State-of-the-art techniques to enhance biomethane/biogas production in thermophilic anaerobic digestion

Anaerobic digestion (AD) is one of the promising technologies for organic waste management, which also supports green energy recovery in the form of biogas. As compared to mesophilic anaerobic digestion (MAD), thermophilic anaerobic digestion (TAD) is considered highly efficient due to accelerated rate-limiting steps in the AD i.e., hydrolysis and methanogenesis, and disruption of contaminants at high temperature. The literature is thoroughly reviewed with the specific key-words on popular platforms including Scopus, Researchgate and Google Scholar and discussed in the present article. It is found that TAD is often encountered with process instability due to the accumulation of high volatile fatty acids and elevated ammonia levels that adversely affect the methanogens and ultimately reduce the methane formation. This inadequacy could be overcome by following several techniques, such as co-digestion of mixed feedstocks of different characteristics, addition of micro-nutrients and conductive materials, and incorporation of exogenous microbial strains of interest into the thermophilic consortium. Despite these conventional methods, various state-of-the-art strategies including integration of AD with bioelectrochemical systems and genetic manipulation in particular microorganisms involved in the process, are now gathering attention that could help to enhance the TAD process performance. However, these methods have been limited to the MAD process only. Thus, this article mainly discusses the conventional and emerging approaches to alleviate the challenges confronted in the TAD process that could improve the process stability. Further, the merits of individual approaches, along with key discrepancies, are also discussed in this review article. The novelty of this article lies in the emphasis of advanced techniques to stabilize TAD process for enhanced biogas/ biomethane production.