Appraisal of chlorine contact tank modelling practices

Abstract

With new water directives imposing strict regulations to reduce the footprint of treatment operations and contaminant levels, a performance review of water treatment facilities, including Chlorine Contact Tanks (CCTs) is required. This paper includes a critical appraisal of the international literature on CCT modelling practices to date, aiming to assist the identification of areas requiring further development, in particular, relating to the computational modelling capability and availability of tools to assist hydraulic design and optimisation studies of CCTs. It notes that the hydraulic optimisation practice of poorly designed tanks commenced with experimental studies undertaken in the 1960s and 1970s, which involved mainly two types of studies, namely in situ tracer tests and laboratory physical modelling. The former has traditionally been conducted to diagnose the hydraulic performance of existing CCTs, typically based on results such as Residence Time Distribution (RTD) curves and values of the Hydraulic Efficiency Indicators (HEIs). The latter has been useful in trial and error testing of the impact of certain design modifications on those results, with suggestions for later improvements of the field scale unit. In the 1980s mathematical and numerical modelling studies started to be used to assist CCT investigations, offering a greater level of detail in a more cost-effective manner than equivalent experimentally based investigations. With the growth of computing power and the popularisation of computational models, the 1990s saw the development and application of Computational Fluid Dynamics (CFD) tools to simulate the hydraulic performance of CCTs, sometimes independently of experimentation, other than by using available data to calibrate and validate modelling predictions. This has led to the current scenario of CFD models being invaluable assistive tools in optimisation studies of CCTs, with the experimentation practice continuing to allow for specific diagnostics and to supply data for the calibration and validation of CFD modelling results. The vast majority of CCT modelling studies published to date have focused on simulating CCT hydrodynamic and conservative solute transport processes. The chlorination kinetics and Disinfection By-Product (DBP) formation have rarely been contemplated in computer modelling studies of CCTs. Commercially available CFD models have not traditionally been applied with this purpose, while research studies undertaken using open source codes to produce tailor-made applications are rare. Aspects that could benefit from further understanding and/or development include the impact of scale when conducting experimentation with prototypes, adequate turbulence closure for a given situation, adequate numerical schemes vis-à-vis CFD model complexity vis-à-vis cost-benefit ratio of simulations and the inclusion of state of the art chlorination kinetics and DBP formation modelling in the CFD tools that can assist modern design and retrofit studies of CCTs.