Mixer removes ice from small water storage tank within one week
South of Laramie Water and Sewer District (SLWSD) is a consecutive system that purchases water from the city of Laramie, Wyo. Since SLWSD was formed in 1996, managing icy conditions and maintaining water quality in their only tank—a 300,000 gal pedisphere—has been a concern.
“This tank is at the end of our system,” said Foster White, operation manager of SLWSD. “While we have only 220 active meter services, we have nearly 230,000 gal in volume in our pipes to provide for fire flow as well as regular consumption. It can take up to a week for water to make it through our system and into the tank, and this makes managing water quality inside the tank a challenge.”
Ice & Water Age Issues
In winter, icing issues inside the tank were severe.
“We often see daytime temperatures no higher than 10 to 20°F and nights well below zero for an entire month,” White said. “We would commonly find a girdle of ice 4 to 6 in. thick and 2 to 3 ft wide where the water level around the outer tank wall fluctuated.”
The ladder access tube passes through the center of the tank and a “pineapple ring” of ice 4 to 6 in. thick would form around the access tube and float up and down with the tank level. The scraping action of this ice ring scoured the paint, requiring repairs every time a tank inspection was done by divers. The damage was increasing to the point, that in a few years, the tank would have to be taken offline, drained, sand blasted and repainted. A repair of that magnitude would require the tank to be out of service for 3 to 4 weeks and would create a severe problem for a system with only one tank.
Another major concern for SLWSD was water quality. With the tank at the end of the distribution system, water age was high. “Because water temperature can vary so much from season to season, we were concerned about thermal stratification and high water temperatures accelerating residual loss and disinfection byproduct formation,” White said. “We wanted to make sure we went with a mixing technology with a proven track record.”
New Mixer Installed
SLWSD contacted PAX Water Technologies  after learning about the effectiveness of their tank mixing technology from the Utility Division in Casper, Wyo. A PAX Water Mixer (PWM100 )—a new mixing technology for small water storage tanks—was installed in early March 2013. The tank had large masses of ice floating in the water and clinging to the walls. Temperatures were still well below freezing for many hours of the day.
“We were impressed by how easy the mixer was to install and operate,” White said. “It’s a drop-in system that can be easily retrieved by an operator, so we didn’t have to worry about paying for a diver.” Temperature probes were also installed in the tank to record the effect of the mixer on the temperature profile in the tank.
Within the first 24 hours, ice within the tank had already shrunk in size and extent. With each fill and drain cycle, new water entered the tank and was circulated by the mixer to melt the volume of ice. After about a week, the tank was ice-free. This visual demonstration of the power of the mixer was all the operators needed to be convinced that they had selected the right mixing system.
“We were very impressed with this new technology,” White said. “This mixer seemed ideally suited to a tank and system of our size, and the ability to easily install and remove it ourselves is a big advantage.”
Peter S. Fiske is CEO of PAX Water Technologies. Fiske can be reached at [email protected] or 866.729.6493.