Chemical Engineering Journal 140 (2008) 110–121 A CFD methodology for the design of sedimentation tanks in potable water treatment Case study: The influence of a feed flow control baffle Athanasia M. Goula, Margaritis Kostoglou, Thodoris D. Karapantsios ∗ , Anastasios I. Zouboulis Division of Chemical Technology, School of Chemistry, Aristotle University, University Box 116, 541 24 Thessaloniki, Greece Received 14 June 2007; received in revised form 13 September 2007; accepted 14 September 2007 Abstract Computational fluid dynamics simulations are employed to assess the effect of adding a vertical baffle at the feed section of a full-scale sedimentation tank for the improvement of solids settling in potable water treatment. A general CFD-based simulation strategy is developed based on the specific features and conditions met in practice for potable water treatment. The linearity of the particle conservation equations allows separate calculations for each particle size class – but performed for all classes of interest – leading to the uncoupling of the CFD problem from a particular inlet particle size distribution. The usually unknown and difficult to be measured particle density is found by matching the theoretical to the easily measured experimental total settling efficiency. The proposed strategy is computationally much more efficient than the corresponding strategies used for the simulation of wastewater treatment. This work compares simulations from a standard and a baffle-equipped tank. It is found that the baffle decreases the inlet recirculation zone and enhances the settling of solids by directing them towards the bottom of the tank with high velocities. It is noteworthy that even small differences in the particle velocity can cause large changes in the percent of settled particles; in this work, the overall solids removal efficiency increased when using the baffle from 90.4 to 98.6% leading to a reduction of the effluent solids concentration of approximately 85%. © 2007 Elsevier B.V. All rights reserved. Keywords: Baffle; Computational fluid dynamics; Numerical modelling; Sedimentation tank; Water treatment 1. Introduction Sedimentation is perhaps the oldest and most common water treatment process. The principle of allowing turbid water to settle before it is drunk can be traced back to ancient times. In modern times a proper understanding of sedimentation tank behavior is essential for proper tank design and operation. Generally, sedimentation tanks are characterized by interesting hydrody- namic phenomena, such as density waterfalls, bottom currents and surface return currents, and are also sensitive to temperature fluctuations and wind effects. On the surface, a sedimentation tank appears to be a simple phase separating device, but down under an intricate balance of forces is present. Many factors clearly affect the capacity and performance of a sedimentation tank: surface and solids loading rates, tank type, solids removal mechanism, inlet design, weir placement and loading rate, etc. To account for them, present-day designs are ∗ Corresponding author. Tel.: +30 2310 997772; fax: +30 2310 997759. E-mail address: karapant@chem.auth.gr (T.D. Karapantsios). typically oversizing the settling tanks. In that way, designers hope to cope with the poor design that is responsible for unde- sired and unpredictable system disturbances, which may be of hydraulic, biological or physico-chemical origin. To improve the design of process equipment while avoid- ing tedious and time consuming experiments computational fluid dynamics (CFD) calculations have been employed during the last decades. Fluid flow patterns inside process equipment may be predicted by solving the partial differential equations that describe the conservation of mass and momentum. The geometry of sedimentation tanks makes analytical solutions of these equations impossible, so usually numerical solutions are implemented using computational fluid dynamics packages. The advent of fast computers has improved the accessibility of CFD, which appears as an effective tool with great potential. Regard- ing sedimentation tanks, CFD may be used first for optimizing the design and retrofitting to improve effluent quality and under- flow solids concentration. Second, it may increase the basic understanding of internal processes and their interactions. This knowledge can again be used for process optimization. The lat- ter concerns the cost-effectiveness of a validated CFD model 1385-8947/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2007.09.022