification, hydrothermal gasification, and microbial electrolysis cells (MEC), show some of the highest water intensities. Hydrothermal gasification and MECs both approach or exceed 15 L/kg H₂ due to the aqueous nature of their feedstocks, wet biomass and wastewater, respectively [131,132]. While they valorize waste streams and can reduce environmental pollution, their high water-usage levels and technological immaturity may limit their widespread adoption without further innovation in water recycling and process integration.

5.4. Air-Pollutant Emissions from Hydrogen Production Technologies

While hydrogen is often promoted as a clean energy carrier, its environmental benefits are heavily dependent on the production pathway. Figure 15 highlights the relative severity of air-pollutant emissions, such as NOₓ, SO₂, CO, particulate matter (PM), and volatile organic compounds (VOCs), across various hydrogen production technologies. During biowaste conversion to hydrogen, various volatile organic compounds (VOCs) can be produced, including light hydrocarbons (e.g., benzene and toluene), oxygenated compounds (e.g., acetic acid and formaldehyde), and phenolic species, particularly during pyrolysis and hydrothermal processes [137,138]. If chlorine-containing materials are present in the feedstock, halogenated compounds, such as chloromethane (CH₃Cl) and hydrogen chloride (HCl), may also form [139,140]. These VOCs pose environmental and operational concerns, including toxicity, corrosion, and secondary-pollutant formation.

To mitigate their impacts, measures such as feedstock pretreatment, optimized process conditions, and gas-cleaning systems (e.g., activated carbon and thermal oxidation) are essential [141]. A better understanding of VOC formation pathways is crucial for developing cleaner and more sustainable hydrogen production technologies from biowaste.

Fossil-fuel-based technologies, particularly coal gasification and steam methane reforming, demonstrate the highest relative air pollution intensities, with scores of approximately 5 and 4 on a normalized severity scale, respectively. These processes emit significant quantities of NOₓ, SO₂, and PM due to high-temperature combustion and the impurities present in fossil feedstocks [142]. Even though these methods dominate global hydrogen production today, they are also responsible for a disproportionate share of air quality degradation, especially in regions with limited emission controls [124].

Grid-based electrolysis exhibits moderate pollutant emissions (a score of ~3), reflecting the embedded emissions of the electricity mix. In regions where fossil fuels dominate the grid, indirect emissions from electrolysis can rival those of SMR in both greenhouse gases and air pollutants [126]. In contrast, renewable electrolysis (solar/wind powered) scores near the bottom of the scale (~0.5), as it avoids combustion altogether and relies on clean, non-emitting electricity sources. These results affirm the environmental superiority of renewable-powered electrolysis when air quality and public health are considered.

Biomass gasification and methane pyrolysis yield intermediate emission scores (~2), but their implications differ significantly. Biomass gasification can emit PM and VOCs due to incomplete combustion and volatile organics in the biomass feedstock [131]. Methane pyrolysis, on the other hand, avoids direct air pollutant emissions by decomposing methane thermally to hydrogen and solid carbon, though upstream fugitive methane emissions from natural gas supply chains remain a concern [137].

Photoelectrochemical (PEC) water splitting, biophotolysis, dark fermentation, and photofermentation all display relatively low air pollution profiles (scores of ~1), primarily because they rely on ambient-temperature biological or photonic processes that do not involve combustion. However, their environmental advantage is counterbalanced by technical immaturity, low hydrogen yields, and the requirement for controlled cultivation environments [139]. The emission impact is often indirect, stemming from energy use in lighting, heating, or downstream processing.

Waste-based technologies, including plasma gasification, hydrothermal gasification, and microbial electrolysis cells (MECs), present nuanced environmental profiles. Plasma gasification scores high (~3) due to the generation of airborne pollutants, like NOₓ and heavy-metal-laden particulates, during waste incineration at extreme temperatures [131]. Hydrothermal gasification and MECs, while operating in aqueous environments, score modestly (~1.5), reflecting some air emissions from auxiliary systems and the handling of wet organic feedstocks. However, their capacities to valorize wastewater and wet biomass gives them environmental relevance in circular economy strategies.

