Numerical investigations of the hydrodynamics and the oxygen mass‐transfer in aerated tanks

The present work concentrates on numerical investigations of the hydrodynamics, the oxygen mass‐transfer, and the biochemical reactions in a pilot‐plant‐scale aeration tank, engaging the Euler‐Euler method and the Activated Sludge Model No. 1. The oxygen concentration, the ammonium concentration, and the nitrate concentration are discussed using results from the experimental investigations at the pilot‐plant‐scale aeration tank in the waste‐water‐treatment plant Ebersbach carried out by the HZDR.


Introduction
Aeration tanks represent a major energy consumer in municipal waste-water-treatment plants. To increase their efficiency, numerical investigations are conducted to capture the hydrodynamics, the oxygen mass-transfer, and the biochemical reactions in the aeration tank. Numerical computations offer the opportunity to test system configurations and operating modes without cost-intensive reconstruction work. To generate a reliable foundation for these investigations, measurement data from a pilot-plant-scale aeration tank are compared with the results of the numerical computations.

Numerical Model
To compute the hydrodynamics, the oxygen mass-transfer, and the biochemical reactions in the pilot-plant-scale aeration tank with a stationary simulation the model of Meier [1] is adapted. This model engages the Euler-Euler method and calculates the activated sludge as continuous phase and air as mono-disperse phase within ANSYS CFX. The biochemical reactions in the activated sludge are calculated using the Activated Sludge Model No. 1 [2]. In the experimental setup, the inflowing activated sludge is taken from the denitrification region out of the real-scale aeration tank in the Ebersbach waste-water-treatment plant. This waste-watertreatment plant uses an aeration tank with preceding denitrification. In contrast to the simulations by Meier [1], in this case the composition of the inflowing activated sludge differs from that of waste water. For this reason, the composition of the chemical oxygen demand (COD) in the activated sludge of the denitrification tank is required. In order to determine this composition, both a sample waste-water-treatment plant is simulated using the software SIMBA, and the simulation of the aeration tank of the waste-watertreatment plant in Schwerte from Meier [1] are considered.  The determined compositions deviate less than 6% for the heterotrophic biomass and the particulate products and less than 2% for all other components. From these results, the arithmetic average is calculated and used as composition of the inflowing activated sludge for the pilot-plant-scale aeration tank. The calculated composition is given in table 1.
The simulations considered below are calculated using the experimental data of the COD, the inlet volumetric air flow ratė V air , the inlet ammonium concentration c ammonium , the inlet nitrate concentration c nitrate , and the bubble size d shown in table 2.

Results
The following figures show the results of the numerical calculations with an volumetric air flow rate ofV air = 10 m 3 h −1 and a bubble diameter of d = 3.38 mm. Figure 1 (a) shows the streamlines of the air bubbles and the flow structure of the activated sludge as vectors. The air rises from the diffuser surfaces to the top of the tank and the bubbles are distracted to the middle of the tank. The flow structure of the activated sludge can be described by two vortexes. Figure 1 (b) shows the dissolved oxygen concentration. A higher dissolved oxygen concentration can be recognized in the second vortex. Figure 1 (c) shows the ammonium concentration and figure 1 (d) the nitrate concentration. A high ammonium inlet concentration and an almost complete ammonium degradation appears already in the first vortex. Consequently, the nitrate concentration rises in the first vortex and is higher in the second vortex. This correlates with the results of the dissolved oxygen concentration in figure 1 (b). The nitrification process consumes oxygen to oxidize ammonium to nitrate. Hence, both ammonium and oxygen are degraded, while nitrate is produced. Due to the almost complete ammonium degradation in the first vortex, the oxygen concentration is higher in the second vortex.

Conclusion and future works
Numerical calculations of the hydrodynamics, the oxygen mass-transfer, and the biochemical reactions in pilot-plant-scale aeration tank following the Euler-Euler method are presented. To characterize the inflowing activated sludge, the composition of the activated sludge after the denitrification of the real-scale aeration tank with preceding denitrification is determined. The flow structure in the pilot-plant-scale aeration tank is discussed, as well as the dissolved oxygen concentration, the ammonium concentration, and the nitrate concentration. The computed results show the expected behavior. In further work, simulations for the optimization of the aeration tank shall be the focus.