This article employs mixture model and discrete phase model to explore the flow and activated sludge distribution in a horizontal secondary sedimentation tank (SST). The influences of sludge diameter, density, and entrance velocity are investigated and cross checked with two models. A stagnation region (5 m ≤ x ≤ 9 m) appears at the bottom hopper, where sludge tends to accumulate. Around 4.5 m is the appropriate depth for this tank. Particles with higher density or larger diameter benefit to the separation of sludge. The minimum diameter 50 μm for sludge of 1,050 kg/m3 at an entrance velocity 0.10 m/s is a threshold to guarantee the separation. Lower entrance velocity helps the sedimentation of sludge. The critical diameter for particle with the same density increases with entrance velocity. Different densities cause different behaviors of settled sludge after returning to the tank. Threshold values of diameter, density of sludge particles, and entrance velocity are presented for the first time for SSTs with the similar structure. All these will contribute to the design and optimization of SSTs.
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References
1.
AdamsE.W., and RodiW. (1990). Modeling flow and mixing in sedimentation tanks. J. Hydraul. Eng. 116, 895.
2.
Al-SammarraeeM., ChanA., SalimS.M., and MahabaleswarU.S. (2009). Large-eddy simulations of particle sedimentation in a longitudinal sedimentation basin of a water treatment plant. Part I: Particle settling performance. Chem. Eng. J., 152, 307.
3.
AsgharzadehH., FiroozabadiB., and AfshinH. (2012). Experimental and numerical simulation of the effect of particles on flow structures in secondary sedimentation tanks. J. Appl. Fluid Mech. 5, 15.
4.
CelikI.B., GhiaU., RoacheP.J., FreitasC.J., ColemanH., and RaadP.E. (2008). Procedure for estimation and reporting of uncertainty due to discretization in CFD applications. J. Fluids Eng. 130, 078001.
5.
DingT., ChenH., and ChengS. (2016). Structural optimization of sedimentation tank based on CFD simulation. 2016 International Conference on Computational Modeling, Simulation and Applied Mathematics (CMSAM 2016), Bangkok, Thailand.
6.
EkamaG.A., and MaraisG.V.R. (2004). Assessing the applicability of the 1-D flux theory to full-scale secondary settling tank design with a 2D hydrodynamics model. Water Res. 38, 495.
7.
(2006). FLUENT 6.3 User's Guide. Lebanon, NH: Fluent, Inc.
8.
GaoH., and StenstromM.K. (2019). Generalizing the effects of the baffling structures on the buoyancy-induced turbulence in secondary settling tanks with eleven different geometries using CFD models. Chem. Eng. Res. Des. 143, 215.
9.
GhawiA.G., and KrisJ. (2011). Improvement performance of secondary clarifiers by a computational fluid dynamics model. Slovak J. Civil Eng. XIX, 1.
10.
GongM., XanthosS., RamalingamK., FillosJ., BeckmannK., DeurA., and McDorquodaleJ.A.Development of a flocculation sub-model for a 3-D CFD model based on rectangular settling tanks. Water Sci. Technol. 2011, 63, 213.
11.
GoulaA.M., KostoglouM., KarapantsiosT.D., and ZouboulisA. (2008). A CFD methodology for the design of sedimentation tanks in potable water treatment case study: The influence of a feed flow control baffle. Chem. Eng. J. 140, 110.
12.
GriborioA. (2004). Secondary Clarifier Modeling: A Multi-Process Approach. Ph.D. thesis, University of New Orleans.
13.
HolendaB., PásztorI., KárpátiÁ., and RédeyÁ. (2006). Comparison of one-dimensional secondary settling tank models. Eur. Water Manage. Online, 1.
14.
LarsenP. (1977). On the Hydraulics of Rectangular Settling Basins: Experimental and Theoretical Studies. Lund, Sweden: Department of Water Resources Engineering, Lund Institute of Technology, University of Lund.
15.
LaunderB.E., and SpaldingD.B. (1974). The numerical computation of turbulent Flows. Comp. Meth. Appl. Mech. Eng. 3, 269.
16.
PatankarS.V. (1980). Numerical Heat Transfer and Fluid Flow. New York: McGraw Hill.
PatzigerM., and KissK. (2015). Analysis of suspended solids transport processes in primary settling tanks. Water Sci. Technol. 72, 1.
19.
PlószB.G., NopensI., RiegerL., GriborioA., De ClercqJ., VanrolleghemP.A., DaiggerG.T., TakácsI., WicksJ., and EkamaG.A. (2012). A critical review of clarifier modelling: state-of-the-art and engineering practices. Proceedings of WWTmod2012-3rd IWA/WEF Wastewater Treatment Modeling Seminar, Mont-Sainte-Anne, Québec, Canada, p. 27.
20.
RaminE., WágnerD.S., YdeL., BinningP.J., RasmussenM.R., MikkelsenP.S., and PlószB.G. (2014). A new settling velocity model to describe secondary sedimentation. Water Res. 66, 447.
21.
RoacheP.J. (2009). Perspective: Validation—what does it mean? J. Fluids Eng. 131, 034503.
22.
SchillerL., and NaumanA.Z. (1935). A drag coefficient correlation. Z. Ver. Deutsch. 77, 318.
23.
StamouA.I., TheodoridisG., and XanthopoulosK. (2009). Design of secondary settling tanks using a CFD model. J. Environ. Eng. 135, 551.
24.
TarpagkouR., and PantokratorasA. (2013). CFD methodology for sedimentations: The effects of secondary phase on fluid phase using DPM coupled calculations. Appl. Math. Model, 37, 3478.
25.
WangX., SunR., AoX., ZhouZ., and LangJ. (2013). Eulerian-Eulerian solid-liquid two-phase flow of sandstone wastewater in a hydropower station rectangular sedimentation tank. Eur. J. Environ. Civil Eng. 17, 700.
26.
XanthosS., GongM., RamalingamK., FillosJ., DeurA., BeckmannK., and McCorquodaleJ.A. (2011). Performance assessment of secondary settling tanks using CFD modeling. Water Resour. Manag. 25, 1169.
27.
ZhouS., and McCorquodaleJ.A. (1992). Mathematical modeling of a circular clarifier. Can. J. Civil Eng. 19, 365.
