The researchers examined two semi-solid olive pomace streams collected from industrial mills in Jordan during the 2025 olive season. Both came from two-phase extraction systems that produced alperujo with approximately 60% moisture. The hot-pressed material underwent malaxation and pressing at about 35 °C with process-water addition, while the cold-pressed material was processed below 27 °C without added heat. Municipal wastewater came from a predominantly domestic-sewage treatment plant in Zarqa Governorate that received no olive-mill or livestock effluent. The researchers combined physicochemical characterization of the substrates with batch digestion experiments and six mathematical models of methane-production kinetics.

Hot and cold pressing produced residues with measurably different compositions. Hot-pressed pomace contained 15.22% lipid on a dry-weight basis, compared with 14.53% in cold-pressed material, and had a higher chemical oxygen demand: 18,480 mg/L rather than 14,212 mg/L. Cold-pressed pomace contained more cellulose, hemicellulose and lignin—36.54%, 21.29% and 17.55%, respectively—than hot-pressed pomace, at 32.60%, 18.53% and 15.10%. Hot-pressed material also contained more measured phenolic compounds, 8.27 mg gallic-acid equivalents per gVS, compared with 6.53 mg/gVS in the cold-pressed residue. These contrasts were associated with the higher yield and faster conversion observed in the hot-pressed treatments.

Adding wastewater changed several conditions that could constrain digestion of the pomace. At a 70:30 olive-pomace-to-wastewater ratio on a volatile-solids basis, wastewater lowered the initial carbon-to-nitrogen ratio of the hot-pressed mixture from 31.89 to 25.12 and the cold-pressed mixture from 33.24 to 26.27. It also reduced the initial oil concentration from 6.8 to 5.1 g/L in the hot-pressed treatment and from 6.4 to 4.7 g/L in the cold-pressed treatment. The final pH remained near neutral at 7.18 for hot-pressed pomace with wastewater and 7.12 for cold-pressed pomace with wastewater, compared with 6.75 and 6.68 in the respective mono-digestions. Wastewater alone produced 195 ± 8 NmL CH₄/gVS in a separate control.

The co-digestion gains were larger than a simple addition of the two substrates could explain. Based on the individual yields and their volatile-solids weighting, the expected additive methane yields were 251 NmL/gVS for the hot-pressed mixture and 234 NmL/gVS for the cold-pressed mixture. The measured yields of 420 and 385 NmL/gVS were 67% and 65% higher, giving synergy indices of 1.67 and 1.65. Biodegradability rose from 53–58% during mono-digestion to 74–79% during co-digestion, while volatile-solids removal increased from 51–55% to 67–71% and chemical oxygen demand removal from 55–58% to 72–77%. The methane content of the biogas also increased from 59.8–61.5% in the mono-digestions to 64.9–66.7% in the mixtures.

Kinetic analysis showed that wastewater co-digestion was associated with faster conversion as well as a higher final yield. The estimated hydrolysis constant increased from 0.072 to 0.127 day⁻¹ for the hot-pressed treatment and from 0.061 to 0.098 day⁻¹ for the cold-pressed treatment. The fitted maximum methane-production rate rose from 15.2 to 28.5 NmL CH₄/gVS/day for hot-pressed material and from 13.8 to 23.1 NmL CH₄/gVS/day for cold-pressed material. Lag periods shortened from 3.5 to 1.8 days and from 4.0 to 2.4 days, respectively. Relative to mono-digestion, co-digestion therefore enhanced the production-rate constants by factors of 1.61–1.88, a larger proportional change than the roughly 1.53–1.54 increase in final yield.

Among the three conventional single-pool models, the Modified Gompertz model described the methane-production curves most successfully. It produced coefficients of determination from 0.958 to 0.980 and the lowest root-mean-square error and Akaike information criterion for every treatment. A dual-pool first-order model performed best for both co-digestion mixtures, reaching R² values of 0.985 and 0.981. Its advantage was consistent rather than decisive because the difference in Akaike information criterion from the Modified Gompertz model was below the study’s threshold of 10 for strong support. The dual-pool structure nevertheless represented two degradability classes, with rate constants that differed by roughly fourfold in the mixtures: rapidly degradable wastewater-derived organics and more slowly hydrolysing pomace material.

