Wastewater treatment plants (WWTPs) are key infrastructures for ensuring a proper protection of the environment. Mathematical modelling and simulation of activated sludge systems have gained increasing attention in control engineering within the last years. The most widely used models to describe the biochemical processes that take place in a WWTP are the Activated Sludge Models 1-3. Due to their increased complexity, these models are often unsuited for control systems design. The aim of this paper is to develop reduced order models which can adequately describe the biochemical processes for the purpose of on-line control design. The models developed contain a minimum number of variables and parameters that allow for model identification and validation based on available on-line measurements.