Clara City Joins the Membrane Wave
Reverse-Osmosis Successful in Removing Nitrite
From the Winter 2003-04 Waterline, the quarterly newsletter of the Minnesota Department of Health Public Water Supply Unit, Waterline, Minnesota Department of Health
After years of having to supply bottled water to pregnant women and families with infants because of nitrite in its water, Clara City has taken care of the problem with a reverse-osmosis treatment plant, the third used by a municipality in the state and possibly the first anywhere to use the process for reduction of nitrite. "I had no idea of how it would work," said Clara City public works superintendent Roger Knapper of the plant, which went on-line in August of 2002, "but now we're producing great water."
Reverse-osmosis was arrived at as a solution only after the city and its engineering firm explored other alternatives, ranging from a different source of water to other treatment alternatives, including breakpoint chlorination. A decade ago, the concept of membrane filtration may have been as unfeasible as the other options. However, with the cost of membranes coming down, along with the chemical requirements for such treatment, many see this as the wave of the future, including several other communities in the west-central and southwestern portion of Minnesota, where the search for good water is often difficult.
The nitrite is caused by high levels of ammonia in the groundwater, possibly the result of an ancient forest. Knapper said some of the drill bits used in test borings came up with pieces of wood, indicating the existence of underground trees that are decomposing and causing the high ammonia.
"The ammonia is causing the nitrite," said John Graupman, an environmental engineer with Bolton & Menk, Inc. of Mankato, Minnesota, the consulting engineer on the new treatment facility, along with Rodeberg & Berryman, Inc. of Montevideo, Minnesota. Graupman said the ammonia levels are from 6 to 8 parts per million and that nitrite occurs as a bacteriological reaction. "It's almost impossible to stop," he explains. "Typically nitrite is not a problem because ammonia essentially converts to nitrate. Nitrite is an intermediate product and essentially because their system had such low levels of bacteria, it got caught up in an intermediate stage. It didn't convert to nitrate fast enough.
"The city tried to clean its lines, thinking that the bacteria was growing in the lines. They tried a variety of things, but nothing really had an overall long-term effect."
The city's previous treatment had been a gravity filter to reduce iron. It's sources were a pair of wells at the water plant and two others a few miles to the west. The wells at the plant were low in ammonia but couldn't produce enough water to make blending an option. Besides, as Knapper explained, the former plant was not set up to run two wells simultaneously.
Along with Rodeberg & Berryman, the city made an extensive search for another water source with lower concentrations of ammonia. Ten separate wells were drilled at various locations, none of which had sufficient quality or quantity for use by the city. Knapper said that test drilling done to the east showed "great water but not enough of it." In addition, wells in this location would have required a new transmission line.
Another possibility considered was breakpoint chlorination, adding chlorine to the water until the demand is satisfied. Because of the ammonia, Clara City had been having trouble maintaining a chlorine residual as the free chlorine converted to chloramines. Graupman said that adding chlorine to ammonia would cause a reaction that breaks down the ammonia. "The thought was that if you chlorinated, the ammonia would be removed and wouldn't be able to convert over to nitrite." The problem, he said, was that breakpoint chlorination would require approximately 7 parts of chlorine per part of ammonia, too much to make it feasible.
With other options exhausted, the city and its engineers made the decision to build a new water treatment plant, adjacent to the existing one, with pressure filters followed by reverse-osmosis treatment. The project would also include a new well, tower, and approximately two miles of additional water mains.
Reverse osmosis has been used in many ways, including desalination of sea water on ships. Industries, including bottling companies have also employed the technology. "They get treated water from the tap, so they start with fairly consistent water" Graupman points out, "where we're taking well waterless consistent and a little more difficult to handle."
The first reverse-osmosis system for a water plant in Minnesota went on-line in Madison in 1999 to remove sulfate as well as soften the water. Later that year, Lincoln-Pipestone Rural Water System completed a reverse-osmosis system for nitrate reduction.
Graupman was the construction engineer for the Madison project and says they have learned from this and other experiences. "In Madison, the anti-scalants were not very advanced at that stage. They also required an acid to drive the pH down, to keep the scale dissolved. Madison expected a life of three-to-five years on their membranes and is currently just over five years, even with the pre-treatment problems that placed an added burden on the membranes. Were impressed with how the membranes have worked."
Compared to conventional treatment, a reverse-osmosis plant is more expensive. However, with the capital and operating costs coming down, more cities are going with reverse-osmosis, particularly those in water-challenged areas. Graupman is involved in the design of two other plants now under constructionone in Lucan, about 30 miles south of Clara City, to remove radium and the other in Mountain Lake in southern Minnesota to reduce sulfate and total hardness.
