Impact of 5-hydroxy methyl furfural on continuous hydrogen production from galactose and glucose feedstock with periodic recovery
Introduction
Anaerobic dark fermentation is a sustainable biohydrogen production method because it utilizes a wide range of biomass as feedstock [1,2]. High-rate continuous hydrogen production can be achieved by optimizing critical factors such as pH, temperature, substrate composition, microbial source, hydraulic retention time (HRT), and bioreactor design and configuration [3]. The next promising feedstock for large-scale fermentative hydrogen production is algal biomass, which is rich in carbohydrates and present in abundance around the world [4,5]. However, hydrolysis/pretreatment of algal biomass generates a wide range of by-products such as organic acids, and furan aldehydes [6,7]. The furan derivatives mainly consist of 2-furaldehyde (furfural) and 5-hydroxymethyl furfural (HMF), which inhibit the anaerobic microorganisms. It has been reported that these chemicals adversely affect DNA unwinding, cell membrane integrity, specific cell growth rate, and activity of the glycolytic and fermentative enzymes [5,8]. Immobilization is one of the crucial techniques, which is always preferable to use in batch/continuous hydrogen production, to increase the productivity and determined by stable HRT and organic loading rate (OLR) [9]. Hybrid immobilization technique have the major advantage that provide proper physical and chemical strength, that can provide the survival critical conditions, and it can increase the microbial biomass, provide the stability on reactor operations [10]. On the other hand, immobilized technique was used to mitigate the 5-HMF toxicity, as described in our previous report [11].
Glucose, generally used as the model substrate, is efficiently utilized by fermentative microorganisms. On the other hand, galactose, which is an analogue of the glucose molecule that has the hydroxyl group at the fourth carbon in opposite orientation, has attracted attention because of its wide availability in macroalgae [12]. Park et al. reported that the batch test using glucose and galactose as a single substrate each resulted in higher hydrogen production potential, although the lag phase of galactose was high [13]. Moreover, galactose rich wastewater has been recently attracted by the researchers due to its bioavailability, galactose not only present in the macroalgae, it also present in many plant seeds, oil seed and also its present in the dairy wastewater industries [14,15]. Recently, hydrogen production was achieved from galactose, which indicated that galactose would be an attractive feedstock for biohydrogen fermentation owing to its abundance [16].
Therefore, in this study, we attempted the quantitative interpretation of 5-HMF concentration on biohydrogen production through CSTR operation by a mixed culture using galactose and glucose as the substrates. Additionally, a recovery strategy was implemented by switching the substrate and the associated microbial correspondence was studied.
Section snippets
Inoculum and media preparation
Granular anaerobic sludge was collected from the local brewery industry wastewater treatment plant (Gyeongsan city, South Korea). The initial pH was 6.8. Volatile suspended solids (VSS), and chemical oxygen demand (COD) were 12.4, and 22.6 g/L, respectively. It was dried at 100 °C in a hot air oven, ground well into a powder, mixed (10% w/v) with sodium alginate 2% (w/v), activated carbon (2% w/v), chitosan (1% w/v), and SiO2 (1% w/v). Finally, mixed solution were dropping into CaCl2 (2% w/v)
Continuous hydrogen production performance with stepwise 5-HMF injections
The CSTR was operated for 120 days for 6 h of HRT. At the beginning of the experiment, the reactor was operated for 12 h HRT for at least three to four consecutive days to achieve stable hydrogen production with steady-state operation. The reactor HRT was then switched to 6 h to enhance the hydrogen production performance and achieve rapid granular formation, in accordance with our previous optimization study [14]. Continuous biohydrogen production was maintained for the entire operating period
Conclusions
In this study, we examined the threshold and recovery strategy of 5-HMF inhibition on high-rate continuous hydrogen production. At 5-HMF concentrations below 0.9 g/L, hydrogen production was stimulated, while concentrations over the threshold inhibited the process significantly. Galactose-dominated feedstock was affected more by the high-levels of 5-HMF as compared to glucose-dominated feedstock. However, 5-HMF inhibition was readily overcome by the removal of 5-HMF from the feedstock.
Acknowledgements
This work was supported by National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2019M3E6A1065160) and Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (No. 20188550000540).
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