Hydrogen (H2) is a desirable transportable fuel for the carbon-neutral future because, when burned, it emits water vapour as the only exhaust gas and it is an ideal fuel for fuel cells. Biological H2 (Bio-H2) generation from the fermentation of organics is regarded as a promising method for sustainable H2 production as it does not require any non-renewable resources and the feed stock is readily available. However, low H2 yields limit the application of fermentative H2 production. Therefore, the overall objective of this thesis was to optimize the H2 yield from organics fermentation. The approach that was taken in this investigation involved the following disciplines: 1. enrichment of stable H2 yielding cultures from environmental sources, 2. microbial community structure studies of the enriched cultures and, 3. the configuration of reactor systems for reducing dissolved H2 concentrations.
The culture selection and enrichment process has successfully enriched a H2 producing culture from the leachate of an anaerobic digester. The H2 yield of this culture was 2.16 ± 0.05 mol H2/mol glucose. A passaging procedure was developed to maintain the enriched culture in sequential batches for over 2 years. The stability of the culture was reflected in the consistency of the H2 yield over this period.
Phylogenetic analysis of the enriched culture showed that the culture was dominated by bacteria, within which 99.8% of the overall population belongs to the genus of Thermoanaerobacterium. FISH analysis using a published probe confirmed that over 90% of the overall population was dominated by one specie Thermoanaerobacterium thermosaccharolyticum, which is a reported H2 producer.
The first set of reactor studies explored the use of H2 permeable membranes that could serve as both a substratum and as H2 sink, submerged into the culture. The submerged H2 permeability of a Carbon Template Molecular Sieve Silica (CTMSS), γ-alumina nanofiltration and Polytetrafluoroethylene (PTFE) membranes were measured. Diffusion modeling indicated that the permeability of the most permeable membrane, PTFE, was sufficient to maintain dissolved H2 concentrations below 20Pa from an active fermenting culture settled directly onto the membrane. Actual trials with the membranes highlighted that water fouled the membrane pores. However, trials with CTMSS membrane, which was most resistant to water fouling, showed that H2 produced by the culture was extracted through the membrane rather than accumulating in the liquid phase or in the headspace of the reactor during the first 24 hours of the culture, prior to water fouling.
The ultimate reactor study explored the use of membranes to protect biomass from the shear forces associated with sparging to remove dissolved H2. A CSTR reactor was modified by adding a sparging recycle circuit where rigid stainless steel sintered tubes were used to retain biomass in the reactor while H2 ladened media was sparged with N2:CO2 mixture in a separate chamber. The flow direction was reversed regularly, to avoid biomass fouling the membranes. Such a bioreactor system is called perfusion system. Fermentation experiments with the enriched culture showed that by controlling the recirculation rates and glucose feeding rates, the dissolved H2 concentration within the fermentation system could be reduced to less than 3000Pa, as verified with direct measurements of dissolved H2. A maximum H2 yield of 2.67 ± 0.08 mol H2/mol glucose was observed.
The developed fermentation model, which uses the dissolved H2 concentration as the key regulating factor, predicted the glucose fermentation products observed during the continuous feeding operations except butyrate.
The outcomes of this thesis indicate that dissolved H2 concentrations play an important regulatory role in fermentation processes and therefore H2 yields can be greatly improved if dissolved H2 concentrations can be maintained at low levels. The advanced reactor systems developed in this thesis, especially the perfusion system, have great potential to promote H2 production.