Physical Modeling of Mixing in Water Storage Tanks

Deterioration of water quality in finished water storage facilities is a major concern for water utilities. Problems include loss of disinfectant residual due to hydraulic short-circuiting, poor mixing and circulation, poor turnover time, and excessive detention time. The mixing processes in these storage tanks are primarily driven by the momentum of the inflow; they are complex, three-dimensional, unsteady, and difficult to predict. As a result, there are few guidelines on how to design storage tanks to promote effective mixing. Experiments on water storage tanks of typical configurations using an innovative three-dimensional laser-induced fluorescence (3DLIF) system that allows non-intrusive measurements of the entire tracer concentration field in the tank are described in this report. Various inlet geometries, inflow rates, and density differences are modeled. The experiments take into account jet momentum and also buoyancy effects that may be caused by density differences between the inlet water and tank water temperatures. The range of buoyancies studied was representative of those that commonly occur in practice. For each experiment, the extent of mixing in the tank is quantified as a function of time and a time for mixing was determined. This allows comparison of the effectiveness of mixing for various conditions. Many utilities are presently upgrading and rehabilitating water storage tanks to meet increasingly stringent regulations. There is presently little rational guidance on how to design tanks to promote mixing, however, and little consistency in the design of the thousands of tanks nationwide. The results should have significant impact on the design of water storage tanks, and will enable prediction of the time required for mixing under various conditions. Mixing can be accomplished cheaply with proper understanding of the effects of nozzle design and can be accomplished without the need for elaborate mixing devices.