This article summarizes tracer studies performed at 32 water treatment plants in Washington State ranging from 5 to 5,400 gpm, with contact basins from 200 gal to 1.5 mil gal. Multiple tracer studies were performed at each plant. The goal of the tracer studies was to determine the baffling efficiency (BE) of each basin by determining T10/T, where T10 is the time at which 10% of a tracer is measured at the outlet of a basin, and T is the hydraulic residence time. The tracer studies indicated that most facilities historically used BE values higher than what the tracer studies indicated for calculating compliance with disinfection regulations. The majority of the basins were rectangular, baffled basins. The tracer studies indicated that standard design parameters, such as length‐to‐width ratio, were not as accurate in predicting basin performance compared with calculating baffled channel cross‐sectional velocity. This effect was pronounced where cross‐sectional flow velocity was below the Reynolds laminar flow threshold. For circular, baffled basins, a relationship between cross‐sectional velocity and BE was also evident. The study also identified issues with plants having parallel, chlorine contact basins where uneven flow splitting between basins created an overall system BE less than the BE of individual basins.
Interacting with operators on site has proved a valuable way to assess turbidity data collection practices and identify areas for training and process changes to optimize plant operations and filter performance.
Is it time to move on from using test strips and color wheels to monitor chlorine residuals? In the State of Washington, different residual test methods produced different results for the same sampled water, casting the reliability of traditional approaches into question.
C ontinuous chlorine disinfection provides multiple benefits to enhance drinking water safety, including protection against waterborne microbial pathogens in drinking water supplies. Microbial pathogens are generally classified into three primary groups: viruses, bacteria, and protozoa. Such pathogens can cause acute and chronic illnesses-even death. At water treatment plants, filtration and disinfection are the primary barriers against pathogens in source water. According to the AWWA Disinfection Committee, chlorine has been and continues to be the most-used disinfectant in US drinking water treatment plants (https://doi.org/10.1002/awwa.1648).Chlorine is also commonly added in water distribution systems as a secondary barrier to protect against microbes. Maintaining a residual level of chlorine in the distribution system also provides the following benefits:
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