This paper presents a new model for the dynamic prediction of water quality in rivers using in-series continuously stirred tank reactors (CSTR) and process transformations based on the IWA RWQM1. The transport model introduces a new parameter f (fraction of solids that are not retained in the in-series reactors) that splits hydraulic and solids retention times, regulating the transport of particulate substances along the river stretch. The characterization of the model components is based on elemental mass fractions, and closed mass balance is guaranteed for each process transformation. The resulting model has been implemented in the new simulator CalHidra 2.0 and applied to a branch of the Tajo River in the Madrid area. The spatial profiles of ammonium, nitrates, and dissolved oxygen in different seasonal periods have been used to calibrate the model. The proposed model also has shown its usefulness for exploring the evolution of the water quality in the Tajo River under different scenarios.

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