The study compared measured co-digestion performance with a volatile-solids-weighted expectation derived from the mono-digestion and wastewater-only results. The yield-based synergy indices were 1.67 for hot-pressed pomace with wastewater and 1.65 for cold-pressed pomace with wastewater. Similar comparisons for kinetic descriptors showed synergy indices of 1.69 and 1.45 for the hydrolysis constants and 2.01 and 1.75 for the maximum methane-production rates. The measured lag phases were 44% and 33% shorter than the corresponding additive expectations. The stronger kinetic than yield-based enhancement suggests that the interaction was expressed principally in the speed of conversion under the tested batch conditions, rather than as a comparable increase in the final extent of digestion.

Three complementary mechanisms were proposed to explain the observed synergy. Wastewater supplied nitrogen and buffering capacity to carbon-rich, weakly buffered pomace digestions; it diluted phenolic and lipid compounds that can inhibit methanogens or disrupt lipid degradation; and it introduced a microbial community that could contribute cellulolytic, lipolytic and fatty-acid-oxidising functions. These explanations were consistent with the simultaneous improvements in pH stability, hydrolysis, methane-production rate and yield, but they were not measured directly. The investigators conducted no 16S rRNA sequencing, metagenomic analysis or enzyme assay, so the study could not quantitatively separate microbial bioaugmentation from nutrient balancing, buffering and inhibitor dilution.

The findings come from batch tests using one co-digestion ratio, an inoculum-to-substrate ratio of 2:1 on a volatile-solids basis and mesophilic incubation at 37 ± 1 °C. The design demonstrates the measured benefit of combining wastewater with olive pomace under those laboratory conditions, but it does not establish the optimal mixing ratio, long-term reactor stability or industrial-scale performance. Strong correlations among ammonia-nitrogen, hydrolysis constants, production rates and fitted methane potential also do not establish causation, because the treatments differed simultaneously in oil content, phenolics, lignocellulosic composition and dilution. Further work would be required to test multiple mixing ratios, continuous-reactor operation, microbial community profiling and sterilized-wastewater controls, as well as to determine the optimal carbon-to-nitrogen ratio and organic loading rate.

Subject of Research: Batch anaerobic co-digestion of hot- and cold-pressed olive pomace with municipal wastewater

Article Title: Effect of hot and cold pressing systems on methane yield and kinetic behavior during anaerobic co-digestion of olive pomace with wastewater

Article References: Alkhrissat, T. (2026). Effect of hot and cold pressing systems on methane yield and kinetic behavior during anaerobic co-digestion of olive pomace with wastewater. Case Studies in Chemical and Environmental Engineering, 14, Article 101507. https://doi.org/10.1016/j.cscee.2026.101507

Image Credits: AI Generated

DOI: 10.1016/j.cscee.2026.101507

Keywords: olive pomace, anaerobic co-digestion, municipal wastewater, methane yield, biogas, hydrolysis kinetics, olive-oil residues, biochemical methane potential, Jordan, wastewater treatment, Effect, cold

The research, led by Gebrehiwot Kunom Hagos of Mekelle University in Ethiopia together with Wondalem Misganaw Golie of the Ethiopian Defense University and Gebremeskel Kahsay Atsbha of Raya University, set out to demonstrate what the authors call a dual-function biorefinery. Rather than treating waste conversion and gas cleaning as separate engineering problems, the team coupled them into a single process chain. Organic waste is first co-digested to produce biogas, and that same gas is then scrubbed through tailored adsorption beds until it approaches the purity of commercial natural gas. The concept is deliberately frugal: every major input, from the feedstock to the sorbent, is a locally generated waste or low-value material.

The feedstock side of the experiment paired cow dung with banana peel, an agricultural residue rich in readily degradable carbohydrates. Anaerobic co-digestion of the two streams was carried out at ambient conditions, and the researchers systematically varied the mixing ratio to find the composition that maximized methane yield. The sweet spot turned out to be a blend of 60 percent cow dung and 40 percent banana peel by volume. Under those conditions the digester produced gas containing 71.23 percent methane, 27.89 percent carbon dioxide, and 1470 parts per million of hydrogen sulfide. That starting composition is typical of agricultural biogas, which is precisely why the team chose it: any upgrading technology that works on this gas should be broadly applicable.

