THE INFLUENCE ON THE FLOW FIELD AND PERFORMANCE OF A SEDIMENTATION TANK FOR POTABLE WATER TREATMENT DUE TO LOW (WINTER) AND HIGH (SUMMER) TEMPERATURES

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1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 THE INFLUENCE ON THE FLOW FIELD AND PERFORMANCE OF A SEDIMENTATION TANK FOR POTABLE WATER TREATMENT DUE TO LOW (WINTER) AND HIGH (SUMMER) TEMPERATURES ROZA TARPAGKOU 1, *, ASTERIOS PANTOKRATORAS 1, AND NIKOLAOS PAPADAKIS 2 1 Laboratory of Hydraulics and Hydraulic Structures Democritus University of Thrace, Department of Civil Engineering, V. Sofias 12 GR Xanthi, Greece and 2 Laboratory of Hygiene, Faculty of Medicine, Aristotle University of Thessaloniki 54124,Greece *Corresponding author: Tel address: rtarpag@civil.duth.gr ABSTRACT A Computational Fluid Dynamics (CFD) model is used in order to simulate the hydrodynamics and flow behaviour in an actual sedimentation tank for potable water treatment. The main objective of the present investigation is to evaluate the influence of temperature variations on the flow field and as a consequence on the efficiency of the tank. A series of experimental measurements in that sedimentation tank of temperature variation between the free surface and the bottom and between the influent and the effluent of the tank has been also conducted. The operation of the tank has been examined for low (winter) and high (summer) temperatures. The combination of the density variations with the horizontal stream produces a mixed convection problem. This mixed convection problem is very complicated because the variation of water density at low temperatures ( C) is strongly non linear (parabolic). A series of numerical simulations that reproduced the experimental data are presented. All simulations have been conducted for a transient, turbulence flow field and the equations describing the water thermophysical properties (density, viscosity, thermal conductivity and specific heat) have been incorporated into the CFD code ANSYS FLUENT using the programming language C. The momentum exchange between the primary and the secondary phase (particles) is taken into account, using a Lagrangian method (Discrete Phase Model) with two-way coupled calculations. Contours of stream function and concentration are presented, as well as velocity profiles, for parabolic (winter) and linear (summer) density region. It was found that the generated density and turbidity currents have a strong influence on the tank flow. At high temperatures new vortices are generated and the particles are transferred by the density currents. As the water temperature decreases the flow is influenced significantly, the turbidity rises and the efficiency of the tank decreases. Keywords: computational fluid dynamics, sedimentation, heat transfer, non linear density, multiphase flow 1. INTRODUCTION Sedimentation is a physical treatment process that utilizes gravity to separate suspended particles and solids from raw water. There are many important considerations that directly affect the design of the sedimentation process such as the variations in the plant flow rate, the types of the tank inlet and outlet, the applied method for the sludge removal, the cost and geometric configuration of the tank, the suspended particles settling velocity and finally the temperature variations (Kawamura 1991). Temperature variations of water in

