Hydrogen fermentation of food waste by alkali-shock pretreatment: microbial community analysis and limitation of continuous operation.

In the study, at first, batch tests were performed to investigate the effect of alkali-shock on H2 production from food waste (FW). After alkali-pretreatment of FW at pH 9.0-13.0, the FW was cultivated under mesophilic condition at pH 6.0 for 30 h without external inoculum addition. The amount of H2 production from FW pretreated at pH 11.0 and 12.0 was higher than that achieved in other pretreatment pH. The main metabolite was butyrate, and Clostridium were dominant at pH 11.0 and 12.0. Meanwhile, lactate was the main metabolite with Enterococcus and Streptococcus being the dominant genus at other pretreatment pH. When the batch process was switched to a continuous mode, H2 production was significantly dropped due to the increased activity of H2-consumers. The reliability of alkali-pretreatment at pH 11.0 was proven by repeating the scale-up batch process, recording 1.57±0.11 mol H2/mol hexose(added) (17±2LH2/kg FW) and 4.39±0.32LH2/L/d.

Effect of the accuracy of pH control on hydrogen fermentation.

pH, known as the most important parameter in H2 fermentation, cannot be precisely controlled in a scaled-up fermenter as in a lab fermenter. In the preset work, to assess the effect of pH control accuracy on H2 fermentation, the pH was controlled at 6.0±0.1, 6.0±0.3, 6.0±0.5, 6.0±0.7, and 6.0±0.9 during batch fermentation of food waste. Up to deviation of ±0.3, a high H2 yield of 1.67-1.73 mol H2/mol hexose(added) was attained with producing butyrate as a major metabolite (>70% of total organic acids produced). A huge drop of H2 production, however, was observed at deviation >±0.5 with lowered substrate utilization and increased production of lactate. Next generation sequencing results showed that Clostridium was found to be the dominant genus (76.4% of total number of sequences) at deviation of ±0.1, whereas the dominant genus was changed to lactic acid bacteria such as Streptococcus and Lactobacillus with increase of deviation value.