Most continuous studies for hydrogen production have investigated hydrogen production using a completely mixed system. Although various organic loading rates have been investigated, conversion efficiencies have not been equal to the theoretical value of 0.467 L H2/g COD for acetic acid fermentation (Majizat et al., 1997; Lin and Chang, 1999; Fang and Liu, 2002). Some have tried to try increase hydrogen yields in a CSTR system by applying a vacuum to the head space of the reactor (Kataoka et al., 1997), by sparging with nitrogen gas (Mizuno et al, 2000) or by vigorously stirring to allow the dissolved hydrogen to escape into the gas phase (Lamed et al., 1988). Another process aimed at increasing hydrogen production was investigated using various immobilized microorganisms (Bacillus licheniformis, Clostridium butyricum, a mixed microbial culture) on brick dust, calcium alginate beads or polyacrylamide gel (Suzuki and Karube, 1983; Kumar and Sharma, 1995).
Two-phase or two-stage systems have potential as processes to achieve greater extraction of hydrogen from organic wastewaters. One example is a two-phase process using a combined culture system of anaerobic fermentative bacteria and photosynthetic bacteria to produce the most hydrogen from organic carbon sources. In this manner, metabolites, such as VFAs produced during anaerobic hydrogen fermentation, can be further degraded with photosynthetic bacteria, and more hydrogen is produced during this process (Kataoka et al., 1997). However, using the photosynthetic bacteria may cause additional waste recycling; thus, increasing the treatment needs of host facilities (Henley et al., 2003). High biomass-retaining reactors also have potential for economical production of hydrogen from anaerobic fermentation (Cheong and Hansen, 2006).
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