As the name implies, reverse osmosis works against the natural process of osmosis by using pressure to force water through a semipermeable membrane that allows water molecules to pass through, while discharging undesirable elements, such as nitrate, nitrite, sulfate, hardness, radium, and arsenic. Put another way, reverse osmosis is essentially a high-pressure filter that removes contaminants as small as molecules.
The water pressure is increased to approximately 160 pounds per square inch as it enters the reverse-osmosis system, forcing the water through the membrane, which removes up to 99 percent of the dissolved solids, including nitrite, nitrate, hardness, and sulfate.
The Clara City plant has 12 housings that contain six membranes in each housing. On the average, each element produces about three gallons per minute (gpm) of treated water.
Brian Wise, an application engineering manager for GE Osmonics of Minnetonka, Minnesota, the firm that installed the reverse-osmosis equipment in Clara City, explained how the system works. "Three hundred gpm is fed into the system. In the first stage, we have six housings with six membranes in each housing. So there's 36 membrane elements in that first stage. So, of the 300 gpm that's fed into it, the first stage in total will produce 113 gallons per minute of purified water. So there is 187 gpm left over of slightly concentrated water. It's all the stuff that is rejected from the membranes."
From here, the remaining water makes a second pass through four housings (a total of 24 membrane elements). This stage produces approximately 75 gallons per minute from the 187 it was fed, leaving 112 gpm.
"That stream is even more concentrated with rejected materials-minerals, nitrates, calcium, et cetera. The third stage, which has two housings, will produce about 37 gpm. What's left over is 75 gpm, and that is the final wastestream that goes to the wastewater treatment plant."
Along with the contaminants, about 25 percent of the water (75 out of 300 gpm) is separated and discharged into the wastewater, leaving the reverse-osmosis system with a recovery rate around 75 percent.
Wise says they strive to find "a happy medium between recovering most of what you feed into the system yet not go too far where your mineral precipitation becomes a concern. This will affect the life of the membranes."
Blending is another way of increasing the overall recovery rate while reducing the burden on the reverse-osmosis membranes. "Blending reduces operating costs because you're not treating 100 percent," says Graupman, adding that this also addresses an issue with corrosion. "If you send out water that it too pure, it tends to be more corrosive."
The Clara City plant reverse-osmosis system has 72 membrane elements in 12 different housings.
Pre-treatment includes chlorine to kill bacteria, potassium permanganate, polyphosphate as a corrosion inhibitor, caustic soda, and a polymer to induce a longer run on the sand filters that precede the reverse osmosis.
An anti-scalant is also added to the water at this point as a means of protecting the reverse-osmosis membranes. "When you get scale on a membrane, you can clean it to some degree, but you run the risk of permanent fouling of the membrane," says Graupman, adding that sodium bisulfate is added to remove the chlorine, which will harm the membranes. "Chlorine is aggressive to the membranes and attacks them, creating holes, versus traditional fouling, which is more of a plugging due to scale and other particles. Fouling can be cleaned to various degrees of success, while chlorine damage is irreversible. Luckily, the low levels of chlorine used in the filtration process have little impact."
All of the water goes through the pressure filters, three eight-foot diameter vessels about 10 feet tall consisting of 12 inches of anthracite on top of 18 inches of greensand.
Most of the water then goes to the reverse-osmosis system. Knapper says they normally use an 80-20 blend although "it may be 85-15 or 90-10."
The blended water is then treated with chlorine and fluoride before being pumped into the distribution system.
In addition to bringing the nitrite down to undetectable levels, the plant has reduced the hardness of the finished water to between 9 and 12 grains, causing many of the city's residents to shut off their home softeners.
The new facilitywhich includes a laboratory that allows them to check the levels of iron, manganese, chlorine, and fluoride as well as the pH of the wateris designed to operate automatically when the operators are not present. A computer operating system is used to control the various motors and equipment and can also be assessed by computers at remote sites. An alarm dialer is also provided to call the operators should anything fail with the plant.
In addition to the plant, Clara City built a new 200,000 gallon water tower, dug a new well, and installed about two miles of new water mains for a total project cost of $3.2 million. The project was funded through a $2.6 million low-interest loan from the Minnesota Public Facilities Authority and a $500,000 grant.
Construction began in Fall 2001. The treatment plant and watermain improvements were completed in the fall of 2002 with the tower completed in spring 2003.
To help pay back the revolving loan, city water rates have increased from $1.25 per 1,000 gallons to $3 per 1,000 gallons. The surcharge was also increased by $2.50 to $12.50.
Water Treatment Plant: American General Contractors, Inc., Valley City, North Dakota
Tower: Maguire Iron, Sioux Falls, South Dakota
Watermain: Rickert Excavating, Brownton, Minnesota
Wells: Thein Well Company, Clara City, Minnesota
Bolton & Menk, Inc., Mankato, Minnesota
Rodeberg & Berryman, Inc., Montevideo, Minnesota
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