Coupling electron bifurcation and interspecies electron transfer to mitigate ammonia and acids inhibition

The twin concurrent challenges of ammonia and volatile fatty acids (VFA) inhibition is commonly encountered in anaerobic digestion (AD). The stability of the community structure and metabolism in the microbial flora is paramount for sustained high production of methane. Till date most of the studies have focused on the modification of the technical parameters of a digester, and very few studies have reported the modulation of the metabolism and/or the bioenergetics of the microbes involved. Stimulation of interspecies electron transfer (IET) and strengthening of the syntrophic association between bacteria and methanogens has the potential to improve methane yield under ammonia and VFA-stressed condition. Electron bifurcation (EB) is an energy conservation strategy adopted by microbes to minimize energy loss during unfavourable and stressed conditions. The current review aims to draw attention of the scientific community on the potential of EB coupled DIET for enhancement of methane production, particularly under high ammonia and VFA inhibition. The review discusses the molecular mechanisms involved in ammonia and VFA inhibition, and mitigation strategies centered around interspecies electron transfer (IET). The energy coupling pathways have been examined in the perspective of inhibitory challenges accorded by high ammonia and VFA accumulation. The article concludes with critical assessment of the mitigation strategies while outlining future research directions to augment the understanding of the metabolic processes involved in EB coupled DIET for AD. This is the first review to highlight the potential of stimulating an EB coupled DIET for ammonia-VFA stressed AD.