Environmental Engineering Reference
In-Depth Information
substrate conversion efficiency of approximately 28 percent with readily-fermented sucrose
and glucose [98, 99]. Moreover, thermophilic systems are calculated to consume
unacceptably great quantities of energy to maintain high temperatures [5]. To become
economically attractive, therefore, research in mesophilic dark fermentations must explore the
feasibility of using organic waste streams and less-expensive organic substrates such as
lignocellulose; pioneering efforts in this direction are few to date, but have already achieved
early success and should be encouraged further [34].
2.7.4. Research priorities
. 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 112 production by dark fermentation, for fermentations are
constrained to generate mixtures of gases and are typically subject to strong end-product
inhibition [94]. Investigation of this problem is underway using hollow fiber membrane
technologies, resulting in a 15 percent improvement in 112 yield, as well as using other
synthetic polymer membranes, but further advances are achievable [5].
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 [34].
Further gains in fermentative 112 production will also require optimization of bioreactor
designs; for example, the highest rate shown in Table 5 involved the use of activated carbon
fixed-bed bioreactors that allowed retention of the 112-generating bacteria [5].
3. Research Priorities
Because of the diversity of approaches to biohydrogen production (direct and indirect
photolysis, photofermentation, the water-gas shift pathway, and dark fermentation), specific
research priorities have been summarized at the end of each section above. These emerging
technologies have been carefully investigated for future practicability, and all face
challenging problems that are nevertheless approachable by creative use of metabolic and
chemical engineering. Given continued investment, each of the major biohydrogen pathways
should be able to find a niche in a future sustainable-energy economy by delivering
competitively-priced H2 at commercial scales.
4. Commercialization
While biohydrogen systems exist at the pilot scale that can produce H2 continuously
from direct photolysis [44], indirect photolysis [12], photofermentation [19], the water-gas
shift reaction [90], and dark fermentation [5], no commercial systems are yet available, and
many questions regarding the practical applications of biohydrogen remain to be answered. In
particular, it is not yet clear whether biohydrogen systems can be integrated with hydrogen
fuel cell technologies to generate electricity at practical scales [5]. A major limitation to
