Proceedings of 2013 IAHR Congress © 2013 Tsinghua University Press, Beijing ABSTRACT: With the aim of optimising contact tank design through numerical model simulations, results are presented herein of an experimental and computational fluid dynamics (CFD) study in a scaled laboratory model. Three-dimensional numerical simulations of flow and transport characteristics were conducted using a Reynolds Averaged Navier-Stokes equation approach. Experimental results were obtained through Acoustic Doppler Velocimetry measurements and a series of conservative tracer experiments. Focus is given on turbulent structures and undesirable flow patterns that lead to a reduced disinfection efficiency, through phenomena such as short circuiting and recirculation zones. The laboratory data analysis indicates extensive three-dimensionality as a result of the current inlet geometry with a confirmed negative impact on the disinfection performance of the contact tank model, as demonstrated by Residence Time Distribution curves. Disinfection performance is evaluated through hydraulic efficiency indicators commonly used in the industry to monitor field-scale disinfection facilities. Correlations between CFD and experimental data confirm the adequate reproduction of hydrodynamic conditions and reinforce the predictive capabilities of numerical models as tools to simulate field scale tanks or optimize existing designs. KEY WORDS: Contact Tanks, Acoustic Doppler Velocimetry, Tracer Experimentation, RANS, Disinfection Performance 1 INTRODUCTION Serpentine contact tank units suggest plug flow to be the ideal hydrodynamic condition at which disinfection performance is maximized (Falconer and Tebbutt, 1986; Markse and Boyle, 1973). Under such flow conditions, disinfectant transport becomes ideal by remaining in the tank for a uniform time interval whilst achieving the desired disinfection. However, previous studies (e.g. Teixeira, 1993) indicate that flow exhibits a residence time distribution (RTD) which can be significantly different from what is dictated by plug flow. The shape of the tracer RTD curves can provide an insight into the hydrodynamic and mixing conditions, as explained by Levenspiel (1999). For example, the residence time at 10% cumulative RTD constitutes a crucial indicator (t 10 ) for the evaluation of disinfection efficiency and design suitability at water treatment works. Digression from plug flow can be attributed to the complex hydrodynamics, namely short-circuiting and recirculation zone formation (Kim et al., 2010). Short-circuiting occurs when particles pass through a reactor quicker than the theoretical hydraulic residence time. Recirculation zones not only promote short-circuiting (since they occupy a considerable part of the tank volume) but they also trap solutes and particles (or pathogens), which are then retained in the tank for a longer period than the theoretical hydraulic residence time. The occurrence of such flow patterns has a detrimental effect on the overall efficiency, because contact times of pathogens with the CFD Study of Flow and Transport Characteristics in Baffled Disinfection Tanks Athanasios Angeloudis Research Student, School of Engineering, Cardiff University, Cardiff CF24 3AA, UK, Email: angeloudisa@cf.ac.uk Thorsten Stoesser Professor, School of Engineering, Cardiff University, Cardiff CF24 3AA, UK. Email: stoesser@cf.ac.uk Dongjin Kim Research Assosiate, School of Civil and Environmental Engineering ,Georgia Institute of Technology , Atlanta, GA 30332,USA. E-mail: dkim46@gatech.edu Roger Alexander Falconer Professor, School of Engineering, Cardiff University, Cardiff CF24 3AA, UK. Email: FalconerRA@cf.ac.uk