WWTP Process Tank Modelling: The Integration of Advanced Hydrodynamicand Microbiological Models

Jesper Laursen

    Research output: Book/ReportPh.D. thesis

    385 Downloads (Pure)


    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.

    After a general introduction to the activated sludge tank as a system, 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.

    The first case study involves the modeling of an activated sludge tank undergoing a special controlling strategy with the intention minimizing the sludge loading on the subsequent secondary settlers during storm events. The applied model is a two-phase model, where the sedimentation of sludge flocs is modeled through an altering of the inter-phase friction forces occurring between sludge flocs and wastewater. The purpose of the simulations is to optimize the flow conditions during the sedimentation phase of the process tank. This is done through a numerical testing of different inlet configurations that are all constructed with a decrease in inlet impulses and the thereby derived rotation flow in the tank in mind.

    Case two concerns the modeling of a hydrodynamically and biologically complex OCO process tank. Since the OCO-tank is constructed with the promotion of parallel aerobic and anoxic zones in mind, its special construction poses strict demands to the hydrodynamic model. In case study three the model is extended to a three-phase model where also the injection of air bubbles during the aeration process is modeled. The aeration of sludge is controlled through a simple expression for the reoxygenation of the wastewater phase as a function of the local volume fraction of air and the concentration of soluble oxygen. A simple model for the bulk consumption of oxygen is linked to the reoxygenation expression in order to model measured oxygen concentrations in the suspension.

    In the final case study, the ASM3 is integrated in the hydrodynamic model and the model is calibrated and validated against both online and manual measurements performed in the tank. The model calibration is performed by systematically changing the standard values of the model.

    Finally an assessment of the models strengths and weaknesses are conducted and proposals for future extensions to the model are given.
    Original languageEnglish
    Place of PublicationAalborg
    PublisherDepartment of Civil Engineering, Aalborg University
    Number of pages245
    Publication statusPublished - 2007
    SeriesDCE Thesis


    • Wastewater Treatment Plants
    • Hydrodynamic models
    • Navier-Stokes equations
    • Sludge
    • Activated sludge tank
    • Sludge Floc
    • WWTP Modelling
    • Plume Theory

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