With the raw biogas characterized, attention shifted to the scrubbing stage. The heart of the system is bentonite clay, a naturally abundant smectite mineral whose layered structure and cation-exchange capacity make it a versatile adsorbent. In its raw form, however, bentonite is not especially selective for acidic gases. The researchers modified the clay in two complementary ways. Carbide slag, which consists largely of calcium hydroxide, was used to functionalize the material for hydrogen sulfide capture, exploiting the strong affinity between sulfide species and alkaline calcium sites. A second modification integrated monoethanolamine, a well-known amine absorbent, into the clay matrix to target carbon dioxide. The result is a tailored adsorbent in which the mineral scaffold hosts two distinct chemistries, each tuned to one of the two principal contaminants.

The physical configuration of the adsorption bed matters as much as its chemistry, and this is where the study’s statistical rigor comes in. The team used response surface methodology, a design-of-experiments framework that maps how multiple variables jointly influence an outcome, to optimize the upgrading process. Using a Box–Behnken design implemented in Design-Expert version 13, they explored three numeric factors: clay particle size ranging from 0.1 to 2 millimeters, gas flow rate from 0.1 to 2 liters per minute, and bed height from 10 to 50 centimeters. The responses tracked were the outlet concentrations of methane, carbon dioxide, and hydrogen sulfide, with statistical significance accepted at a probability level below 0.05.

The regression models that emerged from the analysis were strikingly strong. The coefficients of determination reached 99.99 percent for methane, 99.99 percent for carbon dioxide, and 99.80 percent for hydrogen sulfide, meaning the fitted equations explained essentially all of the variation in the experimental data. Such high values indicate that the three chosen factors, and their interactions, capture the behavior of the adsorption column almost completely, which in turn means the model can be used to predict performance across the entire experimental space rather than only at the tested points. For process engineers, that predictability is what turns a laboratory demonstration into a designable technology.

The optimum operating point combined the smallest clay particle size tested, 0.10 millimeters, with a bed height of 30 centimeters and a gas flow rate of 0.10 liters per minute. At those settings the gas leaving the column contained 96.22 percent methane, up from 71.23 percent at the inlet, while carbon dioxide had fallen from 27.89 percent to 3.65 percent and hydrogen sulfide from 1470 parts per million to just 71. The upgraded gas achieved a calorific value of 34.6392 megajoules per cubic meter, a figure that places it firmly in the territory of high-quality biomethane suitable for cooking, electricity generation, and, in principle, injection into gas grids. The trends behind those numbers are physically intuitive: finer particles expose more adsorption surface, a taller bed provides longer contact time, and a slower flow rate gives the gas more opportunity to interact with the reactive sites before breaking through.

What makes the result notable is not merely the purity achieved but the economics implied by the materials involved. Bentonite deposits are widespread and inexpensive, and Ethiopia’s Afar region in particular hosts significant reserves that have already found local industrial applications. Carbide slag is a disposal liability for acetylene plants, and converting it into a gas-cleaning reagent turns one waste stream into the remedy for another. Monoethanolamine is the priciest component, but it is used in modest quantities as an integration agent rather than as the bulk sorbent. Compared with conventional upgrading routes such as water scrubbing, pressure swing adsorption with engineered zeolites, membrane separation, or amine scrubbing in dedicated columns, all of which demand capital equipment and operating expertise, a packed bed of modified local clay represents a dramatically lower barrier to entry for small-scale and rural installations.

The study also situates itself within a broader shift in how biogas is conceptualized. Instead of viewing the digester as a single-purpose machine that turns manure into a mediocre fuel, the dual-function framing treats the whole plant as a biorefinery in which waste treatment, fuel production, and gas purification are integrated stages of one circular process. The digestate left after anaerobic digestion retains fertilizer value, the banana peel that would otherwise rot is converted into energy, and the sorbent bed polishes the product to a standard that unlocks higher-value uses. Each step consumes something the surrounding economy discards, which is exactly the logic that circular-economy advocates have been pressing for in the biogas sector.

