(571a) Water Quality Analysis Related to Finished Water Mixing in Water Storage Tanks

The degradation of finished water in storage tanks is of great concern to water utilities. It is widely assumed that tank mixing enhances water quality, and slows the creation of disinfectant byproducts (DBP's). In an attempt to enhance water quality and to anticipate future, tighter regulations on DBPs, the Birmingham Water Works Board (BWWB) is conducting a two-part study to examine storage tank mixing effectiveness by comparing tanks with turbulent jet mixing devices and ones without any mixing enhancement.

Part 1.

Two tanks of identical volume, one equipped with a mixing device and the other sporting no such modification, are compared in this study. The two tanks, located less than sixty feet apart, are supplied finished water from the same source at essentially the same pressure. Both tanks maintain roughly identical volumes as the tanks fluctuate through the day and both tanks are fitted with temperature sensors spaced at equidistant heights inside the tanks. Fluoride will be used as a tracer to measure mixing rates. Temperature, fluoride tracer concentration, and disinfection byproducts concentration will all be measured from different elevations in the tank. In addition, the relationship between drawdown and mixing rates will be examined by sampling the tanks elevation ranges from peak demand (lowest volume) to full volume. The data collected will give a clear picture of the effectiveness of the mixing apparatus as compared to the tank with no such apparatus. The data is collected over a twelve (12) month period to account for seasonal variation.

Part 2

Additional tanks throughout the distribution system utilize mixing devices in an attempt to mix stored water and reduce water age. These devices are found in all tank types (standpipe, reservoir and elevated). Nine (9) storage tanks scattered throughout the BWWB distribution system, sporting mixing devices, will be sampled for temperature, free chlorine, total haloacetic acid (HAA's), and total trihalomethanes (TTHM's) at varying heights. These parameters will be measured over a twelve (12) month period to account for seasonal variation. A properly mixed tank should show little variation top to bottom of the sampled parameters.

The combined information from this project should give a detailed comparison between tanks with mixers and tanks without mixers thus demonstrating the effectiveness of the turbulent jet mixers. Data will be analyzed using Systat (Version 11), Sigmaplot (Version 10), and Microsoft Excel.