Effects of Inlet Position and Baffle Configuration onHydraulic Performance of Primary Settling Tanks

A. Tamayol1; B. Firoozabadi2; and G. Ahmadi3

Abstract: Circulation regions always exist in the settling tanks. These regions reduce the tank’s performance and decrease effectivevolume of the tank. Recirculation zones would also result in short-circuiting and high flow mixing problems. Inlet position would have aneffect on these, too. Use of good baffle configuration may increase performance of settling tanks. One method for comparison performanceof different tanks with each other is to use flow through curves �FTCs�. In this note using FTCs, effects of inlet position and baffleconfiguration on the hydraulic performance of primary settling tanks are studied. The best position of baffle is also determined.

DOI: 10.1061/�ASCE�0733-9429�2008�134:7�1004�

CE Database subject headings: Two-dimensional analysis; Numerical analysis; Inlets, waterways; Turbulence; Settling tanks.

Introduction

Sedimentation by gravity is one of the most common appliedtreatments for removal of suspended solids from water or waste-water in refinery plants. Settling tanks are one of the main parts ofa treatment plant especially in refinement of sediment-laden wa-ters. In these tanks sediment-laden waters will move through thelength of the tank with a low speed giving the effluent enoughtime for sedimentation of solid particles. Investment for settlingtanks is about 30% of the total cost of a treatment plant. Theirperformance would directly or indirectly influence the perfor-mance of other parts and water quality. The design of tanks withhigh deposition rate and hydraulic efficiency is important and wasthe subject of many theoretical, experimental, and numericalinvestigations.

Sedimentation tanks are divided into two main categories:Type and concentration of sludge and particles that are availablein them. Primary settling tanks have a low influent concentration.Flow field in them is not influenced much by concentration fieldand buoyancy effects can be negligible. Disinfection tanks alsohave the same characteristics and these tanks can be studied withthe same procedure. However, secondary or final-settling tankshave a higher influent concentration. They usually are placed afterprimary tanks and activation tanks. So they usually contain acti-vated sludge and as a result of this, size of particles would growand flow field is influenced by concentration distribution.

For a tank with high performance, it is essential to have a

1Ph.D. Student, Dept. of Mechanical Engineering, Shiraz Univ.,Shiraz, Iran.

2Associate Professor, School of Mechanical Engineering, Sharif Univ.of Technology, Azadi Blv., PO Box 11365-8639, Tehran, Iran.

3Professor, Dept. of Mechanical and Aeronautical Engineering,Clarkson Univ., Potsdam, NY.

Note. Discussion open until December 1, 2008. Separate discussionsmust be submitted for individual papers. To extend the closing date byone month, a written request must be filed with the ASCE ManagingEditor. The manuscript for this technical note was submitted for reviewand possible publication on August 11, 2005; approved on May 8, 2007.This technical note is part of the Journal of Hydraulic Engineering, Vol.134, No. 7, July 1, 2008. ©ASCE, ISSN 0733-9429/2008/7-1004–1009/

$25.00.

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uniform and calm flow field. This would help particles settledown with a constant velocity during less time. In general, circu-lation zones would always take place in settling tanks. Existenceof these regions may have different effects. These dead zoneswould reduce the effective volume of the tanks. Also it may causea short circuiting problem between the inlet and outlet of the tank.In this case some part of flow would exit the tank without anysedimentation. It also would create regions with high turbulenceintensity, which would not only reduce the possibility of particledeposition, but also may cause resuspension problem.

Design of settling tanks is mostly based on detention time andassumption of uniform flow and uniform settling velocity of par-ticles. These assumptions are not real and neglecting of hydrody-namic phenomena, which would occur in the settling tanks, mayresult in an incorrect design. There are some ways to reduce thesize of these dead zones, which would increase the performance.One is to use a suitable baffle configuration. It must be noted thatthe use of baffles without enough caution would result in tankswith the worse performance than the tank without a baffle. Abaffle’s cost is also high. These make it essential to investigate thebest position and size of the baffles in settling tanks.