Caveats remain, as they do with any laboratory-scale optimization. The experiments were conducted at ambient conditions with controlled flow rates, and real digesters produce gas whose composition and humidity fluctuate with feedstock and season. The longevity of the modified clay, its regenerability, and its performance over extended breakthrough cycles will need to be demonstrated at pilot scale before the approach can be certified for continuous operation. The presence of 71 parts per million of residual hydrogen sulfide, while a reduction of more than 95 percent, may still exceed the tolerance of some sensitive fuel cells and pipeline specifications, suggesting that a polishing stage could be required for the most demanding applications. Even so, the core demonstration stands: a methane content above 96 percent, achieved with ground clay and industrial byproducts, optimized through a transparent statistical framework, and grounded in materials that a rural workshop could plausibly source. If follow-up work confirms durability at scale, the humble mineral underfoot may prove to be one of the cheapest keys yet to making renewable gas genuinely renewable in practice.

Subject of Research: Biogas upgrading by adsorption of hydrogen sulfide and carbon dioxide on modified bentonite clay following anaerobic co-digestion of organic waste

Article Title: Dual-function biorefinery: simultaneous organic waste treatment and biogas quality improvement via tailored clay adsorption

Article References: Hagos, G. K., Golie, W. M., & Atsbha, G. K. (2026). Dual-function biorefinery: simultaneous organic waste treatment and biogas quality improvement via tailored clay adsorption. Clean Technologies and Environmental Policy, 28(10), Article 268. https://doi.org/10.1007/s10098-026-03626-1

Image Credits: AI Generated

DOI: 10.1007/s10098-026-03626-1

Keywords: biogas upgrading, bentonite clay, adsorption, anaerobic co-digestion, hydrogen sulfide removal, carbon dioxide capture, carbide slag, monoethanolamine, response surface methodology, biomethane, circular economy, renewable energy

The core challenge the researchers set out to address is the asymmetry between the two substrates. Sewage sludge is a complex matrix in which much of the organic material is locked inside microbial cells and wrapped in extracellular polymeric substances, making hydrolysis the rate-limiting step of the entire digestion chain. Food waste, by contrast, is rich in readily fermentable carbohydrates that acidogenic bacteria convert into volatile fatty acids almost immediately. When the two are co-digested without intervention, the food waste acidifies rapidly while the sludge has barely begun to release its own organic content, and the resulting acid load can suppress or even collapse the methanogenic community. Pretreatment strategies have therefore become a major focus of research, with the goal of pre-disintegrating the sludge and buffering the early acidification phase before the mixture ever reaches the digester.

The pretreatment tested by the team combines two reagents with complementary chemistry. Nano zero-valent iron, or nZVI, consists of iron particles at the nanometer scale that act as strong electron donors and can promote the disintegration of sludge flocs and the disruption of cell membranes, releasing soluble organic matter that hydrolytic enzymes and fermenters can then access. Sodium percarbonate, or SPC, is a solid source of hydrogen peroxide that decomposes to release reactive oxygen species capable of oxidizing recalcitrant sludge structures, while its carbonate by-product simultaneously raises the alkalinity of the mixture. The combination is attractive from a practical standpoint because both reagents are relatively inexpensive, easy to dose, and free of the heavy-metal contamination concerns associated with some other advanced oxidation catalysts.

The quantitative results reported in the study are striking. After the nZVI/SPC pretreatment, the initial soluble chemical oxygen demand of the substrate mixture reached 22,483.6 milligrams per liter, compared with only 8,493.7 milligrams per liter in the untreated control. Soluble chemical oxygen demand is a standard proxy for how much organic material has been transferred from the particulate phase into the dissolved phase where microbes can consume it, so a nearly threefold increase signals a dramatic acceleration of substrate disintegration. The researchers also observed markedly higher release of soluble proteins and polysaccharides, the two dominant classes of organic macromolecules in sludge, confirming that the pretreatment had effectively cracked open the cellular and polymeric barriers that normally shield this material from digestion.

The downstream consequences for methane production were equally impressive. The cumulative methane yield in the pretreated co-digestion reached 412.5 milliliters per gram of volatile solids, a figure 55.2 percent higher than the control. The maximum methane production rate climbed to 48.6 milliliters per gram of volatile solids per day, and the lag phase, the initial period during which the microbial community acclimates before gas production accelerates, shortened to just 0.52 days. In anaerobic digestion engineering, a short lag phase is highly valuable because it indicates that the microbial consortium can begin productive work almost immediately, which translates into shorter hydraulic residence times and smaller, cheaper reactors for the same throughput of waste.

