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

Abstract

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, E L , was estimated using a statistical method and numerical optimization based on experimental data from three tracer studies. With the first method, the values of E L varied within the range 1082-1860 m 2 /h and 695-1355 m 2 /h, respectively, in section 1 and section 2 of the aerobic zone. Except for one case, deviations of the corresponding numerically optimized values of E L did not exceed 14%. The maximum specific rates of nitrification (r n,max,20 = 4.6 g N/(kg VSS⋅h)) and phosphate uptake (r Pupt,max,20 = 13.5 g P/(kg VSS⋅h)) at T = 20 o 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.

Mathematical Modelling and Computer Simulation of Activated Sludge Systems

“…The estimated E L value was approximately 245 m 2 /h (±15%) for the following conditions: width 2-20 m, height 2.4-6 m, length 28-500 m, RAS recirculation ratio 0.7-1.5, hydraulic retention time 1.3-8 h. In the study of Iida (1988), tracer tests were carried out to evaluate mixing conditions in six full-scale aeration tanks with different volumes, geometries, flow rates and air supply intensity. Significantly higher dispersion in fullscale aeration tanks was observed in the tracer studies of Makinia and Wells (2005) (E L = 1100-1500 m 2 /h) and Zima et al (2009) (E L = 910-1690 m 2 /h). The estimated values of E L ranged from 208 to 826 m 2 /h with the mean = 476 m 2 /h.…”

“…the mean square error test of convergence (Makinia and Wells 2005;Zima et al 2009). The two techniques outlined above (methods of moments, Laplace transforms) may not be appropriate when sludge recirculation occurs in activated sludge reactors.…”