Flow, transport and disinfection performance in small- and full-scale contact tanks

The hydrodynamics and mixing processes in small- and full-scale baffled disinfection tanks are studied experimentally and numerically. Velocity and tracer transport measurements are carried out to quantify the hydrodynamics and to obtain reliable data used to validate a three-dimensional computational fluid dynamics (CFD) model. The flow in the tank under investigation is extensively three-dimensional due to the existing inlet condition of the tank, resulting in short-circuiting and internal recirculation, particularly in the first three compartments. Near the inlet the tracer residence time distribution curve analysis and Hydraulic Efficiency Indicators (HEIs) suggest poor disinfection performance. Further away from the inlet, the flow recovers to a two-dimensional flow and the HEIs improve until the exit of the tank. The computational results demonstrate good agreement between the predicted hydrodynamics and tracer transport with the corresponding experimental data. The numerical model is then employed to investigate the effects of up-scaling of laboratory model findings to a full-scale contact tank. Despite the Froude-Reynolds conflict the full-scale contact tank exhibits similar behaviour to the small-scale tank. The effect of the tank geometry on the disinfection efficiency is demonstrated, highlighting the negative impact of flow three-dimensionality on pathogen inactivation.