Flow in the settling tanks is not a high Reynolds number tur-bulent flow. The Reynolds number based on the depth on the tankmostly is in the range of 10,000–80,000. Circulation zones withcurvature of streamlines always exist in settling tanks. So turbu-lence modeling may affect the predicted results especially nearthe zone where streamlines are curved. Predicted turbulence in-tensity would have many effects on scouring of particles from thebed.

Determination of the flow and concentration fields can beachieved either experimentally or theoretically by means of math-ematical methods. Experimental determination of the flow field isdifficult and mostly expensive. For this reason, simplified theo-retical models have been used. Although the early researchers likeDobbins �1944� and Camp �1946� were aware of the importanceof turbulent mixing and recirculation zones, they were not able toprovide adequate solutions due to the lack of suitable hydrody-namics and turbulence models.

Advanced research presented by Larsen �1977� created a newarea in later investigations. Imam et al. �1983� solved flow equa-

tions with a constant turbulent eddy diffusivity assumption. Celik

and Rodi �1985� and Stamou et al. �1990� used k–� turbulencemodel with flow equations to predict flow field in settling tanks.Stamou et al. �1990� and Adams and Rodi �1990� investigated thediscretization accuracy effects on the predicted results. Stamouet al. �2001� used computed fluid dynamics for increasing perfor-mance of a real settling tank. Ashjari and Firoozabadi �2003� useda nonlinear k–� model for the prediction of flow and tank perfor-mances. Tamayol and Firoozabadi �2006� studied the effects ofdifferent turbulent models on the flow field. Tamayol et al.�2006b� also studied the effects of different inlet positions on theflow field and efficiency of settling tanks.

In the present work, using a renormalization-group �RNG�k–� model, the effects of different baffle configurations on theperformance of settling tanks are studied. As complete standardsare not available for the design of a good baffle configuration, thebest position of the baffle is determined through numerical simu-lations, which could be used in design. The effect of differentinlet places on the hydraulic efficiency of a rectangular primarysettling tank is also studied.

Geometry Specification

Two different geometries are used in this work. In general, flowfield in settling tanks is three dimensional. The degree of impor-tance of the three-dimensional effects is related to the positions ofthe inlet and outlet of a basin and their width. In the presentinvestigation, the inlet and outlet are assumed to uniformly spreadthe width of the tanks so the three-dimensional effects can beneglected and, for simplicity, two-dimensional models are usedfor simulations.

The effects of different positions of the inlet opening are stud-ied on a geometry, which is similar to a real settling tank thatexists in the city of Sarnia, Ont., Canada. The geometry is shownin Fig. 1 and a second geometry is a test model that is shown inFig. 2. This tank is available in Karlsruhe University and some ofthe previous research is done on this test case �Stamou et al.1990�. A study of a suitable baffle position is done on this tank.

Fig. 1. Simplified geometry of the tank in Sarnia with hi=1.35 m andh=2.7 m and L=41 m

Fig. 2. Geometry of the Karlsruhe tank with hi=2 cm andh=11 cm and L=250 cm

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Two cases are rectangular. Neglecting three-dimensional effectsof geometry and for simplicity, cases are assumed to be twodimensional.

Flow through Curves and Hydraulic Efficiency

Design of settling tanks is based on detention time �DT� of par-ticles and flow in the tank. Theoretical detention time �TTH� of atank is TTH=V /QD, where QD�volume flow rate of the tank andV�volume of the tank. Hydraulic efficiency is also related to DTof the tank. If DT is greater than the time that is needed forparticles to arrive at the bed of the tank then the particles wouldsettle, otherwise particles may exit the tank with the effluent. Realdetention time �TRe� of a tank is lower than TTH of it, resultingfrom a deviation from the plug flow.

