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

^{1}

^{2}

^{3}

^{*}

## 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

## References

- Lee, K.H.; Kim, H.; KuK, J.W.; Chung, J.D.; Park, S.; Kwon, E.E. Micro-bubble flow simulation of dissolved air flotation process for water treatment using computational fluid dynamics technique. Environ. Pollut.
**2020**, 256, 112050. [Google Scholar] [CrossRef] [PubMed] - Saththasivam, J.; Loganathan, K.; Sarp, S. An overview of oil–water separation using gas flotation systems. Chemosphere
**2016**, 144, 671–680. [Google Scholar] [CrossRef] [PubMed] - Laamanen, C.A.; Ross, G.M.; Scott, J.A. Flotation harvesting of microalgae. Renew. Sust. Energ. Rev.
**2016**, 58, 75–86. [Google Scholar] [CrossRef] - Behina, J.; Bahrami, S. Modeling an industrial dissolved air flotation tank used for separating oil from wastewater. Chem. Eng. Process.
**2012**, 59, 1–8. [Google Scholar] [CrossRef] - Moruzzi, R.B.; Reali, M.A.P. Characterization of micro-bubble size distribution and flow configuration in DAF contact zone by a non-intrusive image analysis system and tracer tests. Water Sci. Technol.
**2010**, 61, 253–262. [Google Scholar] [CrossRef] - Edzwald, J.K. Developments of high rate dissolved air flotation for drinking water treatment. J. Water Supply Res. T.
**2007**, 44, 2077–2106. [Google Scholar] [CrossRef] - Edzwald, J.K.; Tobiason, J.E.; Amato, T.; Maggi, L.J. Integrating high rate dissolved air flotation technology into plant design. J. AWWA
**1999**, 91, 41–53. [Google Scholar] [CrossRef] - Kiuru, H.J. Development of dissolved air flotation technology from the first generation to the newest (third) one (DAF in turbulent flow conditions). Water Sci. Technol.
**2001**, 43, 1–7. [Google Scholar] [CrossRef] - Lundh, M.; Jönsson, L.; Dahlquist, J. The flow structure in the separation zone of a DAF pilot plant and the relation with bubble concentration. Water Sci. Technol.
**2001**, 43, 185–194. [Google Scholar] [CrossRef] - Lakghomi, B.; Lawryshyn, Y.; Hofmann, R. Importance of flow stratification and bubble aggregation in the separation zone of a dissolved air flotation tank. Water Res.
**2012**, 46, 4468–4476. [Google Scholar] [CrossRef] - Rodrigues, J.P.; Batista, J.N.M.; Béttega, R. Application of population balance equations and interaction models in CFD simulation of the bubble distribution in dissolved air flotation. Colloids Surf. A Physicochem. Eng. Asp.
**2019**, 577, 723–732. [Google Scholar] [CrossRef] - Edzwald, J.K. Dissolved air flotation and me. Water Res.
**2010**, 44, 2077–2106. [Google Scholar] [CrossRef] - Bondelind, M.; Sasic, S.; Pettersson, T.J.R.; Karapantsios, T.D.; Kostoglou, M.; Bergdahl, L. Setting up a numerical model of a DAF tank: Turbulence, geometry, and bubble size. J. Environ. Eng.
**2010**, 136, 1424–1434. [Google Scholar] [CrossRef] - Kwon, S.B.; Park, N.S.; Lee, S.J.; Ahn, H.W.; Wang, C.K. Examining the effect of length/width ratio on the hydro-dynamic behaviour in a DAF system using CFD and ADV techniques. Water Sci. Technol.
**2006**, 53, 141–149. [Google Scholar] [CrossRef] - Zimmerman, W.B.; Tesař, V.; Bandulasena, H.C.H.; Omotowa, O.A. Efficiency of An Aerator Driven by Fluidic Oscillation. Part I: Laboratory Bench Scale Studies; University of Sheffield: Sheffield, UK, 2009. [Google Scholar]
- Wang, G.; Ge, L.; Mitra, S.; Evans, G.M.; Joshi, J.B.; Chen, S. A review of CFD modelling studies on the flotation process. Miner. Eng.
**2018**, 127, 153–177. [Google Scholar] [CrossRef] - Zimmerman, W.B.; Tesař, V.; Butler, S.; Bandulasena, H.H. Microbubble Generation. Recent Pat. Eng.
**2008**, 2, 1–8. [Google Scholar] [CrossRef] [Green Version] - Ying, K.; Al-Mashhadani, M.K.H.; Hanotu, J.O.; Gilmour, D.J.; Zimmerman, W.B. Enhanced mass transfer in microbubble driven airlift bioreactor for microalgal culture. Engineering.
**2013**, 5, 735–743. [Google Scholar] [CrossRef] [Green Version] - Tesař, V. Mechanisms of fluidic microbubble generation part II: Suppressing the conjunctions. Chem. Eng. Sci.
**2014**, 116, 849–856. [Google Scholar] [CrossRef] - Hanotu, J.; Bandulasena, H.C.H.; Chiu, T.Y.; Zimmerman, W.B. Oil emulsion separation with fluidic oscillator generated microbubbles. Int. J. Multiph. Flow.
**2013**, 56, 119–125. [Google Scholar] [CrossRef] - Zimmerman, W.B.; Tesař, V.; Bandulasena, H.C.H.; Omotowa, O.A. Efficiency of an aerator driven by fluidic oscillation. Part II: Pilot scale trials with flexible membrane diffusers. Chem. Eng. Sci.
**2010**, 95, 1–30. [Google Scholar] - Nakayama, A.; Kuwahara, F.; Kamiya, Y. A two-dimensional numerical procedure for a three dimensional internal flow through a complex passage with a small depth (its application to numerical analysis of fluidic oscillators). Int. J. Numer. Methods Heat Fluid Flow.
**2005**, 15, 863–871. [Google Scholar] [CrossRef] - Tang, L.; Zhang, S.; Zhang, X.; Ma, L.; Pu, B. A review of axial vibration tool development and application for friction-reduction in extended reach wells. J. Pet. Sci. Eng.
**2021**, 199, 108348. [Google Scholar] [CrossRef] - Coanda, H. Device for Deflecting a Stream of Elastic Fluid Projected into an Elastic Fluid. U.S. Patent 2,052,869, 1 September 1936. [Google Scholar]
- Tesař, V.; Hung, C.H.; Zimmerman, W.B. No-moving-part hybrid-synthetic jet actuator. Sens. Actuator A-Phys.
**2006**, 125, 159–169. [Google Scholar] [CrossRef] [Green Version] - Bondelind, M.; Sasic, S.; Kostoglou, M.; Bergdahl, L.; Pettersson, T.J.R. Single- and two-phase numerical models of dissolved air flotation: Comparison of 2D and 3D simulations. Colloids Surf. A-Physicochem. Eng. Asp.
**2010**, 365, 137–144. [Google Scholar] [CrossRef] - Chen, A.; Wang, Z.; Yang, J. Influence of bubble size on the fluid dynamic behavior of a DAF tank: A 3D numerical investigation. Colloids Surf. A-Physicochem. Eng. Asp.
**2016**, 495, 200–207. [Google Scholar] [CrossRef] - Rodrigues, J.P.; Béttega, R. Evaluation of multiphase CFD models for Dissolved Air Flotation (DAF) process. Colloids Surf. A-Physicochem. Eng. Asp.
**2018**, 539, 116–123. [Google Scholar] [CrossRef]

**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 |

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |

© 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**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