WWTP Process Tank Modelling: The Integration of Advanced Hydrodynamic and Microbiological Models

The present thesis considers numerical modeling of activated sludge tanks on municipal wastewater treatment plants. Focus is aimed at integrated modeling where the detailed microbiological model the Activated Sludge Model 3 (ASM3) is combined with a detailed hydrodynamic model based on a numerical solution of the Navier-Stokes equations in a multiphase scheme.

The activated sludge tank model is gradually setup in separate stages. The individual sub-processes that are often occurring in activated sludge tanks are initially investigated individually, with the purpose of obtaining a better understanding before the final integrated model is setup. In the sub-process investigations focus is addressed especially at aeration by bottom mounted diffusers and mechanical mixing of the activated sludge suspension via slowly rotating hydrofoil shaped propellers. These two sub-processes deliver the main part of the supplied energy to the activated sludge tank, and for this reason they are important for the mixing conditions in the tank. For other important processes occurring in the activated sludge tank, existing models and measurements conducted by other investigators have given basis to the model. This applies to especially the ASM3, the sedimentation model and the rheology model. The main task has for these models mainly been to perform a suitable integration into the existing commercial Navier-Stokes solver.

After completion of the sub-process models, the last part of the thesis, where the integrated process tank model is tested on three examples of activated sludge systems, is initiated. The three case studies are introduced with an increasing degree of model complexity. All three cases are take basis in Danish municipal wastewater treatment plants.