For the determination of the performance of a tank, two meth-ods can be used. One is to determine the concentration field ev-erywhere in the tank and the other one is to use flow throughcurves �FTCs�. The second method is simpler in an experimentalinvestigation. It also gives some good information about mixingand short-circuiting degrees of the tank. For the second approach,dye, or something similar, is injected in the inlet for a time ofabout 10% percent of TTH and the concentration is measured atthe outlet and is plotted versus time. For a better comparison, theconcentration and time axes are nondimensionalized by C0, whichis mean concentration of the tank and is calculated by dividing thetotal mass of injected dye �Min� by the volume of the tank �V�,C0=Min /V, and TTH, respectively. Nondimensional time is shownwith �t�.

Characteristics and parameters of FTC, which can be used fordetermination of different aspects of hydraulic filed and effi-ciency, are divided into categories:1. Parameters that can be used for the investigation of a short-

circuiting problem are t0 and t10. Indices are a percentage ofthe injected material that passed the outlet. Higher values oft0 and t10 mean less possibility of short circuiting.

2. Parameters for determining the degree of mixing are mea-sured from the width of the curve. These parameters aret75– t25, t90– t10, and t90 / t10. When these values are higherit means the flow in the tank is highly mixed and is notsuitable.

3. For the prediction of performance t50 and time when theconcentration at the outlet is maximum �tmax� are used.Higher values of these parameters mean higher performanceof the tank. It must be noted that the best way for the pre-diction of the efficiency is to determine concentration field.

Table 1. Reattachment Length Calculated by Different Models in theKarlsruhe Tank �Tamayol et al. 2005a�

Turbulence model Grid size Reattachment length

Standard k–� 35�75 0.61

Standard k–� 150�68 0.612

RNG k–� 75�35 0.63

RNG k–� 150�68 0.65

RSM 75�35 0.64

RSM 150�68 0.65

Nonlinear k–� 75�35 0.675

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Governing Equations

The governing equations are conservative equations of mass andmomentum. Closing the governing equations, turbulence modelsfor calculation of Reynolds stresses are also added to those equa-tions. The mass conservation equation for water �fluid� is a simplecontinuity equation. As the flow field is assumed to be two di-mensional, two momentum equations in the x and y directions,length and height of the tank, respectively, are solved.

These equations are written with the Bousinesq approxima-tion. For modeling of turbulence, the RNG k–� model is used andturbulent viscosity is computed from a differential equation. Thismodel is a modified model for Reynolds number turbulent flowsthat are not high and for a case with curved streamlines as incirculation regions.

Previous works have shown �Tamayol et al. 2005a� that theRNG k–� model is precise enough to capture the curvaturestreamlines. Those results are presented in Table 1 and show thereattachment length of the Karlsruhe tank calculated by differentturbulence models. The experimental reattachment length re-ported by Stamou et al. �1990� is 0.6–0.7. The modified terms inthe � equation of RNG k–� model enhance the dissipation rate inthe vicinity of the stagnation region. This mechanism prevents theaugmentation of turbulent kinetic energy in this region �Lee1997�.

The mass conservation equation for particles is

�Ci

�t+

�uCi

�x+

�vCi

�y=

�

�x� �t

�c

�Ci

�x� +

�

�y� �t

�c

�Ci

�y+ VSiCi� �1�

which is named as a concentration equation; C�concentration;and �c�turbulent Schmidt number and is taken to be equal to 1�Stamou et al. 1990�. Vs stands for final settling velocity of par-ticles and must be calculated later using another equation. In theprimary settling tanks a discrete model is used for Vs. In thecalculation of FTC, Vs is set equal to zero.

Symmetry condition is used for a free surface boundary con-dition. In the present work, effects of wind and small ripples onthe flow field are neglected. Values of all properties are known atthe inlet and at the outlet values are interpolated from the com-putational domain. Walls are treated as nonpenetrative bound-aries. No slip condition for velocity and zero concentrationgradient in the concentration equation are used. Standard wall

Fig. 3. Comparison of velocity distribution from present work withformer experimental and numerical simulations for simple case forSi /h=0.25

functions are used for turbulence modeling.

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A finite-volume approach is used for the solution of the gov-erning equations. second-order upwind scheme is selected for dis-cretizing of the governing equations. The SIMPLEC algorithm isused for pressure–velocity coupling. Finally, governing equationswith specified boundary condition are solved using the FLUENTcode �2001�, which is a validated code and can be used for thiscase. For the determination of important parameters in FTC a userdefined function is added to the software. Inlet boundary condi-tions for the concentration equation is also added to the codeusing a user defined function.