2 the tank is one of the most important parameters that affects the flow field and as a consequence the sedimentation process efficiency. The combination of the density variations with the horizontal stream produce a mixed convection problem. This mixed convection problem is very complicated especially at low winter temperatures ( C) where the variation of water density is strongly non linear (parabolic). The main aim of the present paper is to investigate the 2D hydrodynamics and flow behaviour of a sedimentation tank for potable water treatment, using the Computational Fluid Dynamics (CFD) methods offered by the commercial software ANSYS, at low (<10 0 C) and high water temperatures. Many researchers have been focused in the study of sedimentation tanks for wastewater treatment. Larsen (1977) was probably the first who applied a CFD model to several secondary clarifiers. McCorquodale and Zhou (1993) investigated the effect of various solids and hydraulic loads on circular clarifier performance. In a primary sedimentation tank, where the solids concentration is limited and discrete settling prevails Imam et al. (1983) applied a fixed settling velocity and used an averaged particle velocity. Stamou et al. (1983) simulated the flow in a primary sedimentation tank using a 2D model in which the momentum and solid concentration equations were solved. Adams and Rodi (1990) used the same model and did extensive investigations on the inlet arrangements and the flow through curves. Liu et al. (2010) conduct measurements and simulations to achieve the optimal design of a settling tank. Water temperature is an important factor that affects the efficiency of a sedimentation tank. Wells and LaLiberte (2008) mentioned that uniform flow does not occur in sedimentation tanks at low temperatures.taebi-harandy mentioned that even temperature differences of only C seemed enough to induce a density current. The low temperature region were density is non linear has been studied by some researchers such as Pantokratoras (2008) but for other applications different than sedimentation tanks. So far, many researchers have used CFD simulations to study water flow and solids removal in settling tanks for sewage water treatment. However, there are not many works in the literature in CFD modelling of sedimentation tanks for potable water treatment. Moreover, according to the authors best knowledge, the effect of non linear density at low water temperatures on a sedimentation tank for potable water treatment, and the way that influence the flow field and hence the process efficiency, has never been investigated previously. This work study the influence of temperature variation within a sedimentation tank for potable water treatment. Two cases have been examined. One at the lowest winter temperature where the density variation is parabolic, and another at summer where the density is a linear function of temperature. The equations describing the water thermophysical properties (density, viscosity, thermal conductivity and specific heat) have been incorporated into the CFD code using the programming language C. A series of experimental measurements in the sedimentation tank for potable water treatment plant of the city of Thessaloniki has been conducted. The measurements concerned the temperature variation between the free surface and the bottom and between the influent and the effluent of the tank. The velocity and concentration profiles for these cases were investigated, in combination with contours of stream, density and temperature distribution within the sedimentation tank. The present work fully considered the interaction between the liquid and solid phase (and vice-versa). Buoyant forces and the effect of the producing density currents were also taken into account.

3 2. MATHEMATICAL MODEL The model used was based on a multiphase flow using an Euler - Lagrange approach. The Lagrangian approach provides a more detailed and realistic modelling of particle deposition because the equation that describes the particle motion is solved for each particle moving through the field of random fluid eddies. In this approach, the fluid is treated as a continuum and the discrete (particle) phase is treated in a natural Lagrangian manner, which may or may not have any coupling effect with the carrying fluid momentum (in the proposed model with coupling effect). Fluid phase is treated as a continuum by solving the Navier-Stokes equations, that is, the equations of conservation of mass, momentum and energy equation. The flow is turbulent and for turbulence closure the RNG k ε model is applied. The thermophysical properties of the fluid are functions of temperature and are given by the following equations {(1)-(5)}. These equations have been incorporated into the CFD code ANSYS FLUENT using the programming language C. At each iteration that occurs during the solution process, the temperature is updated at each time step, and this results in the updating of the following functions of the thermophysical properties of the fluid in each computational cell. This enables the realistic simulation of the properties and as a consequence the behavior of the fluid in nature. The density of water is a function of temperature, salinity and pressure. In this paper the International Equation of State for Seawater (Fofonoff 1985) is used for the calculation of density. The equation of state for seawater is valid for temperatures from -2 to 40 0 C, salinities from 0 o / oo to 40 o / oo and pressures from 1 to maximum oceanic pressure in bars. In this paper all calculations have been made for atmospheric pressure (1 bar) and zero salinity (pure water). The following equation is used: ( T, s, p) 1/ V( T, s, p) where V(T,s,p) is the specific volume. The specific volume is calculated by: (1) V( T, s, p) V( T, s,0)[1 p / K ( T, s, p)] (2) The form for specific heat that is used in the present paper is given by Fofonoff and Millard (1983) : C s T p A Bs Cs D Es Fs p 3/2 3/2 P(,, ) ( ) ( G Hs Is ) p ( J Ks Ms ) p 3/2 2 3/2 3 The expression used for dynamic viscosity μ (T,s,p) has been proposed by Matthaus (1970). The equation is valid for 0 0 C< T <30 0 C, 0 o / oo < s <36 0 o / oo and 1 to 1000 bars, but for larger ranges of T, s and p a slightly greater error is produced. The viscosity is given as: b (3) ( T, s, p) ( ) T ( ) T ( ) T ( ) p ( ) p ( ) s ( p p ) T ( T T T ) s (4) Thermal conductivity k(t,s,p) in cal/smc is given by Caldwell (1985) as: (0.003) T ( ) T k( T, s, p) (0.0653) p (0.0029) s (5)