The 13 hydrogen production pathways exhibit diverse environmental profiles, depending on feedstock, energy source, and conversion efficiency. Fossil-fuel-based methods, such as steam methane reforming (SMR) and coal gasification, remain the most carbon-intensive, with high GHG emissions, substantial energy consumption, and significant air pollutant release, despite moderate water usage. Partial oxidation and autothermal reforming follow similar patterns, though slightly more efficient when integrated with carbon capture systems. In contrast, electrolysis using renewable energy (green hydrogen) shows minimal GHG emissions and air pollutants but tends to have high electricity and water demands, raising concerns about resource use in water-scarce regions. Biomass-based routes, such as pyrolysis, gasification, and hydrothermal gasification (HTG), offer carbon neutrality potential and moderate water use but can emit particulates and volatile organic compounds if not well controlled. Photobiological and photoelectrochemical methods present very low emissions but currently suffer from low conversion efficiency and high land/resource requirements. Plasma reforming and solar thermochemical processes are still emerging, with uncertain full-scale environmental footprints but potential for low emissions if powered by renewables.

To reduce environmental impacts, future efforts should prioritize (1) scaling up low-emission technologies, like green electrolysis and biomass gasification, with improved controls; (2) optimizing water use efficiency, especially in water-dependent pathways; (3) integrating carbon capture and storage (CCS) in fossil-based routes; and (4) supporting technoeconomic assessments and lifecycle analyses to guide the selection of context-appropriate hydrogen pathways. Environmental sustainability must remain central to hydrogen strategy development to align with broader climate and resource goals.

Figure 16 shows the environmental impact heatmap, which provides a comparative analysis of thirteen hydrogen production technologies across four key environmental indicators: greenhouse gas (GHG) emissions, energy use, water use, and air pollutant emissions. Fossil-fuel-based methods, such as steam methane reforming (SMR) and coal gasification, display the highest environmental burden, with elevated GHG and air pollutant emissions and substantial energy demands. Electrolysis using grid electricity also shows high energy use and water consumption, though its emission profile can vary based on the electricity mix. In contrast, electrolysis powered by renewable energy (green hydrogen) demonstrates excellent environmental performance with minimal emissions and air pollutants, though it still faces challenges related to water and energy resource efficiency.

Emerging and biology-based methods, including dark fermentation, photofermentation, biophotolysis, and microbial electrolysis cells (MECs), generally exhibit low GHG emissions and air pollutants but moderate energy and water demands due to their low conversion efficiencies and operational constraints. Biomass-based routes, such as gasification, pyrolysis, and hydrothermal gasification (HTG), offer a middle ground, with potential for carbon neutrality and manageable pollutant levels, depending on feedstock and system optimization. Plasma gasification and photoelectrochemical (PEC) processes are also promising, showing moderate impact levels while offering opportunities for integration with renewable power. Overall, the heatmap underscores the critical need to prioritize clean, resource-efficient hydrogen technologies while continuing to improve energy efficiency and reduce water intensity, especially in scalable and decentralized applications.

6. Socioeconomic and Regulatory Considerations for WtH Deployment

The large-scale deployment of Waste-to-Hydrogen (WtH) technologies depends not only on technological maturity but also on their economic feasibility, public acceptance, and the regulatory environment in which they operate.

Technoeconomic Analysis (TEA)

Technoeconomic performance is a key determinant of hydrogen production feasibility across scales and sectors. The primary cost drivers include capital expenditure (CapEx, USD/kW), operational expenditure (OpEx, USD/kg H₂), and the levelized cost of hydrogen (LCOH, USD/kg H₂), a function of both capital and operational factors, amortized over the system lifetime and output. This section presents a comparative evaluation of hydrogen production pathways based on the current literature and benchmark data (see

Table 3. Technoeconomic comparison of hydrogen production technologies [29,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160].

Technology	CapEx (USD/kW)	OpEx (USD/kg H2)	LCOH (USD/kg H2)	Technology Status/Notes
SMR	900–1200	0.8–1.2	1.0–2.0	Mature, cost-effective, and high GHG emissions unless combined with CCS
Coal Gasification	1300–1800	1.2–1.8	1.5–2.5	High emissions and water use; feasible in coal-rich regions with CCS
Electrolysis (Grid)	1500–2000	2.5–3.5	4.0–6.0	Dependent on the grid mix; high operating costs
Electrolysis (Renewable)	1800–2200	2.0–3.0	3.5–5.0	Cleanest option; increasingly competitive as renewable costs fall
Biomass Gasification	1800–2500	1.5–2.2	2.5–4.0	Carbon-neutral potential; scalable
Pyrolysis	2000–2500	1.8–2.5	3.0–4.5	Produces solid carbon; still in early deployment
PEC	3500–4500	2.5–3.5	6.0–8.0	Experimental solar-driven technology; scalability issues
Dark Fermentation	2500–3500	2.2–3.0	4.5–6.5	Biowaste conversion: low yields and limited scalability
Photofermentation	3000–4000	2.5–3.5	6.0–8.0	Light-driven microbes; low efficiency
Biophotolysis	3200–4500	3.0–4.0	7.0–9.0	Early-stage alga-based process; low productivity
Plasma Gasification	3000–3800	2.5–3.5	5.5–7.0	High energy demand; useful for plastic and waste conversion
Hydrothermal Gasification	2000–3000	2.0–2.8	4.0–6.0	Ideal for wet biomass; catalyst and efficiency R&D are ongoing

).

