Microbiology of anaerobic biohydrogen production

Anaerobic hydrogen production can be divided into two main categories: one uses photosynthetic bacteria cultured under anaerobic or semi-anaerobic conditions in light and the other uses anaerobic bacteria that produce hydrogen via fermentation metabolism in dark conditions (Benemann, 1996). Hydrogen production by the dark fermentation process is much simpler than the photo-biological process, and the fermentation process generates hydrogen from a large number of carbohydrates frequently obtained as refuse or waste products (Nandi and Sengupta, 1998).

When only considering theoretical hydrogen yields, the photo-biological process using photosynthetic bacteria and algae has good economic potential. However, the process requires large surface areas for photo-bioreactors to achieve the most efficient solar conversion, and its fermented broth adds to the existing water pollution problems (Zaborsky, 1998). One of the merits of anaerobic fermentative hydrogen production is a higher hydrogen synthesis rate compared with many other biological processes, as shown in Fig. 23.5 (Levin et al., 2004). The anaerobic fermentation system may have a more practical application by producing hydrogen on-site with a reduced reactor size.

Hydrogen production by anaerobic fermentation has been studied for a large group of pure fermentative bacteria; Escherichia coli, Enterobacter

to o

H Direct photolysis □ Indirect photolysis M Photo-fermentation

□ Mesophilic dark fermentation, pure strain

□ Mesophilic dark fermentation, undefined

H Thermophilic dark fermentation, undefined

□ Thermophilic dark fermentation, pure strain

Comparison of the rates of hydrogen synthesis in different biological production processes.

H Direct photolysis □ Indirect photolysis M Photo-fermentation

□ Mesophilic dark fermentation, pure strain

□ Mesophilic dark fermentation, undefined

H Thermophilic dark fermentation, undefined

□ Thermophilic dark fermentation, pure strain

Comparison of the rates of hydrogen synthesis in different biological production processes.

aerogenes, Clostridium butyricum, Clostridium acetobutyricum and Clostridium perfringens have been found to produce hydrogen under anaerobic conditions. Escherichia coli and Enterobacter aerogenes are facultative anaerobes, utilizing both glucose and lactose as carbon sources, and producing hydrogen as one of the fermentation by-products (Zajic et al., 1978; Minton and Clarke, 1989). The most effective hydrogen production (2 mol H2/mol glucose) was observed upon fermentation of glucose in the presence of Clostridium butyricum (Zeikus, 1980; Heyndrickx et al., 1987; Taguchi et al., 1992). Clostridium species are Gram-positive, spore-forming, rod-shaped bacteria. When stressed, these bacteria produce spores that tolerate extreme conditions that vegetative bacteria cannot (Reimann et al., 1996). Their endospores are very resistant to heat or harmful chemicals, including acids, and cannot be destroyed easily (Reimann et al., 1996).

23.10.4 Treatments to enrich hydrogen-producing bacteria from mixed anaerobes

During anaerobic digestion, methanogenic or sulfate-reducing bacteria consume hydrogen produced by acidogenic bacteria, contributing negatively to bio-hydrogen production. Effectively extracting hydrogen from an anaerobic bioreactor depends on special procedures that block out the co-degradable chains (Adams and Stiefel, 1998). Heat treatment of sludge used as an inoculum is one feasible method of increasing hydrogen production by inhibiting the activity of non-spore-forming hydrogenotrophic bacteria. Lay (2000) and Okamoto et al. (2000) used wet heat treatment of anaerobic digester sludge, whereas van Ginkel et al. (2001) used dry heat treatment of compost and soils to inactivate hydrogen-consuming microorganisms and to select hydrogen-producing bacteria. Cheong and Hansen (2006) achieved maximum specific hydrogen production of up to 380 mL H2/g cell mass after 96 h using inocula that had been enriched at pH 3.

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