The City of Bradenton is located on the west coast of central Florida and like many Sunbelt cities, it has had to manage extraordinary growth while always striving to improve water quality to its 15,000 customers. With the City’s primary source of water being the Bill Evers reservoir, the City’s choice of secondary disinfectant is monochloramine which helps minimize the generation of disinfection byproducts. However, Bradenton’s warm climate and extensive water distribution and storage system which includes five potable water storage tanks can create water quality management headaches for staff.
While monochloramines are a proven strategy for limiting disinfection byproduct issues, the natural degradation of monochloramine – especially in warm climates – allows for the presence of free-ammonia which can become a food source for various types of nitrifying bacteria. To combat these bacteria and any other potential pathogens, the City would employ a program of tank dumps and flushing to keep “fresh” disinfectant levels in the distribution systems. As with many water utilities who utilities these programs, the constant monitoring, water wastage and unplanned operator callouts to adjust disinfectant levels can become costly for the utility.
In 2012, the City became an early adopter of tank mixing technology by installing water tank mixers in each of the five water storage tanks. Water tanks can become areas of water quality degradation as the system detention time allows the water in the tanks to form layers or strata of common temperature and disinfectant concentrations. Layers with very high temperature and low chloramine concentration can allow for pathogen growth that initiate a cycle of water quality destruction. The PAX tank mixers that were installed are engineered to de-stratify the water in the tanks and allow for the incoming more highly chloraminated water to mix with the bulk volume of the tank (Figure 1).