Start-up strategy for continuous fermentative hydrogen production: Early switchover from batch to continuous operation

The start-up period for fermentative hydrogen production was investigated using a continuous-flow stirred-tank reactor (CSTR) inoculated with heat-treated anaerobic sludge. To germinate endospores of H2H2-producing bacteria (HPB), continuous feeding was postponed until H2H2 production from the initial batch feeding reached a given level. In the first trial, continuous feeding started when the initial H2H2 production was equivalent to 0.5molH2/molhexoseadded. After the switchover from batch to continuous operation, H2H2 production was sustained for 12 h; however, it decreased with propionate production as the operation continued. We hypothesized that propionic acid bacteria (PAB), whose abundance had been reduced by heat-shock but not completely eliminated, grew during the initial batch operation mode and resulted in the failure of the following continuous mode. PAB such as Selenomonas   spp. were detected only in an unsuccessful start-up based on 16S rDNA analysis by PCR denaturing gradient gel electrophoresis, which agrees with our hypothesis. To avoid H2H2-consuming propionate production, an earlier switchover from batch to continuous operation, at 0.2molH2/molhexoseadded, was tested in the second trial. In this case, H2H2 production fluctuated during the 10 days of continuous operation but eventually stabilized at 0.8molH2/molhexoseadded. Alcohol production data showed that the early fluctuation in the second trial resulted from a selfdetoxification mechanism of HPB.