Steam methane reforming (SMR) remains the most cost-effective and widely deployed technology. Recent estimates place its CapEx between USD 900 and USD 1200/kW and OpEx between USD 0.8 and USD 1.2/kg H₂, with a resulting LCOH of USD 1.0–2.0/kg H₂, depending on the plant scale and natural gas price [144,145]. These costs make SMR the baseline against which emerging technologies are typically assessed. However, in the absence of carbon capture and storage (CCS), SMR is associated with high greenhouse gas (GHG) emissions, which could trigger carbon penalties under future regulatory frameworks [146].

Coal gasification, another mature thermochemical route, has a CapEx ranging from USD 1300 to USD 1800/kW and an OpEx of USD 1.2–1.8/kg H₂, yielding an LCOH of USD 1.5–2.5/kg H₂ [147]. Although cost-effective, its adoption is regionally dependent on coal availability and is increasingly constrained by environmental concerns, including high CO₂ emissions and water usage rates [148].

In contrast, electrolysis-based technologies, particularly proton exchange membrane (PEM) and alkaline electrolysis, present significantly higher production costs, primarily due to electricity consumption and capital intensity. Grid-powered electrolysis typically has a CapEx in the range of USD 1500–2000/kW, an OpEx of USD 2.5–3.5/kg H₂, and an LCOH between USD 4.0 and USD 6.0/kg H₂ [149,150]. When powered by renewable energy, OpEx is slightly reduced (USD 2.0–3.0/kg H₂) due to declining solar and wind electricity prices, but CapEx remains elevated (USD 1800–2200/kW), yielding an LCOH of USD 3.5–5.0/kg H₂ [151,152]. Although the costs are still above those of fossil-based routes, renewable electrolysis is expected to become cost-competitive by 2030, as capital costs fall, and electrolyzer lifespans increase [151].

Biomass-based thermochemical processes, such as biomass gasification, present mid-range economics, with CapEx between USD 1800 and USD 2500/kW and an OpEx of USD 1.5–2.2/kg H₂, resulting in LCOH values of USD 2.5–4.0/kg H₂ [153,154]. These systems benefit from waste-to-energy synergies and lower carbon footprints but face challenges in feedstock logistics and gas cleanup. Pyrolysis, which can co-produce solid carbon, exhibits a similar CapEx (USD 2000–2500/kW) but a higher OpEx (USD 1.8–2.5/kg H₂), leading to LCOH values of USD 3.0–4.5/kg H₂ [155].

Photodriven and biological methods, including photoelectrochemical (PEC) water splitting, dark fermentation, photofermentation, and biophotolysis, remain at early development stages and reflect higher cost structures. PEC systems have among the highest CapEx (USD 3500–4500/kW) and LCOH (USD 6.0–8.0/kg H₂) values, largely due to low solar-to-hydrogen conversion efficiencies and short device lifetimes [156]. Similarly, biological pathways, such as dark fermentation and photofermentation, exhibit a CapEx of USD 2500–4000/kW and an OpEx of USD 2.2–3.5/kg H₂, producing hydrogen at USD 4.5–8.0/kg H₂ [157,158]. These routes remain economically uncompetitive at current technology-readiness levels.

Plasma gasification and hydrothermal gasification, though less conventional, show promise for waste valorization and wet biomass processing, respectively. Plasma gasification systems have a CapEx between USD 3000–3800/kW and an LCOH ranging from USD 5.5 to 7.0/kg H₂, with high electricity demand as a limiting factor [29]. Hydrothermal gasification offers an LCOH of USD 4.0–6.0/kg H₂, supported by a CapEx of USD 2000–3000/kW and an OpEx of USD 2.0–2.8/kg H₂ [159], but requires further R&D to improve catalyst stability and reaction efficiency.

Microbial electrolysis cells (MECs) represent a hybrid platform for simultaneous hydrogen production and wastewater treatment. Although still under research, reported CapEx values (USD 2500–3500/kW) and OpEx values (USD 2.2–3.0/kg H₂) suggest an LCOH of USD 5.0–6.5/kg H₂ [160], making them suitable for niche applications where integrated wastewater remediation is valued.

