Sequential production of hydrogen and methane by anaerobic digestion of organic wastes: a review

Abstract

Energy and waste disposal issues are calling for advanced recycling methods such as conversion of organic waste into biohydrogen and biomethane. Here we review factors that influence yields, such as pH, temperature, substrate composition, biocatalyst, nutrient content, volatile fatty acids concentration, organic loading rate, hydraulic retention time and C/N ratio. The optimum pH is 5.5–6 for hydrogen production, and 6.8–7.2 for methane production. Hydrogen yield improved highly after reducing the retention time from 72 to 20 h. The highest methane productivity was achieved with C/N ratio of 16–27. We also discuss methods to improve efficiency such as co-digestion, pre-treatment, application of additives and optimal digester design. Co-digestion synergizes the effects on microbial communities, balances the nutrients, reduces the inhibitory effects and improves the economic viability. Co-digestion has enhanced the productivity by 25–400% compared to mono-digestion. Acid pre-treatment is the best method for lignocellulose hydrolysis, followed by enzyme pre-treatment. Microwave pre-treatment enhances the biomethane production 4–7 times. The batch mode improves the substrate degradation efficiency and hydrogen production by 25% compared to the continuous mode. The addition of trace metals alters the hydrogenase activity during anaerobic fermentation. Reaction kinetics and metabolomics, bioaugmentation, digestate recirculation, frequent feeding and development of bioreactor systems for two-stage anaerobic digestion are also presented.