The Activated Sludge Model No. 3 (ASM3) can predict oxygen consumption, sludge production, nitrification and denitrification of activated sludge systems. It relates to the Activated Sludge Model No. 1 (ASM1) and corrects for some defects of ASM1. In addition to ASM1, ASM3 includes storage of organic substrates as a new process. The lysis (decay) process is exchanged for an endogenous respiration process. ASM3 is provided as a reference in a form which can be implemented in a computer code without further adjustments. Typical kinetic and stoichiometric parameters are provided for 10°C and 20°C together with the composition of a typical primary effluent in terms of the model components.