Environmental Engineering Reference
In-Depth Information
2.5.2. Substrate range.
An attractive feature of photofermentation is the high substrate-
conversion efficiencies of many anoxygenic phototrophic bacteria, as well as their abilities to
use a wide variety of substrates for growth and H2 production. Malate, lactate, other organic
acids, sugars, and even some alcohols are used readily. The greatest potential value of
photofermentation for H2 production, however, depends on the use of complex substrates
such as those found in mixed organic wastes. In pursuit of this goal, numerous approaches
have been attempted, and early success has been achieved with use of dairy wastewater
blended with malate, sugar refinery wastewater also blended with malate, tofu wastewater,
wastewater of a lactic acid fermentation plant, and olive mill wastewater. An especially
promising approach for H2 production from wastewater involves the fermentative
pretreatment of wastes to generate small organic acids such as lactate and malate favorable
for H2 production [19].
2.5.3. Rate and efficiency
. In general, H2 production by photofermentation occurs more
rapidly when cells are immobilized or on a solid matrix; currently, the most rapid rates have
been obtained when the cells are immobilized in porous activated glass. In addition, some
substrates support more rapid H2 production than others. If an ideal laboratory system could
be scaled up without diminishing the rate of H2 synthesis, rates of 3.6-4.0 liters H
2
per liter
immobilized culture per hour would result, corresponding to 0.145-0.161 millimoles H
2
per
liter per hour (Table 5) [5].
Photofermentation generates H2 at the expense not only of sunlight energy but also of
organic substrates. Furthermore, the H2 is produced by nitrogenase, with the result that much
of the input energy is diverted to N2 fixation. The result is that photofermentation has a
calculated efficiency that is significantly lower than the theoretical efficiency obtainable by
direct photolysis. However, great hope lies in the possibility that photofermentation may be
adapted to the use of organic wastes, raising the practical efficiency a great deal [34].
In calculating efficiency, two components are typically considered: the substrate
conversion efficiency and the sunlight conversion efficiency. The substrate conversion
efficiency describes the percentage of a substrate utilized for H
2
production rather than
biosynthesis or growth, according to the equation:
CxHyOz + (2x - z)H
2
O - (y/2 + 2x - 2)H
2
+ xCO
2
Although purple non-sulfur bacteria can use a wide variety of substrates for
photoheterotrophic growth, only some of these are suitable for H
2
production. Substrate
conversion efficiencies vary by strain; those with the highest values for the well-studied
Rhodobacter sphaeroides include malate, lactate, and butyrate, ranging from 50-100 percent
[19].
The light conversion efficiency, in turn, is the ratio of the total energy (heat of
combustion) value of the H2 that has been obtained to the total light energy input to the
photobioreactor. For photofermentative H
2
production, light energy conversion efficiencies
range from 1-5 percent on average [19] substrates, is the development of cultivation
techniques and/or organisms that allow the use of organic wastes. Improved understanding of
the energy flow within the photofermentative H2- producing metabolism, including the
mechanisms by which organic substrates improve H
2
- production activity, would complement
these efforts greatly and should be quite achievable through available metabolic engineering
techniques. The nature of the wastes used will then inform the design of cultivation
