Environmental Engineering Reference
In-Depth Information
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Photofermentation.
The most immediate priority for photofermentative 112
production, given its high (nitrogenase-based) energy requirement as well as its need
for organic 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 H2-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 conditions, which form the next priority. Genetic modifications of
antennae to limit self- shading and to allow utilization of a greater portion of the
solar spectrum remain a high priority for all photobiological H2-production
techniques.
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Water-Gas Shift Mediated H2 Production
. A major challenge for the water-gas shift
reaction is the achievement of efficient transfer of synthesis gas into aqueous
solution, as the CO must be available to the bacteria at sufficient concentrations to
allow efficient metabolism. Genetically, in addition, greater understanding of the
interaction between CODH and the hydrogenase might indicate features amenable to
optimization, and investigation of the O2-tolerant R. gelatinosus hydrogenase to
understand the basis for its resilience would be valuable to all efforts to engineer
improved O2-tolerance in hydrogenases.
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Dark fermentation
. Dark fermentation is a promising avenue for biohydrogen
production that could benefit greatly from progress in a few key areas. Improvement
in gas separation technology, as well as gas removal from cultures during
fermentation, are uniquely crucial to economically feasible H2 production by dark
fermentation, for fermentations are constrained to generate mixtures of gases and are
typically subject to strong end-product inhibition. Investigation of this problem is
underway using hollow fiber membrane technologies, resulting in a 15 percent
improvement in H2 yield, as well as using other synthetic polymer membranes, but
further advances are achievable. Interestingly, both of these problems may also be
addressed through genetic engineering. Because most fermentation pathways are
well-understood at both the genetic and physiological levels, and because many of
the most attractive fermentative organisms are genetically tractable, great potential
exists to enhance, diminish, or even to alter the products of specific fermentative
pathways through metabolic engineering, as well as to diminish the sensitivity of
crucial enzymes to end-product inhibition. Further gains in fermentative H2
production will also require optimization of bioreactor designs.
K.
B
IODESULFURIZATION OF
F
OSSIL
F
UELS
While a number of issues described above have been fully addressed, additional effort is
still needed to expand the substrate specificity of the desulfurization process to include
smaller compounds, allowing efficient desulfurization of gasoline, as well as to include
compounds with sterically obstructed S atoms, allowing more complete desulfurization of all
fossil fuels. In addition, metabolic engineering to increase the availability of FMNH2 to the
