Modelling solute transport in water disinfection systems: Effects of temperature gradient on the hydraulic and disinfection efficiency of serpentine chlorine contact tanks

Chlorine residual plays a key role in determining the quality of treated water and wastewater. One of the most critical factors affecting chlorine decay rates is flow and ambient temperature. Detailed knowledge of temperature impacts on the efficiency and performance of chlorine contact tanks will enable optimum design and operation of water and wastewater treatment infrastructures. This paper develops a robust and computationally efficient three-dimensional numerical simulation model using Reynolds-averaged Navier-Stokes equations (RANS) with k-ε turbulence closure model. A non-reactive tracer transport model is developed by implementing three-dimensional advection-diffusion equation. The Chlorine decay processes are simulated using Reynolds-averaged species transport model. Temperature effects on density and viscosity is simulated through Millero and, Poisson and Vogel equations, respectively. Eight scenarios with variation in inflow and ambient temperature are simulated in this study. The residence time distribution (RTD) and hydraulic efficiency indexes are determined for the simulation scenarios. It is shown that small fluctuation in inflow and ambient temperature cause a significant change in chlorine concentration and performance of disinfection tank. The analysis of numerical simulations indicated that increase in ambient and inflow temperature can increase chlorine decay by up to 75 %, leading to undesirable disinfection consequences and disruption of water treatment processes. The numerical model developed within this study was successfully validated against experimental measurements and it is shown to be robust and efficient tool to determine optimum inflow and ambient temperature configurations for high-efficiency water treatment processes and to prevent microorganism residual and by-products disinfection formation. The computational framework presented in this study can inform optimum design of water and wastewater treatment processes.