Organic Waste-to-Hydrogen via Anaerobic Digestion and Steam Reforming

Organic Waste-to-Hydrogen via Anaerobic Digestion and Steam Reforming

Abstract

The global imperative for decarbonized energy systems has positioned hydrogen (H₂) as a pivotal clean fuel, yet its predominant production from fossil sources emits ~830 million tonnes of carbon dioxide (CO₂) annually. The integrated anaerobic digestion and steam reforming (AD-SR) pathway offers a sustainable alternative, converting organic waste—such as food scraps, agricultural residues, municipal solid waste (MSW), and livestock manure—into H₂ via methane-rich biogas. This review critically evaluates AD-SR, synthesizing over 50 studies from 2022-2025 to assess process efficiencies (50-70% H₂ yield), technological advancements, and challenges. Anaerobic digestion (AD) degrades organic matter into biogas (50-70% methane, CH₄), while steam reforming (SR) catalytically transforms biogas into H₂-rich syngas, achieving yields of 0.1-0.3 kg H₂/kg dry waste. Key optimizations include nickel (Ni)-based catalysts for cost-effective SR, steam-to-carbon ratios >2 to mitigate coking, and hybrid reforming for CO₂ utilization, enhancing yields by 10-20%. Challenges encompass biogas impurities (e.g., hydrogen sulfide, H₂S, poisoning catalysts), high energy demands for SR (~800°C), and economic hurdles, with levelized cost of hydrogen (LCOH) at 1.5-5 USD/kg. Environmental benefits are significant, reducing greenhouse gas (GHG) emissions by 70-90% compared to fossil H₂, with potential negative emissions via carbon capture and storage (CCS). Case studies, such as food waste processing in China (0.15 kg H₂/kg) and manure in Canada (8.11 kg H₂/h), demonstrate feasibility. Future directions emphasize AI-driven process control, low-temperature electrocatalytic reforming, and policy incentives like carbon pricing to bridge economic gaps. This review underscores AD-SR’s potential as a scalable, green H₂ production route, advocating for interdisciplinary research and regulatory support to achieve net-zero goals and circular waste management.

Keywords: Hydrogen Production, Anaerobic Digestion (AD), Steam Reforming (SR), Organic Waste, Biogas, Sustainability, Carbon Capture and Storage (CCS), Levelized Cost of Hydrogen (LCOH), Greenhouse Gas (GHG) Emissions, Circular Economy.

1.     Introduction

Hydrogen (H₂) has assumed a pivotal role in the global pursuit of sustainable energy systems, functioning as an adaptable zero-emission energy carrier capable of decarbonizing hard-to-abate sectors such as heavy industry, long-haul transportation, and power generation (Elazab et al., 2025). According to the International Energy Agency (2024), world hydrogen consumption exceeded 97 million tonnes (Mt) in 2023 and is expected to draw to 100Mt in 2024, although mainly due to conventional uses in refining and the chemicals business to date. However, more than 99 percent of this demand is met using fossil fuel-derived hydrogen that emits roughly 830 Mt of carbon dioxide emissions per year in an amount comparable to the total emissions by the United Kingdom and Indonesia combined. In the IEA Net Zero Emissions by 2050 Scenario (NZE Scenario), the need to produce hydrogen with low emission intensity in a cost-effective way that is climate-consistent leads the demand to soar in 2030 (130 Mt) and 2050 (more than 500 Mt) necessitating a paradigm shift to low-emission production routes.

Though policy frameworks have spurred investment, especially through the European Union and the U.S. delegated acts and the U.S. Inflation Reduction Act, the pace of implementation has been slow and regulatory uncertainties still prevail; thus, project development has been delayed. These delays risk undoing the efforts to meet high goals with which these frameworks are aimed at tackling (Stokes, 2024; Kasyanenko et al., 2025).

The need to have sustainable production of hydrogen is also compounded by the fact that the organic waste management sector is in a crisis which is increasing by the day. Globally, food waste alone generates 9.3 billion tonnes of CO₂-equivalent (GtCO₂e) emissions annually, accounting for 8-10% of anthropogenic greenhouse gas emissions comparable to the total emissions from the U.S. and EU combined in 2017 (Dwyer, 2023; Rai et al., 2025).

According to the Food Waste Index Report 2024 by the United Nations Environment Programme (UNEP), 1.05 billion tonnes of food is estimated to be discarded each year, with households contributing 60 % (631 Mt), food services 28 % (290 Mt) and retail 12 % (131 Mt). Such waste increases the amount of methane produced by landfills that produce between 8 and 10 % of greenhouse gas emissions globally. In addition to the environmental damage, it wastes resources that measure 45 trillion gallons of water and territory that is larger than the surface area of China (UNEP, 2024). In the United States the main source of methane is organic waste, with 66.2 Mt of food wastes generated in the year 2019, 40 % generated at household and 60 % in services and retail. At the same time, food waste has both social justice and economic implications as there are around 800 million people struggling with hunger and one-third of the generated food, which would cost 400- to 500-billion dollars, is going to waste.

Transforming some type of organic waste, including food scraps, agricultural residues, sewage sludge, and livestock manure to hydrogen through integrative approaches, e.g., anaerobic digestion (AD) followed by steam reforming (SR), has a twofold value: wasted materials are valorised and low-emission energy is obtained. The AD-SR pathway utilizes microbial degradation in AD to produce methane-rich biogas (typically 50–70 % CH₄, 30–50 % CO₂) that is subsequently hydrogenated in SR, yielding H₂-rich syngas via the endothermic reaction. Such an approach corresponds to the concepts of a circular economy and Sustainable Development Goal (SDG) 12.3 to reduce food wasted by 50 % by 2030 (Mazzanti et al., 2025).

