Abstract
Three-dimensional numerical simulations are performed to evaluate the effect of porous baffles on the efficiency of water treatment contact tanks. A second-order accurate numerical model is employed for the solutions of unsteady flow and tracer transport through the porous baffles. The flow through the porous medium is characterized while using the Darcy-Forchheimer relationship. Large Eddy Simulation (LES) model is used to simulate the instantaneous mixing of the tracer in the chambers of the contact tank. Three different porosities are considered to evaluate the effect of porosity on the hydraulic and mixing efficiencies of the contact tank. Simulated time-averaged flow field shows that porous baffles that are placed at the entrance of each chamber could successfully mitigate short-circuiting and yield plug-flow conditions through the system for low porosities. Flow in the contact tank becomes laminar as the flow velocities decrease due to viscous effects and inertial resistance in the porous zone. For this case, the tracer is transported with bulk flow through the system and leaves the contact tank with a high peak seen in the Residence Time Distribution (RTD) plot. Porous layer increases the hydraulic efficiency of the conventional design from “poor” to “good” according to the baffling factor and increases the overall efficiency from “compromising” to “good” according to the AD index. Comparison of the performance of the porous layer with the previously developed slot-baffle design shows that the slot-baffle design increases the efficiency of the tank with increasing dispersion effects, whereas the porous design increases hydraulic efficiency and reduces the dispersion effects. While the porous design reduces energy efficiency by 33% due to a drastic increase in drag in the flow through porous zone, the slot-baffle design increases the energy efficiency of the conventional design by 67%.
Highlights
Due to urbanization, increasing population, and the use of energy intensive treatment methods, there is an increasing trend in the cost of water treatment in urban population centers
The objective of the present study is to numerically investigate the effect of porous baffles on the flow and transport characteristics in a widely used contact tank geometry that is reported in the literature
Flow features are analyzed for each tank design by means of time-averaged flow field obtained from the Large Eddy Simulation (LES) studies
Summary
Due to urbanization, increasing population, and the use of energy intensive treatment methods, there is an increasing trend in the cost of water treatment in urban population centers. The last stage of the treatment process in a water treatment plant consists of disinfection of water by chemicals in a contact tank system. Energy and chemical dose requirements during the disinfection process are proportional to the efficiency of the contact tank design. The use of vertical baffles in the tank are a common design alternative to increase the contact time. Conventional tank designs suffer from high short-circuiting and low mixing due to the formation of dead zones and high velocity flow regions in the contact system. Researchers have focused on the enhancement of the efficiency of contact tanks to reduce the required chemical dosage that is used in treatment
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