Perhaps the most technically significant finding concerns process stability during the acidification surge. Volatile fatty acids in the pretreated reactors peaked at 8,124.6 milligrams per liter on day seven, a concentration that in many co-digestion systems would be associated with severe acid stress and methane inhibition. Yet the pH in the reactors remained between 6.99 and 7.41 throughout, comfortably within the optimal window for methanogens. The authors attribute this resilience to effective buffering by carbonate species derived from the sodium percarbonate, which neutralize the acid wave as it forms. In effect, the pretreatment allows the system to accumulate a large pool of volatile fatty acids, which are the direct precursors of methane, without ever crossing the pH threshold at which those intermediates become toxic to the archaea that convert them into biogas.

Biogas quality also improved in ways that matter for real-world deployment. The maximum methane content of the produced gas reached 71.2 percent, well above the roughly 50 to 65 percent typical of conventional digesters, which reduces the burden of downstream biogas upgrading. At the same time, the peak hydrogen sulfide concentration decreased by 49.6 percent. Hydrogen sulfide is a corrosive and toxic contaminant that must be scrubbed from biogas before it can be used in engines or fuel cells, and iron species are known to bind sulfide as insoluble iron sulfide. The nZVI in the pretreatment therefore appears to deliver a double benefit: it accelerates the digestion chemistry while simultaneously polishing the gas stream at its source.

Microbial community analysis provided mechanistic depth to these performance gains. The pretreated reactors showed enrichment of hydrolytic and syntrophic bacteria, including members of the genera Clostridium, Syntrophomonas, and Syntrophobacter. Clostridium species are prolific fermenters that break down sugars, proteins, and other polymers into volatile fatty acids and alcohols, while Syntrophomonas and Syntrophobacter specialize in the energetically demanding oxidation of longer-chain fatty acids and propionate, reactions that only proceed when paired with hydrogen-scavenging methanogens. On the archaeal side, the community was enriched in Methanosaeta and Methanobacterium, representing the acetoclastic and hydrogenotrophic methanogenic pathways respectively. This combination indicates that the pretreatment cultivated a tightly coupled food web in which every intermediate, from complex polymers down to acetate and hydrogen, had a dedicated microbial specialist waiting to consume it.

The broader significance of the work lies in its integration of three historically separate concerns: substrate availability, acid-base management, and microbial ecology. Many pretreatment studies have focused solely on increasing solubilization, only to find that the extra soluble material simply worsens acidification. By pairing an oxidative disintegration agent with an alkalinity source in a single step, the nZVI/SPC approach addresses both sides of the problem at once, and the microbial data confirm that the resulting environment favors the exact organisms needed to convert the released organics into methane. The study was supported by the Natural Science Foundation of Shandong Province and the China Postdoctoral Science Foundation, and the authors suggest that the strategy offers a feasible pathway for efficient resource recovery from multiple organic waste streams simultaneously.

For wastewater treatment plants and biogas operators, the findings point toward a relatively low-barrier upgrade path. Both nZVI and sodium percarbonate can be dosed as solids or slurries into existing pretreatment tanks, and the improved buffering could reduce the need for external alkalinity addition, which is a recurring operating cost in food waste co-digestion. The higher methane content and lower hydrogen sulfide burden further cut downstream gas-cleaning expenses. As cities worldwide seek to divert organic waste from landfills and squeeze more energy from their wastewater infrastructure, pretreatments of this kind may prove to be among the most cost-effective levers available, turning two problematic waste streams into a cleaner, richer source of renewable fuel.

Subject of Research: Nano zero-valent iron and sodium percarbonate pretreatment to enhance anaerobic co-digestion of sewage sludge and food waste

Article Title: New insights into nZVI/SPC pretreatment for enhanced anaerobic co-digestion of sewage sludge and food waste: substrate disintegration, acidification buffering and microbial responses

Article References: Kong, Y., Zhao, J., & Liu, J. (2026). New insights into nZVI/SPC pretreatment for enhanced anaerobic co-digestion of sewage sludge and food waste: substrate disintegration, acidification buffering and microbial responses. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38291-w

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38291-w

Keywords: anaerobic co-digestion, sewage sludge, food waste, nano zero-valent iron, sodium percarbonate, methane production, biogas, volatile fatty acids, acidification buffering, microbial community, hydrogen sulfide, waste-to-energy