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Abstract
Design, operation, control and derating of modern activated sludge plant require good modelling effort. The completely‐mixed tank in series (CSTR‐in‐series) concept has been widely used to represent nonideal flow in the actual activated sludge process. However there appears to be lack of a generalised approach to model the process using this concept. In this paper, a systematic procedure to model the process using the CSTR‐in‐series concept is developed. Using equisized CSTRs, the number of CSTR‐in‐series to represent the actual process may be determined through investigation of the steady‐state and dynamic responses of the continuous process. This procedure is proven to be simple and the process can be modelled with reasonable accuracy. Based on simulation results and validation of model with pilot‐scale experimental data, it is recommended that two CSTR‐in‐series model should be employed to adequately represent the activated sludge process for carbon removal of a typical municipal wastewater plant to a satisfactory effluent quality. The transient behaviour of biomass, substrate and dissolved oxygen concentration at various spatial positions along the reactor represented by the serial number of the CSTR's may also be usefully predicted.