Vol.:(0123456789) 1 3 Environmental Chemistry Letters https://doi.org/10.1007/s10311-020-01122-6 REVIEW Sequential production of hydrogen and methane by anaerobic digestion of organic wastes: a review Salma Aathika Abdur Rawoof 1  · P. Senthil Kumar 2  · Dai‑Viet N. Vo 3  · Sivanesan Subramanian 1 Received: 1 October 2020 / Accepted: 13 October 2020 © Springer Nature Switzerland AG 2020 Abstract Energy and waste disposal issues are calling for advanced recycling methods such as conversion of organic waste into bio- hydrogen and biomethane. Here we review factors that infuence 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 efciency such as co-digestion, pre-treatment, application of additives and optimal digester design. Co-digestion synergizes the efects on microbial communities, balances the nutrients, reduces the inhibitory efects 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 efciency and hydrogen production by 25% compared to the continuous mode. The addition of trace metals alters the hydrogenase activ- ity 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. Keywords Biohydrogen · Biomethane · Organic waste · Anaerobic fermentation · Pre-treatment · Co-digestion Abbreviations NADH Nicotinamide adenine dinucleotide hydride NADP + Nicotinamide adenine dinucleotide phosphate ATP Adenosine triphosphate FAD + Flavin adenine dinucleotide S/I Substrate to inoculum ratio COD Chemical oxygen demand C/N Carbon/Nitrogen C/N/P/S Carbon/Nitrogen/Phosphorus/Sulphur ND Not Described Introduction Globally, approximately 78% of energy demand is met by the utilization of hydrocarbon fuels like coal, natural gas and oil (International Energy Agency 2020). When fossil fuels undergo combustion, they emit pollutants such as oxides of nitrogen, oxides of sulphur, oxides of carbon, particu- late matters, organic compounds and ash that cause green- house gas emissions, thereby resulting in climate change and global warming (Watts et al. 2017). It has been observed that the demand for energy has increased exponentially and will continue to increase rapidly due to the increase in population, civilization and industrialization. Fuels derived from hydrocarbons have been the major energy source with oil accounting up to 32%, coal 27% and natural gas 20%, whereas, renewable energy such as combustible biomass accounts to 10%, nuclear power 6%, hydroelectric power 2%, and only 0.8% is attained combinely from wind, solar and geothermal energy (Anoop et al. 2017; Banks et al. 2007). As a consequence of growing global population, industriali- zation and modernization, the generation of solid waste has also increased immensely, and its disposal has become an * P. Senthil Kumar senthilchem8582@gmail.com * Sivanesan Subramanian sivanesh1963@gmail.com 1 Department of Applied Science and Technology, AC Tech, Anna University, Chennai, India 2 Department of Chemical Engineering, Sri Sivasubramaniya Nadar College of Engineering, Chennai, India 3 Center of Excellence for Green Energy and Environmental Nanomaterials (CE@GrEEN), Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam