Biological Hydrogen Production as a Sustainable Green Technology for Pollution Prevention
Bruce Logan, Dept. of Civil and Environmental Engineering;
Mary Ann Bruns, Dept. of Agronomy and Soil Science
Funding: National Science Foundation
(January 2002 - December 2005)
Hydrogen gas can fill an important role in the "greening" of the global energy and industrial base. Hydrogen is not a greenhouse gas, it has 2.4 times the energy content of methane (mass basis) and its reaction with oxygen in fuel cells produces only harmless water. Not only can pollutants from fuels used in high-temperature combustion engines be avoided using hydrogen-based fuel cells, but the elimination of combustion also avoids the generation of NOx. As a result of these advantages of hydrogen-based fuel cells, there is a global transition occurring to hydrogen-based technologies. However, hydrogen is currently produced mostly from fossil fuels, an inherently non-sustainable technology.
Sustainable technologies must rely upon biomass-based technologies because these processes use renewable resources. Unfortunately, biomass-based technologies can increase production of wastewaters containing high concentrations of organic matter. If treated by aerobic processes in conventional wastewater treatment plants, these wastewaters represent a drain on the economic health of biomass-based renewable technologies and the environment. However, these wastewaters can also be viewed as a resource and not as a waste. High-strength wastewaters with a high sugar content are an important potential resource for biological hydrogen production.
For this project we will develop a new technology to create a biologically-based source of clean hydrogen gas which avoids pollution via wastewater generation. The main barrier to efficient conversion of dissolved organic matter in wastewater is preventing interspecies hydrogen transfer so that hydrogen generated through fermentation processes is not lost to anaerobic microorganisms. To prevent interspecies transfer, the microbes responsible for fermentation will be separated from those responsible for methane production using a novel two-tank bioreactor. In the first reactor we will use a unique approach to control cell detention time so that slow-growing methanogens will not be able to exist in the first tank and will therefore be unable to significantly degrade the hydrogen produced from fermentation. The reactor will be operated in a mode that has been shown to limit biofouling in suspended growth reactors.
The high hydrogen-content gas will be produced by seeding the fermentation reactor with a hydrogen-producing culture using a heat-shock process. Once established, this biomass will be maintained by control of cell detention time; biomass will be transferred to the second tank but only at a controlled rate.
We will also examine the impact of elevated hydrogen gas production on microbial community structure (i.e. the presence of various hydrogen-consuming bacterial species in the reactor). To examine these ecological responses, we will examine shifts in microbial community structure using 16S rDNA analysis.
The three specific tasks that will be accomplished in this project are therefore:
Examination of maximum hydrogen production versus thermodynamic predictions, using batch and continuous flow reactors;
Construction and testing of a novel bioreactor for hydrogen production (fermentation step only);
Community profiling of the microorganisms in the hydrogen-producing bioreactor.