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In this paper we present a novel average model for alternating activated sludge models. This kind of processes, where nitrification-denitrification operations occur, are represented by two consecutive dynamics: an aerobic phase followed by an anaerobic one. In particular, we study a reduced AS model for nitrogen removal. The resulting average continuous model captures the governing (average) behavior of switching dynamics, without considering the switching behavior. The continuous nonlinear AS models thus obtained can be used straightforwardly for analysis, control design and estimation purposes.