Three-dimensional numerical modeling of particle concentrations in a counter flow lamella separator A. Morin 1, J.-F. Figue 2, J. Schaffner 3* and J. Steinhardt 3 1 Hydroconcept, ZA Trappes Elancourt, 46 avenue des frères Lumière, 78190 Trappes, France 2 Laboratoire du Génie des Procédés - Faculté d œnologie, Université de Bordeaux II, 351 Cours de la Libération, 33405 Talence, France 3 Steinhardt Wassertechnik, Roederweg 10, 65232 Taunusstein, Germany *Corresponding author, e-mail joerg.schaffner@steinhardt.de ABSTRACT The investigated HydroM.E.S.I. particle separator is a lamella based treatment system for the separation of particular pollutants from stormwater runoffs or mixed sewage overflows. The water is cleaned mechanically by gravimetric settlement of small particles (< 100 µm diameter) between the lamellas before it is discharged to connected waters or infiltration areas. To calculate and improve the flow distribution and the effectiveness of the lamella system numerical investigations were carried out by using the three-dimensional computational fluid dynamics model CFX. The influence of the lamella chambers geometry on the flow distribution was analyzed as well as the hydraulic inflow conditions and the sedimentation area under the lamellas. These results were published in Morin et al. (2008). Further numerical investigations were focused on the distribution of particle concentrations inside the particle separator. The particle behavior was also analyzed for a varied inflow opening height where the concentrations of three different, predefined, particles between the lamellas and on the bottom of the tank were compared. These investigations were continued with the implementation of cross and lateral directed drainage channels above the lamella plates. KEYWORDS Stormwater treatment, particle separation, lamella system, numerical modeling, particle concentrations. INTRODUCTION The aim of this paper is to present the function of a particle separator for the treatment of stormwater, mixed sewage runoffs or overflows and investigations on hydraulic details and boundary conditions of the lamella system by using three-dimensional computational fluid dynamics. Fine particles smaller 100 µm diameter dominate the suspended phase of stormwater and represent between 66 and 85 % of the total mass with mean diameters ranging from 25 to 44 µm. Since the nineties many investigations have proven that the main part of stormwater pollution is bound to small suspended solid particles. Approximately 80 % of the COD and BOD5 are bound to solid particles with a diameter smaller than 100 micrometers, which charge the treatment plant as well as the receiving waters. (Asley et al., 2004) (Chebbo, 1992) (Kirchheim, 2005) (Saget, 1994) These sediments are well suited for the gravimetric settlement in a particle separator. It is possible to achieve a removal of 70 80 % of the suspended solids in the stormwater if their
properties like density and sinking velocity are taken into account as well as the characteristics of the connected catchment when designing the lamella system. (Andral et al., 1999) (Gromaire-Mertz, 1998) The investigated HydroM.E.S.I. ( Matières en Suspension Intercepteur ) particle separator is a lamella based treatment system for the separation of particular pollutants from storm water runoffs before they enter connected waters or infiltration areas. (Steinhardt GmbH, 2008) The stormwater is cleaned by the gravimetric separation and settling of solids in a counter flow system. The inflow is divided into smaller parts when passing the arrangement of several parallel lamellas which are inclined in a 45 degree angle. Therefore the upward velocity of the stormwater between the lamellas is reduced and a laminar flow is generated. Not caught particles Coalescence Ve Vs Caught particles Sedimentation Deposits Figure 1. Gravimetric settlement of suspended solids. Suspended particles having, due to their density, a higher settlement velocity than the upward general flow velocity will be caught on the lamellas. There they will settle down and form clusters of sediments. When these clusters overcome the resistance of the lamellas and the flow velocity they run down to the bottom of the lamella chamber where they remain for the rest of the storm event. Floatable sediments will be caught by a scum board at the end of the lamella system where they settle down after the rain event. (Figure 1) The calculation of the number of lamellas is depending on the inflow, the size of the lamellas and the sinking velocity of the particles which have to be treated. The sinking velocity v s is calculated with the formula of Stokes (1) and is a function of the particle density ρ p, the fluid density ρ f, the particle diameter d, the gravity g and the kinematical viscosity η. 2 ( ρ p ρ f ) g d v s = (1) 18 η FUNCTION OF THE LAMELLA SYSTEM After or during rain events the runoff enters the inflow chamber by passing the rake bar screen and the discharge throttle, which ensures a constant inflow into the lamella system. The pumping and flushing sump collects the rain water before it flows across the inflow shield into the sedimentation chamber. The inflow shield guides and distributes the flow
