Impact of hydraulic retention time on the production of volatile fatty acids from lignocellulosic feedstock by acidogenic fermentation

Abstract

The olive oil industry generates large quantities of olive mill solid waste, which can be used as biorefinery feedstock for the extraction of high value-added compounds. This research explores the relationship between hydraulic retention time (HRT) variations, and key process parameters, i.e., substrate solubilization/hydrolysis, volatile fatty acids (VFAs) production, and the microbial population dynamics. This research proposes the production and accumulation of VFAs through acidogenic fermentation at controlled pH 5 of olive mill solid waste. The influence of HRT on microbial structure dynamics during hydrolysis and acidogenesis stages was evaluated. Optimal hydrolysis was achieved with 14-day HRT, while the maximum acidogenesis was observed with a 21-day HRT, reaching 32 ± 5 % and 39 ± 3 %, respectively. VFAs concentration up to 13,421 ± 240 mg O2·L−1 was achieved at 21-day HRT, mainly as acetic, butyric, and propionic acids. A positive correlation was found between the relative abundance of hydrolytic species g_Ruminococcus and f_Sphaerochaetaceae, and the hydrolysis and accumulation of VFAs. Increasing HRT up to 21-days showed beginnings of destabilization, despite a further increase in the accumulation of VFAs.