# CFD Simulation of Aeration and Mixing Processes in a Full-Scale Oxidation Ditch

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methodology

^{−4}.

#### 2.1. Computational Geometry

^{3}was used in this study. The 3D geometry of the aeration basin and the submerged agitators were designed as a representative of the real oxidation ditch system and implemented in this CFD analysis. However, some considerable simplifications have been done because of the complexity of the geometry. These simplifications help to reduce computational requirements to a great extent. Although ODs are usually operated in continuous mode, the inlet and outlet of the tank were avoided in the computation. According to the study of Stamou, the inlet and outlet of an OD affect the flow field locally, and hence they were not considered in the numerical simulation [12]. The 3D view of the computational geometry of OD is shown in Figure 2.

#### 2.2. Mesh Grid

^{+}value of the production grids (the selected ones) at the wall of the racetrack without flowmakers (momentum sources) is y

^{+}= 16.6 and for the racetrack with flowmakers is y

^{+}= 23.3. However, the SST model in ANSYS CFX uses automatic wall functions, i.e., both high-Reynolds number grids with y + values > 11 and low-Re grids with y + <2 are possible. Also, in the transition zone, the model works consistently with the equations. The automatic wall functions work with all turbulence models based on the omega equations.

#### 2.3. Boundary Conditions

^{3}/s was set in each aeration channel. In order to model the air injection from the 3D fine bubble diffuser geometry to the tank, the aeration subdomain was generated. Mass source term was used to introduce air to the system by means of the continuity equation. The value of air mass source is calculated from:

#### 2.4. Simulation of Submerged Agitators

#### 2.5. Setting Up the Multiphase Flow Model

_{k}represents the interaction force with the other phases, where index k denotes the corresponding phase. Mass transport from phase k to phase l indicated by the term ṁ

_{kl}. Two closure models must be used to define τ and F

_{k.}τ used describing rheology of phase and complex closure models must be implemented if the non-Newtonian fluid present in the system. Solid particles in wastewater were not considered in this study, and pure water was used.

## 3. Results and Discussions

#### 3.1. Momentum Source Term Approach

#### 3.2. Transient Rotor-Stator Model

#### 3.3. Positioning of the Flowmakers

**C**. Another one is clearance,

**C**, which describes the distance between a flowmaker and the first row of the air diffuser grid. In addition to them, it is crucial to keep minimum distance,

_{f}**C**, between the last row of a diffuser grid and the beginning of the following tank curve [23].

_{M}_{w}represent the channel width and the water depth, respectively.

_{C}indicates the length of the channel which is 55.26 m.

_{f}was 9.26 m for both flowmakers in the original layout, while C was 10.94 m and 10.79 m for flowmaker 1 and flowmaker 2, respectively. Therefore, clearance, C

_{f}, was shifted 0.7 m back in case 1 compared to the original one. On the contrary, the distance between the flowmakers and the first row of air diffusers was set to 9 m. Modified parameters are summed up in Table 3.

#### 3.4. Contribution of Aeration to the Mixing Process without Flowmakers

## 4. Summary and Conclusions

- The momentum source term approach was used due to its reliability and high computing speed. The minimum required liquid velocity was reached with this model and adequate mixing was determined. This approach predicts lower water velocity in comparison with the transient rotor-stator model. This is due to the fact that the tangential velocity components were not considered during flowmaker modelling.
- Despite the excessive computing resource requirements, the transient rotor-stator model accurately predicted the fluid flow pattern in the OD. The flowmakers were able to generate the required thrust in order to obtain sufficient bulk flow. Better mixing performance was determined with this model. Also, the normal forces that act on the blades were monitored and the normal force fluctuations were in the allowed range during the simulation.
- Grundfos best practice guidelines were taken into account for the correct positioning of the flowmakers. Higher thrust fluctuations were determined when the flowmaker position was close to the first row of the diffuser grid. Meanwhile, the uneven distribution of water velocity was observed due to the tank curves and it might be dangerous for the blades. Therefore, the rear clearance is also an important parameter for the correct positioning of the flowmakers. When the distance between the flowmaker and the diffuser grid was 9.96 m, relatively low thrust fluctuations were monitored. As a result, it is the recommended position of the flowmaker for this study.
- The contribution of the aeration process to the mixing was also investigated by removing the flowmakers. Inadequate mixing was monitored throughout the OD due to the recirculation zones. Zones of recirculation were determined due to the high aeration rate. There were dead zones around the rounded ends of the oxidation ditch. Also, the diffuser grid arrangement was not suitable to get adequate mixing without the flowmakers. Therefore, it is necessary to use the flowmaker in order to achieve effective bulk flow.
- Fine bubble diffusers were used instead of the more energy-intensive surface aerators and approximately 57% reduction of energy consumption was achieved in the aeration process.

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**Fixed bottom aeration grid and agitators [5].

**Figure 4.**Water velocity contour (

**a,c**) and its vectorial representation (

**b,d**) on a horizontal plane at 0.5 m and 6 m (from top to bottom) above the tank floor.

**Figure 7.**Water velocity contour (

**a,c**) and water velocity vectors (

**b,d**) on a horizontal plane at 0.5 m and 6 m (from top to bottom) above the tank floor.

**Figure 9.**Normal force on the blades of the flowmakers in axial (Z) and radial (X and Y) directions.

**Figure 10.**Normal force on the blades in axial (Z) and radial (X and Y) directions after stopping the flowmakers.

**Figure 13.**Water velocity contour (

**a**,

**c**) and water velocity vectors (

**b**,

**d**) on a horizontal plane at 0.5 and 6 m (from top to bottom) above the tank floor.

Computational Geometry | Number of Nodes | Number of Elements |
---|---|---|

Tank | 1,557,159 | 7,083,099 |

Agitator | 440,350 | 2,306,427 |

Mesh Independence Study | Total Number of Elements (Millions) |
---|---|

Mesh A | 8.41 |

Mesh B | 11.63 |

Mesh C | 20.47 |

Parameters | Case 1 | Case 2 | ||
---|---|---|---|---|

Flowmaker 1 | Flowmaker 2 | Flowmaker 1 | Flowmaker 2 | |

${C}_{f}$ | 9.96 m | 9.96 m | 9 m | 9 m |

C | 10.24 m | 10.09 m | 11.20 m | 11.05 m |

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**MDPI and ACS Style**

Höhne, T.; Mamedov, T.
CFD Simulation of Aeration and Mixing Processes in a Full-Scale Oxidation Ditch. *Energies* **2020**, *13*, 1633.
https://doi.org/10.3390/en13071633

**AMA Style**

Höhne T, Mamedov T.
CFD Simulation of Aeration and Mixing Processes in a Full-Scale Oxidation Ditch. *Energies*. 2020; 13(7):1633.
https://doi.org/10.3390/en13071633

**Chicago/Turabian Style**

Höhne, Thomas, and Tural Mamedov.
2020. "CFD Simulation of Aeration and Mixing Processes in a Full-Scale Oxidation Ditch" *Energies* 13, no. 7: 1633.
https://doi.org/10.3390/en13071633