Optimizing continuous medium-chain fatty acid production from biohydrogenic palm oil mill effluent: Operational parameters and microbial dynamics

Abstract

Palm oil mill effluent (POME) was dark fermented into biohydrogen for clean energy production with volatile fatty acids (VFAs) as a byproduct. The VFAs remaining 70% in hydrogenic POME offer potential substrate for conversion into valuable medium-chain fatty acids (MCFA), presenting an opportunity for resource recovery and environmental mitigation. This study investigated the influence of varying operating parameters on continuous MCFA production from biohydrogenic POME through a stirred tank reactor. Optimizing MCFA production necessitates a 5.4 g/L ethanol load supplemented with 2.6 g/L NaHCO3 and 1.5 L-CO2/L/d while maintaining a pH of 6.5 and a temperature of 35 °C. Under optimized conditions, a maximum MCFA production of 2.2 g-COD/L was achieved, consisting primarily of 1.8 g-COD/L caproic acid, 0.2 g-COD/L heptanoic acid, and 0.2 g-COD/L caprylic acid, translating to a total MCFA yield of 0.5 g-COD/g-COD and MCFA selectivity of 51% with Caproiciproducens sp. (45%), Lactobacillus sp. (21%), and Clostridium sp. (14%) as predominant bacteria. Notably, CO2 loading significantly enhances MCFA production, particularly caproic and heptanoic acids. Remarkably, the system maintained stable MCFA production for an impressive 220 days. The microbial community in the reactor effectively removed total solids, volatile solids, chemical oxygen demand (COD), suspended solids of 42%, 61%, 31%, and 95%, respectively. Key bacteria like Caproiciproducens sp., C. kluyveri, Clostridium sp., and Sporanaerobacter sp. possess most of the enzymes required for the reverse β-oxidation pathway, an energy-efficient pathway for MCFA generation. This study makes a significant contribution to the field of biowaste valorization, specifically focusing on anaerobic fermentation for bio-based chemical production. Despite challenges related to butyric acid accumulation and inhibition, these findings offer promising potential for scaling up the fermentation process.