4 3. DESCRIPTION OF PHYSICAL PROBLEM AND NUMERICAL APPLICATION 3.1 Physical problem In the present paper the circular sedimentation tank for potable water treatment plant of the city of Thessaloniki was investigated (Fig.1). The plant receives raw water from Aliakmon river and its capacity is m 3 /d. The employed processes include preozonation, coagulation flocculation, sedimentation, filtration through sand, active carbon adsorption, ph correction and chlorination. Figure 1. Cross-section of flocculation-sedimentation tank for potable water treatment plant of the city of Thessaloniki 3.2 Experimental procedure Experiments of water temperature were carried out in the above sedimentation tank. These tests were made to understand the effect of low temperature on density currents. Temperature distribution has been measured in five positions: one at the inlet of the tank, three at the middle radius position of the tank for tank depth: y 1=0.07m (surface), y 2=2.01m (middle), y 3=4.11m (bottom) (these distances are measured below free surface) and finally one at the outlet channel. Thermistors have been installed in these five positions. Each thermistor was attached to a computer data logger that recorded temperature continuously during the study periods. The study period for winter temperature measurements was 14 months (November 2011 until January 2013) 24 hours/day. The thermistors were calibrated in the laboratory with a typical error of ±0.2 o C. The position of each thermistor and the geometry of the 2D-axisymmetric (modeled) sedimentation tank are shown at Fig.2. The numerical model has been applied to an extreme winter day in the nonlinear (parabolic) density region (case 1) and in a typical summer day where the density distribution is linear (case 2). Case 1 corresponds to extremely low temperatures, especially at the free surface, and at the middle of the tank where water has the maximum density.

5 Figure 2. A schematic sketch of the experimental setup This case allows the study of the parabolic density region where density increase over the temperature, in contrast to case 2 where density decrease linearly over the temperature. The temperature initial conditions are shown in Table 1. initial conditions 3.4 Model validation inlet channel Table 1. Simulated cases free surface (y=0.07m) Temperature C middle (y=2.01m) bottom (y=4.11m) outlet channel case case Specific experimental quantitative results of temperature (K) at three different positions at the middle radius position of the tank for tank depth: surface (y 1=0.07m), middle (y 2=2.01m), and bottom (y 3=4.11m), are compared with the corresponding numerical results, in order to validate the reliability of the numerical simulations for case 1. The validation was made for a continuous 24hr operation of the tank, with simulated time 9 days. The numerical simulations show a good match with the experimental data. The maximum average percentage differences are: 0.06% for surface, 0.11% for middle and 0.09% for bottom of the tank. In addition to the above validation test, another validation test has been conducted in order to examine the reliability of the ANSYS code. This was achieved by simulating the experimental data of Liu et al. (2010). The numerical velocity profiles are compared with the corresponding experimental results. The comparison concerns a flow with Re= 22,239 (case 3 from the work of Liu et al. 2010). The numerical profiles and the corresponding experimental data are illustrated in Fig. 3.

6 Figure 3. Comparison of the experimental data (Liu et al. 2010) with the numerical results of ANSYS code. 4. RESULTS AND DISCUSSIONS The most important macroscopic observation drawn from this study is the influence of temperature distribution on the flow field. In Fig. 4 streamlines for time 30 min for each case are depicted. In case 1 a large eddy has been created and after 30 min has been expanded up to the peripheral weir. In case 2 two recirculation eddies have been created between the baffle and the outlet of the tank. Figure 4. Streamlines and concentration contours for simulation time 30 min, for each case.

