Abstract A set of simplified dynamic models was developed for the activated sludge process (primary settler, biological reactor, and final settler). The models incorporate variable gains, variable time constants, and fixed dead times. Because the parameters may be easily evaluated on site, the models will be particularly useful for process analysis, control system design, and automatic control of a particular plant. Two different cases were studied; one for a plant in which the steady state concentration of suspended solids in the final effluent is directly proportional to the mixed liquor suspended solids, and one in which it is inversely proportional. Due to this difference in steady state gains the final performance of the system is quite different for the two cases although the dynamics are very similar. Therefore, care should be taken in characterizing the final settler.