The selection of the correct pH level is crucial to hydrogen production, due to the effects of pH on the hydrogenase activity or specific metabolic pathways (Dabrock et al., 1992). At pH values lower than 6.3 or higher than 7.8, the methanogenic rate may decrease or stop. Using a low pH environment to inhibit methane production in the acidogenic phase and to obtain dominant microbes for hydrogen production from sludge is considered feasible. Van Ginkel et al. (2001) reported the highest rate of hydrogen production occurred at an initial pH of 5.5, with a sucrose substrate concentration of 7.5 g COD/L, in batch experiments using composts as the inoculum. Lee et al. (2002) investigated the effects of initial pH in the batch culture using sucrose from mixed micro-flora. Their results show that maximum specific hydrogen production yields were achieved at an initial pH of 9.0, whereas small amounts of hydrogen production were observed at pH values of 5.0 and 5.5. These experiments investigated the effects of initial pH on hydrogen production without a well-maintained buffering capacity for preventing a sharp decrease of pH. Some investigators reported effects of pH on the continuous production of hydrogen by mixed culture, the optimal values being 5.5, 4.0-4.5 and 4.7-5.7 for fermentation of glucose (Fang and Liu, 2002), sucrose (Ren et al., 1995), and starch (Lay, 2000), respectively.
HRT is also a key factor for enhanced hydrogen production. Some investigators have observed the effect of HRT for hydrogen production along with the effect of pH on anaerobic continuous systems. When fed with glucose or sugary synthetic wastewaters, the reported maximum hydrogen production was 1.0-2.1 mol H2/mol hexose at a short SRT and low pH (Ueno et al, 1996; Lin and Chang, 1999; Sung et al, 2002).
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