Collectively, this analysis highlights a clear cost–performance hierarchy. SMR and coal gasification remain the most cost-competitive under current economic conditions, while electrolysis powered by renewables is approaching competitiveness, with long-term cost decline trajectories. Most biological and photochemical pathways, although environmentally attractive, require substantial cost reductions, particularly in capital expenditures (CapEx) to become feasible. Future deployment will likely depend on policy incentives, scalability investments, and co-product valorization strategies to bridge the gap between cost and sustainability.

7. Global Hydrogen Production Policies: Current Landscape and Future Directions

Hydrogen is increasingly recognized as a critical vector in global decarbonization strategies. Governments worldwide are advancing policies to scale up hydrogen production, infrastructure, and market integration, particularly as a part of their commitments to the Paris Agreement and long-term net-zero emission targets. While current hydrogen production remains heavily reliant on fossil-based methods, principally steam methane reforming (SMR), future policy frameworks are shifting focus toward low-carbon and renewable hydrogen pathways, including those derived from biowaste and other circular economy feedstocks.

7.1. Global Policy Landscape and Strategic Frameworks

At the international level, hydrogen has become a focal point in climate and energy diplomacy. Multilateral bodies, such as the International Energy Agency (IEA), the International Renewable Energy Agency (IRENA), and the Hydrogen Council, have developed roadmaps emphasizing the role of hydrogen in energy transition scenarios. The IEA’s Net Zero by 2050 roadmap projects global hydrogen demand to more than double by 2030 and to reach 530 Mt by 2050, with at least 50% derived from low-carbon sources [161]. Concurrently, IRENA emphasizes the integration of green hydrogen—produced via electrolysis powered by renewables—and biomass-derived hydrogen as a pathway for both emission reduction and energy security [162].

Many countries have adopted national hydrogen strategies with clearly defined production goals, investment frameworks, and technology priorities. As of 2024, over 50 countries have released or are developing hydrogen roadmaps [163]. These strategies are broadly aligned with three priorities: (1) reducing the carbon intensity of hydrogen production, (2) scaling up domestic manufacturing and infrastructure, and (3) developing export capabilities in response to emerging international demand.

7.2. National and Regional Policy Mechanisms

The European Union (EU): The EU’s Hydrogen Strategy, launched in 2020, aims to produce up to 10 million tonnes of renewable hydrogen annually by 2030, underpinned by the REPowerEU plan and Fit-for-55 legislative package [164]. The strategy emphasizes green hydrogen but acknowledges the transitional role of low-carbon (blue) hydrogen. Key instruments include the Innovation Fund, Hydrogen Bank, and Contracts for Difference (CfDs), alongside regulatory support through the Renewable Energy Directive (RED III).

Germany has committed over EUR 9 billion to its National Hydrogen Strategy, prioritizing electrolysis capacity, R&D, and international hydrogen trade corridors. France, Spain, and the Netherlands have similarly invested in gigawatt-scale renewable hydrogen projects and biogenic hydrogen from biomass and waste [165].

The Asia–Pacific Region: Japan and South Korea are early hydrogen adopters, with strategies emphasizing fuel cell technology and hydrogen imports. Japan’s updated Basic Hydrogen Strategy (2023) sets a target of 20 Mt of hydrogen consumption annually by 2050, supporting both gray and green hydrogen sources in the near term [166]. South Korea’s Hydrogen Economy Roadmap targets 6.2 Mt of hydrogen consumption by 2040 and supports green hydrogen derived from biowaste, hydropower, and solar.

China, the world’s largest hydrogen producer, is transitioning from coal-based hydrogen to low-carbon pathways. The national Hydrogen Energy Industry Plan (2021–2035) sets goals for 100,000–200,000 tonnes of renewable hydrogen production annually by 2025, with growing emphasis on electrolysis and biomass gasification [167].

North America: The United States has adopted a technology-neutral approach with the Inflation Reduction Act (IRA, 2022), offering substantial tax credits, under Section 45V, of up to USD 3/kg H₂ for clean hydrogen production based on lifecycle emissions. The U.S. National Clean Hydrogen Strategy and Roadmap (2023) outlines targets to produce 10 Mt/year by 2030 and 50 Mt/year by 2050, with pathways including biowaste conversion, methane pyrolysis, and renewable electrolysis [168]. Similarly, Canada’s Hydrogen Strategy prioritizes regional production hubs, with attention to biomass-rich provinces and circular economy models.

