We Investigate a model for the treatment of wastewater in the activated sludge process. The process is based on the aeration of wastewater with flocculating biological growth, followed by the separation of treated wastewater from biological growth. Part of this growth is wasted, and the remainder returned to the system. The biochemical model consists of two types of bacteria, sludge bacteria and sewage bacteria, and two types of ciliated protozoa, free-swimming ciliates and ciliates attached to sludge floes. A combination of steady-state analysis, path following techniques and numerical integration of the governing equations are used to study the dynamics of this system in a network of two coupled reactors arranged in a series. We compare the treatment efficiency for a single tank system with that of a two-reactor cascade. In the latter scenario the total residence time is fixed and the residence time in the first reactor is taken to be a design parameter. Process parameters that ensure optimal performance are discussed.