In this work, the nonlinear behavior of a bioreactor coupled to a separator is analyzed. The main focus is on the effect of kinetics on the stability and multiplicity of the coupled system using a cybernetic modeling approach. One example of such a system is the activated sludge process. In this work we consider the growth of a culture of Pseudomonas putida in a medium containing phenol and glucose. We propose a cybernetic model which accounts for the regulatory features in the microorganism in a qualitative way in the aerator. These regulatory features control the rate of uptake of the substrates by the cells in the aerator. We use kinetics with self-substrate inhibition and cross-substrate inhibition effects. We observe a strong dependence of the window of multiplicity on kinetics. We depict that introduction of the more detailed kinetics into the model considerably widens the multiplicity region and helps the experimentalist to design the experiments. We use flocculation kinetics in the bioreactor which is the key for the settling of the suspended solids. The more realistic limiting flux condition is also introduced in the settler. We depict that under such conditions the resulting model of the coupled system exhibits nongrowth associated maintenance phenomena which occur at low dilution rates. The model equations governing the system behavior are ordinary differential equations and algebraic equations. In this work we vary the dilution rate and study the behavior of the system. We observe a shift in the uptake rates of the substrates as we vary the dilution rate. A two parameter continuation is also carried out to investigate the effect of other operating parameters on the nonlinear behavior. © 2009 American Chemical Society.