CH₄ + H₂O ⇌ CO + 3H₂ (ΔH = +206 kJ/mol), often coupled with water-gas shift

(WGS: CO + H₂O ⇌ CO₂ + H₂) for enhanced yields (Zhu et al., 2021).

The integrated synthesis of biogas via anaerobic digestion (AD) and hydrogen via steam-reforming (SR) has achieved an operational efficiency of 50-70 % for H₂ production, with H₂ yields ranging from 0.1 to 0.3 kg H₂ per kg of dry waste, as demonstrated by studies undertaken with food waste in China and manure in Canada (Seglah et al., 2023). There is still, however, significant impediments. Biogas impurities, especially hydrogen sulphide (up to 5000 ppm) may poison catalysts; and higher temperature ranges (700-900) require large energy inputs; and economic viability depends on levelized costs of hydrogen (LCOH) which range between 2 and 5 USD/kg with the scale and subsidy levels determining viability. Recent progress, such as the hybrid steam–dry reforming of biogas-derived CO₂, has improved carbon utilisation by 10-15 % while diminishing coking risks (Werkneh, 2022). A critical appraisal, however, indicates that although AD-SR reduces GHG emissions by up to 80 % relative to fossil-based hydrogen, lifecycle assessments (LCAs) frequently overlook emissions from feedstock collection and carbon management downstream, potentially overstating environmental benefits (Bird et al., 2011).

Nishimura et al. (2024) and Chattanathan et al. (2014) examined the combination of AD and biogas reforming, achieving hydrogen yields of 2.5-3.5 mol H₂ per mol CH₄ using Ni-based catalysts; nonetheless, catalytic deactivation by biogas impurities was problematic. In South Korea, AD–SR systems configured from food waste demonstrated conversion efficiencies of 60-70 %, yet scalability was constrained by seasonal feedstock variability (Rawoof et al., 2021). That paper also emphasised the energy intensive character of SR and the necessity to pretreat feedstocks to increase digestibility. To maximize conversion and energy recovery, AD has been studied as a biohythane production method, but no studies have comprehensively explored its capability to integrate with injection in order to enable potentially overlooked synergistic negative emissions opportunities via carbon capture. Usoro and Umoh (2025) assessed animal waste steam reforming, reporting efficiencies comparable to natural-gas reforming (85-95% CH₄ conversion), but highlighting heightened impurity challenges in biogas feeds.

This review undertakes a critical synthesis of recent developments in AD-SR for H₂ production. The analysis will analyse the following: the fundamental aspects of a process with a specific focus on kinetics and thermodynamics; examining the issues related to integration, especially mitigation of impurities and energy balances, an analysis of the effect of the feedstock, the comparative yield analysis, economic and environmental metrics through LCAs and TEAs, and proposed optimisations, including control through AI and hybrid reforms. The review further highlights that, although AD-SR possesses large-scale potential capable of supplying approximately 16 Mt H₂ equivalent by 2030, as reported by several announced projects the disparity between the announced capacities (520 GW electrolysis equivalent) and the few projects that have achieved FIDs (only 20 GW) underscores the imperative for accelerated policy intervention to address current demand–supply mismatches.

Anaerobic digestion (AD) refers to a biological process that involves converting of complex organized substances into biogas in an oxygen-free environment. Such a metabolic cascade is comprised of four mutually dependent stages hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Achinas et al., 2020). Polymers which are carbohydrates, proteins, and lipids are dissolved by extracellular enzymes to monomeric form during hydrolysis. In the acidogenic stage, these released constituents are metabolised into volatile fatty acids (VFAs), alcohols, hydrogen (H₂), and carbon dioxide (CO₂). VFAs are subsequently converted to acetate and further H₂ and CO₂ in the acetogenic phase. The final methanogenic phase, via the acetoclastic or hydrogenotrophic pathway, produces methane (CH₄) from acetate or from the reduction of H₂ and CO₂, respectively. This whole process is reliant on syntrophic microbial consortia with a high sensibility of environmental change with methanogens being extremely sensitive. Overall AD performance for biogas production from organic solid waste (OSW) ranges from 0.3 to 0.6 m³/kg volatile solids (VS), with biogas typically containing 48–65 % CH₄ and 36–41 % CO₂ (Harirchi et al., 2022). Nonetheless, yeast downstream accumulation can suppress methanogenesis, thereby attenuating yields by up to 50 %.

2. Anaerobic Digestion for Biogas Production from Organic Waste

The composition of the feedstock dominantly controls the performance of AD. Food waste, characterised by elevated volatile solids content (70–90 %), often affords superior yields (0.4–0.6 m³/kg VS) through rapid degradability. The low carbon-to- nitrogen (C/N) ratio of this material 15 or less, however, entails the prospects of ammonia inhibition and unstable process. Conversely, agricultural residues such as crop straw require extensive pretreatment, including alkaline hydrolysis or thermal pyrolysis, to disrupt lignocellulosic structures and thereby enhance biogas yield by 20 – 50 % while simultaneously elevating operational expenditure (Bong et al., 2018). Adequate nutrient balance and indigenous inocula can be provided by livestock manure, which has yield ranges of 0.2 - 0.4 m³/kg VS, but is limited in digestibility by its high lignin content. Heterogeneous mix, Municipal OSW, median yields of 0.30 to 0.50 m/kg VS, and sorting was required to remove inorganic material and inhibitory compounds (Kadam et al., 2023). Co-digestion, in particular, food waste and manure, is becoming a viable option on the one hand to get carbon-to-nitrogen ratios within the desirable limit of 20 to 30, making it friendly to methanogenic processes in the one hand and synergistic in terms of nutrients on the other (Mhlanga et al., 2023). An illustrative case is the report by Mhlanga et al. (2023) that demonstrates combined cafeteria waste and cow dung achieving 435 mL/g VS with 73% CH₄ production. Nevertheless, variable input quality introduces a degree of uncertainty, capable of producing output fluctuations ranging from 10 % to 30 % (El-Mashad & Zhang, 2007).

