Because water has become a major natural resource, it is important to address several important issues that may lead to its better use and conservation. From a process system engineering point of view, the approaches that could be taken for improving the quality of treated domestic and industrial water streams are related to the modeling, simulation, optimization, and control of waste water treatment plants (WWTPs). Although there have been important contributions from the modeling, simulation, and control research communities, practically there is not reported research work addressing the nonlinear analysis, and its control or operability implications, on large scale WWTPs. Such studies could provide some indications about potential operability problems and suggest potential ways of avoiding them. Because in the traditional biological treatment of polluted streams there are embedded complex interactions due to the highly nonlinear nature of the kinetic relationships and because the traditional activated sludge process involves process recycle streams, strong interactions among the main processing variables should be expected. In this paper, we address the bifurcation analysis of an industrial scale WWTP with the aim of learning how a typical WWTP process features complex interactions mainly among potential controlled, manipulated, and disturbance variables. It was found that nonlinear behavior was only present when considering disturbances in the main flow rate of the polluted stream. The rest of analyzed bifurcation variables led to rather smooth behavior. This behavior seems to be an indication that WWTP plants could be properly controlled with simple proportional, integral, and derivative (PID) controllers making probably unnecessary the use of advanced control systems. The model used in this work reflects operating conditions closely resembling those found in a real WWTP..