Impact on the microbial population during biological volatile fatty acid production from olive mill solid waste

Volatile fatty acids (VFAs) revalorisation from waste products are key in achieving industrial sustainability and circular economic goals. Hence, the objective of this work was to correlate the adaptability of the microbial community in olive mill solid waste (OMSW) anaerobic fermenta-tion processes, to the production of VFAs under different pH conditions, i.e. under acidic (pH 4 &5), neutral (pH 6 & 7) and alkaline conditions (pH 9 & 10). At neutral conditions, anaerobic digestion exhibited minimal accumulation of VFAs, as they were primarily biotransformed to methane, where no significant changes in the microbial community were observed. At acidic conditions, a diverse profile of VFAs were present in the reactors, although the VFA production was limited to around 20 % of fed OMSW. Despite the low accumulation, the VFA profile at pH 5 was more complex than those at alkaline conditions, accounting propionic acid as the main VFA compound produced at pH 5 (60 % of the total VFAs). Acidic conditions entailed a shift in the microbial composition compared to the initial inoculum, although the reactors maintained similar diversity indices. At alkaline conditions, around 50 % of the fed OMSW was accumulated as VFAs, mainly as acetic acid. Overall, a lower diversity and higher dominance corresponded to a less diverse VFAs profile, such as the preponderance of acetic acid correlated with a microbial di-versity decrease and the increased dominance of Tissirella.

Automated summary

The adaptability of the microbial community in the anaerobic fermentation process of olive mill solid waste (OMSW) was found to be correlated with the production of VFAs under different pH conditions. Neutral conditions led to minimal VFA accumulation, while acidic conditions resulted in a more complex VFA profile and a shift in microbial composition. Alkaline conditions exhibited higher VFA accumulation, primarily in the form of acetic acid.