Maximizing the Capacity of Wastewater Treatment Processes - Study of Autothermal Thermophilic Aerobic Digestion

Several authors have pointed out the need to identify the optimum operating conditions of autothermal thermophilic aerobic digestion (ATAD). The main hypothesis of the present ongoing study is that the improvement of the operating conditions opens the potential for increased plant capacity (m(3)/d) of established ATAD designs by decreasing the batch time, while complying with treatment goals and maintaining energy requirements nearly unchanged. The variables defining the operating conditions considered in this study are oxygen concentration, temperature, and flowrate. Conventional ATAD systems make use of invariable air supply, unregulated temperature without pre-heating of the influent sludge, and one single volume change per day. These characteristics have been shown to negatively influence the energy requirements of the process and its overall capacity. The methodology selected in this paper to maximize the capacity of existing ATAD systems by altering the operating conditions is dynamic optimization. In order to solve the optimization problem, a mathematical ATAD reactor model is necessary. For this purpose, the dynamic model proposed by Kovacs and Mihaltz (2002) was chosen as a starting point of our investigation. The existing model includes a differential mass balance and takes into consideration temperature-dependent kinetic parameters, but it lacks a differential energy balance. The latter is critical for the purpose of this study, as it affects the overall system dynamics and it allows to predict temperature fluctuations within the reactors' system and their effect on the sludge oxidation rate and overall performance of the treatment. The chosen model is currently being extended with a differential energy balance as reported here and built in the MATLAB environment.