The operation relies on narrowly regulated process parameters to be efficient. A stable pH level of 6.8-7.2 favours stable methanogenesis; a drift chain towards acidic can jeopardise the growth of microbes and inevitably, digestion (Wani & Parveez, 2025). Temperature regimes likewise exert a major influence on digestion kinetics: thermophilic (50 to 60 °C) conditions enhance process rates by approximately two- to threefold, yet they simultaneously elevate energy demand by 20 to 30 % and heighten the risk of ammonia toxicity.

The productivity in terms of volumes of biogas is also sensitive to hydraulic retention time (HRT) that should be enough to allow the best growth of microbes without greatly sacrificing the economic viability. The satisfactory range of HRT is between 15 to 30 days and feeding loads higher than 5 kg VS/m³/day might jeopardize the quality of methane due to substrate overloading (Meyer, 2010).

Innovative designs particularly dual-stage anaerobic digestion has been introduced to decouple acidogenic and methanogenic phases in order to maximise H₂ production (Akindolire et al., 2022). Under this configuration, the first stage produces H₂-rich gas at a rate equivalent to approximately 2 mol H₂/mol glucose, albeit at the cost of overall CH₄ yields owing to methane prioritisation in second-stage reactors; consequently, the total H₂ recovery is sufficiently modest to warrant further engineering refinements.

Research remains limited on biogas composition, yet impurities such as H₂S (0 to 5000 ppm), siloxanes, and ammonia can become inhibitory when present in significant concentrations, underscoring the necessity for effective upstream purification strategies prior to biogas utilisation in solid-reactor systems. Recent advances employing machine-learning methodologies, including tree-based predictive models derived from eight years of industrial data, have been shown to enhance process stability, thereby reducing operational upsets by approximately 15 to 25 % (Vali et al., 2023).

Lastly, adopting or not wet digestion will influence spending both capital and operating costs. Dry digestion operating at total solids concentrations exceeding 20 % offers significant water-savings benefits but is commonly constrained by reduced diffusion rates; consequently, dry systems typically achieve methane production that is 10 to 20% lower than their wet counterparts (Eduok, 2015; Alalawy et al., 2025).

Table 1: Biogas Yields from Various Organic Wastes

Figure 1 illustrate the anaerobic digestion pathway following the four major step of hydrolysis, acidogenesis, acetogenesis and methanogenesis

Figure 1: Anaerobic digestion pathways

Source: Rea (2014)

3. Steam Reforming of Biogas for Hydrogen Production

The steam reforming (SR) of the biogas, which here is a methane-rich gas produced by the anaerobic fermentation of organic waste (Singh et al., 2024), is a renewable process to produce hydrogen that relies on the exothermic reaction leveraging the reaction CH₄ + H₂O ⇌ CO + 3H₂ (ΔH = +206 kJ/mol) followed by the water-gas shift (WGS: CO + H₂O ⇌ CO₂ + H₂, ΔH = -41 kJ/mol) to yield H₂-rich syngas (typically 70-75% H₂ on a dry basis) (US EPA, 2019).  In this regard, biogas steam reforming is significantly different in classical natural-gas reforming due to its higher CO₂ content (30-50 %), which can act as a diluent and also allow hybrid reactor systems, e.g. bi- and tri-reforming (steam and dry: CH₄ + CO₂ ⇌ 2CO + 2H₂, ΔH = +247 kJ/mol) and tri-reforming (an oxidative addition of O₂ that can promote partial oxidation), potentially improving carbon utilization by 10-20% while reducing coking (Ingole & Dhawale, 2021).

Thermodynamically, steam reforming is endothermic and, therefore, temperatures of 700 to 900 °C and low (5 bar or less) pressure conditions are preferred since they facilitate the optimal production of hydrogen by a shift in the equilibrium. Moreover, replacing steam-to-carbon ratios higher than 1.5 does not allow carbon deposition to be done using the Boudouard reaction (2CO ⇌ CO₂ + C) or the cracking of methane (CH₄ ⇌ C + 2H₂) (Urrejola et al., 2011). Such variable composition of biogas 50-70 % methane poses some operational challenges; namely, elevated CO₂ weakens reactants and hence reduces the conversion of methane by 5-15% as compared to pure methane feeds. Therefore, high steam-to-carbon ratios usually need to be above two (Bharathiraja et al., 2018) in order to generate conversion efficiencies of 85 to 95 %. Measurements in experiments support them: at 773 K and a ratio of S/C of 2, a membrane reactor having Pd-Ag foam structure in Ru-Ni showed 74% methane conversion, 95% hydrogen recovery, and an overall yield of 55%, leaving an exergy efficiency of 85 % and a final purity of more than 99.999 % hydrogen (Habib et al., 2021). Though the syngas carrier gases, hydrogen, and methane, have varying molar volumes and densities, their specific energy intensity during the SR (0.048 to 0.075 MWh/kg H₂) is of similar value to that of dry reforming (0.072 to 0.079 MWh/ kg) (Singh et al., 2025). This comparison proves the effectiveness of SR, but also proves that process heat recovery and heat integration are needed.