7 In Fig.5 two x-velocity profiles (one for each case) at 5 different x positions of the tank are presented. In general from the position x 3 to the outlet, there are two picks in the velocity profiles. One near the bottom and another smaller near the surface. Between those two picks a velocity reduction takes place approximately at the y center of the tank. This phenomenon can be explained by observing the recirculation eddies from Fig.4. The flow changes direction and at these positions, near the y center of the tank, the x velocity takes negatives values. The reason for the higher pick at the velocity profile near the bottom is the presence of particles. A large amount of particles due to baffle, move fast towards the bottom and push the flow with them. The smaller pick near the surface, as has been observed by Wells and LaLiberte (2008) too, caused by the vertical velocity which increases due to surface cooling. At position x 1 it is seen that the three profiles show large differences. At position x 2 and x 3 that divergence is bigger at the bottom of the tank and decreases at the free surface. Figure 5. Velocity profiles (u, y) at different distances (x) for time t=30 min, for each case. Observing Fig.4 (concentration profiles) in comparison with Fig.5, at case 1 particles are moving all over the tank and the turbidity is much greater than that in case 2 where the particles do not rise above the middle of the tank. The presence of particles affect the fluid velocity due to the momentum exchange between the primary (water) and the secondary phase (solid particles) and vice-versa (Tarpagkou and Pantokratoras 2013). 5. CONCLUSIONS A series of experimental measurements in an actual sedimentation tank, for potable water, of temperature variation has been conducted. The flow and the temperature field of the tank has been simulated with the ASYS software, taking into account the non-linear density variation at low water temperatures. The predictions of the numerical simulations agreed very well with the experimental measurements in the tank. Two cases have been examined in order to capture the full scale of non-linear, and linear density region. The velocity profiles for these cases ware investigated, in combination with contours of stream and concentration within the sedimentation tank. The interaction between the liquid and

8 solid phase (and vice-versa), buoyant forces and the thermophysical properties of the fluid as a function of temperature were included in the model. In addition the following conclusions have been derived: 1. The general conclusion concerning velocities is that in all cases water velocity is higher near the bottom compared to surface, except at the tank inlet. At the region between tank centre to the outlet the velocity profiles show a special behaviour which is as follows: there are two picks, one at the bottom, and another near the surface, and a serious reduction of velocity between the two picks. 2. In low water temperatures and more specifically in the non-linear density region the coagulation procedure weakens, the water viscosity increases, the drag increases and as a consequence sedimentation velocity decreases and for that reason the turbidity at the free surface increases. 3. The tank efficiency with buoyancy in the linear density region is around 80%. At low temperatures in the non-linear density region drops even more and reaches the 50%. ACKNOWLEDGMENTS The financial support provided by Thessaloniki Water Supply and Sewerage co (EYATH) SA is gratefully acknowledged. The authors would like to thank K. Zampetoglou, G. Seretoudi, A. Soupila, A. Papaionannou and E. Samara for their contribution. REFERENCES 1. Adams E.W., Rodi W. (1990). "Modelling flow and mixing in sedimentation tanks." J. Hydraul. Eng.-ASCE, 116(.), Caldewell D., Separation of seawater by Soret diffusion. Deep-sea Res., 32(8), Fofonoff N. P, Millard R.C. (1983). "Algorithms for computation of fundamental properties of sea water." UNESCO Tech. Pap. Mar. Sci Fofonoff N.P. (1985). "Physical properties of seawater: a new salinity scale and equation of state for seawater." J. Geophys. Res., 90(.), Kawamura S. (1991). "Integrated design of water treatment facilities." John Wiley & Sons. 6. Larsen P. (1977). "On the hydraulics of rectangular settling basins." Rep. No. 1001, Dept. of Water Research Engineering, Lund Institute of Technology, Lund, Sweden. 7. Liu B., Ma J., Luo L., Bai Y., Wang S., Zhang J. (2010). "Two-Dimensional LDV Measurement, Modeling, and Optimal Design of Rectangular Primary Settling Tanks." J. Environ. Eng.-ASCE, 136(.), Matthaus W. (1970). Monatsber. Dtsch. Akad. Wiss. Berlin 12 (11-12), McCorquodale J.A., Zhou S. (1993). "Effects of hydraulic and solids loading on clarifier performance." J. Hydraul. Res., 31(.), Pantokratoras A. (2003). "Vertical penetration of double-diffusive water plumes discharged vertically downwards." J. Hydraul. Eng.-ASCE, 129(7), Stamou A.I., Adams E.A., Rodi W. (1989). "Numerical modelling of flow and settling in primary rectangular clarifiers." J. Hydraul. Res., 27(.), Taebi-Harandy A., Schroeder E.D. (2000). "Formation of density currents in secondary clarifier." Water Res. 34(4), Tarpagkou R., Pantokratoras A. (2013). "CFD methodology for sedimentation tanks: The effect of secondary phase on fluid phase using DPM coupled calculations." Appl. Math. Model., 37(5), Wells S. A., LaLiberte D. M. (2008). "Winter Temperature Gradients in Circular Clarifiers." Water Environ. Res.,70(7),