Results and Discussion

A settling tank with the simplified geometry of the tank, whichexists in Sarnia, Ont., Canada, is picked for studying effects ofdifferent inlet positions. Inlet of the tank in reality is from thesurface. Three different positions for inlet openings are used incomputations. The flow Reynolds number based on the depth of

Fig. 4. Streamlines for Si /h=0.75

Fig. 5. Streamlines for Si /h=0.5

Fig. 6. Streamlines for Si /h=0.25

Fig. 7. FTC for different values of Si /h

the tank is 62,000. Different computational grids are used forcalculations, eventually a grid with the size of 600�160 is foundwhere results have an acceptable independency from the grid. InFig. 3, the predicted results of velocity are compared to experi-mental and numerical results of Stamou et al. �1990�. The reasonsfor the differences between the experimental and numerical re-sults are described by Stamou et al. �1990�. The most importantreasons are simplifications that are used for geometry modelingand a lack of accuracy in instruments used for measuring velocityin the real tank.

For better judgment, in Figs. 4–6 only circulation zones fordifferent values of Si /h are shown, instead of the whole flow field.When the inlet is from the surface of the tank �Si /h=0.75�, onerecirculation region with length of about 9.3 m exists, which oc-cupy 10% of the total volume of the tank. In Fig. 5 two vorticesare shown for Si /h=0.5 with a maximum length of 4.2 m, whichspoils only 5% of the total volume of the tank. For Si /h=0.25,shown in Fig. 6, one circulation zone with a length of 8.1, whichoccupies 9% of the total volume exists. From these, it is predict-able that the case with Si /h=0.5 would have better performance.

In Fig. 7 FTC diagrams for different values of Si /h are shown.TTH of this tank is 1 ,782.6 s. Different important values of dia-grams are available in Table 2. It can be seen that Si /h=0.5 hashigher values of t0 and t10, which means that short circuiting islower in this case. Values of t75– t25, t90– t10, and t90 / t10, whichshow that degrees of flow mixing are lower than in other cases.Finally, higher values of t50 and tmax show higher hydraulic effi-ciency. This result was also predictable from the flow field andstreamlines. The inlet position with Si /h=0.5�best and the casewith Si /h=0.25, which is the inlet from the bottom of the tank isbetter than the other one.

Another way for increasing the performance of a settling tankis to use baffle in the tank. In this part, second geometry shown inFig. 2 is used. In this part Si /h is set equal to 0.91, which meansthat the inlet opening is placed near the surface. The Reynoldsnumber is equal to 10,000 based on the inlet opening height.Different grids were used for calculations and finally a 75�35grid used for the computation of the results.

Six different positions and two different sizes for baffles areused for this part. Positions of baffles are determined with respectto streamlines of the same tank without any baffle �WT�. Stream-lines of the tank without a baffle are shown in Fig. 8. The reat-tachment length is also shown and is equal to 0.66 m. From Fig.8, baffles are placed in three distances from the inlet. First, nearthe inlet and near the circulation zone at x=0.35 m; second, nearthe outlet at x=1.8 m; and third at x=0.9 m, which is near the

Table 2. Important Values from FTC for First Geometry

Inlet depth ratio t0 t10 t75– t2

Si /h=0.75 0.56 0.70 0.32

Si /h=0.5 0.68 0.84 0.17

Si /h=0.25 0.61 0.78 0.20

Fig. 8. Streamlines of case without baffle �reattachmentlength�0.66 m�

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middle of the tank. Baffles are placed on the bottom or attached tothe surface of the tank. These geometries are shown in Fig. 9.

When we use a baffle in the tank two circulation regionswould exist. One under the inlet of the tank, the same as case WT,and another one would exist behind the baffle. The size of bothcirculation zones relates to the position and height of the baffle. Inthis investigation, baffle height are taken 1.5 and 2.5 times of theinlet opening.

