| Activated sludge systems are the most common method of municipal wastewater treatment. Many interacting biological, physical and chemical processes in the system make optimization and mathematical modeling of the operation complex. One of the primary shortcomings of models is that they focus only on a selected aspect of the activated sludge system (e.g., biology) while making crude assumptions for other aspects (e.g., hydraulics). The focus of this research was to develop a comprehensive dynamic model of activated sludge reactors including biological processes, reactor hydraulics, oxygen transfer and temperature.; The International Association on Water Quality Activated Sludge Model No. 1 provided a framework for description of the biological processes, such as carbon oxidation and nitrogen removal. Hydrodynamics of the reactor were modeled using the one-dimensional advection-dispersion equation instead of widely accepted, but rarely realistic, tanks-in-series approach. Oxygen transfer, expressed by the overall oxygen transfer coefficient, was related linearly to the air flowrate. The temperature model was based on a theoretical energy balance over the reactor.; Experiments were conducted at the Rock Creek Wastewater Treatment Plant in Hillsboro, Oregon, USA. Batch tests were performed for estimating kinetic and stoichiometric coefficients. The dispersion coefficient and the overall oxygen transfer coefficient were determined from dye studies and off-gas measurements, respectively. The model was verified using data from steady-state and dynamic experiments. The dispersion model generated much more realistic longitudinal profiles of all measured parameters, i.e., ammonia, nitrate + nitrite, dissolved oxygen concentrations, and total oxygen uptake rates. Sensitivity analyses revealed that the impact of the parameters on model predictions changed along the longitudinal axis of the reactor. The specific growth rate of autotrophs was the most sensitive model coefficient. The temperature model showed that processes associated strictly with wastewater treatment, i.e., biological processes (heating) and aeration (cooling), play a significant role in the energy budget for wastewater treatment facilities located in western Oregon.; Several model applications were also demonstrated, such as the impact of alternating aerobic/anoxic conditions on nitrogen removal, the combined effects of temperature and hydraulic retention time on nitrification, and the impact of temperature and sludge retention time on sludge characteristics. |