7.3. Policy Support for Biowaste-to-Hydrogen Technologies

Although most national strategies focus initially on electrolysis and SMR with CCS, there is growing policy attention toward biowaste-to-hydrogen pathways due to their dual benefits of clean energy generation and waste valorization. In the EU, RED III classifies advanced biohydrogen under renewable fuels of non-biological origin (RFNBOs), eligible for renewable energy quotas and CfDs [169]. Germany’s Bioeconomy Strategy further integrates biohydrogen from agricultural residues and municipal waste streams.

The U.S. Department of Energy (DOE) has funded several biohydrogen research and development (R&D) initiatives under its Bioenergy Technologies Office (BETO), supporting the dark fermentation, microbial electrolysis, and hydrothermal gasification of food waste and sewage sludge [170]. The California Low-Carbon Fuel Standard (LCFS) also provides a favorable market signal for biowaste-derived hydrogen by crediting its carbon intensity reduction value.

In developing economies, particularly in Sub-Saharan Africa, Southeast Asia, and Latin America, donor-funded initiatives and development banks (e.g., the World Bank, GIZ, and ADB) are increasingly promoting biowaste-to-hydrogen solutions to address both energy poverty and environmental degradation from unmanaged organic waste. However, these markets still face policy gaps in terms of subsidies, regulatory standards, and off-take agreements for biohydrogen. The studies included in this review varied significantly in design, methodology, and reporting, which imposes several limitations on the interpretation of the results. First, there was substantial heterogeneity in feedstock types, conversion technologies, and operating conditions, making direct comparisons difficult. Second, many studies lacked consistent reporting of key metrics, such as hydrogen yield units, standard deviations, or confidence intervals, limiting the ability to assess statistical reliability. Third, most of the evaluated technologies, particularly biological and electrochemical routes, remain at the lab or pilot scale, reducing the external validity and generalizability of findings to commercial settings. Additionally, geographic clustering of studies in certain regions (e.g., China, India, and the EU) may reflect regional policy drivers and infrastructure contexts, limiting the transferability of conclusions globally. Finally, potential publication bias could not be ruled out, as studies with favorable outcomes are more likely to be published in peer-reviewed journals.

8. Challenges and Future Research Directions

The transition to a sustainable hydrogen economy necessitates intensive research and development of biowaste-to-hydrogen technologies that efficiently utilize biomass, organic residues, and industrial waste. These technologies span thermochemical, biochemical, physicochemical, and hybrid processes. Future research must focus not only on overcoming technical bottlenecks but also on aligning environmental, economic, and societal dimensions with circular economy principles.

8.1. Technological Prospects

8.1.1. Thermochemical Routes

Gasification remains a cornerstone of thermochemical hydrogen production. Future research should prioritize the development of tar-resistant, sinter-resistant, and coking-tolerant catalysts to ensure long-term operational stability. Advanced reactor designs that enable better syngas cleanup and heat integration (e.g., dual fluidized beds) are also critical. The integration of renewable energy sources, such as solar-driven steam generation, can substantially reduce the carbon intensity of gasification processes. Supercritical Water Gasification (SCWG), with its unique capacity to treat wet biomass without drying, warrants scale-up studies focusing on corrosion-resistant materials, reactor pressure control, and feedstock preconditioning to mitigate salt deposition and char formation.

Plasma Gasification, a high-temperature, electricity-intensive process capable of handling toxic and heterogeneous waste, offers unique advantages in hazardous waste management and hydrogen-rich syngas generation. Research should focus on reducing energy input through plasma-assisted catalysis, improving the quality of syngas, and developing lifecycle models to assess energy and material efficiency.

Pyrolysis, while widely explored for bio-oil and char production, still faces challenges in maximizing hydrogen yield. Future studies should investigate catalytic and non-catalytic approaches for in-situ steam reforming of pyrolysis vapors. Solar-assisted pyrolysis and microwave-based systems are promising for reducing external energy demand. Additionally, the design of continuous pyrolysis reactors and integration with downstream hydrogen separation systems (e.g., membrane reformers) can improve scalability and efficiency.

SMR of Biogas, especially when derived from anerobic digestion of organic waste, offers a partially renewable route to hydrogen. Future innovation should focus on CO₂ removal and sulfur-resistant catalysts to improve SMR efficiency. Coupling with carbon capture and storage (CCS) and renewable heat sources (e.g., solar concentrators) could render the process near-zero-emission.

