Numerical simulation of flow and tracer transport in a disinfection contact tank

Chlorine is extensively used to disinfect drinking water supply. However, the design of chlorine contact tank is usually based on the volume displacement criterion, paying attention that detention time would be at least of half an hour. This criterion relies upon the assumption that plug-flow conditions hold along the tank. However, the existence and arrangement of baffles in the tank, and the tank inlet and outlet configurations can result in a much more complex flow pattern, where turbulent mixing, dead-zones and short-circuiting exist. This situation could result in an inefficient use of chlorine due to the formation of potentially carcinogenic compounds, which originate from residual chlorine in the tank. Further some bypassing allows some out-coming flow with less of the standard amount of contact with the chlorine dose. Therefore, understanding hydrodynamics and mass-transfer characteristics within a contact tank is very useful. The objective of the paper is to present the preliminary results of a numerical study undertaken to investigate hydrodynamics and turbulent transport and mixing inside a contact tank. Two-dimensional steady-state and time-variable numerical simulations were performed with Multiphysics 3.2aTM in a contact tank geometry, which is that experimentally studied by Shiono and Teixeira (Shiono and Teixeira, 2000). Tracer transport was analyzed in the case of a burst of concentration. Both velocity profiles and flowthrough curves were generally in good agreement with experimental data. The model results pointed out the presence of recirculation areas near the baffles and the corners formed by the baffles and the tank walls. These areas introduced dead-zones in the tank deviating flow patterns from ideal plug-flow conditions.