Biohydrogen Production from Anaerobic Fermentation - Biotechnology in China: Biofuels and Bioenergy - page 148

2.3 Hydrogen Fermentation Processes
Biohydrogen fermentation can be conducted in either batch or continuous mode.
Batch fermentation has been shown to be suitable for initial optimization studies,
but large-scale operation would likely have to be performed on a continuous or at
least semicontinuous (fed or sequencing batch) basis. In China, many studies have
employed CSTRs, upflow anaerobic sludge blanket (UASB) reactors, expanded
granular sludge bed (EGSB) reactors, and anaerobic baffled reactors (ABRs) for
biohydrogen production.
CSTRs have been the most widely applied bioreactor systems by far. They are
simple to construct, easy to operate, and offer effective homogenous mixing. In 1992,
Ren et al. patented a CSTR design (patent number ZL92114474.1) for continuous
biohydrogen production from wastewater. The bioreactor can be operated at high
concentrations of volatile suspended solids (VSS) ranging from 10 to 30 g/L, with a
maximum hydrogen yield and a maximum production rate of 10.4-11.4 m 3 /m 3 /day
and 36-40 mL/g VSS/h, respectively. Compared with CSTRs, UASB reactors retain
large amounts of granular sludge, and are operated for shorter hydraulic retention
times (HRTs) and thus higher organic loading rates (OLRs). Mu and Yu [ 39 ] treated
sucrose-rich synthetic wastewater in a UASB reactor using hydrogen-producing
granular sludge with a diameter of 1.0-3.5 mm, an average density of 1.036 g/mL,
and a settling velocity ranging from 32 to 75 m/h. Guo et al. [ 40 ] employed an EGSB
reactor with granular activated carbon as the carrier of active sludge for hydrogen
production from molasses-containing wastewater. Without pH control, the reactor
operated at 35 C, exhibited good hydrogen production performance with a yield of
3.47 mol H 2 /mol sucrose, and a specific production rate of 3.16 mmol H 2 /g VSS/h.
A similar EGSB reactor was also used to treat starch-containing wastewater for
hydrogen production, with a maximum hydrogen production rate of 1.64 L/L/day at
an OLR of 1.0 g starch/L/day, pH 4.42, and HRT of 4 h, and the average chemical
oxygen demand (COD) removal rate was 31.1% at an OLR of 0.125 g starch/L/day
and HRT of 24 h [ 41 ]. Li et al. [ 42 ] investigated hydrogen production from molasses-
containing wastewater using a three-compartment ABR with an effective volume of
27.48 L. At the stable operational conditions, the ABR exhibited a hydrogen yield of
32.51 L/day, a specific hydrogen production rate of 0.13 L/g molasses VSS/day, and
a substrate conversion rate of 0.13 L/g COD.
Apparently, a short HRT is preferred to reduce capital investment in bioreactors
[ 43 ]. Chu et al. [ 44 ] found that the population of hydrogen-producing bacteria
decreased gradually with the increase of the HRT: 9.2 9 10 8 , 8.2 9 10 8 ,
2.8 9 10 8 , and 6.2910 7 cells per milliliter at HRTs of 6, 8, 12, and 14 h,
respectively. At the optimal HRT of 6-8 h, the hydrogen yield was 1.4-1.5 mol/mol
glucose. On the other hand, a high OLR can reduce energy consumption for
hydrogen production. Yu et al. [ 45 ] investigated hydrogen production from high-
concentration winery wastewater with COD concentrations of 14-36 g/L, and
found the specific hydrogen production rate increased with the increase of the
COD concentration, and the maximum hydrogen production rate of 9.33 L H 2 /g