ReviewFermentative hydrogen production in anaerobic membrane bioreactors: A review
Introduction
Hydrogen represents one of the highly attractive directions in alternative energy research (Winter, 2009). It is an environmentally gentle compound which can be formed by several biological ways including both the light-dependent and dark fermentative processes (Show et al., 2012). Nowadays, considering practicality aspects, the latter class seems more feasible and therefore not only receives high scientific attention in laboratories but also there is a remarkable, ongoing progress towards scaling-up. As a result, a couple of pilot plants have recently been established (La Licata et al., 2011, Lin et al., 2011) and demonstration as well as full-scale facilities may be expected (Guo et al., 2010). Although fermentative hydrogen production is undoubtedly promising and it is developing step by step to a level of real field applications, scientists need to spend additional efforts to enhance the overall process efficiency, preferentially by using waste materials (Sinha and Pandey, 2011). In particular, from the upstream point of view, further advancements are essential to attain better generation rates and yields so that hydrogen can be made more competitive with other energy carriers e.g. in economical terms (Hallenbeck and Ghosh, 2009). Nevertheless, it has been shown that the fate of biohydrogen is also dependent on the successfulness of the downstream technology which may contribute to the intensification of the production side (Bakonyi et al., 2013).
Hence, various biological and engineering approaches have been suggested with the aims mentioned, such as the construction of more sufficient and robust hydrogen producer microorganisms (metabolic- and genetic engineering), fermentation optimization and bioreactor design (Guo et al., 2010). All of these approaches possess high importance because strains require proper surroundings (e.g. pH, temperature, H2 partial pressure, mass transfer, etc.) to express their advantageous properties (Wang and Wan, 2009). Moreover, since bioreactors are the places of the microbiological hydrogen conversion, their quality features such as type and configuration significantly affect the applications reliability. In the last decade, as a response to the demand for biosystems with upgraded hydrogen generation performance, several researchers have started to deal with the novel and innovative way of combining traditional hydrogen fermenters with membrane technology. Recently, our group comprehensively assessed the integration possibilities of membranes and bioreactors for biohydrogen recovery and enrichment in gas separation membrane bioreactors (Bakonyi et al., 2013) or in other words, in hydrogen extractive membrane bioreactors (Ramírez-Morales et al., 2013). This is one particular way to establish membrane-based systems for fermentative hydrogen technology. Another one is the design of anaerobic bioreactors employing membranes in the liquid phase, which are in the scope of the present paper. Although a couple of review papers have recently been published on anaerobic membranes bioreactors (AnMBRs) (Lin et al., 2013, Ozgun et al., 2013, Singhania et al., 2012, Smith et al., 2012) and their potential for hydrogen production was enlightened (Gallucci et al., 2013, Jung et al., 2011), H2 production in systems combining liquid filtration membranes has not specifically been addressed and evaluated so far.
Therefore, this work attempts to overview the progress on the anaerobic membrane bioreactors used in the fermentative hydrogen technology. Firstly, the main features of conventional, anaerobic membrane bioreactors are presented. Thereafter, several main process considerations (retention time, nutrient loading, membrane related issues) affecting the performance of anaerobic hydrogen producing membrane bioreactors (AnHPMBR) are discussed. Finally, the feasibility of AnMBRs for biological hydrogen generation in comparison to the traditional CSTRs will be evaluated.
Section snippets
General features of AnMBR systems
AnMBRs have been used for a long time in different fields, mostly in waste water treatment for process intensification purposes even at full-scale plants (Judd, 2008).
Integrated systems assisted by membranes – being either aerobic or anaerobic and regardless the purpose of use – can be distinguished as external loop (Fig. 1A) and submerged (Fig. 1B) bioreactors (Yang et al., 2006). In the former case, as indicated in Fig. 1A, the liquid filtration membrane module is linked to the reactor from
The effect of solid- and hydraulic retention times in AnHPMBRs
Hydrogen bioproduction by continuous cultures is frequently carried out in well-mixed vessels in which proliferation of microorganisms is determined by the dilution rate applied, presenting a potential risk for biomass washout (Li and Fang, 2007, Show et al., 2008). Therefore, decoupling hydraulic- (HRT) and solid/biomass retention times (SRT) in anaerobic, hydrogen producing bioreactors (Table 1) possesses several benefits.
Preserving cells in continuous bioreactors can be accomplished in
Conclusion
The present review on anaerobic membrane bioreactors – despite the limited number of relevant papers – indicates that these integrated systems are attractive for biohydrogen production and can be considered as alternative solutions to the most common CSTR applications. However, more research dedication is needed for the further development of the field e.g. to get a better understanding about the interrelationship of bioreactors and t coupled membranes, which is a key factor to achieve better
Acknowledgements
This work was supported by the European Union and financed by the European Social Fund in the frame of the TAMOP-4.2.2/A-11/1/KONV-2012-0071 project and the János Bolyai Research Scholarship of the Hungarian Academy of Sciences.
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