# Perforated Baffles for the Optimization of Disinfection Treatment

^{*}

## Abstract

**:**

## 1. Introduction

_{10}/τ can be used for the global assessment of the hydraulic efficiency of contact tanks, where t

_{10}is the time necessary for the passage through the outlet section of the tank of 10% of the tracer mass injected and τ is the theoretical mean residence time based on the assumption of plug-flow conditions. The Morrill index (Mo = t

_{90}/t

_{10}) and the dispersion index (σ) may be used to evaluate the mixing performance of a contact system, where t

_{90}is the time necessary for the passage of 90% of the mass of the tracer injected, while σ is the dispersion index of RTD curve [7].

## 2. Numerical Model

#### 2.1. Computational Domain

#### 2.2. Mathematical Formulation

_{i}and x

_{i}are velocity and position in the i-th direction, t is time, ρ is fluid density, p is pressure, ν is the kinematic viscosity, f

_{i}represents the body force per unit of mass in the i-th direction, $\overline{{u}_{j}}$ is the velocity component along the j-direction (x, y and z), x

_{j}represents the Cartesian coordinate, $\overline{C}$ is the filtered concentration, D is molecular diffusivity, negligible compared to turbulent diffusivity D

_{t}. The turbulent diffusivity can be computed as Dt = μ

_{t}/S

_{c}, where μ

_{t}is eddy viscosity calculated in (4) while S

_{c}is the turbulent Schmidt number which is set to 1000 in order to maintain the same ratio between the water molecular viscosity and the tracer mass diffusivity used in the experiments by Kim et al. [26,29].

_{t}is the subgrid-scale turbulent viscosity, L

_{s}is the mixing length for subgrid scales, ${\overline{S}}_{ij}$ is the rate-of-strain tensor for the resolved scale defined by:

_{s}and ${S}_{ij}^{d}$ are respectively defined as:

_{w}is the WALE constant equal to 0.325 and V is the volume of the computational cell.

_{init}is the injected tracer concentration (C

_{init}= 1), T

_{release}is the injection period, θ = t/τ is the dimensionless time and τ is the MRT [13].

#### 2.3. Boundary Conditions

## 3. Results and Discussion

#### 3.1. Perforated Baffles Geometry

#### 3.2. Flow Analysis

#### 3.3. Tracer Analysis

_{10}/τ which, as suggested by the US EPA (United States Environmental Protection Agency) [31], can be used as a parameter to classify hydraulic performance of a contact tank based on the values reported in Table 2.

_{90}/t

_{10}) and dispersion (σ

^{2}) coefficients should be close to 2 and 0, respectively. As previously mentioned, t

_{10}/τ and t

_{90}/τ represent the normalized times, starting from the introduction of the tracer, respectively necessary for the passage of 10% and 90% of the mass of the tracer injected through the monitoring section. The dispersion index is instead defined as:

_{t}

^{2}is the variance of the RTD curve and t

_{g}is the normalized time to reach the centroid of the effluent curve [32].

_{10}/τ = 0.509), while Design D14 allows the highest mixing efficiency (Mo = 4.127 and σ

^{2}= 1.268). Figure 6 compares the CRTD curve of conventional solid baffles design with the CRTD obtained for D12 and D14 perforated designs and shows that the differences between the two perforated baffles designs are so small that the two curves almost perfectly overlap.

_{10}/τ value with the perforation percentage. For low perforation values, corresponding to very small holes sizes, the baffling condition is very similar to that of the conventional solid baffles design. As the perforation percentage increases, the mixing efficiency increases, reaching the maximum value for D12 design that corresponds to an average baffling condition. By further increasing the perforation, there is a reduction in reactor performance because high quantities of tracer pass through the larger holes, causing short-circuit effects that lower the HRT.