under the lamellas and creates an area where the already settled particles will not be lifted again. (Figure 2) Ground view Figure 2. Functional diagram HydroM.E.S.I. Lamella separator, overview. In an empty tank the lamellas are in vertical position; with the rising water level the float attached to the first lamella rises and lifts the lamellas to the 45 degree working position. The water then runs through the lamellas and flows towards the outlet. Before it passes the scum board the flushing reservoir for the cleaning of the tank bottom is filled. (Figure 3) Cross view Figure 3. Functional diagram HydroM.E.S.I. Lamella separator, cross view. The treatment of the polluted rainwater happens between the lamellas as explained in figure 1. After the rain event, when the inflow to the lamella system has stopped, the tank is drained by the pump into the sewer system. The lamellas fall into vertical position which causes the wet
solids attached to the lamella surface to slide down to the bottom. When the stormwater tank is fully emptied the flushing gate is opened and the tank bottom is cleaned by a highly turbulent wave. The inflow shield which consists of a flexible bottom part is opened by the flush wave before the wave is reaching the flushing sump. The flushing volume with the deposits is then pumped into the sewer system. NUMERICAL MODEL In the year 2006 numerical investigations were carried out in cooperation with the Laboratoire de Génie des Procédés et Environnement at the University of Bordeaux by applying the threedimensional model CFX on the particle separator. (Morin, 2006) (Morin et al., 2007) CFX solves the Navier-Stokes equations with the Volume of Fluid Method using the k- epsilon turbulence model. Flows near walls were modelled with a wall function-based model, called scalable wall function. (CFX, 2004) To set up the numerical model the HydroM.E.S.I. particle separator build in Villefranche-sur- Saône Gleizé in France, which is used for the treatment of stormwater in an industrial area was chosen as a real example. (Table 1) Table 1. Geometrie data of particle separator. Inflow [m³/s] 0.1 Lamella height/width [m] 1.50 / 3.18 Material / Stretching factor Trapezoid PVC / 1.31 Declination [ ] 45 Distance between lamellas [m] 0.09 Particle diameter [µm] 23 Settling velocity [m/h] 1.0 Lamella chamber length /width [m] 7.90 / 3.50 No. of lamellas [-] 77 Declined projected lamella surface [m²] 335.80 The structured numerical grid was refined at the inflow, the inflow plate and at the free water surface in the lamella chamber. To represent the thin lamella body a further refinement of the grid would have been necessary. This would lead to a very large number of grid cells and long calculation times. To overcome this problem the grid cells representing the lamellas were defined to be impervious at one surface side. No flows rectangular to this surface were possible while all other surfaces of the cells were kept pervious for the flow. By this means very small lamellas were created and the number of grid cells was kept small. (Figure 4) Figure 4. Numerical grid of the stormwater tank with the lamella separator. The upper boundary condition was set up as a constant inflow of Q = 0.1 m³/s and the lower boundary at the outflow was a constant pressure boundary for a free outflow.
INVESTIGATIONS AND RESULTS For the investigation of the particle concentrations a two-phase flow was chosen to calculate the movement of the particles inside the lamella separator. The modelling was carried out with sediment particles as a continuous phase with local concentrations and sinking velocities in water. Based on a literature research three particles diameters with different densities were chosen to analyse varied scenarios of particle behaviour. (Table 2) Table 2. Particle diameter and densities. Particle no. d [µm] ρ [kg/m³] Vs [cm/s] 1 50 2300 0.14 2 200 1200 0.33 3 300 2000 0.42 Analysis of the inflow area - Height of the inflow opening The first investigations did analyse the inflow conditions to the lamella chamber. The inflow opening was varied for a lamella chamber with a ratio of L/W = 3 and a constant width of the inflow plate of 2.72 m. Figure 5 shows the inflow area to the lamella chamber. The height of the opening above the curved plate was varied between 0.10 0.50 m. 0.30 m h = 0.1 0.5 m 1.00 m 0.30 m 0.70 m Figure 5. Geometry of the inflow area. 1.00 m The cross view of the lamella chamber shows the formation of a large vortex in the first third of the chamber. The incoming water flows downwards to the bottom of the tank and disturbs the sedimentation zone. Already settled particles will then be lifted. After two thirds of the chamber length it is raised and enters the lamella structure. Thereby the sedimentation zone is very small. For the small opening heights the flow is also reflected at the tank wall and forms a back flow into one third of the chamber length which also affects the sedimentation zone negatively. (Figure 6) For an opening height of h = 0.10 m a inflow velocity of v = 0.36 m/s was calculated.