The technological heart of Steam reforming (SR) is catalytic systems, and nickel-based oxide carriers (in many cases Ni/Al₂O ₃) remain in economically dominant for cost-effectiveness and activity, yet susceptible to deactivation via sintering (>600°C), poisoning (H₂S >5 ppm), and coke formation (Wu et al., 2020). The activity sequence of single-metal catalysts is Rh > Ru > Ni > Ir = Pd = Pt > Co > Fe, but nickel is low cost so it is optimized. Additions of promoters like La or Mg enhance dispersion and coke suppression leading to up to 50-70 % reductions; 5 % CeO₂ in Ni/CeO₂ -Al₂O₃suppressed coke down by 70 % (Kim et al., 2020). Bimetallic formulations (Ni-Mg, Ni-Co, Ni-Ru) provide a potential synergistic effect, NiCeSnRh/Al₂O₃ being one of them that proved to be more immune to sulphur than a similar (monometallic) formulation (Nogales-Delgado et al., 2023). Synthesis route is also very important. The activity level of wet impregnation is often very high but subject to the problem of agglomeration, and most likely to achieve uniformly dispersed phases to enhance durability, sol-gel synthesis is generally used (Yergaziyeva et al., 2021; Tang et al., 2025). Although such noble metals as Rh can yield coke and surface-poison resistance with exemplary performances, their 10-100-fold increased costs preclude viable scale-up, which is another reason why efficient Ni-based advancements are critical.

Recently, microwave plasma SR has been developed with a current output of 239 g H₂/h at 36.8 g /kWh yield and 74.3% CH₄ conversion of a synthetic biogas feed consisting of 70 wt % CH₄ (Hrycak et al., 2023). This mechanism exceeded dry reforming by about 1.5-fold in the situation of least H₂O addition to reduce soot formation. However, the overall energy payoff of 40 g/kWh was lower than DOE targets (60 g/kWh) thus pointing to the limitation of the scalability that manifests itself in relation to the low power level used (10 kW). Control of impurities is also significant; efficient removal of H₂S to levels of <0.1 ppm through ZnO or other forms of desulfurization processes reduce Ni-poisoning activities and siloxanes require adsorption by activated carbon (Fan et al., 2010). Pressure swing adsorption (PSA) or Pd-containing membranes are typically used in downstream purification to produce H₂ with purity of >99.9 % but this purification carries energy costs of 10 to 20 % (Duncan et al., 2014). Figure 2 is a line chart showing yield (mol% vs. temperature (100-900°C)

Figure 2: Reforming reaction pathways

Source: Bion et al. (2010)

4. Integrated AD-SR Systems Process Design and Optimisation

A combined anaerobic digestion (AD) and SR system constitutes a closed-loop system that can be used to produce hydrogen out of organic waste with AD generating biogas (e.g., food waste or manure) which is upgraded using SR. The overall efficiency associated with this setup is usually 50 to 70 % with an output of 0.1 to 0.3 kg H₂ /kg dry waste (Park et al., 2020). Activated development Process often includes AD reactor, biogas clearing, (desulfurisation, dehumidification), SR unit, wet-dry hybrid SR, methanation (where available) and hydrogen separate by pressure swing adsorption (PSA) or membrane capsules. The SR exhaust (800-900^oC) is used as a source of heat in the preheating of AD digester (35-55^oC) thus eliminating 20-30 % of energy requirement. Nevertheless, variations in operating regimes of the two stages can provide difficulties: AD runs either in the batch or semi-continuous modes, whereas SR runs continuously. Therefore, biogas storage buffers are needed to stabilise feeds, lack of which can lead to losses in efficiency of up to 10-15 %.

There are various parameters in optimisation. Mesophilic (35^o C) or thermophilic (55 ^oC) AD optimises the biogas production (0.4-0.6 m³/kg VS of food waste), but requires steady-state operating conditions, while SR has best results at steam to carbon ratio (S/C = 2-3) and a reaction temperature of 900 ^oC to achieve high purity and hydrogen yields that are additionally boosted. Hybrid dry reforming is a technology that adds biogas-CO₂ and increases hydrogen production by around 15 % and makes it responsive to net-negative emissions in combination with carbon dioxide capture and storage (CCS). In Aspen Plus simulation studies, even a small change in the methane concentration in biogas can produce high change in hydrogen production; that is, a difference of 10% produces an 8-12% difference in hydrogen composition. The dry reforming in combination with optimal psychrophilic AD (20 ^oC) is expected to produce 55 mol% H₂ in manure (Kopac & Demirel, 2025). According to Seglah et al. (2023), 2073 kt H₂ can be recovered through food waste (2007-2030), equivalent to the potential to produce 3728 GWh electricity and to avoid 17% of national electricity demand and prevent 3060 kt CO₂ eq (8% of emissions).

According to the empirical techno-economic modelling, at carbon capture and storage (CCS), the production of H₂ using biomethane steam reforming (SR) has a levelized cost of 1.84 – 2.88 euros/kg that compares to grey hydrogen in the non-tax scenario (1.47 euros/kg) and to 2.92 – 3.84 euros/kg with consideration of carbon taxes (Yagüe et al. 2024). Biomethane SR with CCS is superior to hydrogen electrolyzed or natural gas synthesized through the steam reforming process comparatively either when there is a CO₂ tax or through the use of fossil fuels as feedstocks in such latter processes.

