Chemical Engineering Journal 152 (2009) 307–314 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Large-eddy simulations of particle sedimentation in a longitudinal sedimentation basin of a water treatment plant. Part I: Particle settling performance M. Al-Sammarraee, A. Chan ∗ , S.M. Salim, U.S. Mahabaleswar Department of Chemical and Environmental Engineering, University of Nottingham (Malaysia Campus), Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia article info Article history: Received 7 August 2008 Received in revised form 10 April 2009 Accepted 27 April 2009 Keywords: Water treatment plant Longitudinal sedimentation tank Large-eddy simulations Particle settling Sedimentation abstract The process of particle sedimentation in a three-dimensional longitudinal basin in a water treatment plant is studied computationally. The fluid mixture mimics typical contaminated water in which the contaminants are represented by a spectrum of 13 different particle sizes. The processes of sedimentation of all classes of particles are simulated. The flow patterns, the particle settling velocities, the effectiveness of the particle removal from the stream and the particle concentrations distributions along the basin are estimated using large-eddy simulations (LES). Moreover the particles settling phenomenon and efficiency for different sizes of particles and the overall sedimentation efficiency of the basin are calculated. Results show that longitudinal tanks are more efficient in dealing with larger particles compared to smaller particles in terms of settling. The influent is flushed downwards to the tank bottom. The flow then creates a large recirculation eddy near the sump area. Smaller recirculation regions, which are important to sedimentations, are also found near the entry and near the exit weir. Turbulence is generated near the inlet and outlet weir of the basin which also tends to inhibit settling. As a result, smaller particles tend to distribute more evenly in the basin while the larger particles settle quickly near the sump. The flow paths of the smaller particles also show that there is necessity in altering the geometries to enhance settling, especially near these turbulent regions. It also shows that there is a great deal of interactions with the sedimentation tank geometry which has significant improvements on the design aspects of these basins. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. 1. Introduction Water supplied for public use must be potable from the stand- point of its chemical, physical and biological characteristics [1–4]. Drinking water should preferably be obtained from a source free from pollution or contaminants. The raw water normally avail- able from surface water sources is, however, not directly suitable for drinking. The main objective of water treatment process is to produce safe and potable drinking water [1–5]. To such end, many water treatment processes have been developed and used for decades, such as coagulation–flocculation units, sedimentation basins, slow sand filtration, rapid sand filtration and disinfection units [1–4,6]. Sedimentation tanks are one of the most important compo- nents and the workhorses of any water purification plants. It is thus crucial for the sedimentation tank to operate at its full potential. It is not only the physico-chemical aspects of flocculation that is important. Hydraulics plays a prominent part [1–2,7]. Overdesign ∗ Corresponding author. Tel.: +60 3 89248144; fax: +60 3 89248017. E-mail address: andy.chan@nottingham.edu.my (A. Chan). of sedimentation tank is common, leading not only to unnecessary capital expenditure, but also to water wastage in the form of exces- sive sludge. Improper and inadequate design cause overloading of filters, and lead to frequent backwashing, which in turn waste a sig- nificant percentage of treated water. Since treatment tanks tend to last a few decades, most do not incorporate the latest developments in technology to deal with these issues [1–5,8,9]. In this respect, good understanding of the various hydraulic processes within water treatment is essential for good design. Sedimentation is a solid–liquid separation process, in which particles settle under the force of gravity. Particles with density greater than that of water deviate from the streamline of fluid flow by gravity and settle on the bottom of sedimentation tank [1–5,8,9]. At the same time these particles undergo various hydro- dynamic and physical processes due to the shear forces in water flow which eventually affect the aggregation process and their removal efficiency [10–13]. Hence the sedimentation tank perfor- mance is strongly influenced by effects such as density driven flow, gravity sedimentation and flocculation and thickening. In turn the velocity and density patterns in tanks influence these processes and are therefore of great interest to design engineers [1–5,12,13]. 1385-8947/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2009.04.062