Evaluation of volatile fatty acids (VFAs) production in thermophilic and mesophilic anaerobic digestion of oil palm empty fruit bunch (OPEFB)

Abstract

Palm oil is one of the leading global commodities, and Indonesia is one of the largest palm oil producers in the world. One of the solid wastes generated from oil palm industrial activities is oil palm empty fruit bunches (OPEFB). The annual accumulation of OPEFB in Indonesia is 126,317.54 tons/year, and it increases with increasing palm oil production. The cellulosic content of OPEFB is a potential substrate for anaerobic digestion to produce volatile fatty acids (VFAs). This study used chemical pretreatment (NaOH 8% w/V) for OPEFB delignification before the anaerobic digestion for VFAs production. Digested cow manure (DCM) was used as the inoculum. Process optimization of producing VFAs is challenging because VFAs act as intermediate products in the metabolic pathway of anaerobic digestion. In general, the temperature can affect the activity of hydrolytic and acidogenic facultative microorganisms. It increases the hydrolysis rate of organic polymers to more soluble forms and improves the performance of anaerobic reactors for the formation of VFAs. This study examines the effect of temperature on the anaerobic digestion process of OPEFB for VFAs production in a batch reactor. The temperature varied in mesophilic (29 °C) and thermophilic (55 °C) conditions. This study showed that anaerobic digestion in the thermophilic conditions produces almost thrice VFAs compared to the mesophilic process. The highest accumulation of VFAs in both reactors occurred on the seventh day, i.e., 3311.57 ± 89 mg.L−1 at ORP -163 ± 16 mV in the thermophilic reactor and 1307 ± 42 mg.L−1 at ORP -224 ± 32 mV in the mesophilic reactor. Acetic acid was the dominant VFA in each reactor. The VFA production in thermophilic reactor (RT) reached 59% of the stoichiometric potential. Although the thermophilic condition improves the VFA yield compared to the mesophilic condition, further study is still needed to increase the VFA production from OPEFB.