Anaerobic biohydrogen production by a fluidized granular bed bioreactor under thermophilic condition
There is now a critical need for development of full-scale practical application of fermentation technologies for energy generation (e.g. hydrogen production) that would be dependent on carbon neutral fuels such as biomass or wastewaters containing organic materials. Thermophilic fermentative biohydrogen production was studied in the anaerobic fluidized bed reactor (AFBR) operated at 65ºC with sucrose as a substrate. Theoretically, the maximum hydrogen yield (HY) is 4 mol H2.mol-1 glucose when glucose is completely metabolized to acetate, H2 and CO2. But somehow, under most bioreactor design and operation conditions the maximum possible hydrogen yield (HY) has generally been observed not to exceed or reach 70-100% of the maximum theoretical hydrogen yield. In this study the application of external work in the form of high temperatures, high dilution rates and high rates of de-gassed effluent recycling were investigated as a means to overcome the thermodynamic constrains preventing the simultaneous achievement of high hydrogen yield (HY) and hydrogen productivity (HP) in an AFBR reactor. Bacterial granulation was successfully induced under a thermophilic temperature of 65 oC within a period ranging from 7 to 14 days. The bacterial granules consisted of a multispecies bacterial consortium comprised of thermophilic clostridial and enterobacter species. At a hydraulic retention time (HRT) of 1.67 h and effluent recycle rate of 3.5 L min-1, hydrogen production rate (HPR) of 32.7 L H2/h and hydrogen yield (HY) of 3.91 mol H2/ mol glucose were achieved. The design and operation of our bench scale AFBR system has also resulted in HYs greater than 4 mol H2/mol glucose. The maximum substrate conversion efficiency was 95%. However, it was noted that at very low HRTs (< 1h) the bioreactor substrate conversion efficiency dropped to 55%. This work demonstrated that the application of external work to a bioreactor in the form of high temperatures, high dilution rates and high rates of de-gassed effluent recycling could be used to overcome the thermodynamic constraints preventing the simultaneous achievement of high HYs and high HPs.