Inhibition of biohydrogen production by undissociated acetic and butyric acids.

Glucose fermentation to hydrogen results in the production of acetic and butyric acids. The inhibitory effect of these acids on hydrogen yield was examined by either adding these acids into the feed of continuous flow reactors (external acids), or by increasing glucose concentrations to increase the concentrations of acids produced by the bacteria (self-produced). Acids added to the feed at a concentration of 25 mM decreased H2 yields by 13% (acetic) and 22% (butyric), and 60 mM (pH 5.0) of either acid decreased H2 production by >93% (undissociated acid concentrations). H2 yields were constant at 2.0 +/- 0.2 mol H2/mol glucose for an influent glucose concentration of 10-30 g/L. At 40 g glucose/L, H2 yields decreased to 1.6 +/- 0.1 mol H2/mol glucose, and a switch to solventogenesis occurred. A total undissociated acid concentration of 19 mM (self-produced acids) was found to be a threshold concentration for significantly decreasing H2 yields and initiating solventogenesis. Hydrogen yields were inhibited more by self-produced acids (produced at high glucose feed concentrations) than by similar concentrations of externally added acids (lower glucose feed concentrations). These results show the reason hydrogen yields can be maximized by using lower glucose feed concentrations is that the concentrations of self-produced volatile acids (particularly butyric acid) are minimized.