Following the case of animal wastes, empirical case studies indicate that the general decreasing trend of the H₂ yield as a result of the type of the feedstock including poultry manure (400.8 L/ day of 100 m³), cattle (396 L /day of 100 m³), and swine (381.6 L/day of 100 m³). The related capex costs are estimated to be about 7 to 10 million US dollars; full-scale production costs are estimated to be US$0.074/kWh equivalent. At these estimates, NPV would be US$659 million and benefit cost ratio 3.43. It is estimated that decentralized operations will alleviate logistical pressures but will raise operating and maintenance cost by about 15 to 20%. Waste streams management and manure using co-digestion schemes have been observed to increase nutrient equilibrium, hence driving H₂ production by between 20 % and 30 % (Olugasa et al. 2025).

Significant issues in the operation pertain to propensity of feedstocks to contaminants such as H₂S that rapidly undermine catalytic activity and must be remedied promptly; and to the overall energy balance, which is normally poor and can be addressed by upstream purification (CH₄ purity 92 % post-scrubbing) and autonomously-generated control measures (AI charge reduction by 15 %) (Werkneh 2022). A biogas SR reference plant in Portugal operated under zero-emission conditions when available renewable electricity meets 100% of total energy required to support all aspects of the broad process; under carbon-negative conditions biogas SR can thus compete with electrolysis at least (Werkneh 2022).

Table 2: Integrated AD-SR Efficiencies

Theoretically, integrated AD-SR may scale up to 1-10 kt H₂/yr, but the commercial opportunities are still relatively low because of the variability of feedstock or the added cost of CCS integration (about 20 %) (Yagüe et al., 2024). In this respect, policy tools that implement financial incentives are a critical factor of the viability of the sector.

5. Feedstocks and Their Impact

Feedstock choice is a critical parameter that governs the performance of the AD-SR pathway to hydrogen production, adjusting both the production of biogas and its composition, and the efficiency of the resulting syngas reforming process. Organic wastes also have a distinct variation in their biochemical composition (carbohydrates, proteins, lipids and lignocellulosic) that determines digestibility, methane (orange about 50-70 %), and trash such as H₂S or siloxanes that can force extra post processing to preserve SR catalysts (Gotore et al., 2025). The food waste is described as highly moist (70-90 %) and VS (80-95 %) forms of waste ideally degradable, however, they can ultimately lead to acidification as a result of low C/N (<15), therefore, decreasing methanogenesis and lowering biogas yields by 20-30% unless co-digested. Conversely, agricultural residues that are rich in lignocellulose (e.g., crop straw) are much more abundant (worldwide ~3 billion tonnes/year) yet need to undergo pretreatment (i.e., thermal hydrolysis at 150-180 °C) to open up recalcitrant structures thereby increasing yields by 25-50% at rates of 0.5-1 MJ/kg (Wang et al., 2023). Animal manure presents well-balanced nutrients (C/N 15-25) and inherent microbial inoculation, providing stable process of AD, however, its high content of fibres results in reduced biogas production (0.2-0.4 m³/kg VS), which can be replenished by co-digestion with food waste by 30 to 100%. The sewage sludge, extracted at the wastewater treatment facilities (WWTPs), despite being highly rich in organics, also contains heavy metals and pathogens and, therefore, its H₂S levels (up to 2000 ppm) also demand effective desulfurization before its use in SR. The passive LFG (40-60 % CH₄) is also an AD output that exhibits a similar composition variability based on waste age and thus is considered incompatible with meeting consistent SR feedstock demands, but can support high methane conversion in simulations (up to 99 %) (Li et al., 2021).

Seglah et al. (2023) determine the feedstock-specific yields, as well as environmental assessments. In Ghana, a scenario analysis extrapolates 2073 kt H₂ over 2007-2030 using ADSR: annual production increases to 119 kt by 2030 with the potential to meet 17% of national electricity demand, so long as collection logistics are addressed. Variations between animal wastes are significant: poultry manure has the highest amount of H₂ /day (400.8 L) 100m³, which is even more than cattle (396 L) and swine (381.6 L): this can mainly be attributed to higher amounts of VS. Its efficiencies are near-high temperature natural-gas SR levels (85 to 95 % CH₄ conversion) and are lower or equal to other streams of biomass. Biomass feedstocks are used in cow manure where psychrophilic AD at 20 o C is combined with dry reforming to achieve 8.11 kg H₂/h with 48 kg/h biogas at 72.85 % effectiveness, which exceeds the steam processes (Hajizadeh et al., 2022). Transformation of landfill-gas with SMR attains 0.14-0.19 kg H₂ /Nm₃ at S/C = 3; yields are higher using CH₄: CO₂higher ratios (60:40), but energy intensities are higher (0.048-0.075 MWh/kg H₂) also. There is process instability associated with feedstock heterogeneity, especially in the regional differences in food-waste composition. Higher fat wastes such as high-fat wastes increase H₂S and precipitate Ni catalysts in hours, which highlights the necessity of the pretreatment. It is thus vital to pretreat, and co-digest: alkaline pretreatment of sludges yields higher biomethane (40%) and manure and food-waste mixing yield better C/N (2030) and CH4 (75%) (Elalami et al., 2020). Pretreatment however can also involve environmental trade-offs, including metal build up in sludge that restricts digestate use. Altogether, food waste gives high yield (100 to 200 gH₂/kg dry) but needs the careful control over the inhibitors, whereas manures are more stable at operation with lower output (50-150 g/kg dry). Therefore, site-specific evaluations continue to play critical roles in the viability of AD-SR.

