In summer 2005, Redwood City experienced several nitrification episodes. Multilevel water samples revealed that cool water at the bottom of the tank had plenty of disinfectant, while warm water at the top had little or no residual, as well as elevated nitrite. After scrapping several unsuccessful operational changes, including deep-cycling and valving modifications, the city decided to install a PAX Water Mixer in the most problematic tank. Thermal stratification was completely eliminated in the first few hours, and days following installation, temperatures became uniform throughout the tank, chlorine levels stabilized and nitrite readings plummeted. Since 2006, chlorine residuals have remained consistent and Redwood City has experienced no further nitrification episodes.