Photobiohydrogen production and high-performance photobioreactor - Solar Energy Sciences and Engineering Applications - page 362

between microorganisms and substrates, it is difficult to ensure long-term operation
and high photobiohydrogen production performance due to low biomass caused by
wash-out. Therefore, ever-increasing attention has recently been turned to the pho-
tobiohydrogen production by immobilized microbial cells for increasing the biomass
concentration. As such, not only the volumetric productivity and stability and light
utilization efficiency are increased, but also the ability to recover and reuse the cell
mass is also enhanced (Das and Veziroglu, 2001). Typically, cells can be immobilized
by cell entrapment and biofilm to dramatically increase the biomass concentration in
the photobioreactor.
11.4.3.1 Cell entrapment
The features provided by cell entrapment include the lower cost and easy operation.
As the cells are entrapped, an anaerobic environment is automatically created. More-
over, the entrapment materials allow the system to be stably operated at relatively high
flow rate without suffering from cell wash-out. However, it should be pointed out that
the cell entrapment requires strict entrapment materials, high mechanical strength,
sufficient light supply and low mass transfer resistance, which are currently unsatis-
fied with the state-of-the-art technique. Therefore, many researchers have attempted
to solve these problems for improving the photobiohydrogen production by the cell
entrapment technique.
A variety of support materials such as agar, agarose, alginates, pectin and car-
rageenan have been tested (von Felten et al., 1985). The results showed that agar was
the best immobilizing agent according to the photobiohydrogen production rate and
stability, which were also superior to the suspension culture. The photobiohydrogen
production performance with the purple nonsulphur bacterium Rhodopseudomonas
palustris DSM 131 immobilized by agar, agarose, carageenan and sodium alginate were
also explored (Fissler et al., 1995). It was shown that the cells immobilized in agar,
agarose and carageenan yielded low photobiohydrogen production performance as
compared to suspended cells due partly to cell damage at the temperature of 45-50 ◦ C
during the immobilization process in these matrices. However, with sodium alginate
beads produced at room temperature, a higher photobiohydrogen production perfor-
mance than suspended cell was achieved. In addition, this work also indicated that
the reduction of gel bead diameter can increase the surface-to-volume ratio, enable
the easy access of the substrate to the cells and an increase in the light supply per
immobilized cell, both of which improved the hydrogen yield.
As mentioned earlier, the mechanical strength and light supply of the entrapment
technique are poor. Planchard et al. (1989) entrapped cells in a planar agar matrix
bounded by a microporous membrane filter. The addition of a microporous membrane
significantly enhanced the mechanical strength. However, although the mechanical
strength is enhanced by this microporous membrane, the light supply is still inefficient.
In order to improve the light supply, therefore, a gel layer/microporous membrane
structure with an inner optical fiber illumination device was developed (Mignot et al.,
1989), which can guide light from an external source to the entrapped cells, suppressing
thermic effect and ensuring a homogeneous illumination and a large active surface.
Another method to utilize optical fibers was to directly immobilize cells onto the surface
of optical fibers using alginate gel (Yamada et al., 1996). In this design, the optical fibers