Sunday, April 29, 2007

Bioreactors for molecular hydrogen production by purple nonsulfur bacterium

Sergei A. Markov, Biology, Austin Peay State University, College Avenue, Clarksville, TN 340044

Two types of laboratory-scale bioreactors were designed for H2 production by purple nonsulfur bacterium.  Bioreactors employed a unique type of hydrogenase activity found in some photosynthetic bacteria that functions in darkness to shift CO (and H2O) into H2 (and CO2).  Mass transport of gaseous CO into an aqueous bacterial suspension was the rate-limiting step and the main challenge for bioreactor design.  Hollow-fiber and bubble-train bioreactors employing immobilized and suspended bacteria have proven effective to enhance mass transfer of CO.  The hollow-fiber bioreactor was designed such that both a growth medium and CO (10% in N2) passes from the inside of the fibers to the outside within the bioreactor.  Bacteria were immobilized to the outer surface of the hollow fibers.  Hydrogen production from CO at an average rate of 125 ml∙ g cdw-1∙ h-1 (maximum rate of 700 ml∙ g cdw-1∙ h-1) was observed for more than 8 months.  The bubble-train bioreactor was built using PVC tubing, wound helically on a vertical cylindrical supporting structure.  Small bubbles containing CO were injected continuously through a needle/septum connection from the gas reservoir (20% CO).  Up to 140 ml∙ g cdw-1∙ h-1 of H2 production activity was observed using this bioreactor for more than 10 days.