Table 3: H₂ Yields and Impacts from Key Feedstocks

Figure 3 illustrate feedstock biochemical compositions

Figure 3: Feedstock composition

Source: Seglah et al. (2023)

Critically, while diverse feedstocks enable regional adaptability, inconsistent quality demands advanced monitoring (e.g., AI prediction with 98% accuracy for yields), as variability can reduce overall efficiency by 10-20% (Seglah et al., 2025).

6. Challenges and Limitations

AD-SR pathway is still faced with complex limitations on technical, economic, environmental, and operational aspects, hence, inhibiting it to be extensively implemented in commerce. The first shortcoming is seen in technology as Ni-based catalytic membranes are prone to biogas impurities, such as the presence of hydrogen sulfide (H₂S, 0 - 2000 ppm in biogas produced by sludge). The chemisorption of H₂S decreases catalytic activity by 50-80 percent and requires minimum desulphurization to <0.1 ppm through ZnO beds or activated carbons, which increases OPEX by 10-15 percent (Jellali et al., 2021). Simultaneously, Boudouard or C-C cracking products (coke) have a distinct potential of blocking reactors and deactivating catalysts (e.g., loss of 70% activity in 100 h), which, however, is alleviated by high S/C ratios (>3) as well as inflating energy requirement (0.075 MWh/kg H₂) (Meng et al., 2007). The required refractory construction and large heat input (must be supplied using the biogas combustion, usually 750 to 950 °C) requires high processing temperatures (750 to 950 °C), which provide net conversion efficiencies in the range of 40 to 50 % (not including CO₂recovery) (Meng et al., 2007). The activity of methanogenesis can be stopped by the process instability that can be manifested by VFA accumulation when food waste is fed into the system, or pH reduction in case of high loading (Zimmermann et al., 2013). High temperatures exacerbate feedstock variability as well due to seasonal compositional variations in manure leading to CH₄ concentration variability by 10 to 20 % and thus disrupting AD-SR balance, and inorganics ingress in heterogeneous municipal wastes (García-Gómez et al., 2021).

The financial limitations are hampering. The AD-SR pathway is not competitive in most unsubsidized markets due to high CAPEX (e.g. a 274-kg/year H₂ plant using animal waste requires a 10million USD investment) and the LCOH high (1.39-2.88 USD/kg) is not competitive until its BCR becomes above 4 (i.e. the payback of an investment is in excess of eight years with a BCR of 3.43 at large scale) (Seglah et al., 2025). An example would be a study analysing food waste in Ghana, which showed favourable net present value (i.e., $659M) and suggested collection/transport charges and policy inaccuracies as the key obstacles to feasibility (Seglah et al., 2025).

There are further complications to this in environmental terms. Negative emissions required due to biogenic CO₂emissions attributed to LFG SR (4.94 to 13.28 kg/kg H₂) add extra costs to the process of CCS, increasing overall costs by additional 20-30 % (Grobelak et al., 2024). There is also the introduction of digestate management which brings about metals leached by sludges in the soil (Grobelak et al., 2024). There is an observable difference between anaerobic digestion (AD) and steam reforming (SR) based on their batch and continuous operating modes, posing difficulty in attaining scalability. The scale-up of AD will require the substantial storage to cater to the temporal changes in feedstock supply, resulting in up to 1-5 % of methane losses and related emission of greenhouse gases (GHG). According to analytical models, it is feasible to boost carbon dioxide use in a hybrid scheme that consists of dry reforming and steam reforming by approximately 15% and reduce coking tendency (Hajizadeh et al., 2022). Similar progress on membrane reactors with a palladium catalytic surface has allowed operation at less severe conditions (400 to 600 ^oC), at which CO₂ purities exceeding 99.9 % are considered; however, the sensitivity of those reactor systems to hydrogen sulphide poisoning is an operational issue. The latest report by Seglah et al. (2025) examines the potential of catalytic material like Ni-CeO₂ that reduces the coke formation by 70% and the implementation of artificial intelligence to optimize the process and hence increase the accuracy of prediction to 98%. Lifecycle assessment (LCA) analyses often overlook the upstream logistics of the collection of wastes collected, which can underestimate the total footprint between 15 and 25% (Singh et al., 2025). In turn, policy interventions, especially carbon taxes, are needed to address the shortfalls of using anaerobic-digestion-based-hydrogen production and to propel an international collective use of hydrogen produced by anaerobic digestion.

Table 4: Key Challenges in AD-SR Systems

7. Case Studies and Examples

Hydrogen production via the AD-SR pathway of organic wastes has been shown to be practical with a range of pilot- and full-scale plants globally providing relevant data on operation, profile performance and scalability. Most of these case studies focus on anaerobic digestion biogas production and its upgrade by successive stages to renewable natural gas (RNG) or direct reforming to hydrogen-a configuration that highlights both synergistic opportunities and practical challenges both on the ground. It is important to note that even though methane-rich biogas is the most important commodity in energy use, the addition of steam reforming to hydrogen is still at an infant stage and limited most of the time by the cost of the catalyst as well as controlling the impurities levels.

