The chemistry at the heart of the study is fermentation, the same family of microbial processes that turns sugars into alcohol or yogurt into existence. In dark fermentation, hydrogen-producing bacteria break down complex organic matter into hydrogen, organic acids, and alcohols without needing light or oxygen. Photo fermentation, the second stage, uses photosynthetic bacteria that capture light energy to generate ATP, which they then spend to ferment the organic acids left behind by the first stage into more hydrogen. Each stage alone is inefficient, converting only a fraction of the feedstock into gas, but the two-stage combination recovers substantially more hydrogen because the fatty acids that would otherwise accumulate as waste in dark fermentation become the fuel for the second act. This sequential logic formed the backbone of the experimental design.

The team collected fresh leachate from the Al Multaqa landfill in Al Amerat, Oman’s first engineered landfill, which receives roughly 200 cubic meters of leachate per day, along with household food waste and sewage sludge from a local treatment plant to serve as the microbial inoculum. Characterization of the leachate revealed a chemical oxygen demand of about 43,000 milligrams per liter and a biochemical oxygen demand of roughly 27,000 milligrams per liter, values indicating a highly biodegradable, organic-rich liquid drawn from a young landfill cell. Iron and magnesium concentrations fell within ranges previously reported as favorable for hydrogen-producing microbes. The food waste was composed of about 50 percent fruit and vegetables, 25 percent protein, and 20 percent carbohydrate, a composition typical of Omani kitchen discards, which collectively represent more than 50 million Omani rials in wasted value each year.

Eight experimental schemes probed the variables that matter most. Four tested different ratios of landfill leachate to food waste, from pure leachate to pure food waste, under dark fermentation at 30 degrees Celsius and a pH of 5.5. The sweet spot turned out to be a 75:25 leachate-to-food-waste blend, which yielded 2.507 milliliters of hydrogen per liter of medium at a rate of 0.021 milliliters per liter per hour, roughly double the output of either feedstock alone. The explanation lies in the carbon-to-nitrogen ratio: leachate brings abundant nitrogen that supports microbial growth, while a modest dose of carbon-rich food waste supplies the electrons that ultimately end up in hydrogen gas. Acetic and propionic acids dominated the fatty acid profile of the best-performing scheme, confirming that the microbial community was operating along productive metabolic pathways rather than being stalled by acid accumulation.

Pretreatment emerged as a make-or-break variable. When the researchers skipped the heat treatment of the inoculum and feedstock, hydrogen production fell to zero even though fatty acid concentrations soared, a signature of acid accumulation poisoning the hydrogen-producing metabolism. Similarly, omitting the centrifugation and sterilization of the dark fermentation effluent before feeding it to the photo fermentation stage cut hydrogen output and left a heavy load of unconverted acids. In the single-stage experiments using only food waste, smaller medium volumes outperformed larger ones: a scheme with 40 percent more substrate and inoculum produced 28.74 percent less hydrogen per liter, echoing earlier reports that smaller reactors avoid substrate inhibition. Hydrogen purity in the collected gas ranged from under 5 percent to over 60 percent depending on the blend and treatment regime.

With technical feasibility established at bench scale, the team scaled the concept to a hypothetical plant processing 50 cubic meters of leachate per day alongside collected food waste, using literature benchmarks of 200 to 400 milliliters of hydrogen per liter for dark fermentation and 700 to 1000 milliliters per liter for photo fermentation. Four plant configurations were modeled: two-stage fermentation with full pretreatment, two-stage with treatment only before the photo stage, two-stage with treatment only before the dark stage, and single-stage dark fermentation of food waste alone. Plant capacities ranged from 2.7 to 12.13 kilograms of hydrogen per day, and capital costs clustered around 25.5 to 25.8 million US dollars, dominated by bioreactors and gas compression equipment.

The economics told a striking story. All four scenarios produced a positive net present value over a ten-year lifetime at a 5 percent discount rate, but Scenario 2, the two-stage system with treatment applied only before photo fermentation, came out on top with an NPV of 45.06 million US dollars and cumulative revenue of roughly 80.65 million dollars. Revenue flows from four streams: hydrogen sold at approximately 12 dollars per kilogram, carbon dioxide captured during gas separation, biofuel derived from volatile fatty acids, and compost recovered from the microfiltration stage. Operating costs were driven primarily by energy and maintenance, with heating vessels alone consuming 45.6 percent of total energy demand. Notably, the feedstocks themselves cost nothing; in fact, facilities can collect tipping fees for accepting waste, a structural advantage over electrolysis plants that must purchase purified water and electricity.

