Numerical Investigation of the number of baffles effect on the efficiency of primary sedimentation tank in true dimensions

Authors
M. javadi rad
M. shahrokhi 1
T. rajaee 2
1 Assistant Prof. of Department of Civil Engineering, Ghyasedin University
2 Associate Prof. of Department of Civil Engineering, Qom University
Abstract
Numerical Investigation of the number of baffles effect on the efficiency of primary sedimentation tank in true dimensions.

Settling tanks of wastewater treatment plants are the most important components so that about one-third of the cost of a wastewater treatment plant construction is concerned to these tanks. In addition, the regulations referred to design of settling tanks are not enough. According to these, the adverse factors in settling tanks is very important to design and improve their performance. These factors in primary sedimentation tanks are including circulation zones, the phenomenon of short circuiting and non-uniform flow. One of the most important ways to improve flow conditions and increase the efficiency of settling tanks, is modifying the geometry of the tank through the installation of baffles. In this investigation, the effect of baffle structures on the hydraulic efficiency of primary sedimentation tanks has been investigated by three-dimensional numerical modeling in Flow-3D software. In this study, the optimum number of baffles is studied to increase hydraulic efficiency. Settling tank in true size and simplified model primary settling tank treatment plant south of Tehran. The geometry Specifications of tank include of length tank (L) from the main inlet to the overflow output of 60 m, a width of tank (W) 12.9 m, water depth in normal mode (H) 3 m. the input flow rate to the tank 650 l/s. Model geometry using AutoCAD software and three-dimensional shape is drawn. In this study, from three mesh blocks to mesh geometry model is used. All three of these blocks in all directions are fully in touch with and the type of are linked blocks. Intended for three blocks, the size of the mesh is selected respectively, 5, 7 and 10 cm monotonically in each direction. In numerical modeling, the turbulence model used to solve turbulent flow and to solve pressure the GMRES method is used. In addition, the VOF technique to show the behavior of fluid on free surface flow and FAVOR technique have been used to simulate surfaces and geometric boundaries. Verification of numerical simulation results with former experimental data properly acknowledges the numerical results. It is already known the optimal location of the first baffle.

The results show that baffle causes the uniformity flow and increase removal efficiency of the primary sedimentation tanks. To determine the optimum number of baffles, the comparison results between no baffle tank and optimized cases with one, two and three baffle done. Using more baffles, in ideal conditions, causes suppression of the jet flow and more chances the suspended particles deposition. The addition of new baffles in suitable locations reduces the maximum velocity amount, the size of the circulation zones and kinetic energy and create uniform velocity vectors inside the settling zone. Volume circulation zones by using one, two and three baffles compared to non-baffle decreased 4.18, 4.44 and 4.56% of the total tank volume, respectively. Finally, the results of the FTC method for several cases indicated that using number of baffles lead to increasing the performance of the sedimentation tanks.

Keywords

Subjects

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Modares Civil Engineering journal
Volume 17, Issue 5
November and December 2017
Pages 49-59