Case Study 1: New York State Anaerobic Digestion Projects for Organic Waste Valorisation

Several anaerobic digestion plants are operating anaerobic of organic material resulting in the production of RNG in New York which could in turn be converted to hydrogen through the process of steam reforming. The Newtown Creek Water Resource Recovery Facility, located in Brooklyn, was subjected to a massive revamp early in the year 2023. It now co-digests 5 % city food waste and sewage sludge, upgrading biogas to RNG which is injected into the National Grid to heat over 5,000 homes. With the production of 400,000 MMBTU/year of RNG, the facility could avert up to 600,000 tons of GHG emissions per year, in case the facility is city-wide. Although with an AD efficiency of 60 - 70 % as methane content, steam reforming is not integrated, predictive modelling suggests a H₂ production of 0.15 - 0.2 kg/kg dry waste at 800 °C with nickel catalysts, but due to the H₂S content of sludge (up to 1000 ppm), ZnO desulfurization is required and introduces additional costs of 10 to 15 % (Jenkins et al., 2025). An analogous is the Regional Digester in Buffalo, with 45,000 tons/year of food wastes converted at farms and retailers to the RNG to power 900 houses, and digestate to feed 1,000 acres of farmland.

Biogas yields are traditionally reported in standard cubic meter, though not per MMBTU, but are roughly 0.31-0.5 m³ biogas/kg VS, especially when it comes to the extraction of the methane produced in landfills (Erraji et al., 2023). However, the pronounced seasonal variations in waste stream would require proper storage capacities when anaerobic digestion is combined with the gas purification units.

Case Study 2: Biological Hydrogen Production from Cheese Whey and Tofu Waste via Fermentation

This generation of biological hydrogen using waste cheese whey and tofu is a good example of production by fermentation-based production. By optimizing the pH between 5.5 and 6.5, Patel et al. (2016) used Clostridium sp. IODB-O3 on cheese whey (5 % lactose) to obtain 6.35 mol H₂ / mol lactose, better than before. It proved to have a high level of efficiency in removing COD and minimal expenditure in operating at dairy processing. Direct dark fermentation processing gave significant yields, but sequential processing of residual biogas followed theoretically could increase hydrogen production overall by a further 20 % although fermentative VFA buildup severely limits scale. Similarly, Lay et al. (2013) mixed tofu-processing waste effluent and sludge (20 % v/v) to achieve 2.3 mol H₂ /mol glucose, hydraulic retention time of 4 h and 12 L H₂ /L/day and helped in recycling wastewater and reducing CH₄ emissions. However, the economic viability depends on such combination measures that suppress the inhibitory impact of VFAs and reformed hydrogen with a low-cost of merely 3-4 USD/kg has been forecasted at the optimal condition.

Case Study 3: Industrial-Scale Hydrogen from Waste Molasses in China

In China, a large-scale production of hydrogen out of the waste molasses is another case study. Ren et al. (2006) had a conversion rate of biogas of 347 m3/day after 120 days operation period with 0.7-0.75 m3/kg COD removed. Unit output exceeded conventional biogas plants, the capital investment of 21 million Chinese yuan because high metabolically intensive processes were fermented and the high operation costs were entailed. Concurrent SR of co-generated methane has the potential to boost the H₂ yield by 30-40 %, but early investment and the need to develop reuse of highly advanced wastewater highlight the cost barrier, especially in developing markets. The positive effect on the environment is the mitigation of CO₂ by upcycling of waste.

Case Study 4 -Boulder Colorado Waste-to-Energy through AD and Proposed Reforming

National Renewable Energy Laboratory (NREL) modelling of Boulder reveals a 78,300-tonne per year residual municipal solid waste (MSW) stream which is processed in an ADcombustion hybrid, delivering 45 million kWh a year (5.6 MW installed capacity) of the fuel at a treatment cost of US58 tonne (Funk et al., 2013). This pathway results in up to 25 million kg/yr reduction of fossil carbon dioxide emissions as compared to conventional coal-fired power plant electricity generation. The AD process also negates the landfill-generated methane emissions to a mere 600 kg per annum as opposed to an estimated 677,000 kg per annum of methane that will be produced at a conventional landfill site.

8. Economic Analysis and Environmental Analysis

The AD -SR pathway has significant potential promise regarding sustainable production of hydrogen, but thorough lifecycle assessments (LCAs) and techno-economics analyses (TEAs) produce several strengths and shortcomings. The technology essentially reduces overall GHG emissions by quite a significant amount, posing a 70 - 90 % decrease compared to fossil hydrogen, due to biogenic carbon capture. Still, upstream collection systems and downstream CO₂mitigation processes are still absolutely necessary. In economic terms, the estimates of LCOH have shown range between 1.5 to 5 USD/kg of hydrogen, which is competitive with the current regimes of subsidization and limited by the high capital expenditures.

LCAs show that per kilogram of hydrogen produced using AD-SR, there is a footprint of environmental impact of 1 -3 kg CO₂e, which is significant compared to the conventional grey hydrogen, which has the environment to take 10 - 12 kg CO₂e per kg. Research suggests that AD will, in Boulder, result in an annual avoidance of 20-25 million kg of fossil carbon dioxide with an insignificant amount of associated NOx and SO₂ ancillary emissions (Funk et al., 2013). The proposed AD plants in New York City, at a larger scale to prevent 600,000 tonnes of GHG emissions annually, exemplify a potential that the given technology holds, although the pre-treatment necessary to eliminate siloxanes and H₂S (so the gas could be purified to be removed) comes with the energy costs, estimated at about 5-10 % (EPA, 2023a). The low water demand only needs 0.5- 1 m³ /ton of waste and thermophilic AD can raise the amount by about 20 %.

