Biohydrogen production kinetics from cacao pod husk hydrolysate in dark fermentations: Effect of pretreatment, substrate concentration, and inoculum

Abstract

This study aims to explore the impact of pretreatment on cacao pod husk (CPH) and investigate the kinetics of biohydrogen production in a dark fermentation (DF) batch reactor with consideration for substrate and inoculum concentration. As the first study focused on biohydrogen production from CPH, the Taguchi orthogonal array was utilized to determine the optimal pretreatment condition for powdered CPH. This study examined different substrate concentrations (3–100 g VS/L) and inoculum concentrations (5 and 10 % v/v). The study showed that biohydrogen production was significantly increased by 74.4 ± 0.1 % by pretreatment with 5.5 % HCl compared to the untreated CPH as control. Furthermore, the optimal performance was achieved by 35 g VS/L of CPH in the mixture with inoculated by 10 % v/v inoculum, resulting in a biohydrogen production and concentration of 257.05 ± 7.97 mL/L and 23.87 ± 0.15 %, respectively. The inoculum's microbial community analysis using Illumina 16S sequencing showed a dominance of the Ethanoligenes (30.4 %), which was positively correlated with the profiles of hydrogen production and the proportion of VFAs. The Aiba model with an exponential substrate concentration and inhibitory constant (R2 of 0.93) was statistically the most precise mathematical model to predict the specific growth rate during the fermentative biohydrogen production by CPH substrate.