A recent trend in anaerobic digestion (AD): Enhancement of microbiome and digestibility of feedstocks via abiotic stress factors for biomethanation

Abstract

Anaerobic digestion (AD) is a potential environment-benign approach to stabilize biowaste and subsequently generate biomethane. The complete utilization of biowaste in AD relies on substrate composition, microbial activity, and abiotic stress factors (ASFs). The recent research trend on the ASFs applications (including salinity, micronutrients, micro-aeration, and organic substances) to improve digestibility and biomethanation from different feedstocks has increased. Previous reviews have emphasized only the effect of operational parameters (such as temperature, pH, digestion times, and co-substrates) on AD efficiency. However, no review discussed the recent ASFs application in AD. Thus, this review intends to fully cover the effects of ASFs on substrate digestibility, biomethanation, microbial shifts, functional enzymes, and metabolic pathways. Salinity stress was reported to enhance biomethane (by 13–49%), microbial abundance (Methanosaeta and Methanosarcina), enzymes (including hydrolases, dehydrogenases, and co-enzyme F420), and metabolic pathways (solute transfer and ATP synthesis) at the lab-scale. The micro-aeration stress enhanced biomethanation owing to increased activity of hydrolases, volatile fatty acids production/oxidation, and high sulfide mitigation (upto 90%) at both lab. and pilot-scale. Micronutrient stress improved biomethanation by 10–50% and 30–65% at the lab. and pilot-scale, respectively, due to high direct interspecies electron transport which increases the abundance of Methanobrevibacter and Methanosarcina. Antibiotics augmented the biowaste solubilization and biomethane production upto 70 and 46%, respectively. Binary ASFs application in AD also facilitated the feedstock digestibility and biomethanation. However, the integration of multiple ASFs to attain high biomethanation and a holistic understanding of synergistic mechanisms needs more research.