The soluble acid metabolites or volatile fatty acids (VFA) formed during the aci — dogenic process help in understanding the metabolic pathway [18]. The following equations show variable soluble acid metabolites generation during acidogenic fermentation.

2CH3- COOH + 2CO2 + 4H2 CH3-CH2-CH2-TOOH + 2CO2 + 2H2 2CH3-CH2-COOH + 2H2O COOHCH2CH2OCOOH + CO2 CH3-CH2OH + CO2

Depending on the pathway used by the microorganism and the corresponding end-products, H2 yields are variable. Products formed from pyruvate such as acetate, butyrate, butanol, acetone, lactate or ethanol determine the theoretical yield of H2 [3]. In obligate anaerobes, pyruvate is converted to H2 from the reduced Fd by the action of hydrogenase resulting in maximum yield of 2 mol H2/mole glucose. Two additional moles of H2 can be produced from NADH produced during glycoly­sis, where NADH is oxidized by Fd reduction by NADH:ferredoxin oxidoreductase (NFOR) [3]. Further, H2 can be produced from the reduced Fd by hydrogenase. The highest theoretical yield of 4 mol H2/mole glucose can be obtained when acetate or acetone is the fermentation end-product. Two molecules of formate are produced from two pyruvate molecules where a theoretical maximum yield of 2 mol H2/mole of glucose can be obtained. In the case of butyrate as the fermentation end-product, the maximum theoretical yield is 2 mol H2/mole glucose. When alcohols are the end-products, lower yields of H2 are obtained as alcohols contain additional H2 atoms that have not been converted to H2 gas [3]. The presence of higher concentra­tions of propionic acid or solventogenesis is generally not considered to be feasible for H2 production.