Combining Computational Fluid Dynamics (CFD) with a Biokinetic Model for Predicting Ammonia and Phosphate Behavior in Aeration Tanks

The aim of this study was to use computational fluid dynamics (CFD) for predicting behavior of reactive pollutants (ammonia and phosphate) in the aerobic zone of the bioreactor located at the “Wschod” wastewater treatment plant (WWTP) in Gdansk (Poland). The one-dimensional (1-D) advection-dispersion equation (ADE) was combined with simple biokinetic models incorporating the Monod-type expressions as source terms for the two pollutants. The problem was numerically solved by a multi-step splitting technique algorithm. The dispersion coefficient, EL, was estimated using a statistical method and numerical optimization based on experimental data from three tracer studies. With the first method, the values of EL varied within the range 1082-1860 m2/h and 695-1355 m2/h, respectively, in section 1 and section 2 of the aerobic zone. Except for one case, deviations of the corresponding numerically optimized values of EL did not exceed 14%. The maximum specific rates of nitrification (rn,max,20 = 4.6 g N/(kg VSS·h)) and phosphate uptake (rPupt,max,20 = 13.5 g P/(kg VSS·h)) at T = 20 °C were determined based on laboratory batch experiments. With minor adjustments of the kinetic parameters, the model was capable of accurately predicting the longitudinal profiles of ammonia and phosphate in the aerobic zone and the simulation results were presented using the actual bioreactor geometry as a background.