Development of a sulfate reducing packed bed bioreactor for use in a sustainable hydrogen production process
McMahon, Matthew James Lee
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A two-stage process is proposed that is based on the biological production of H2S from organic waste and its subsequent overall conversion to H2 via an exothermic reaction. The current study examined the first step of this process, namely, the design and operation of a packed bed bioreactor with high volumetric H2S production (mol/m3.d) and its comparison to analogous methanogenic technology. A novel method of inoculum design was developed by evaluating the kinetics and immobilization potential of Desulfovibrio desulfuricans (ATCC 7757) and a sulfate reducing bacteria (SRB) consortium. The consortium’s kinetics, as measured by the specific rate of sulfate reduction (1.2 g SO42-/g CDW.h), were approximately twice as fast as those of D. desulfuricans. The pure strain however exhibited superior immobilization potential. Studies revealed that a mixed inoculum containing 96 % D. desulfuricans and 4 % consortium facilitated the rapid immobilization of a highly active SRB biomass and contributed to improved bioreactor performance. Diatomaceous earth (DE) pellets, porous glass beads, polyurethane foam, and bone char were evaluated as potential carrier materials for SRB immobilization. The DE pellets immobilized the most biomass, were well suited for use at the industrial scale, and were thus employed in all continuous flow bioreactor experiments. Using the designed inoculum and DE pellets, a 615 mL bioreactor achieved a volumetric productivity of 493 mol H2S/m3.d (at D = 1.6 h-1) and a dissolved sulfide concentration of 9.9 mM. This occurred after 8 d of operation and represents a tenfold reduction in the required start-up period compared to similar bioreactors in the literature. An N2 strip gas was later used to remove the dissolved sulfide to the gas phase and enhance sulfate conversion. Shifting the medium pH from 7 to 6 increased the fraction of strippable sulfide and improved the strip gas composition from 3.6 to 5.8 mol % H2S. The strip gas to liquid feed ratio (G/L, m3/m3) was investigated in the range of 0-14 and was found to be a suitable basis for scale-up indicating that productivities of up to 830 mol/m3.d were readily achievable. This represents a considerable improvement over current methanogenic bioreactor productivities.