Experimental and Numerical Evaluation of a Porous Baffle Design for Contact Tanks

In water treatment plants, the design of contact tanks is of critical importance for effective disinfection of potable water. The most important design problem in these flow-through systems is the formation of recirculation and flow jet zones, which reduce mixing efficiency of the contact system. To this end, in this study, a porous baffle design is proposed as an alternative to the slot baffle design, which may provide an alternative solution to these problems. Experimental and numerical studies were conducted on a 1:10-scale laboratory model of a prototype water treatment contact tank located in the province of Eskisehir in Turkey. To understand the effectiveness of the proposed porous baffle design, before the use of alternative porous baffle designs, heterogeneous soil-gravel mixtures with specified porosities were prepared and placed inside the baffles such that the porosity of the baffle could alternatively be changed in the flow direction. The purpose here is to investigate the efficiencies of several baffle configurations without using elaborate solid baffle structures that will mimic the performance of the porous baffle. The porous designs were thoroughly investigated, relying on the efficiency indexes obtained from dye tracer experiments. Experimental studies conducted for different porosity distributions indicate that the baffle porosity should increase in the flow direction along the chamber length so that the flow jet entering to the chamber with high momentum should be allowed to pass to the neighboring chamber relatively higher than the flow jet emerging from the chamber. This important observation led to the design of the porous baffle with variable porosity in the flow direction for a controllable fluid transfer between neighboring chambers. The proposed design successfully improves the hydraulic efficiency from poor to average based on the baffling factor measure, and the Morrill ($M_o$) index approaches 2, which is recommended by regulations. To further ascertain the contribution of the proposed baffle design, a computational fluid dynamics (CFD) model was developed for the simulation of turbulent flow through the porous baffle, and it was validated by the experimental studies conducted in this study. The simulated mean velocity profiles and tracer results were in good agreement with the experimental results. Visualization of internal structure of the flow through the tank and the baffles revealed that the momentum of the entering flow jet to the chamber could effectively turn dead zones into active mixing zones in the neighboring chamber. The authors emphasize that in the final design, porous baffle solid structures will be used that will mimic the pore structure characteristics of the design alternatives determined in this study instead of the use of the soil-gravel mixtures in this experimental study.