  In this paper, the stability analysis of an anaerobic digestion process is
presented. The analysis is performed using a simplified mathematical model,
which includes explicit temperature and pH dependence on kinetic growth rates.
A detailed nonlinear and bifurcation analyses are performed in order to study
the effects of temperature and pH parameters on the process behaviour. In
addition, both safety and optimal operation regions of the bioreactor are
established. Based on bifurcation diagrams, it is observed that the washout
condition occurs by combining a fold bifurcation and a transcritical
bifurcation. Consequently, to prevent the washout and guarantee optimal
operation conditions of the bioreactor, a risk criterion oriented to monitor
the bioprocess on-line is proposed, allowing to detect the system
destabilisation.