The total volume of two dead positions mentioned earlier arepresented in Table 3. These values must be compared with thevolume of 567 cm2 occupied by the vortex that exists in the tankwithout a baffle. It can be seen that when the baffle is placed nearthe circulation zone the total size of the circulation region may be

t90– t10 t90 / t10 tmax t50

0.72 2.03 0.75 0.88

0.43 1.51 0.91 0.95

0.51 1.65 0.85 0.89

Table 3. Total Dead Volume for Various Positions and Heights

Bafflepositiona

Baffleheight�cm�

Totalcirculation

volume �cm2�b

1 3 555

1 5 637.5

2 3 648

2 5 807.5

3 3 476

3 5 483

4 3 715.5

4 5 882

5 3 622

5 5 710

6 3 589.5

6 5 655aPositions are shown in Fig. 9.bPer width of the tank.

Fig. 9. Different positions of baffle in the tank �dimensions are inmeters�

Fig. 10. Streamlines of baffle in Position 3 and size 3 cm

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reduced. The best position is in the middle of the circulationregion, because when the baffle is placed there it cuts the circu-lating streamlines. For better judgment, the streamlines for thebaffle with Position 3 and height 3 cm are shown in Fig. 10. Useof baffles without good experience and knowledge would increasethe size of the circulation zones. Because of this problem in pri-mary settling tanks baffles are not used.

From Table 3 it is predictable that some cases must have apoor performance. This is related to the size of dead zone. AsFTC diagrams are not clear enough, Table 4, which contains im-portant data from FTC, is presented. t0 and t10 are important inunderstanding the degree of short circuiting; higher values mean alower degree of short circuiting. Between these cases, Case 1 isbetter than the others from this point of view, but the case withoutthe baffle �WB� and Case 3 are also acceptable. Case 5 is alsoacceptable for this part. tmax and t50 are indicators of efficiency soif they are high it means hydraulic efficiency is also high. Cases 1and 3 are much better than the others in comparison with the WBcase. When the two different values that appear in the last col-umns are lower, that means the flow is more uniform. From thispoint of view, Case 3 is also better than the others.

Finally, a conclusion must be made to use or not use the bafflein primary settling tanks. As it was predictable, using the baffle inmost positions may reduce tank efficiency, but if it is used with agood design it may increase performance. From the results, hav-ing the baffle far from the circulation zone is not acceptable. Thebaffle must be near the circulation zone and better be somewherein that zone to spoil the circulation effect.

Conclusions

The effects of two different methods for increasing the perfor-mance of the primary settling tanks were studied. For comparisonof the performances of different models the FTC method wasused. Two different geometries were also used. The position ofthe inlet opening would affect the performance and flow mixingin the settling tanks. The best position for the inlet is to be placedsomewhere in the middepth of the tank. The inlet from the bottomis better than the inlet from the surface. Another method for in-creasing the hydraulic efficiency is to use a suitable baffle con-figuration. In primary settling tanks the placement of the bafflemust carefully be determined, as it could deteriorate the perfor-

Table 4. Useful Data from FTC Diagrams

Case t0 t10 t75– t25

1-3 0.565 0.791 0.307

1-5 0.551 0.773 0.336

2-3 0.495 0.76 0.327

2-5 0.46 0.715 0.404

3-3 0.533 0.787 0.279

3-5 0.538 0.789 0.289

4-3 0.476 0.725 0.38

4-5 0.425 0.689 0.434

5-3 0.540 0.78 0.295

5-5 0.505 0.755 0.338

6-3 0.509 0.775 0.295

6-5 0.484 0.76 0.324

Without baffle 0.55 0.789 0.284

mance of the tank.

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As complete standards are not available for the design of agood baffle configuration, the best position of the baffle is deter-mined through numerical simulations that can be used in thedesign. The best position of the baffle is somewhere in the circu-lation zone to spoil this circulation region. The results are validfor primary settling tanks, which have a low rate of sedimentationand disinfection basins. In large sedimentation tanks with a highrate of sedimentation it is better to use direct sediment transportmodels for the determination of the best baffle position.