## 4. Conclusions

_{10}/τ = 1 characteristic of the ideal plug-flow condition. The D12 design allowed to pass from a “poor” to an “average” hydraulic condition thus defined according the baffling factor. The perforated baffles configuration also allows increasing the overall efficiency of the contact system by enhancing the mixing effects. In fact, the Morril index passes from values close to six, obtained for the traditional configuration, to values just over four approaching the ideal condition (Mo = 2). By increasing the size of the holes beyond a certain value, there is instead a reduction in the baffling factor since too large holes cause the passage through the entire reactor of a significant portion of tracer in a shorter time than the theoretical hydraulic residence time. The large holes, therefore, do not reduce the detrimental short-circuit effects while allowing an increase in mixing in the tank. It is, therefore, necessary to carry out a preliminary study in order to identify the optimal configuration that allows transforming the dead zones into areas of active mixing while avoiding the short circuit in the tank.

## Author Contributions

## Funding

## Conflicts of Interest

## References

- Angeloudis, A.; Stoesser, T.; Gualtieri, C.; Falconer, R.A. Contact tank design impact on process performance. Environ. Model. Assess.
**2016**, 21, 563–576. [Google Scholar] [CrossRef] [Green Version] - Kim, D.; Kim, D.-I.; Kim, J.; Stoesser, T. Large eddy simulation of flow and tracer transport in multichamber ozone contactors. J. Environ. Eng.
**2010**, 136. [Google Scholar] [CrossRef] [Green Version] - Angeloudis, A.; Stoesser, T.; Falconer, R.A. Predicting the disinfection efficiency range in chlorine contact tanks through a CFD-based approach. Water Res.
**2014**, 60, 118–129. [Google Scholar] [CrossRef] [PubMed] - Wang, H.; Shao, X.; Falconer, R.A. Flow and transport simulation models for prediction of chlorine contact tank flow-through curves. Water Environ. Res.
**2003**, 75, 455–471. [Google Scholar] [CrossRef] - Amini, R.; Taghipour, R.; Mirgolbabaei, H. Numerical assessment of hydrodynamic characteristics in chlorine contact tank. Int. J. Numer. Methods Fluids
**2011**, 67, 885–898. [Google Scholar] [CrossRef] - Zhang, J.; Asce, M.; Tejada-Martínez, A.E.; Zhang, Q. Hydraulic efficiency in RANS of the flow in multichambered contactors. J. Hydraul. Eng.
**2013**. [Google Scholar] [CrossRef] - Teixeira, E.C.; Do, R. Nascimento Siqueira, performance assessment of hydraulic efficiency indexes. J. Environ. Eng.
**2008**, 134, 851–859. [Google Scholar] [CrossRef] - Demirel, E.; Aral, M.M. Unified analysis of multi-chamber contact tanks and mixing efficiency based on vorticity field. part I: Hydrodynamic analysis. Water
**2016**, 8, 495. [Google Scholar] [CrossRef] [Green Version] - Aral, M.M.; Demirel, E. Novel slot-baffle design to improve mixing efficiency and reduce cost of disinfection in drinking water treatment. J. Environ. Eng.
**2017**, 143, 06017006. [Google Scholar] [CrossRef] - Demirel, E.; Aral, M.M. An efficient contact tank design for potable water treatment. Tek. Dergi
**2018**, 29, 8279–8294. [Google Scholar] [CrossRef] - Demirel, E.; Aral, M.M. Liquid sloshing damping in an accelerated tank using a novel slot-baffle design. Water
**2018**, 10, 1565. [Google Scholar] [CrossRef] [Green Version] - Kizilaslan, M.A.; Demirel, E.; Aral, M.M. Effect of porous baffles on the energy performance of contact tanks in water treatment. Water
**2018**, 10, 1084. [Google Scholar] [CrossRef] [Green Version] - Kizilaslan, M.A.; Nasyrlayev, N.; Demirel, E. A perforated baffle design to improve mixing in contact tanks. Water
**2020**, 12, 1022. [Google Scholar] [CrossRef] [Green Version] - Rauen, W.B.; Angeloudis, A.; Falconer, R.A. Appraisal of chlorine contact tank modelling practices. Water Res.
