Short CommunicationEffect of substrate concentration on hydrogen production by photo-fermentation in the pilot-scale baffled bioreactor
Introduction
Bio-hydrogen production has been regarded as a pollution-free bioenergy production process (Hosseini et al., 2015). One of the bio-hydrogen production methods is photo-fermentative hydrogen production which is conducted using photosynthetic bacteria under light-illuminated and anaerobic conditions. Compared to other bio-hydrogen production methods, photo-fermentative hydrogen production has many advantages, such as effective removal of organic pollutants with a wide range of substrates, high substrate conversion efficiency, high hydrogen production rate (HPR, mol H2/m3/d) and hydrogen content (Pattanamanee et al., 2015, Trchounian and Trchounian, 2015, Zhang et al., 2017b).
Among various factors that can significantly affect bio-hydrogen production, (Buitron and Carvajal, 2010, Guo et al., 2015, Hallenbeck and Liu, 2016, Krujatz et al., 2015, Li et al., 2017, Wang et al., 2015), substrate concentration and organic loading rate (OLR, g/L/d) are particularly crucial for continuous bio-hydrogen production (Mariakakis et al., 2011, Zhang et al., 2015). Substrate concentration below the optimal value always leads to a low HPR, hydrogen content, and biomass concentration (Guo et al., 2014, Mariakakis et al., 2011). When substrate concentration is higher than its optimal value, hydrogen producing microorganism could overproduce inhibitory substances, such as volatile fatty acids (VFAs) and ethanol, leading to decreased HPR. A negative correlation was found between HPR and concentration of volatile fatty acids. Akutsu et al. (2009) reported that maximum HPR of 197.77 ± 21.43 mol/m3/d using starch was obtained at a substrate concentration of 30 g/L; however, HPR decreased drastically to 162.05 ± 33.48 mol/m3/d when substrate concentration increased to 50 g/L (Akutsu et al., 2009). Antonopoulou et al. (2011) reported that maximum HPR of 130.80 ± 4.02 mol/m3/d was observed at a substrate concentration of 17.50 ± 2.0 g carbohydrates/L using sweet sorghum extract as substrate. Whereas HPR decreased to 120.54 ± 8.93 mol/m3/d when substrate concentration increased to 20.99 ± 2.0 g carbohydrates/L (Antonopoulou et al., 2011). Hydrogen yield, HPR and hydrogen content are also significantly affected by OLR. HPR increased with OLR, whereas it may decrease when substrate concentration reach a high level (Lee et al., 2014). Zahedi et al. (2013) reported that HPR fell drastically when OLR was higher than 110 g TVS/L/d. Tawfik and Salem (2012) reported that HPR went down when OLR exceed 21.4 g COD/L/d.
Pilot-scale bio-hydrogen production test is necessary prior to a commercial-scale application. At present, there are very few investigations on pilot-scale photo-fermentative bio-hydrogen production (Zhang et al., 2017a). In a previous study, a baffled photo-fermentative hydrogen production reactor (BPHR) was developed and an optimal HRT for bio-hydrogen production was determined (Zhang et al., 2017a). During the feeding process, reaction liquid was rolled up and down along with baffled structure, which improved mixing and consequently improved hydrogen production (Zhang et al., 2015). Up to date, there have been no reports on the effect of substrate concentration on pilot-scale photo-fermentative bio-hydrogen production.
In this study, pilot-scale continuous bio-hydrogen production with mixed microflora HAU-M1 was operated in BPHR system at different substrate concentrations. Effect of substrate concentration on HPR, hydrogen content, and broth characteristics was evaluated. Effect of OLR on HPR was also investigated. Kinetic analysis of hydrogen production process and analysis of variance were further conducted to quantitatively and statistically describe the effect of key factors on the bio-hydrogen production process.
Section snippets
Seed microflora, medium, and bio-hydrogen production process
The seed microflora HAU-M1 was provided by key laboratory of new materials and facilities for renewable energy ministry of agriculture, and cultured as described previously (Lu et al., 2016, Zhang et al., 2017a). The growth medium and photo-fermentative bio-hydrogen production medium were prepared using the same methods as in a previous study (Jiang et al., 2016). Continuous hydrogen production was conducted in a BPHR system as described by Zhang et al., 2017a, Zhang et al., 2017b, using a
Effect of substrate concentration on HPR and hydrogen content
HPR increased first and then decreased with substrate concentration or chamber number (Fig. 1a). HPRs of all the four chambers of BPHR showed increasing trends when substrate concentration increased from 10 g/L to 20 g/L. Further increasing substrate concentration to 25 g/L led to a significant drop in the HPR (Fig. 1a). Maximum HPR values of 88.52 ± 2.4, 168.4 ± 2.4, 202.6 ± 8.8, and 135.0 ± 5.5 mol/m3/d were obtained at a substrate concentration of 20 g/L for chambers #1, #2, #3, and #4, respectively (
Conclusion
Continuous bio-hydrogen production with photosynthetic bacteria HAU-M1 was studied in a pilot-scale baffled photo-fermentative hydrogen production reactor. Substrate concentration showed significant effects on HPR, pH, ORP and residual sugar. The optimal substrate concentration for hydrogen production was 20 g/L in terms of HRP and hydrogen content. A maximum HPR of 148.65 ± 4.19 mol/m3/d was obtained at an OLR of 20 g/L/d during continuous bio-hydrogen production. Hydrogen yield decreased with OLR,
Acknowledgements
This project was supported by Henan Collaborative Innovation Center of Biomass Energy of China, National Natural Science Foundation of China (51676065) and National High Technology Research and Development Program of China (i.e., 863 Program 2012AA051502).
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