Figure 6. Velocity vectors in the vertical cross view, h = 0.10 m. This hydraulic behaviour is also reflected in the particle concentrations. Figure 7 shows the concentration of particles no. 1 chamber after 1200 s. The inflow moves down into first part of sedimentation zone and leads to a concentration of 15 mg/l. The particle concentration only in the rear of the lamella chamber is much higher with 40 mg/l. 15 mg/l 10 mg/l 40 mg/l Figure 7. Concentration of particles no. 1 after 1200 s, h = 0.10 m. For the larger openings the main vortex still exists but the velocity vectors at the bottom of the tank are more parallel than for the small opening heights. The downward movement of the inflow is reduced and leads to a more stable sedimentation zone where the resuspension of the particles will be decreased. The inflow velocity is at 0.0735 m/s. The backflow at the end of the lamella chamber is also reduced which should have a positive effect for the sedimentation of particles. (Figure 8)
Figure 8. Velocity vectors in the vertical cross view, h = 0.50 m. In figure 9 the particle concentration zone of 40 mg/l is now stretched along the complete tank bottom. This means that the flow velocities in the sedimentation zone are reduced and the current of the inflow is above this zone. It is very likely that, due to the increased inflow opening, settled down particles will remain at the bottom and not be lifted again. 10 mg/l 15 mg/l 40 mg/l Figure 9. Concentration of particles no. 1 after 1200 s, h = 0.50 m. Set-up of drainage channels The previous investigations showed that the flow in the first part of the lamella structure is moving up- and downward between the lamellas and is not directed to the end of the lamella chamber. Therefore the single overflow at the end of the chamber seems to be not very effective for the present flow distribution. The set-up of drainage channels above the lamella structure should solve this problem. Therefore a numerical model with a lamella chamber possessing a ratio of L/W = 3, an inflow opening of 0.30 m and a distance of 1.70 m between the tank bottom and the underside of the lamella structure was set up.
Lateral drainage channels The configuration of drainage channels parallel to the longitudinal axis with a height of 0.50 m above the lamellas at the chamber walls lead to an improvement of the flow distribution. Figure 10 shows the distribution of the flow velocities in a width cross section in the middle of the lamella chamber. It is noticeable that the flow is directed to the drainage channels at the sides with a velocity of 5 10 mm/s. Figure 10. Velocity vectors in the width cross view with longitudinal drainage channels. Figure 11 shows a longitudinal view of the velocity distribution. In only two lamella plates a minor downward flow can be noticed.
Downward flow inside lamellas Figure 11. Lateral drainage channels, longitudinal view flow velocities. Figure 12 shows a increased concentration of particles under the lamellas and in the lower part between the lamellas. The highest concentration can be found directly behind the inflow plate. An increased settlement of sediments at the bottom of the storm water tank is noticeable which is evident for all three modelled particle sizes. Figure 12. Concentration of particles no. 2 after 1200 s with lateral drainage channels. Transversal drainage channels In further investigations transversal drainage channels above the lamella field were included in the numerical model. The lengthwise equally arrangement of the drainage channels transversal to the longitudinal axis of the lamella chamber showed good results with a regular flow distribution and flow velocities of 5 10 mm/s between the lamellas. The downward flow, which could be found with lateral drainage channels, takes not place with the transversal drainage channels. (Figure 13) (Morin, 2006)
No downward flow inside lamellas Transversal drainage channels Figure 13. Transversal drainage channels, equally distributed, longitudinal view flow velocities. In figure 9, where not drainage channels were modelled, a concentration distribution in vertical layers was found. In contrast to this, with transversal drainage channels horizontal concentration layers are created. The highest particle concentration takes place in the first part of the lamella chamber behind the inflow plate. (Figure 14) Figure 14. Concentration of particles no. 2 after 1200 s, equal distribution of transversal drainage channels. CONCLUSIONS The hydraulic investigations of the HydroM.E.S.I. particle separator showed the formation of several vortexes under and aside of the lamella structure which interfere the formation of a sediment layer at the bottom of tank. The enlarging of the inflow opening was one answer to this problem. Unsolved by these findings was the problem of the downward flow between the first lamellas. The set-up of lateral or transversal drainage channels led to an improved flow distribution between the lamellas with very low flow velocities beneficial for the gravimetric settlement of particles.