**2012**, 46, 5834–5847. [Google Scholar] [CrossRef] - Zhang, J.; Tejada-Martínez, A.E.; Zhang, Q. Developments in computational fluid dynamics-based modeling for disinfection technologies over the last two decades: A review. Environ. Model. Softw.
**2014**, 58, 71–85. [Google Scholar] [CrossRef] - Angeloudis, A.; Stoesser, T.; Kim, D.; Falconer, R.A. Modelling of flow, transport and disinfection kinetics in contact tanks. Proc. Inst. Civ. Eng. Water Manag.
**2014**, 167, 532–546. [Google Scholar] [CrossRef] - Pfeiffer, V.; Barbeau, B. Validation of a simple method for predicting the disinfection performance in a flow-through contactor. Water Res.
**2014**, 49, 144–156. [Google Scholar] [CrossRef] - Zhang, J.; Xu, X.; Tejada-Martinez, A.; Zhang, Q.; Wicklein, E. Evaluating reactor hydraulics in a cost-effective and environment-friendly way: Numerical tracer study. AWWA Water Sci.
**2019**, 1, e1163. [Google Scholar] [CrossRef] - Okhravi, S.; Eslamian, S.; Fathianpour, N. Assessing the effects of flow distribution on the internal hydraulic behavior of a constructed horizontal subsurface flow wetland using a numerical model and a tracer study. Ecohydrol. Hydrobiol.
**2017**, 17, 264–273. [Google Scholar] [CrossRef] - Shahrokhi, M.; Rostami, F.; Said, M.A.M.; Yazdi, S.R.S.; Syafalni. The effect of number of baffles on the improvement efficiency of primary sedimentation tanks. Appl. Math. Model.
**2012**, 36, 3725–3735. [Google Scholar] [CrossRef] - Zhang, J.; Tejada-Martinez, A.E.; Lei, H.; Zhang, Q. Indicators for technological, environmental and economic sustainability of ozone contactors. Water Res.
**2016**, 101. [Google Scholar] [CrossRef] [PubMed] - Zhang, J.; Tejada-Martínez, A.E.; Zhang, Q. Evaluation of large eddy simulation and RANS for determining hydraulic performance of disinfection systems for water treatment. J. Fluids Eng.
**2014**, 136, 121102. [Google Scholar] [CrossRef] - Angeloudis, A.; Stoesser, T.; Falconer, R.A.; Kim, D. Flow, transport and disinfection performance in small- and full-scale contact tanks. J. Hydro Environ. Res.
**2015**, 9, 15–27. [Google Scholar] [CrossRef] - Kizilaslan, M.A.; Demirel, E.; Aral, M.M. Efficiency enhancement of chlorine contact tanks in water treatment plants: A full-scale application. Processes
**2019**, 7, 551. [Google Scholar] [CrossRef] [Green Version] - Zhang, J.; Tejada-Martínez, A.E.; Zhang, Q. Reynolds-averaged Navier-Stokes Simulation of the flow and tracer transport in a multichambered ozone contactor. J. Environ. Eng.
**2013**, 139, 450–454. [Google Scholar] [CrossRef] - Kim, D.; Elovitz, M.; Roberts, P.J.W.; Kim, J.H. Using 3D LIF to investigate and improve performance of a multichamber ozone contactor. J. Am. Water Work. Assoc.
**2010**, 102, 61–70. [Google Scholar] [CrossRef] - Bruno, P.; Di Bella, G.; De Marchis, M. Large Eddy Simulation of Contact Tanks for Disinfection in Drinking Water Treatment. In Direct and Large Eddy Simulation XII; García-Villalba, M., Kuerten, H., Salvetti, M., Eds.; DLES 2019 ERCOFTAC Series; Springer: Cham, Switzerland, 2020; Volume 27, pp. 503–508. [Google Scholar] [CrossRef]
- De Marchis, M. Large eddy simulations of roughened channel flows: Estimation of the energy losses using the slope of the roughness. Comput. Fluids
**2016**, 140, 148–157. [Google Scholar] [CrossRef] - Kim, D.; Stoesser, T.; Kim, J.H. Modeling aspects of flow and solute transport simulations in water disinfection tanks. Appl. Math. Model.
**2013**, 37, 8039–8050. [Google Scholar] [CrossRef] - Zhang, J.; Pierre, K.C.; Tejada-Martinez, A.E. Impacts of flow and tracer release unsteadiness on tracer analysis of water and wastewater treatment facilities. J. Hydraul. Eng.
**2019**, 145. [Google Scholar] [CrossRef] - United States Environmental Protection Agency. Disinfection Profiling and Benchmarking Guidance Manual; Appendix A Rep. No. EPA 816-R-03-004 EPA; USEPA: Cincinnati, OH, USA, 1999.
- Marske, D.M.; Boyle, J.D. Chlorine contact chamber design—A field evaluation. Water Sew. Work.
**1973**, 120, 70–77. [Google Scholar]

