Hydrogen production from a thermophilic alkaline waste activated sludge fermenter: Effects of solid retention time (SRT).

This study aims to investigate the effects of solid retention times (SRTs) on hydrogen production via thermophilic alkaline fermentation of waste activated sludge. The reactor was subjected to a SRT from 10 to 6 days during approximately 82 days of operation. The results revealed that SRT had minor influence on hydrolysis and hydrolysis efficiency in different phases were from 48.11% to 50.55%. Nevertheless, the efficiency of acidogenesis process was highly related to SRT and longer SRT could enhance the acidogenesis. On the other hand, acidogenesis efficiency was also related to H2 partial pressure and high H2 partial pressure negatively affected the acidogenesis. Thus, the maximum acidification was achieved in phase 1 (21.29%) resulting in the maximum H2 yield in phase 1 (95.94 mL/g VSS; SRT = 10 days; H2 partial pressure = 0-18%). Phyla Actinobacteria and Proteobacteria, who are highly related to hydrolytic microbial population, were abundant in all phases that resulted in high hydrolysis extent. H2 production was attributed to the relative high abundance of Clostridia. Thus, this study suggested that longer SRT and lower H2 partial pressure was necessary to improve the H2 yield under alkaline pH condition.