Economic Analysis

The estimated LCOH (scale >100 t/day) of TEAs is 1.5 to 3 USD/kg, the total capital expenditure cost is between 1 and 2 M USD/MW and the operation expenditure between 0.5 and 1 USD/kg H₂. Treatment cost in Boulder is 58/ton and the capital cost 310/ton/year and O&M cost 65/ton/year is 30 years (Funk et al., 2013). The economies of scale are paramount, so small-scale can boost LCOH to 4-6 USD/kg since it is more costly; 21M CNY in molasses H₂ investment was associated with increased yields compared to biogas H₂. With subsidies, payback = 5-10 years and NPV = positive ($659M) (Funk et al., 2013).

Table 4: Environmental and Economic Metrics from Recent Studies

While environmentally superior, economic parity requires carbon pricing (>50 USD/ton CO₂); hybrid models with CCS could lower LCOH by 20-30%.

9. Future Perspectives and Conclusions

Recent developments in the area of catalyst development in the field of process engineering have shown the potential to reduce coke formation seriously as well as to increase sulphur tolerance. Specifically, bimetallic Ni-Co and Ni-CeO₂formulations have been proven to suppress coke formation by 70-80 % and catalytic lifetimes to exceed 200 hours. With the utilisation of biogas CO 2, hybrid reforming (steam + dry) can achieve H₂ yields increases of up to 15-20 %, along with the opportunity to produce negative emissions by combining carbon capture and storage (CCS) (Hajizadeh et al., 2022). At the same time, low-temperature reforming techniques, such as microwave plasma steam reforming, which are capable of running at 74.3% CH₄ conversion and 400-600 ^oC process temperatures, can cut the energy supply by ca. 20 % compared to conventional steam reforming at 800-900 ^oC. However, extension of microwave systems beyond 10 kW is a major challenge (Authors not specified, 2023). In electrocatalytic reforming, the possibility of energy savings is further opened up as long as the process can be practiced below 500 ^oC, where commercial viability is established; although at current efficiencies around 50 % it is below the efficiencies that can be achieved with thermal steam reforming. Lastly, two-stage anaerobic digestion with an anaerobic sub-reactor to decouple H₂ -producing acidogenesis and methanogens may have the advantage of further boosting H₂ production, up to 2 mol H₂ /mol glucose, easing synergy with thermal steam reforming to produce biohythane.

Such processes as digitalization and optimization with the use of tools such as artificial intelligence (AI) and machine learning (ML) entail a significant improvement in the level of control and monitoring of processes. ML models trained with a history of eight years of industrial AD information based on trees have an accuracy of 98% to predict the biogas outcomes thus reducing upsets and making the process of regulating OLR/ pH easy. Monitoring variability in real-time of feedstocks, e.g. CH₄ shifts of 10-20%, stabilizes input streams to provide higher SR performance, ultimately increasing H₂ production by 8-12% (Singh et al., 2025). Aspen Plus process simulations and computational fluid dynamic (CFD) process simulations optimize heat integration, pre-heating the AD digesters with the SR exhaust and reduce energy requirements by 20-30 % making the net efficiency requirements in excess of 60-70 %. Although combining membrane reactors (e.g. Pd-based) to in-situ H₂ separation would raise the total yields to 80-90 %, the effectiveness of such solution under conditions of H₂S-impregnated regime remains a topic of increased research.

The economics cost of H₂ production depend on a decrease in the levelized cost of H₂ (LCOH, which is 1.5-5 USD/ kg) must beat the cost of grey H₂ (1-2 USD/ kg). Prices on carbon costing more than 50 USD/ton CO₂ or an equivalent subsidy to the U.S. Inflation Reduction Act 45V tax credit (max. 3 USD/kg low-carbon H₂) price could reduce payback times on half (5 -7 years). Large scale (>100 t/day) waste stream feedstock use also contributes to LCOH reductions by 20-30 % through economies of scale as seen in projections to Ghana (2073 kt H₂ by 2030). The ratio of global investment in low-emission H₂ (520 GW equivalent capacity announced) already considerably surpasses the final investment decision (20 GW), which is the reason to implement policies to fill the gap. Both adoptions can be induced by such regional frameworks as the EU RED III requiring 42 % renewable H₂ in industry by 2030, especially in the regions with high resources of waste feedstocks.

In spite of the business potential, there are still some drawbacks. Feedstock variability conditions (such as seasonal fluctuations of methane) and contaminants (up to 5000 ppm H₂S) require heavy pretreatment and purification, at an extra cost of about 10 - 20% per cent. The process of digestate management also poses a question on contamination of soil with metal remnants, which needs regulation. Although CCS integration can allow negative emissions, it also adds another 20- 30 per cent to the cost of capital and thus requires low-cost, benign sorbents or downstream use options, one example being conversion of CO₂ to methanol. The issue of social acceptance and the infrastructure relating to H₂ distribution in both the developed and developing countries also falls behind the technological development.

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11. Abbreviations:

·       AD: Anaerobic Digestion

·       SR: Steam Reforming

·       H₂: Hydrogen

·       CO₂: Carbon Dioxide

·       CH₄: Methane

·       MSW: Municipal Solid Waste

·       H₂S: Hydrogen Sulfide

·       LCOH: Levelized Cost of Hydrogen

·       GHG: Greenhouse Gas

·       CCS: Carbon Capture and Storage

·       VS: Volatile Solids

·       WGS: Water-Gas Shift

·       PSA: Pressure Swing Adsorption

·       C/N: Carbon-to-Nitrogen Ratio

·       HRT: Hydraulic Retention Time

·       OLR: Organic Loading Rate

·       LCA: Lifecycle Assessment

·       TEA: Techno-Economic Analysis