**Figure 4.**Time-averaged absolute velocity vectors in the center-plane (z/L = 0.5) of (

**a**) conventional reactor and (

**b**) D2, (

**c**) D8 and (

**d**) D16 perforated designs.

**Figure 5.**Residence time distribution (RTD) curves of (

**a**) conventional, D2, D4, D6 and D8 designs and (

**b**) from D8 to D16 models.

**Figure 8.**Distribution of vertical velocity component across the chamber width of conventional and D12 designs at different depths: (

**a**) y/H = 0.27; (

**b**) y/H = 0.5; (

**c**) y/H = 0.72.

Design | Hole Side (mm) | Number of Holes | Perforation Percentage (%) |
---|---|---|---|

D2 | 2 | 204 | 2.0 |

D4 | 4 | 120 | 4.6 |

D6 | 6 | 126 | 11.0 |

D8 | 8 | 96 | 14.8 |

D10 | 10 | 63 | 15.2 |

D12 | 12 | 48 | 16.7 |

D14 | 14 | 40 | 18.9 |

D16 | 16 | 35 | 21.6 |

Baffling Condition | t_{10}/τ |
---|---|

Unbaffled (mixed flow) | 0.1 |

Poor | 0.3 |

Average | 0.5 |

Superior | 0.7 |

Perfect (plug-flow) | 1.0 |

Design | t_{10}/τ | t_{90}/τ | Mo | σ^{2} |
---|---|---|---|---|

Conventional | 0.347 | 2.000 | 5.758 | 0.105 |

D2 | 0.409 | 2.052 | 5.022 | 1.313 |

D4 | 0.459 | 2.275 | 4.960 | 1.308 |

D6 | 0.472 | 2.152 | 4.560 | 1.300 |

D8 | 0.481 | 2.116 | 4.396 | 1.285 |

D10 | 0.491 | 2.098 | 4.273 | 1.279 |

D12 | 0.509 | 2.116 | 4.161 | 1.272 |

D14 | 0.499 | 2.061 | 4.127 | 1.268 |

D16 | 0.481 | 2.070 | 4.302 | 1.280 |

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

© 2020 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 (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Bruno, P.; Di Bella, G.; De Marchis, M.
Perforated Baffles for the Optimization of Disinfection Treatment. *Water* **2020**, *12*, 3462.
https://doi.org/10.3390/w12123462

**AMA Style**

Bruno P, Di Bella G, De Marchis M.
Perforated Baffles for the Optimization of Disinfection Treatment. *Water*. 2020; 12(12):3462.
https://doi.org/10.3390/w12123462

**Chicago/Turabian Style**

Bruno, Paolo, Gaetano Di Bella, and Mauro De Marchis.
2020. "Perforated Baffles for the Optimization of Disinfection Treatment" *Water* 12, no. 12: 3462.
https://doi.org/10.3390/w12123462