Influence of inoculum-to-substrate ratio on biomethane production via anaerobic digestion of biomass

The influence of the inoculum-to-substrate ratio (ISR) on anaerobic digestion (AD) of biomass in terms of methane yield and microbial community, was explored in this paper. The level of ISR can affect the AD performance in several ways. At extremely low ISR, volatile fatty acids (VFAs) accumuate, while inhibition occur at higher level of ISR. An ISR ranging from 1.0–2.0 was found optimal resulting in higher methane yield, organic matter removal and VFA degradation. Furthermore, a high ISR (2.0–4.0) is favourable to methanogenesis, while a lower ISR (<1.0) is prone to irreversible acidification. The range of ISR can shift the methanogenic pathway of AD to favour an acetoclastic or hydrogenotrophic response, indicated by the enriched group of microorganisms. The genus Methanosaeta (acetoclastic) and Methanobacterium (hydrogenotrophic) are the most enriched methanogens across all ISRs, while Firmicutes, Bacteroidetes, Proteobacteria and Spirochaetae are dominant in the bacterial community. Additionally, the interplay of substrate biodegradability and ISR potentially affects AD performance. Finally, novel equations are developed and proposed for characterizing the quantity of inoculum and substrate.