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SUMMARYIn the present study, a three-dimensional numerical approach is applied to simulate the velocity field and tracer experimental studies performed in a scaled-down physical model of Embsay chlorine contact tank being used in U.K. It is found that the computational fluid dynamics (CFD) model could accurately predict the hydrodynamics of the tank so that, using the developed CFD models, further investigations are conducted in six tanks with different number of baffles to explore the effect of their numbers on the hydraulic characteristics of the unit. The results show the existence of recirculation areas behind the baffles in a way that the extension of these regions dwindles with increasing the number of baffles, leading the fluid flow to approach plug-flow conditions. Baffle wall arrangement are conducted, to improve the unit's performance in terms of higher effective contact time (lower Morrill Index). The results from the new proposed arrangement with six baffle walls show an improvement of about 13% increase in effective contact time compared with that of the original baffle arrangement.