Biomedical Engineering Reference
In-Depth Information
Recently, significant progress has been made on efficient conversion of various
volatile fatty acids (especially acetic acid) to hydrogen via photofermentation [ 76 ]
or with a microbial electrolysis cell (MEC) using exoelectrogens [ 77 ]. Thus,
combination of dark fermentation and photofermentation or coupling of dark
fermentation and an MEC could convert sugars more efficiently into hydrogen.
4.2 Combination of Dark Fermentation and Photofermentation
Two schemes for the combined process of dark fermentation and photofermentation
have been studied: an integrated process and a sequential process. The integrated
process allows dark-fermentative and photofermentative bacteria to be co-cultured
in one system, saving space and operational costs. However, the first concern with
the co-culture system is the ratio of the two groups of bacteria, because dark-
fermentative bacteria always grow faster than photofermentative bacteria. In 2006,
Fang et al. [ 78 ] co-cultured Clostridium butyricum and Rhodobacter sphaeroides,
and observed a lag phase of 10 h before hydrogen was produced. They further
studied the effect of different ratios of the two bacteria, and found that the maximum
hydrogen yield was 0.6 mL H 2 /mL medium when the ratio of C. butyricum and
R. sphaeroides was 1:5.9. In 2009, Liu et al. [ 79 ] studied a coupling system con-
taining C. butyricum and Rhodopseudomonas faecalis RLD-53, and found that the
maximum hydrogen yield reached 2.45 mL H 2 /mL culture at a ratio of 1:500. They
also studied the combination of suspended E. harbinense B49 and immobilized
R. faecalis RLD-53, in which phosphate buffer solution (PBS) was used to reduce
the effect of pH variation caused by the difference in the growth rates of the two
bacteria. The results indicated that higher PBS concentrations enhanced the ratio of
acetate to ethanol, and the hydrogen yield was increased by two to three times
compared with dark fermentation, reaching 6.32 mol H 2 /mol glucose [ 80 ].
However, owing to the great difference in the growth rate and acid-resistant
capacity between photofermentative and dark-fermentative bacteria, they found
that the system was very difficult to operate.
Compared with the integrated process, the sequential process is easier to
operate and control because the two groups of bacteria are in individual reactors
which can be operated independently [ 81 - 84 ]. With use of two-step coupling
systems, the fermentation products from the dark fermentation are converted into
hydrogen during the photofermentation as illustrated in Table 3 . Tao et al. [ 81 ]
adopted such a sequential process for hydrogen production. They used photo-
trophic R. sphaeroides SH 2 C to produce hydrogen from the effluent of dark
fermentation, achieving a total hydrogen yield of 6.63 mol H 2 /mol sucrose. The
effect of initial pH and C/N ratio on the dark-fermentative hydrogen production
was also determined. Su et al. [ 83 ] applied anaerobic sludge as seed sludge for
dark-fermentative hydrogen production from cassava starch, followed by photo-
fermentation by Rhodopseudomonas palustris. The cumulative hydrogen produc-
tion was 402 mL H 2 /g starch, which was substantially higher than the hydrogen
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