Staples Sticks with Softening
From the Winter 2004-05 Minnesota Department of Health Public Water Supply Unit, Waterline, Minnesota Department of Health
The city of Staples, a community of approximately 3,300 residents in central Minnesota, opened its new water-treatment plant to visitors on Friday, August 27, 2004. Mayor Bruce Nelsen, flanked by Representative Mary Ellen Otremba and Steve Wenzel, the state director of U. S. Department of Agriculture (USDA) Rural Development, did the ceremonial ribbon cutting although the plant has been on-line since early June.
Wenzel praised the project, which included two new wells and a reservoir, as a good example of a partnership between federal, state, and local units of government. The funding came from a $1.39 million grant and loan from USDA Rural Development, a small cities grant of $600,000 from the Minnesota Department of Employment and Economic Development, and a below-market-rate loan of $1.4 million through the states Drinking Water Revolving Loan fund. The bid for the project was $2.963 million compared to the final construction cost of $2.866 million. The total project costincluding land acquisition, engineering and administrative fees, and relocation costswas $3.391 million.
The lime-softening plant replaced a similar facility that had been built as a Works Progress Administration project in 1938. The old plant was showing its age with some of the equipment severely corroded. “I don’t know how it held together,” said operator Gary Thorman, who added that the new facility and wells will have greater capacity “so that we don’t have to run it as long. Now we can meet demand in a shorter amount of time and still have a cushion.”
Along with the previous plant, two wells were abandoned. A third was kept and serves as a backup to the two new wells, each capable of producing 500 gallons per minute (gpm), that were drilled in conjunction with the new plant. Thorman noted that the combined capacity of the three wells at the old plant was 450 gpm.
The plant design is essentially the same although the process now includes recarbonation for pH adjustment by carbon dioxide. The treatment begins with an induced draft aerator to remove the trace organics. Hydrated lime, soda ash, and sodium aluminate are then added to soften the water. The raw-water quality is approximately 250 parts per million (ppm) of hardness and is softened to 100 ppm.
From the aerator and the clarifier, the water passes through the recarbonation basin and then to the two filter beds, which contain 18 inches of anthracite and 12 inches of silica sand over 12 inches of support gravel. A Leopold underdrain system brings the water to the new 350,000-gallon ground reservoir.
Ammonia, to form chloramines, and fluoride for dental protection are added to the water as three high-service pumps send the water into the distribution system.
The plant contains two gravity filters with sand and anthracite.
The new plant has three high-service pumps with a capacity of 500 gallons per minute. At the far left is the backwash pump.
Progressive Consulting Engineers (PCE) of Brooklyn Center, Minnesota, worked with the city to obtain financing for the project and designed the new plant. PCE president Naeem Qureshi says that this could be one of the last lime-softening plants constructed. “Most cities are going to iron and manganese plants,” Qureshi says, noting that softening, combined with the pH adjustment, will result in the removal of iron and manganese.
Staples residents, however, had become accustomed to softened water, a feature the city wanted to keep while also addressing other aesthetic contaminants.
A schematic drawing of the new Staples plant.