Sensitivity analysis showed the discount rate and hydrogen price exert the strongest influence on profitability, while carbon pricing barely moves the needle. A 30 percent increase in the discount rate dropped Scenario 2’s NPV to 41.63 million dollars, while a 30 percent decrease lifted it to 48.90 million. The Oman results sit within a global spectrum of biohydrogen economics, from a modest 4.7 million dollar NPV for rice straw fermentation in Malaysia to industrial-scale biomass systems in Bangladesh and Nigeria exceeding 480 million dollars, suggesting that revenue diversification through biorefinery integration is what separates marginal projects from compelling ones.

The authors are candid about limitations. The experiments were designed as a proof of concept without replicates, and the economic model relied on literature-based hydrogen prices rather than localized Omani market data. Pilot-scale validation will be essential before commercial deployment, since hydrogen productivity at industrial scale often diverges from laboratory performance. Still, the broader implications are hard to ignore: waste streams that cost municipalities money to manage can be converted into a zero-emission energy carrier, compost, and biofuel, aligning circular-economy practice with national climate goals. As global hydrogen strategies mature, the study argues that waste-derived biohydrogen deserves formal recognition within green hydrogen frameworks, supported by feedstock-based subsidies, carbon credit mechanisms, and standardized purity certification. If Oman’s landfills can become hydrogen factories, the model could travel well beyond the Gulf.

Subject of Research: Techno-economic feasibility of biohydrogen production from landfill leachate and food waste via dark and photo fermentation in Oman

Article Title: Economic feasibility and optimization of biohydrogen gas production- a case study in Oman

Article References: Barghash, H., Farai, S. A., AlRashdi, Z., & Okedu, K. E. (2026). Economic feasibility and optimization of biohydrogen gas production- a case study in Oman. Results in Engineering, 32, Article 113232. https://doi.org/10.1016/j.rineng.2026.113232

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113232

Keywords: biohydrogen, landfill leachate, food waste, dark fermentation, photo fermentation, techno-economic analysis, Oman Vision 2040, circular economy, waste-to-energy, net present value, renewable energy, Al Multaqa landfill

The process at the heart of the review is dark fermentation, a biotechnology in which anaerobic bacteria dismantle organic matter, sugars, and waste streams such as food waste, palm oil mill effluent, and agricultural residues, in the absence of light and oxygen. During this microbial dismantling, hydrogen gas is released as a byproduct of the bacteria’s energy metabolism. The appeal is obvious: the feedstock is often waste that would otherwise rot in landfills, the reaction proceeds at ambient to moderately warm temperatures without light energy, and the gas produced is pure energy carrier rather than a carbon-containing fuel. Yet the technology has struggled to escape the laboratory. Yields remain stubbornly low relative to the theoretical maximum, and the microbial communities that do the work are notoriously sensitive to disturbance, swinging between productive and unproductive metabolic states as conditions drift.

The review’s first pillar is a systematic account of the physicochemical and operational variables that govern microbial performance. pH emerges as perhaps the most decisive lever. Fermentative hydrogen producers operate best in a mildly acidic window, typically around pH 5 to 6, where the hydrogenase enzymes responsible for releasing hydrogen gas remain active and where competing microbes that channel carbon into methane or other reduced products are suppressed. Let the pH drift too low and acid accumulation crashes the culture; let it rise and methanogens invade, consuming the hydrogen that the fermenters worked to produce. Temperature is the second master variable, with mesophilic and thermophilic regimes each selecting for distinct microbial consortia and distinct metabolic end products. Thermophilic operation, the authors note, often supports higher hydrogen yields and faster kinetics, but demands more energy input and careful reactor management.

Beyond pH and temperature, the review examines the characteristics of the inoculum itself, the starting microbial community seeded into the reactor. Heat treatment of seed sludge is a common strategy to kill off hydrogen-consuming methanogens while sparing spore-forming hydrogen producers such as Clostridium species. Organic loading rate, the amount of substrate fed per unit reactor volume per day, must be balanced carefully: push it too high and volatile fatty acids accumulate faster than the microbes can process them, acidifying the reactor and stalling hydrogen production; run it too low and the process becomes economically unattractive. Reactor configuration adds another layer of control. Immobilized-cell systems, upflow anaerobic sludge blanket reactors, packed bed biofilm reactors, and membrane bioreactors all retain biomass more effectively than simple suspended-growth tanks, allowing higher cell densities, faster throughput, and greater resilience to washout during shock loads.

