Product inhibition in the acid forming stage of the anaerobic digestion process

The acidification of glucose was studied in a continuous flow stirred tank reactor at a pH of 6.0 and a temperature of 30°C. Together with the glucose, a complete mineral medium was pumped into the reactor vessel, to ensure a carbon limitation, throughout. In various experiments product inhibition was studied with respect to specific growth rate, yield and product distribution. The total amount of inorganic nutrients was varied, in its entirety, at constant glucose input (10g 1 −1). No inhibition took place if the inorganic nutrient concentration was gradually increased, however a sudden large change caused reactor failure. Sulphate reduction to sulfide occurred if sulphate was higher than 140 ppm but this caused no inhibition of the process. Product inhibition by the acids formed, was measured by changing the glucose input concentration. After changing the glucose concentration from 2.5 to 75 g l −1, the maximum dilution rate was lowered from 0.54 to 0.044 h −1. Biomass yield decreased from 16 to 8%. Product distribution however was fairly constant. A semi logarithmic relationship was found between the maximum dilution rate and the acids produced. This relationship is briefly discussed.