Effect of gas sparging on continuous fermentative hydrogen production

The effect of gas sparging on continuous fermentative H 2 production was investigated in completely stirred-tank reactors (CSTR) using internal biogas, N 2 and CO 2 with various flow rates (100, 200, 300 and 400 ml/min). The sparging with external gases of N 2 and CO 2 showed higher H 2 yield than the control of no sparging and internal biogas sparging. It indicated that the decrease of H 2 partial pressure by external gas sparging had a beneficial effect on H 2 fermentation. Especially, CO 2 sparging was more effective in the reactor performance than N 2 sparging, accompanied by higher production of H 2 and butyrate. The best performance was obtained by CO 2 sparging at 300 ml/min, resulting in the highest H 2 yield of 1.68 mol H 2 / mol hexose consumed and the maximum specific H 2 production rate of 6.89 L H 2 / g VSS/day. Compared to N 2 sparging, there might be another positive effect in CO 2 sparging apart from lowering H 2 partial pressure. High CO 2 partial pressure had little effect on H 2 -producing bacteria but inhibitory effect on other microorganisms such as acetogens and lactic acid bacteria which were competitive with H 2 -producing bacteria. Only H 2 -producng bacteria, such as Clostridium tyrobutyricum, C. proteolyticum and C. acidisoli were isolated under CO 2 sparging conditions based on 16S rDNA analysis by PCR-DGGE.