8.1.2. Biochemical Routes

Dark fermentation (DF) continues to receive attention due to its simplicity and ability to process complex waste streams. Research is required to develop genetically engineered strains (e.g., via CRISPR/Cas9) with enhanced tolerance to pH shifts, inhibitors, and high hydrogen partial pressures. Pretreatment strategies, such as ultrasonic, enzymatic, and freeze–thaw methods, can improve hydrolysis rates and substrate availability. Additionally, optimizing reactor hydrodynamics (e.g., packed bed and fluidized bed) can significantly enhance productivity.

Photofermentation (PF) can convert volatile fatty acids and organic acids to hydrogen using photosynthetic bacteria. Major research gaps include low light conversion efficiency and oxygen sensitivity. The development of narrow-band energy-efficient LEDs, engineering of oxygen-tolerant strains (e.g., via synthetic biology), and novel photobioreactor configurations (e.g., internal illumination and optical fiber distribution) are critical to advancing this route.

Biophotolysis, involving green microalgae and cyanobacteria to split water using sunlight, represents a long-term strategy for direct solar hydrogen production. Research must overcome barriers such as low hydrogenase activity, oxygen inhibition, and photoinhibition. Genetic engineering of hydrogenase enzymes, hybrid photobioreactor systems, and improved light-harvesting antenna designs is pivotal to improve feasibility.

Microbial electrolysis cells (MECs) are bioelectrochemical systems that can produce hydrogen from wastewater under mild conditions. Ongoing work is needed to reduce capital costs by developing low-cost high-performance electrode materials (e.g., carbon felt with graphene or metal oxides), reducing internal resistance, and integrating photovoltaic systems for off-grid operations. Modular MEC units should be designed for integration into municipal and industrial wastewater treatment facilities.

8.2. Environmental Prospects

To ensure long-term sustainability, WTH technologies must be aligned with environmental objectives. In situ carbon capture using solid sorbents, such as calcium oxide (CaO) or magnesium-based materials, should be incorporated into high-temperature reactors to reduce CO₂ emissions. In SCWG and SMR systems, quantifying net emissions in various operational scenarios, including the use of renewable heat and power, is essential for full carbon accounting. As the hydrogen economy advances, the results suggest that policy instruments must be aligned with lifecycle climate performance. While current subsidies and policy frameworks often support all electrolysis methods equally, only electrolysis powered by renewable electricity truly offers a zero-carbon solution [171].

Feedstock optimization is also vital. Priority should be given to locally available waste resources, such as agricultural residues, food waste, sewage sludge, and municipal solid waste, which exhibit lower carbon footprints compared to those of energy crops. Co-processing of lignocellulosic waste with other organic fractions can balance carbon-to-nitrogen ratios and improve process yields. In addition, waste-to-hydrogen pathways, including plasma gasification, hydrothermal gasification, and MECs, should be prioritized for funding and infrastructure development. These methods not only reduce emissions but also enhance resource recovery and support circular economy principles.

Lifecycle assessments (LCAs) must be expanded to cover underrepresented systems, such as PF, MECs, plasma gasification, and biophotolysis. Integrated systems (e.g., DF-MEC-AD and SCWG with CCS) should be evaluated for their energy return on investment (EROI), water use, land use, and GHG emissions to facilitate comparison with conventional hydrogen routes.

The technologies with the lowest energy use, SMR, and coal gasification are also the most carbon-intensive unless coupled with CCS, which can reduce emissions by 50–90% but at significant energy and cost penalties [97]. Electrolysis, while energy intensive, offers the cleanest pathway when powered by renewables. Its scalability, however, hinges on the availability of low-cost, low-carbon electricity and improved electrolyzer efficiency. In contrast, biomass-based and microbial methods strike a balance between energy use and sustainability.

Biomass and waste feedstocks are renewable and often carbon neutral or negative when combined with CCS. Nonetheless, these methods face barriers in terms of feedstock logistics, process complexity, and limited technology maturity. The high energy demands of PEC and biological photolysis processes make them unsuitable for near-term deployment, although they represent important areas for long-term R&D. Technologies like MEC offer niche applications with promising environmental profiles, particularly in wastewater-integrated systems.

Water use is becoming a defining sustainability metric for hydrogen technologies, especially in regions experiencing water stress. While electrolysis is often heralded for its decarbonization potential, its moderate-to-high water demand must be weighed carefully in deployment strategies. Conversely, low-water-use technologies, like SMR or methane pyrolysis, are less constrained by water but require rigorous emission mitigation.

Emerging systems, such as MECs and hydrothermal methods, show promise for circular economy applications, integrating waste management and hydrogen production. Yet their high water-requirements emphasize the need for innovations in water recycling and decentralized deployment models. Technologies using organic or wet feedstocks should be evaluated on a case-by-case basis, factoring in water availability, feedstock logistics, and local infrastructure.

