Syntrophy mechanism, microbial population, and process optimization for volatile fatty acids metabolism in anaerobic digestion

The low efficiency and stability of anaerobic digestion (AD) has always hindered its wide application. The accumulation of volatile fatty acids (VFAs) cause acidification and inhibition of anaerobic digester. The conversion of some VFAs (propionate and butyrate) to acetate, CO₂, and H₂ is endergonic under standard conditions, which make it difficult to occur spontaneously in AD system. Thus, the degradation of VFAs is regarded as the rate-limiting step in AD. Studies proved that VFAs could be degraded through the syntrophic cooperation between bacteria and methanogens. Therefore, clear insight into the mechanism of syntrophic methanogenesis as well as the process optimization in syntrophic metabolism of VFAs are essential to improve the efficiency of AD. The present review summarized the roles of syntrophic bacteria and methanogens in AD process, containing metabolism pathways, biochemical reactions, microorganisms characteristics, substrate oxidation, and electron transfer in syntrophic methanogenesis. The main forms and mechanisms of interspecies electron transfer were detailly and deeply described. In addition, the process control and optimization of the syntrophic metabolism of VFAs were also summarized in the review for improving the efficiency of syntrophic methanogenesis. This review provides a deeper understanding of efficiency enhancement of syntrophic metabolism in AD system.