Behavior of acidogenesis during biohydrogen production with formate and glucose as carbon source: Substrate associated dehydrogenase expression
Introduction
Production of microbial catalyzed hydrogen (H2) by acidogenic fermentation process is considered as one of the promising alternatives for sustainable renewable energy as it exhibits eco-friendly merits. The process is viable from practical point of view and can be operated under ambient conditions. It has been attracting increasing attention due to its applicability to different types of wastewaters and high specific productivity [1], [2], [3]. Improving H2 production rate and yield are the critical confronts to sustain economical production. In this regard, various strategies were reported in the literature. A few of them are – selection and pretreatment of microbial consortia, immobilization of consortia, statistical techniques for process optimization, sequencing of bioreactors, bio-electrochemical treatment, multiple process integration and bio-augmentation [3], [4], [5], [6], [7]. Commercial operation still poses certain process impedes like inhibitory effect of un-dissociated volatile fatty acids (VFA). In addition to H2 generation, storage in a safe and reversible manner remains a challenge to researchers. Conventionally, it can be stored with solid absorbent materials such as palladium under pressure, which further adds unwanted weight to vehicles. Use of more stable organic chemical compounds like formic acid with high hydrogen content might overcome the storage problem due to easy delivery and release of H2 by a single step dehydrogenation reaction [8]. Interestingly, microorganisms present in nature contain enzymes which are competent to catalyze the formation/oxidation of formic acid to H2 with high turnover frequencies. This is more efficient than the currently known industrial hydrogenation/dehydrogenation catalysts [9]. Formate is a key metabolite in the energy metabolism of many bacteria. A direct single step decomposition of formate is catalyzed by formate dehydrogenase (FDH) and yields H2 without the generation of by-products except CO2. Because of its low redox potential (−420 mV), it can serve as an electron donor for the anaerobic reduction of fumarate and nitrate or nitrite [10]. Recent studies showed feasibility of H2 production from formate and obtained high H2 production rates using pure cultures [11], [12]. On the other hand, using mixed consortia as biocatalyst for H2 production is a potential and promising option for scale-up especially when wastewater is used as substrate [13]. Therefore, in the present study evaluation of acidogenic behavior of anaerobic biocatalyst for H2 production from formate in comparison with glucose as primary substrates was carried out. The study emphasizes on the function of substrate-linked dehydrogenases in H2 production. Bio-chemical and bio-electrochemical analysis were carried out to comprehensively understand the biological process and evaluate the reactor performance.
Section snippets
Selection and enrichment of the biocatalyst
Anaerobic mixed consortia acquired from an existing commercial full scale wastewater treatment plant was used as biocatalyst in this study. The parent culture was washed (at 1000 rpm for 5 min) about three times to remove particulate material present in the sludge. The supernatant containing biomass was collected and enriched with designed synthetic wastewater (DSW, Glucose – 3 g/l, NH4Cl – 0.5 g/l, KH2PO4 – 0.25 g/l, K2HPO4 – 0.25 g/l, MgCl2 – 0.3 g/l, CoCl2 – 25 mg/l, ZnCl2 – 11.5 mg/l, CuCl2
Biohydrogen production
The efficiency of H2 producing bioreactor was evaluated in terms of HPR and CHP. Experimental data depicted variation in HPR and CHP as a function of operating conditions, substrate metabolism and nature of biocatalyst (Fig. 1). HPR was better when glucose (80 ± 10 ml/h) was used as substrate when compared to formate (60 ± 5 ml/h). Thereafter, HPR declined gradually prior to reaching the baseline at 12th h. The CHP results also showed a maximum value with glucose (180 ± 10 ml) than formate
Conclusions
In this study, a relative comparison of H2 production potential of anaerobic consortia was evaluated using two pure organic substrates. The study revealed the functional role of substrate linked dehydrogenase activity which influenced H2 production. This was evidenced by bio-electrochemical analysis using cyclic voltammograms and Tafel plots. Reactor performance in terms of product formation and substrate degradation were affected by initial pH. Non-linear curve fitting and statistical analysis
Acknowledgments
The authors wish to thank the Director, CSIR-IICT for support and encouragement in carrying out this work. GNN duly acknowledges Council of Scientific and Industrial Research (CSIR) for providing Senior Research Fellowship. Part of the research was funded by CSIR in the form of XII five year plan project on ‘Sustainable Waste Management Technologies for Chemical and Allied Industries’ (SETCA; CSC 0113).
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