Comparison of different models of substrate and product inhibition in anaerobic digestion

In batch-experiments (in lab-scale fluidized bed reactors) the inhibition of acetate- and propionate-degradation by propionate (substrate inhibition) and the inhibition of propionate degradation by acetate (product inhibition) was studied. Various models were compared by fitting them to the experimental data. The importance of independent variation of the acid concentrations and the pH for the experimental design is discussed. The substrate inhibition was best described by a model of inhibition by undissociated acid with an additional independent pH influence. The inhibition by propionic acid was only slight in the practically relevant range of concentrations. However, the propionate degrading bacteria were sensitive against low pH. The product inhibition was best described with the model of competitive inhibition. The inhibition already occurred from an acetate/propionate-ratio of 1 upwards. A complete standstill of the propionate degradation at high acetate/propionate-ratios, as expected because of the proximity to the thermodynamic equilibrium, was not observed.