Photoheterotrophic microalgal hydrogen production using acetate- and butyrate-rich wastewater effluent
Introduction
Biological hydrogen production from organic substrates using phototrophic microorganisms, in relation to wastewater treatment is a promising approach for bioenergy generation [1], [2]. Photoheterotrophic hydrogen production involves endogenous catabolism of organic compounds as a source of reducing power [3]. Bacterial and cyanobacterial species have been studied for photoheterotrophic hydrogen production [4].
Photoheterotrophic hydrogen production using microalgae is attracting great interest because of its potential ability to degrade organic pollutants, which serve as carbon and energy sources during hydrogen production [5], [6]. Microalgal hydrogen production using organic compounds such as acetate, starch and glucose has been reported [7], but application of such substrates for commercial hydrogen production might not be economically feasible [8]. Different wastewaters streams such as municipal wastewater, piggery wastewater and agricultural runoff have been utilized for microalgal cultivation [9], but their use for microalgal hydrogen production has not been thoroughly investigated. Optimization of microalgal photoheterotrophic hydrogen production by replacing synthetic organic substrates with real wastewater effluents would contribute significantly to bioenergy research. The effluent of dark-fermentation is rich in short chain VFAs (volatile fatty acids) such as acetate and butyrate, which can serve as alternative substrates for microalgae photoheterotrophic cultivation and subsequently the hydrogen production [10]. Oxygen evolved during microalgae cultivation due to photosynthetic activity inhibits the oxygen sensitive hydrogenase and hydrogen production [11]. Most research related to phototrophic hydrogen production has therefore been performed under anaerobic conditions [12], but the effect of evolved oxygen on hydrogenase with respect to its activity and hydrogen evolution should be examined when new microalgal strains are being investigated [13].
We investigated heterotrophic hydrogen production by the microalga M. reisseri YSW05 using acetate- and butyrate-rich effluents derived from conventional microbial fermentation to mimic wastewater from municipal wastewater treatment facilities. Parameters such as the effluent dilution, pH and light/dark cycle were optimized to achieve higher biomass production, consumption of acetate and butyrate, and hydrogen production. The influence of oxygen evolved during microalgal photoheterotrophic cultivation on hydrogenase activity was simultaneously evaluated with apparent hydrogen production.
Section snippets
Seed sludge for fermentation and isolation/identification of microalga
Seed sludge used in this study was collected from the anaerobic digesters of a municipal wastewater treatment plant (Wonju, Water Supply and Drainage Center, South Korea). The inoculum was acclimatized to glucose for 1 month in an anaerobic continuous culture at 35 °C with a hydraulic retention time (HRT) of 12 h [14]. We used glucose as the substrate with a concentration of 14,063 mg/L (15,000 mg COD/L).
Microalga was isolated from the effluent of a municipal wastewater treatment plant (Wonju
Effect of effluent dilution on photoheterotrophic microalgal biomass and hydrogen production
Hydrogen production in the combined dark and photoheterotrophic system is illustrated in Fig. 1. During dark-fermentation (first stage), soluble metabolites (i.e., VFAs and alcohols) produced by the bacterial strains were contained in the fermentation effluent. The microalga, M. reisseri was used to assimilate such soluble metabolites from the effluent as organic carbon source for heterotrophic cell growth in the second stage (Supplementary Fig. 2). Bacterial dark-fermentation showed a maximum
Conclusions
Acetate/butyrate rich effluent supported the growth and hydrogen production of microalga M. reisseri. Maximum hydrogen production of 191.2 ± 14.7 mL/L was observed with undiluted effluent at pH 8.0, which correlated with the highest biomass production. Continuous light was more effective for growth and hydrogen production than dark/light. Hydrogenase in this system might be oxygen tolerant as it showed an activity at almost atmospheric oxygen level. Our findings indicate that VFA rich effluents
Acknowledgments
This work was supported by the Mid-career Researcher program (National Research Foundation of Korea, 2013069183), the Yonsei University Research Fund of 2013, and the Waste to Energy Recycling Human Resource Development Project of the Korean Ministry of the Environment.
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Authors worked in the Dept. of Environmental Engineering at Yonsei University while they conducted a part of this study.