Uses mixing technology to provide lower-cost alternative when meeting town’s water quality requirements
For the residents of Concord, Mass., high standards are part of their heritage. The “shot heard round the world,” fired in Concord, began the Revolutionary War and set new standards for democracy. Well-known 19th century writer Henry Thoreau mused about the quality of life at his nearby Walden Pond. In these modern times, Concord residents expect no less than high-quality drinking water, and Water/Sewer Division Superintendent Alan Cathcart is meeting that expectation without spending a lot of extra money. He learned that submersible, grid-powered SolarBee mixers installed in potable water storage tanks eliminate stratification and short-circuiting with very little power consumption—and best of all, reduce operating costs and improve the reliability of his system.
Concord’s water system consists of six groundwater supply wells, one surface water supply and three storage reservoirs for a total capacity of 7.6 million gal. System demands fluctuate from more than 4 million gal per day (mgd) in summer, when tourists visit Concord’s historic sites, to 1.5 mgd in winter. In the future, the town may have to depend more heavily on the surface water supply if it outgrows its well-water supply. For this reason, Cathcart wanted to make sure that the mixer would assist in removing disinfectant byproducts, if needed.
Concord’s experience with SolarBee began with extensive discussions with SolarBee application engineers, including President Joel Bleth. Impressed with SolarBee’s experience and knowledge of potable water systems, Cathcart installed three SolarBee GS-12 submersible mixers: two in an underground storage tank and one in a ground storage tank. The high-volume, low-voltage mixers sit on the bottom of the tank to create a constant direct and induced water flow, ensuring uniform distribution of disinfectant, preventing stratification and providing uniform water age. In effect, the mixers provide major flows at the floor and sides of the entire tank, keeping the disinfectant constant and bacteria in check. And, at 40% of the cost of comparable mixers, GS-12 mixers met Cathcart’s budget requirement. Installation was easy as well; the mixers install through the hatch, with no need to enter or drain the tank.
Before the Concord system employed mixing, water temperature differentiation and resulting stratification made it difficult to control chlorine residual. Water in the underground rectangular tank had a 5°C differential from bottom to the surface, while the ground storage tank had a 3°C differential. To test the mixers’ effectiveness, Cathcart measured water temperature at various depths after the mixers were installed.
Within 24 hours of starting the mixers, water column temperatures converged to within 0.5°C. Fill cycles in both tanks were detectable by observing how bottom temperatures dipped lower than surface waters due to cooler influent water. For several hours following each fill, the GS-12 mixer converged the temperatures, rapidly mixing in the new colder water that entered during the fill cycle. As part of the study, an intentional off/on cycle in the underground tank showed that temperatures began to stratify soon after the mixer was turned off, but again quickly converged when the mixer was turned back on. Furthermore, measurements from the three separate test locations within each tank indicated that water temperatures were uniform both vertically and horizontally, from one end of the tank to the other.
“Water storage is the silent behemoth,” Cathcart said, “but I’ve learned that I can enhance water quality throughout the entire system with efficient and complete mixing.” His strategy meets the town’s high expectation for performance and value and ensures reliable, safe water for the future—a strategy not even a revolutionary would challenge.
http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/SBE8274B_Cathcart.jpgWater/Sewer Div. Superintendent Alan Cathcart installed three new submersible mixers.
http://www.wwdmag.com/sites/default/files/imagecache/article_slider_big/SolarBee%20GS12chart-1%20orig_0.jpgWithin 24 hours of starting the mixers, water column temperatures converged to within 0.5°C.