Table 4. Summary of challenges and future directions.

Pathway	Prospect	Challenges	Future Research Directions
Biomass Gasification	Technological	Tar formation, catalyst deactivation, and feedstock variability	Low-temperature gasification, tar-resistant catalysts, and carbon capture integration
	Environmental	GHG emissions, toxic byproducts, and ash handling	Lifecycle assessment (LCA), clean gas handling, and ash utilization
Methane Pyrolysis	Technological	High energy demand, material degradation, and reactor design complexity	Renewable heat sources, robust reactors, and biogenic methane use
	Environmental	Fossil methane dependency and thermal emissions	Biogas feedstock and green electricity input
Hydrothermal Gasification	Technological	Corrosion, salt deposition, and energy efficiency	Anti-corrosion materials and energy recovery systems
	Environmental	Effluent treatment and CO2 emissions	Closed-loop water reuse and process intensification
Plasma Gasification	Technological	Expensive operation and plasma torch durability	Scalable systems, cost reduction, and decentralized design
	Environmental	Emission control and slag disposal	Advanced gas cleaning and vitrified residue applications
Dark Fermentation	Technological	Low H2 yield, microbial instability, and byproduct inhibition	Microbial engineering, hybrid fermentation, and pretreatment of food waste
	Environmental	Acidic effluents and methane co-production	pH control systems and optimized reactor management
Photofermentation	Technological	Low light efficiency and large surface area needed	High-efficiency photobioreactors and improved photosynthetic organisms
	Environmental	Light source requirement and oxygen sensitivity	Use of solar-driven systems and oxygen-tolerant strains
Sequential Fermentation	Technological	Synchronizing processes and intermediate management	Integrated bioreactors and kinetic modeling
	Environmental	Complex resource use	Combined LCA and water–energy balance optimization
Extraction	Technological	Low hydrogen recovery efficiency and solvent issues	Green solvent systems and integrated recovery
	Environmental	Chemical waste and energy use	Sustainable extraction protocols
MEC (Electrolysis)	Technological	High capital cost, membrane fouling, and low current density	Durable electrodes, anti-fouling strategies, and renewable voltage integration
	Environmental	Energy demand and methane leakage	Solar/wind integration and methanogen suppression
Biophotolysis	Technological	Low H2 rates, O2 inhibition, and enzyme degradation	Engineered algae/cyanobacteria and protective photoreactor designs
	Environmental	Land and water use and fragile ecosystems	Compact bioreactor design and eco-friendly media
SMR (Steam Reforming)	Technological	CO2-intensive and fossil dependency	CCS integration, biogas-based SMR, and reforming catalyst innovation
	Environmental	High GHG emissions and natural gas reliance	Low-carbon feedstock reforming
Transesterification	Technological	Limited direct H2 production and waste glycerol	Integrated H2–glycerol valorization and enzyme catalysts
	Environmental	Chemical waste from catalysts	Eco-friendly biocatalysis and biorefinery integration

shows a summary of the key challenges associated with the current technologies and outlines potential future directions for advancing hydrogen production from organic biomass.

The advancement of a sustainable hydrogen economy requires policies that reflect the environmental performance of technologies throughout their entire lifecycle. Currently, policy frameworks often treat all electrolysis technologies equally, regardless of their source of electricity. As emphasized by Guerra et al. [172], only those powered by renewable electricity provide genuine zero-carbon solutions.

Lifecycle-based GHG thresholds should be established for all publicly supported hydrogen initiatives to prevent greenwashing and ensure that investments drive true sustainability [96]. Additionally, prioritizing funding for waste-to-hydrogen pathways, such as MECs, plasma, hydrothermal gasification, dark fermentation, and photofermentation, can yield multifaceted benefits by reducing waste, lowering emissions, and supporting resource circularity.

As illustrated in Table 3 and Figure 10, Figure 11, Figure 12 and Figure 13, no single hydrogen production technology currently meets all the environmental, economic, and scalability criteria. Fossil-based methods remain dominant but are environmentally untenable without CCS. Renewable electrolysis is the most promising from a decarbonization perspective, but requires significant infrastructural and efficiency improvements. Biomass and microbial pathways strike a feasible intermediate balance, while advanced photodriven and fermentation technologies remain important future R&D targets.

