Wastewater co-digestion raises batch methane yield from olive pomace

Co-digesting olive pomace with municipal wastewater produced substantially more methane than digesting the oil-mill residue alone in a 40-day laboratory study conducted in Jordan. The hot-pressed pomace mixture generated a net methane yield of 420 ± 15 NmL CH₄/g volatile solids, compared with 275 ± 10 NmL/gVS for hot-pressed pomace alone. The corresponding cold-pressed mixture yielded 385 ± 12 NmL/gVS, versus 250 ± 9 NmL/gVS for cold-pressed pomace alone. All treatments and controls were tested in triplicate, and methane volumes were normalized to standard temperature and pressure after background methane from the inoculum was subtracted.

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