Since hydrogen fermentation accompanies organic acids production, it is needed to consider total system combined subsequent treatment methods such as methane fermentation. In hydrogen fermentation, 4 mol of hydrogen is theoretically produced from 1 mol of glucose (Eq. (5.4.3)). When subsequently formed acetate is utilized for methane fermentation and converted to methane, the reaction is shown as follows:

2CH3COOH — 2CH4 + 2CO2 (5.4.5)

C6H12O6 + 2H2O ^ ЗСО2 + 4H2 + 2CH4 (5.4.6)

The sum of high heat value of these products is 2.924 MJ (2,924 kJ). On the other hand, in only methane fermentation, the reaction is shown as follows:

C6H12O6 -► ЗСО2 + 3CH4 (5.4.7)

High heat value of the product from Eq. (5.4.7) is 2.671 MJ (2,671 kJ). It is obvious from these results that the energy yield of hydrogen-methane fermentation increases 10% compared with that of only methane fermentation.