PROCESS MODELING OF AN INDUSTRIAL ACTIVATED SLUDGE SYSTEM FOR AERATION CAPACITY EXPANSION AND IMPROVED OPERATION

An industrial wastewater treatment facility, treating high temperature waste streams (30°C) from fiber staple and resin production processes, was evaluated for aeration system capacity. The facility's existing blowers and static tube diffusers were a capacity-limiting component of the treatment process. Historical data was reviewed, and, based on past loads and discussions with production facility personnel, a design basis was developed to provide increased aeration capacity and flexibility to prevent production limitations. Several load conditions were evaluated to cover the range of expected production scenarios. To evaluate oxygen requirements, three methods were employed:Traditional process oxygen coefficient calculationsComputer process model simulationsIn-situ oxygen uptake rate measurement using off-gas testingFor the process modeling, historical data and assumptions were used to determine appropriate stoichiometric and kinetic parameters, and a preliminary calibration was performed against current conditions. Next, simulations were performed at each of the load conditions to calculate the operating conditions (MCRT, wastage rate, operating MLSS, etc) required to maintain steady state at any given condition and the resulting oxygen requirements under those conditions. These oxygen requirements were compared with the conventional process oxygen coefficient method, and results were ultimately used to select, size, and design an upgraded fine bubble diffused aeration system, a new blower, and a dissolved oxygen control system.Off-gas testing was also performed to evaluate In-situ oxygen uptake rates for comparison to the other approaches for calculating oxygen requirements. Measurements were taken at the influent end, middle, and effluent end of the aeration tank. Loads to the aeration tank were controlled during testing by bleeding in high strength wastewater from a storage tank.The process model slightly overestimated mixed liquor suspended solids (MLSS) concentrations (within 10%) and yielded higher oxygen requirements than other calculation approaches (also within 10%). The difference in MLSS concentrations was likely due to the uncertainty in growth yield and decay rate model parameters, while the oxygen requirements were likely predicted to be higher due to the model's more accurate prediction of nitrification when supplemental urea or ammonia was overfed. The slightly higher oxygen requirements were considered more conservative and were used in sizing the aeration system. The oxygen requirements obtained from the off-gas/OUR testing were less consistent with the other methods. However, these were more specific to the location within the basin at which the samples were taken, as the tank exhibited some degree of plug flow. The assumed division of the basin had too large of an effect on the overall AOR to provide a stand-alone reliable basis of design for AOR. Design of fine bubble aeration equipment was completed in spring 2004, and installation of new pre-manufactured equipment is expected by the end of summer 2004.