Into this well-mapped landscape, the review introduces its genuinely novel perspective: multifunctional green materials, with hydroxyapatite as the star. Hydroxyapatite, with the chemical formula reflecting its calcium, phosphate, and hydroxide components, is the mineral that gives bones and teeth their rigidity. In a fermenter, it performs several jobs at once. Its surfaces serve as attachment sites for bacteria, promoting cell immobilization and the formation of dense, stable granules that resist washout in continuous operation. Its mineral chemistry buffers pH, absorbing and releasing ions in ways that damp the acidification that so often kills hydrogen-producing cultures. And through the controlled release of calcium and phosphate ions, it appears to support microbial metabolic stability, keeping the community locked into hydrogen-producing pathways for longer stretches of operation.

The evidence for this approach is not merely theoretical. The review’s own authors have published experimental work showing that eggshell-derived hydroxyapatite enhanced thermophilic hydrogen production, with the material promoting the formation of biogranules, compact microbial aggregates in which cells packed closely together exchange metabolites efficiently and maintain favorable microenvironments. Other studies cited in the review report that hydroxyapatite fabrication improved hydrogen output from glucose dark fermentation, and that graphene-hydroxyapatite composites boosted productivity from duckweed biomass. The mechanism is elegantly circular: the material that stabilizes the microbes is itself made from waste, so a discarded eggshell becomes the scaffold on which bacteria convert discarded food into fuel.

How the hydroxyapatite is made matters enormously, and the review devotes detailed attention to synthesis routes. Physical methods include pyrolysis and spray techniques that produce fine, well-defined particles. Chemical routes encompass wet chemical precipitation, sol-gel synthesis, hydrothermal treatment, sonochemical processing, and microwave-assisted reactions, each offering control over crystal size, morphology, and surface area, properties that directly determine how well the material buffers pH and hosts bacterial attachment. Biological and green synthesis routes use plant extracts, microbial activity, or biogenic templates to direct mineral formation under mild conditions, avoiding harsh reagents and high energy demands. Most compelling from a sustainability standpoint are the waste-based routes: eggshells, cow and fish bones, mussel and oyster shells, sea urchin spines, calcified algae, and even industrial byproducts such as phosphogypsum and carbide slag have all been converted into hydroxyapatite. Calcination temperature during processing, the review notes, strongly influences the crystallinity and purity of the final product, and higher specific surface area enhances the release of calcium ions, the very property that makes the material useful in fermentation.

This waste-to-material-to-energy chain is where the review’s conceptual contribution becomes clearest. By linking bioprocess engineering with materials science, the authors construct a framework for what they describe as more efficient, resilient, and environmentally friendly biohydrogen systems. The circular economy logic runs in both directions: organic waste feeds the fermenters, and inorganic waste supplies the additives that make the fermenters work better. Nothing in the chain requires virgin mined minerals or energy-intensive synthetic chemistry if biogenic sources are chosen. The review also situates hydroxyapatite within a broader family of additives, from carbonaceous materials and magnetite nanoparticles to nickel and cobalt ferrites, that have been explored to enhance fermentative hydrogen yields, but argues that hydroxyapatite’s combination of buffering capacity, immobilization support, biocompatibility, and waste-derived availability gives it a distinctive edge.

Challenges remain before the vision reaches industrial scale. The review is candid that low yield and process instability still limit the scalability of dark fermentation, and the integration of a solid additive introduces new variables: optimal dosing, particle size selection, long-term material stability, and the economics of recovering or disposing of spent mineral. Techno-economic assessments of dark fermentation systems generally conclude that profitability hinges on cheap feedstock, high productivity, and valorization of the fermentation effluent, which can itself be converted to biomethane, bioplastics, or microbial lipids in cascading processes. Hydroxyapatite strengthens two of those three pillars, feedstock flexibility and productivity, but the full chain must be demonstrated at pilot scale. The review’s framework, mapping process parameters onto material design, gives researchers a structured way to run those demonstrations, and it signals a broader shift in bioenergy thinking: the microbes may be the engine, but increasingly, the materials around them are the engineering that keeps the engine running.

Subject of Research: Enhancement of dark fermentative biohydrogen production using waste-derived hydroxyapatite additives

Article Title: Integrating process parameters and green hydroxyapatite (HAP) strategies for sustainable dark fermentative biohydrogen production

Article References: Zisha, S. Z., Che Man, H., Omar, R., Abdul Rashid, S., Tan, J. P., Abdul Manaf, S. F., Mumtaz, T., & Jamali, N. S. (2026). Integrating process parameters and green hydroxyapatite (HAP) strategies for sustainable dark fermentative biohydrogen production. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38274-x

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38274-x

Keywords: biohydrogen, dark fermentation, hydroxyapatite, waste valorization, circular economy, bioprocess optimization, cell immobilization, pH buffering, anaerobic digestion, sustainable energy, green synthesis, microbial consortium