DEGRADATION OF ACETIC AND PROPIONIC ACIDS IN THE METHANE FERMENTATION

A series of batch digestion experiments was performed to evaluate the effect of elevated acetic acid concentrations on the degradation of propionic acid and vice versa. Flask digesters were incubated at 37-degrees-C with acetic and propionic acids added at up to 2000 and 1500 mg l-1, respectively. It was found that increasing the concentration of either acid from low levels reduced the rate of its utilization and that increasing the acetic acid concentration from 1000 to 2000 mg l-1 significantly inhibited degradation of propionic acid added at 500 mg l-1. Good agreement was obtained from fitting the logistic equation to the measured data and between duplicate runs performed within the experiments. The observed inhibition of propionate degradation by acetate confirms several earlier reports of product inhibition in the literature and emphasizes the importance of controlling acid levels in maintaining satisfactory digester operation.