Scale-Model Studies of Mixing in Drinking Water Storage Tanks

Abstract

Storage tanks and reservoirs are commonly used in drinking water distribution systems to equalize pumping requirements and operating pressures, and to provide emergency water for fire-fighting and pumping outages. Poor mixing in these structures can create pockets of older water that could have negative aesthetic and public health impacts. Experiments were conducted on cylindrical scale-model tanks to determine the effect of various factors on mixing. The time taken to mix the contents of a tank with new water introduced during the fill period was found to be proportional to the initial volume to the two-thirds power divided by the square root of the inflow momentum flux (the product of flow rate and velocity). This time is insensitive to whether the inlet is vertically or horizontally oriented. Whether or not complete mixing occurs depends on the ratio of the momentum to buoyancy fluxes of the inlet jet, similar to past findings for jet discharges to unconfined bodies of water. However, the confined geometry of the tank results in a narrower range of conditions that produce stratification. Finally, a formula is derived to estimate the minimum volume exchange required for a fill-and-draw cycle to ensure complete mixing before the end of the filling period.