A process and system mixes drinking water stored in large water tower type storage tanks, preventing stratification of the water, by generating large mixing s bubbles in the tank's standpipe, causing mixing of layers of water in the tank through turbulence created as the bubbles rise through the tank. Embodiments of the present invention, adapted for use in storage tanks in which the standpipe serves as both water inlet and outlet, detect flow in the standpipe and provide mixing only when the standpipe is not serving as a water outlet.