Numerical and experimental modelling of flow and kinetic processes in serpentine disinfection tanks

New water directives impose strict regulations to reduce the footprint of treatment operations and contaminant levels, which suggest a performance review of water treatment facilities, including disinfection contact tanks. Serpentine contact tank units suggest plug flow to be the optimal hydrodynamic condition at which disinfection performance is maximized. However, previous studies indicate that flow exhibits a residence time distribution (RTD) which can be significantly distorted from what is dictated by plug flow. Over the years, there has been rising concern over the impact of such digressions from optimal hydraulic conditions on microbe inactivation and the regulation of potentially carcinogenic Disinfection By-Products (DBPs). With the growth of computing power and the advancement of computational models, the potential of contact tank water disinfection optimization by means of numerical modelling techniques can be assessed. In this study, Acoustic Doppler Velocity (ADV) and fluorescent tracer dye measurement campaigns are carried out to assess the hydraulic efficiency of a serpentine contact tank physical model and evaluating appropriate indicators. Then, three-dimensional Computational Fluid Dynamics (CFD) models are set up to simulate the hydrodynamic and solute transport processes for a variety of contact tank geometries examining the effects of inlet design, baffling configuration and tank scale. The simulation capability to reproduce the actual conditions is attested through comparisons against available laboratory results. The CFD approach is subsequently refined with appropriately selected kinetic models, describing the processes of disinfectant decay, pathogen inactivation and DBP formation. Results highlight that computational models can become invaluable tools for the simulation of disinfection processes as they can reproduce the conditions encountered experimentally to a satisfactory extent. Moreover, the optimization of hydraulic efficiency, as studied numerically, facilitates more uniform disinfectant contact time which corresponds to greater levels of pathogen inactivation and a more controlled by-product accumulation

Effect of three-dimensional mixing conditions on water treatment reaction process

The performance of water disinfection facilities traditionally relies on Hydraulic Efficiency Indicators (HEIs), extracted from experimentally derived Residence Time Distribution (RTD) curves. This approach has often been undertaken numerically through computational fluid dynamics (CFD) models, which can be calibrated to predict accurately RTDs, enabling the assessment of disinfection facilities prior to the construction of disinfection tanks. However, a significant drawback of the conventional efficiency methodology prescribed for disinfection tanks is associated with the respective indicators, as they are predominantly linked to the internal flow characteristics developed in the reactor, rather than the disinfection chemistry which should be optimized. In this study three-dimensional numerical models were refined to simulate the processes of chlorine decay, pathogen inactivation and the by-product formation in disinfection contact tanks (CTs). The main objective of this study was to examine the effect of three-dimensional mixing on the reaction processes which were modelled through finite-rate kinetic models. Comparisons have been made between pathogen inactivation and disinfection by-product accumulation results produced by a RANS approach against the findings of a Segregated Flow Analysis (SFA) of conservative tracer transport. CFD Results confirm that three-dimensional mixing does have an effect on the reaction processes, which, however, is not apparent through the SFA approach