Pilot-scale biohydrogen production from Napier grass and oil palm frond: Comparison of SSF and pre-HF processes and the influence of trace metal supplementation on taxonomic and enzymatic profiles

This research aimed to develop a prototype process for hydrogen production from Napier grass (NG) and oil palm frond (OPF). First, biohydrogen production was investigated in batch experiments using either the simultaneous saccharification and fermentation (SSF) process or the pre-hydrolysis and fermentation (pre-HF) process under different NG to OPF ratios. The proportions of NG:OPF of 2.5:7.5, 5.0:5.0, and 7.5:2.5 g-VS/L were tested in both SSF and pre-HF. The results showed that the proportion of NG:OPF of 5.0:5.0 g-VS/L (ratio of 1:1) had the highest hydrogen production in both the SSF and pre-HF processes, with hydrogen production of 370 ± 21 mL-H2/L and 993 ± 64 mL-H2/L, respectively. Among these, pre-HF showed higher performance. Next, a semi-continuous experiment in a 100 L continuous stirred tank reactor (CSTR) with a pre-HF process and NG:OPF of 5.0:5.0 g-VS/L at hydraulic retention times (HRTs) of 4 and 3 days, with and without trace metal supplementation, was conducted. The highest hydrogen yield, 50.1 ± 3.4 mL-H2/g-VS, was found at an HRT of 3 days with trace metal supplementation. Key microbes, including Lactococcus sp., Bacteroides sp., Dysgonomonas sp., and Enterobacter sp., showed increased abundance, improving hydrogen production from NG and OPF. The addition of trace elements significantly altered essential enzymes like 6-phosphofructokinase, which is crucial for forming pyruvate in hydrogen production. The medium-based economic analysis revealed that the pre-HF process with trace elements resulted in an overall hydrogen production of 9.32 m3, an improvement of 16.6 % compared to the 7.99 m3 obtained under pre-HF without adding trace elements. Consequently, the Economic Yield (EY) of pre-HF without trace element addition was 0.030 USD-H2/USD-medium, which was enhanced to 0.035 USD-H2/USD-medium by adding trace elements.