@@1. Introduction Mathematical models, which have been regarded as one of the most useful and powerful tools for development and optimization of wastewater treatment processes, are inevitably needed for the quantitative evaluation of the activated sludge processes. Since the ASMs were proposed by International Water Association (IWA) task group, ASMs have been enhanced through introducing two-step model for nitrification process and considering both nitrite and nitrate denitrification processes, or introducing glycogen-accumulating organisms (GAOs) into models. In the enhanced models for nitrogen removal, denitrification was taken by facultative heterotrophic biomass through anoxic respiration either on nitrite or nitrate. However, the facuitative heterotrophic biomass actually include nitrite reducing organisms and nitrate reducing organisms. Additionally, phosphorus-accumulating organisms in those models for biological phosphorus removal, including non-denitrifying phosphorns-accumulating organisms (PAOs) and denitrifying phosphorus accumulating bacteria (DPB), actually were just assumed as one functional group. While the phosphorus removal mechanism of the two kinds of functional organisms is different, it won't be so suitable when these models are applied to describe phosphorus removal under anoxic condition. Furthermore, the theories of growth and decay for heterotrophic and autotrophic organisms are different from that for phosphorus-accumulating organisms and GAOs in the models including GAOs. Therefore, the present study is to develop a sub-microscopic mechanism model for biological nutrient removal, named Fully Coupled Activated Sludge Model (FCASM), which describes the biological processes between macrocosmic and microcosmic field in wastewater treatments.
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