Water Mixing and Renewal in Circular Cross-Section Storage Tanks as Influenced by Configuration and Operational Conditions

Abstract

Storage tanks are essential components of water supply systems to store water to face normal and emergency conditions and to level off required pressures. However, tanks can also be the source of water quality deterioration due to poor mixing and long residence times. In this paper, the effect of the tank configuration and of the operating mode on water mixing and renewal in cylindrical small-scale tanks of typical configurations is assessed. Results show that nearly complete mixing conditions can be attained in the tanks with inlet and outlet pipes located at opposite walls. Short-circuiting and the formation of dead zones is demonstrated in tanks with inlet and outlet pipes located close together. In fill-and-draw mode, water mixing is enhanced by a wide water-level variation, but renewal takes longer than in steady-state conditions. In addition, an empirical formula is proposed for estimating the turnover time in cylindrical tanks operated at a constant level. By identifying the configurations and operating conditions that promote mixing, this paper contributes to better water quality management in existing and new storage tanks in distribution systems.