Notation

The following symbols are used in this technical note:C � concentration;

C0 � average concentration of the tank;c1�, c2� � constants in turbulence model;

H � baffle height;h � depth of the tank;k � turbulent kinetic energy per unit mass;

Q � inlet flow rate;TTH � theoretical detention time;

t � nondimensional time;u � horizontal velocity;

Vs � settling velocity;v � vertical velocity;� � dissipation rate;� � laminar viscosity;�t � turbulent viscosity; and�c � turbulent Schmidt number.

References

Adams, E. W., and Rodi, W. �1990�. “Modeling flow and mixing in sedi-mentation tanks.” J. Hydraul. Eng., 116�7�, 895–913.

Ashjari, M. A., and Firoozabadi, B. �2003�. “Use of nonlinear k–� incalculation of performance of settling tanks.” Proc., 11th Int. Conf. ofMechanical Engineering, 34–43.

Camp, T. R. �1946�. “Sedimentation and the design of settling tanks.”ASCE Trans., Paper No. 2285, 3, 895–936.

Celik, I., and Rodi, W. �1985�. “Prediction of hydrodynamic characteris-tics of rectangular settling tanks.” Int. Symp. on Refined Flow Model-

t90– t10 t90 / t10 tmax t50

0.64 1.81 0.867 0.969

0.691 1.89 0.844 0.962

0.704 1.93 0.851 0.945

0.858 2.20 0.793 0.918

0.662 1.84 0.880 0.955

0.673 1.85 0.876 0.955

0.8 2.10 0.805 0.933

0.9 2.31 0.767 0.916

0.667 1.86 0.867 0.953

0.732 1.97 0.842 0.947

0.68 1.88 0.873 0.955

0.707 1.93 0.860 0.953

0.651 1.83 0.882 0.96

ing and Turbulence Measurements, 641–665.

Dobbins, W. E. �1944�. “Effects of turbulence on sedimentation.” Trans.Am. Soc. Civ. Eng., 109�2218�, 629–656.

Fluent Inc. �2001�. Fluent 5.2.3 user’s manual, Centerra Resource Park,Lebanon.

Imam, E., Mc Corquodale, J. A., and Bewtra, J. K. �1983�. “Numericalmodeling of sedimentation tanks.” J. Hydraul. Eng., 109�12�, 1740–1754.

Larsen, P. �1977�. “On the hydraulics of rectangular settling basins.” Rep.No. 1001, Dept. of Water Resource Engineering, Lund Institute ofTechnology, Lund, Sweden.

Lee, S. �1997�. “Unsteady aerodynamic force prediction on a square cyl-inder using k–� turbulence models.” J. Wind. Eng. Ind. Aerodyn.,67-68�1�, 79–90.

Stamou, A. L., Adams, E. W., and, Rodi, W. �1990�. “Numerical model-ing of flow and settling in primary rectangular clarifiers.” J. Hydraul.

Res., 27�5�, 665–682.

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Stamou, A., Katsiris, I., Georgiadis, S., Kanellopoulou, S., Tzoumerkas,F., and Xenos, D. �2001�. “Improving the efficiency of existing waterprocess tanks using flow through curves �FTCs� and mathematicalmodels.” 7th Conf. on Environmental Science and Technology, 838–845.

Tamayol, A., and Firoozabadi, B. �2006�. “Effects of turbulent modelsand baffle position on hydrodynamics of settling tanks.” ScientiaIranica, 13�3�, 255–260.

Tamayol, A., Firoozabadi, B., and Farhanieh, B. �2005a�. “Calculation ofhydraulic efficiency of primary settling tanks using RNG k–� turbu-lence model.” 5th Conf. of Hydraulic, 15–21 �in Persian�.

Tamayol, A., Nazari, M., Firoozabadi, B., and Nabovati, A. �2005b�.“Numerical modeling and study of effects of inlet position and heightof inlet baffle on the performance of settling tanks. Proc., Fluid Dy-

namics Conf. �in Persian�.

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