Chemical Tank Cleaning Eliminates Town's Chlorine Demand

May 6, 2003
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Finished water quality often declines in storage facilities. Low chlorine residuals, elevated chlorination byproduct concentration, high HPC bacterial counts and increased turbidity are consequences of surface deposits and sediment in clearwells and distribution system tanks. The potential of regular tank cleaning for maintaining water quality is now being increasingly recognized.

Traditional tank cleaning methods are flushing, underwater vacuuming and pressure washing. While these methods have some effect in removing loose debris and sediment, they leave most of the scale, metal oxide deposits and biological growth in place.

Chemical cleaning methods have been widely applied in Europe for years and have become an essential part of preserving water quality in systems which operate with minimal or no chlorine. The benefits are reduced chlorination byproduct levels, lower corrosiveness and better tasting tap water. Recently, patented and NSF Certified chemical cleaning technology under the brand name FLORAN has become available in North America for storage facility cleaning and filter media rehabilitation.

In order to evaluate the effect of chemical surface cleaning on drinking water quality, Floran Technologies and the town of Dustin, Okla., conducted a case study of the town's water supply system. As part of the study, the clearwell was chemically cleaned. Before and after cleaning, turbidity, chlorine residual and TTHM data were collected.

The images show the tank before, during and after cleaning. The FLORAN chemical was applied as a low-pressure mist using a spraying wand to all areas of the tank sidewalls and bottom. The procedure was completed by a three-man crew within three hours. The cleaning exposed the painted steel surface and allowed for an accurate inspection of the tank's white epoxy coating.

After cleaning the clearwell, water collected from the tank and from nearby taps showed improved chlorine residuals. In order to determine the original chlorine demand, the tank was drained, refilled and taken off-line overnight both before and after cleaning. Chlorine residuals were measured at the beginning and the end of the off-line periods. In parallel, water from the tank was incubated in a clean sampling vial and used to determine the chlorine demand of the bulk water itself. Before cleaning, the chlorine residual in the stored water dropped 94 percent from 3.79 mg/L free chlorine to 0.22 mg/L. The chlorine was much more stable after cleaning, dropping only 14 percent from 2.24 mg/L to 1.93 mg/L. The control sample also lost 14 percent, indicating that the remaining chlorine residual was due to reaction of the chlorine with the bulk water.

In addition, the TTHM concentration in the tank water dropped from140 µ g/L to 100 µ g/L. Turbidity was reduced from 5 FAU to <0.5 FAU at the sampling tap, which was attributed primarily to sediment removal. After the encouraging chlorine demand results for the clearwell, the town?s second tank was also chemically cleaned. The FLORAN cleaning program, combined with regular line flushing, has now enabled the town to provide safe drinking water while reducing chlorination by nearly 50 percent.

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