Effect of control parameters on biogas production during the anaerobic digestion of protein-rich substrates

This paper deals with the theoretical study of the anaerobic digestion of protein-rich substrates. Since the success of such digestion raises the issue of the possible release of ammonia, we have carried out a mathematical analysis of an Anaerobic Digestion model specifically developed for the treatment of Microalgae, termed MAD (Microalgae Anaerobic Digestion) in the literature. Our aim is to investigate the qualitative properties of the system via the calculation of its equilibria, their conditions of existence as well as their stability according to the operating parameters values. Simulation results highlight the key parameters acting on model behavior. In particular, it is shown that the control parameters can greatly affect biogas yield and, thus, process performance. The optimal conditions for process operation are then identified. To emphasize the effect of pH on biological reactions, we plot the operating diagram of the MAD model for different fixed pH values. We demonstrate that a rising pH favors the formation of the free ammonia, leading ultimately to methanogen inhibition. Surprisingly, our study highlights the insensitivity of the model to acidic environments, thus limiting its potential applications at low pH. Finally, we investigate the qualitative and quantitative properties of a modified ADM1 (Anaerobic Digestion Model no1) model in the light of those of the MAD model.