Partial pressure of the produced hydrogen is one of the important factors affecting the hydrogen yields. When converting reduced components such as the long chain fatty acids into hydrogen and volatile fatty acids the positive Gibbs free energy makes the system thermodynamically unfavorable. Also positive Gibbs free energy results from the conversion of acetate to hydrogen again making the system unfavorable (Eqs. 10.8, 10.9). So the system becomes extremely sensitive to biohydrogen formation.

n(LCFA) ! (n — 2)LCFA + 2 Acetate + 2 H2 AG° = +48kJ/mol (10.8)

CH3COOH + 2H2O! 4H2 + CO2 AG0 = +104.6kJ/mol (10.9)

Gas sparging can be a good solution for removing the produced hydrogen from the system but for large-scale operations this will raise the process costs. So researchers are working on membrane technologies which are a more efficient and cost effective solution for hydrogen removal from the gas mixture. But after a while on membrane surface biofilm formation could occur and this can favor the methanogens’ activity. Therefore, it is very important to find a technique to purify the hydrogen and to use it directly in fuel cell systems [88].