Incorporating dark fermentation and photofermentation into the strategic outlook enhances the diversity and adaptability of hydrogen production systems. Dark fermentation offers decentralized, low-tech solutions suitable for organic waste management in rural and peri-urban areas. Photofermentation, though technically challenging, holds promise in integrated systems where sunlight and wastewater streams are co-optimized. These technologies can complement more mature pathways by addressing niche applications and enhancing the resilience of the hydrogen supply chain. Thus, a strategic and location-specific mix of hydrogen production technologies, informed by local resource availability, policy frameworks, and environmental priorities, is essential for a sustainable global hydrogen transition.

8.3. Social Acceptance

Public perception of using waste as an energy feedstock varies by region and application. Concerns often stem from the association with pollution, odor, or proximity to residential zones. Acceptance is generally higher when WtH is framed within a circular economy narrative and co-located with existing municipal waste facilities. Public education, stakeholder engagement, and demonstration projects can play vital roles in increasing community buy-in.

8.4. Regulatory Barriers

In many jurisdictions, regulatory frameworks are either outdated or narrowly focused on traditional energy systems, lacking clear pathways for certifying hydrogen derived from waste. This ambiguity can stall permitting, funding, or utility integration. For example, classification of hydrogen as “renewable” may exclude WtH routes, depending on the feedstock origin. Harmonized standards, certification schemes, and lifecycle-based GHG accounting methods are needed to ensure technology-neutral access to policy support mechanisms. Furthermore, inter-agency coordination among waste management, energy, and environmental regulators is necessary to streamline approval and integration processes.

Addressing these economic, social, and regulatory dimensions is crucial for transitioning WtH technologies from lab-scale innovation to commercial deployment. Multi-stakeholder collaborations involving governments, industries, researchers, and communities will be essential to de-risk investments and promote inclusive, sustainable adoption.

8.5. Future Policymaking Recommendations

To unlock the full potential of hydrogen from biowaste, integrated and technology-specific policy frameworks are essential. These should include:

• Carbon pricing and credit mechanisms that reward low-carbon and circular hydrogen production, including from biogenic sources;
• Dedicated investment support for demonstration and scalable projects using biomass gasification, pyrolysis, and fermentation routes;
• Standardized certification systems to track origin and lifecycle emissions of hydrogen, enabling international trade in “green” and “bio-based” hydrogen;
• Inclusion of biowaste-to-hydrogen pathways in national waste management policies, linking clean energy goals with circular economy mandates.

Overall, the policy landscape for hydrogen is rapidly evolving. While early efforts have centered on electrolysis and CCS-based reforming, there is a growing recognition that biowaste-to-hydrogen pathways offer a unique convergence of sustainability, waste reduction, and decentralized energy production. Policymakers should thus ensure that future hydrogen roadmaps and investment frameworks accommodate and actively promote these emerging conversion routes.

9. Conclusions

This review underscores the growing importance of biowaste-to-hydrogen (WtH) technologies as critical components of sustainable energy transitions and integrated waste management systems. By systematically evaluating thirteen conversion pathways—including thermochemical, biochemical, electrochemical, and hybrid processes—the study provides a comprehensive synthesis of the current technological performance, feedstock compatibility, energy efficiency, scalability, and environmental impacts.

Among the technologies reviewed, hydrothermal gasification and dark fermentation emerge as particularly promising for valorizing high-moisture organic waste, offering favorable hydrogen yields while minimizing the need for energy-intensive preprocessing. Steam methane reforming, though fossil-based, remains technologically mature and serves as a benchmark for comparison. Meanwhile, innovative approaches, such as microbial electrolysis cells and sequential fermentation systems, hold future potential, especially within decentralized and circular bioeconomy models.

From an environmental perspective, WtH technologies utilizing organic and municipal waste demonstrate markedly lower greenhouse gas emissions and energy consumption than conventional fossil-derived hydrogen production. However, widespread deployment remains limited by challenges related to scalability, economic feasibility, and the technological maturity of biological and hybrid systems. Moreover, this review highlights data limitations, such as the reliance on fixed-yield assumptions and the lack of large-scale operational data, which constrain robust technoeconomic and lifecycle assessments.

To overcome these barriers, future research should focus on catalyst innovation, process intensification, hybrid system integration, and the development of dynamic, site-specific lifecycle assessment frameworks. Strategic prioritization of technology pathways—based on local feedstock characteristics, infrastructure readiness, and environmental policy landscapes—will be essential to maximize the roles of WtH systems in achieving global decarbonization and circular economy goals.

Ultimately, advancing biowaste-to-hydrogen technologies offers a dual opportunity: decarbonizing hydrogen supply chains and transforming organic waste liabilities to renewable energy assets. This synthesis provides a critical foundation for guiding future R&D, investment, and policymaking toward commercial feasibility and long-term sustainability.