for organic carbon substrates between two organisms in the
same medium also needs to be investigated [9].
Concentration and processing of cell biomass. In an integrated
system, cell biomass from either green agae/cyanobacteria or
photosynthetic bacteria can serve as the substrate for dark
fermentation. The green algal and cyanobacterial cell walls are
made mostly of glycoproteins (sugar-containing proteins), which
are rich in sugars like arabinose, mannose, galactose, and
glucose. Purple photosynthetic bacterial cell walls contain
peptidoglycans (carbohydrate polymers cross-linked by protein,
and other polymers made of carbohydrate protein and lipid).
Pretreatment of cell biomass may be necessary to render it more
suitable for dark fermentation. Methods for cell concentration
and processing will depend on the type of organism used and
how the biological system is integrated [9].
3.1.2.6. Water-gas-shift. Finally, certain photoheterotrophic bacteria
in the family Rhodospirllacae have been found which can grow
in the dark by feeding only upon CO[132]. The oxidation of the CO to
CO2 was determined to follow theWGS reaction (Eq. (10)), but uses
enzymes rather than metal to catalyze the process. Since it occurs at
low temperatures and pressures, thermodynamics favor a high
conversion of CO to CO2 and H2 [132]. Its conversion rate is actually
relatively high compared to other biological processes, but it does
require a CO source and darkness [132].
3.1.2.7. Production rates comparison. Levin et al. [132] compiled a
table comparing hydrogen synthesis rates by different technologies
which is adapted in Table 5. Although there have been some
advances since Levin et al. published their findings in 2004, the
table does provide order of magnitude estimates for the
approximate size of the reactors for hydrogen production. One
of the major challenges to this technology is the slow hydrogen
production rate. For example, a 5 kW PEM fuel cell, sufficient to
provide residential power, requires approximately 119.5 mol H2/h
(95% H2 utilization, 50% efficiency). Therefore a bioreactor ranging
from 1 to 1700 m3 would be required to provide the hydrogen
[132]. The complete system with controls and balance of plant
equipment is not included in the size estimate.
3.2. Hydrogen from water
There has been a great deal of research in splitting water to
make hydrogen and oxygen; in fact its commercial uses date back
to the 1890s [165]. Water splitting can be divided into three
categories: electrolysis, thermolysis, and photoelectrolysis.
3.2.1. Electrolysis
Water splitting in its simplest form uses an electrical current
passing through two electrodes to break water into hydrogen and
oxygen. Commercial low temperature electrolyzers have system
efficiencies of 56–73% (70.1–53.4 kWh/kg H2 at 1 atm and 25 8C)