Laboratory Production of Biogas from Apple Pomace

Abstract

Apple processing generates substantial quantities of pomace waste, comprising approximately 25–30% of the processed fruit dry mass. This inexpensive, non-food-competing residue retains a substantial fermentable carbohydrate content, making it an attractive feedstock for biogas production through anaerobic digestion. This study investigated the effect of total solids (TS) concentration on biogas yield from apple cider pomace (APC) combined with cow manure inoculum (CM), using a laboratory benchtop anaerobic fermentation (LBAF) system. APC/CM mixtures were prepared at three TS loadings—1.5%, 2%, and 3%—and digested in duplicate at 39°C ± 2°C. Cumulative biogas volume was measured over time using a barrier-fluid displacement method. Biogas production followed a sigmoidal trend, with an initial lag phase (0–20 h), a steep exponential phase (approximately 20–60 h), and a subsequent plateau as substrate became limiting. The 2% TS condition yielded the highest cumulative biogas volumes and the greatest biogas fraction measured after CO2 removal, reaching approximately 240–298 mL over approximately 108 h, compared with 104–182 mL for 1.5% TS and 70–77 mL for 3% TS. The lower yield at 3% TS suggests possible substrate overloading that inhibited methanogenic activity, whereas the intermediate performance at 1.5% TS and higher yield at 2% TS indicate that 2% TS was the more favourable loading under the tested conditions. Notable variability was observed between duplicate reactors, particularly at 1.5% and 2% TS, possibly because of inoculum heterogeneity or inconsistent feedstock particle size, whereas the 3% TS replicates showed closer agreement. These findings support the potential of apple pomace as a biogas feedstock and indicate that 2% TS was the more favourable loading under the tested conditions. Future work should address feedstock homogenisation, extend trial durations to allow direct yield comparisons, and characterise methane content to better evaluate process efficiency.