Effects of Scale and Discharge Variation on Similitude and Solute Transport in Water Treatment Tanks

Abstract

This paper addresses the design and experimentation aspects of scaled contact tanks, a key type of treatment units in water works worldwide. The paper covers the effects of scale and discharge variation on the dynamic similitude and hydraulic efficiency of such tanks. The flow patterns tending to complete mixing (CM) and plug flow (PF) were studied, each assessed in a prototype and three further geometrically similar models of L-shaped and serpentine geometries. Tracer experiments were conducted under five discharge values. The sensitivity of each flow pattern to the changing conditions was assessed based on the residence time distribution curves and hydraulic efficiency index values, contrasted for prototypes and their scaled models. Although the CM flow pattern was insensitive to the variation of scale and discharge, within the ranges tested in this study, the PF pattern was affected by both variations when the flow regime was not turbulent in a model tank. A dependency of the dispersion number on the Reynolds number was observed, leading to the proposal of a new model to support the design and upscaling of results in contact tank studies.