# Numerical Investigation on the Dynamic Flow Pattern in a New Wastewater Treatment System

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## Abstract

**:**

## 1. Introduction

^{3}/h throughput. They also illustrated that this kind of microbubble generator is a promising component of wastewater treatment.

## 2. Description of FAF

## 3. Numerical Simulations

#### 3.1. Computational Domains and Boundary Conditions

#### 3.2. Time Step and Computational Time

#### 3.3. Grid Convergence Index

#### 3.4. Turbulence Models and Multiphase Flow

^{2}), ${\overrightarrow{V}}_{pq}$ is interphase velocity (m/s), ${\overrightarrow{F}}_{lift,q}$ is lift force (N), ${\overrightarrow{F}}_{q}$ is external body force (N), ${\overrightarrow{F}}_{wl,q}$ is wall lubrication force (N), ${\overrightarrow{F}}_{vm,q}$ is virtual mass force (N), ${\overrightarrow{F}}_{td,q}$ is turbulent dispersion force (N), ${\overline{\overline{\tau}}}_{i}$ is the stress–strain tensor of phase q.

## 4. Results and Discussion

#### 4.1. Analysis of the Dynamic Flow Pattern

#### 4.2. Switching Mechanism of the Dynamic Flow Pattern

#### 4.3. Effect of the Size of Bubble on Air Distribution

#### 4.4. Effect of the Size of Microporous Diffuser on Air Distribution

## 5. Conclusions

- The flow pattern in the separation zone was dynamic. The upper part of the separation zone contained a wavy flow, and the flow pattern at the lower part periodically switched between clockwise and counterclockwise. This dynamic flow pattern can help to improve bubble removal because it leads to the formation of larger bubbles by increasing the residence time and bubble–bubble contact. Additionally, this flow pattern eliminates the dead zone, which also improves the efficiency of wastewater purification.
- The flow pattern also affected the air distribution, which exhibited a wavy shape in the upper part of the separation zone. The height of the “white water zone” is larger than that of the DAF, which demonstrated that the efficiency of generating bubbles was also improved. It was also found that the height of the “white water zone” almost linearly decreased with the increase in bubble size and microporous diffuser size.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**(

**a**) Geometry of the FAF and DAF tank, (

**b**) velocity profile of separation zone in DAF tank.

**Figure 2.**(

**a**) Schematic of the microbubble generating device composed of fluidic oscillator and microporous diffuser, (

**b**) simulated velocity at one outlet of a fluidic oscillator and the corresponding air switching process.

**Figure 3.**Velocity profile before and after the maximum velocity generated by the fluidic oscillator in the FAF tank. (

**a**) Velocity profile before the maximum velocity, (

**b**) velocity profile at the maximum velocity, and (

**c**) and (

**d**) velocity profile after the maximum velocity.

Information | Adopted Condition |
---|---|

Multiphase model | Euler–Euler |

Turbulence model | Realizable κ-ε |

Gravity | 9.81 m/s^{2} |

Discretization scheme for the momentum equation | 2nd Order Upwind |

Discretization scheme for the volume fraction equation | 1st Order Upwind |

Discretization scheme for the turbulent kinetic energy equation | 2nd Order Upwind |

Discretization scheme for the turbulence dissipation rate equation | 2nd Order Upwind |

Average time-step | 0.002 s |

Total simulated flow time | 200 s |

Wastewater inlet | Velocity inlet |

Oscillating air inlet | Velocity inlet |

Outlet | Pressure outlet |

Walls and baffles | Wall |

Surface of flotation tank | Degassing |

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**MDPI and ACS Style**

Tang, L.; Zhang, S.; Li, M.; Zhang, X.; Wu, Z.; Ma, L. Numerical Investigation on the Dynamic Flow Pattern in a New Wastewater Treatment System. *Water* **2021**, *13*, 1101.
https://doi.org/10.3390/w13081101

**AMA Style**

Tang L, Zhang S, Li M, Zhang X, Wu Z, Ma L. Numerical Investigation on the Dynamic Flow Pattern in a New Wastewater Treatment System. *Water*. 2021; 13(8):1101.
https://doi.org/10.3390/w13081101

**Chicago/Turabian Style**

Tang, Lubo, Shaohe Zhang, Meng Li, Xinxin Zhang, Zhanghui Wu, and Like Ma. 2021. "Numerical Investigation on the Dynamic Flow Pattern in a New Wastewater Treatment System" *Water* 13, no. 8: 1101.
https://doi.org/10.3390/w13081101