The Estimation of Stability Boundaries for an Anaerobic Digestion System

The operation of biochemical reaction systems requires substantial expertise and a good understanding of system dynamics. Due to various inhibition effects, these systems usually possess several equilibrium points. Selecting the initial state of the system is as important as selecting the system's inputs for a proper operation and for achieving the technological goal. This paper presents an anaerobic digestion system with two biochemical reactions, which possesses six equilibria for some ranges of system's inputs. The system's stability boundary separates the set of initial states from which the system converges to the desired operating point and the set of initial states which lead the system to a totally undesired condition, the acidification point. A methodology for estimating this stability boundary is described. Additionally, a procedure for visualizing the extent of the estimated boundary in a lower dimensional space is introduced. The algorithms are simple and provide accurate estimates. Moreover, the results may be displayed as two-dimensional diagrams that can be easily understood by the system's operator.