The modelling of the particle concentrations was carried out with three different particle diameters and densities. The results of these investigations were as follows: For an inflow opening height of 10 cm the concentration of particles behind the inflow plate is very low with 10 15 mg/l. At the back wall the concentration is 40 mg/l which shows that the flow conditions do not lead to a constant distribution of particles along the sedimentation zone. For an inflow opening height of 50 cm the particle concentration behind the inflow plate is higher and not effected by downward flows. The sedimentation zone is equally distributed with a high particle concentration. In the first third of the lamella field areas with virtually no flow velocities or downward flows can be found. The particle concentration in these areas is very low with 5 mg/l. Therefore this part of the separator has a very low efficiency. The implementation of drainage channels changes the distribution of the particle concentration along the tank bottom. The concentration is strongly increased especially behind the inflow plate. The mean concentration along the tank bottom is around 40 mg/l. Therefore large sediment layers can be expected under the lamellas. The usage of drainage channels also leads to a horizontal concentration distribution inside the lamella block. The highest concentrations can be found in the lower part of the lamella block and they are decreasing when moving upward along the lamella plates. The results of the shown investigations led to pending patents concerning the flexible lamella system and the design of the inflow area to the lamella chamber. ACKNOWLEDGEMENT The authors want to thank the Agence de l Eau Seine Normandie, here especially Mrs. Nadine Aires, for the financial support of the project and the help with the final report. REFERENCES Andral, M.C., Montrejaud-Vignolles, M. and Herremans, L. (1999). Répartition granulométrique et caractéristiques hydrodynamiques des rejets solides drainés par les eaux de ruissellement sur chaussée autoroutière, ESI-GTI Pau, ANJOU RECHERCHE Rungis, INPT, Toulouse. Ashley R.M., Betrand-Krajewski, J.-L., Hvitved-Jacobsen, T. and Verbanck, M. (2004). Solids in sewers Characteristics, effects and control of sewer solids and associated pollutants, Scientific and Technical Report, IWA Publishing. CFX 5.0 (2004). Ansys Inc., Canonsburg, USA. Chebbo, G. (1992). Solides des rejets urbains par temps de pluie: caractérisation et traitabilité, Dissertation, Ecole nationale des Ponts et Chaussées, Paris, France. Gromaire-Mertz (1998). La pollution des eaux pluviales urbaines en réseau d assainissement unitaire. Caractéristiques et origines, Dissertation, Ecole nationale des Ponts et Chaussées, Paris, France. Kirchheim, N. (2005) Kanalablagerungen in der Mischwasserkanalisation, Kanalablagerungen in der Mischkanalisation, Deutsche Vereinigung für Wasserwirtschaft, Abwasser und Abfall (DWA), Hennef. Morin, A. (2006). Simulations numériques des écoulements dans un décanteur lamellaire à contre-courant, Report Hydroconcept, France. Morin, A., Figue, J.-F., Schaffner, J. and Steinhardt, J. (2008). Advanced mechanical stormwater treatment Numerical simulations of a counter flow particle separator, 11th International Conference on Urban Drainage, Edinburgh, Scotland, UK. Morin A., Milisic V., Figue J.F. (2007). Simulations numériques des écoulements dans un décanteur lamellaire à contre-courant, Scientific report for the Agence de l Eau Seine Normandie, France.
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