Smooth Transition

Operators in a South Dakota water system guide a plant expansion and switch to membrane filters without service disruption and with the ‘utmost professionalism’
Smooth Transition
Plant operator Shane Bothwell checks amp readings on Cornell wash water recovery pumps with Eaton SVX-9000 variable-frequency drives.

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Reliable. Resourceful. Responsible. Those are good words to describe the people of South Dakota, a frontier state with the fifth lowest population density in the United States.

They’re also good words to describe Bill Sarringar, Ron Ramsey, Shane Bothwell and Al Thomas of the sprawling Mid-Dakota Rural Water System. On their watch, the capacity of the Mid-Dakota Water Treatment Plant is being expanded by 50 percent, from 9 mgd to 13.5 mgd, facilitating new connections and expanding water availability in the system’s 7,000-square-mile, 14-county service area.

The project, budgeted at up to $7 million, is also increasing raw water and treated water storage capacity, ending a three-year moratorium on new connections. “It’s a big responsibility,” says Sarringar, plant manager. “But lots of the time you don’t think about it. You make a mistake and it could affect thousands of people.”


Big territory

Mid-Dakota is typical of the several large regional water systems in South Dakota. It serves a population of about 33,000 in 18 municipalities, two community systems, and more than 2,300 individual rural hookups where the water is vital to livestock feedlots. The expansion will increase those numbers to 24 municipalities, three community systems, and more than 3,000 rural connections.

More than 3,800 miles of distribution lines connect these users to the Oahe plant, just outside Pierre, the state capital. The original plant was built in 1997, funded through the U.S. Bureau of Reclamation and the State of South Dakota. Its deep-bed gravel, sand, and coal media filter cells have been replaced by membrane filtration.

Lake Oahe, a reservoir on the Missouri River above the Oahe Dam, supplies the raw water. “It’s a reliable source,” says Sarringar. “We draw from Corp Bay, which is out of the way of the main channel. Turbidity averages around 4 NTU, and can be as low as 1 NTU in the winter when the lake freezes over. That’s almost unheard of for surface water.”

First, the raw water is pumped to two bolted, glass-lined “grain-bin” type storage tanks at the chemical feed building site that have been expanded from 408,000 gallons to 635,000 gallons by adding “rings” to the bottom. The tanks were raised 30 feet.

From there the water passes through the chemical feed building, where chlorine gas is injected into the raw waterline. The water gravity-flows about four miles to the treatment plant. As it enters the plant, aluminum chlorohydrate is added ahead of a five-stage flocculation basin to form pin floc before the water flows to the filters.

Before the filtered water enters the clearwell, caustic soda is added for pH adjustment. Chlorine and anhydrous ammonia are injected to form chloramines, a longer-lasting disinfectant to maintain the required chlorine residual (less than 4 ppm) throughout the lengthy distribution system; fluoride is also added. A 30-inch mainline stretches for some 120 miles into the service area, with smaller lines branching off to customers, or customer systems.


Major improvements

The current construction project essentially replaces the multi-media filtration system with new ZeeWeed ultrafiltration membranes from Zenon (GE Water & Process Technologies), while adding much-needed storage capacity. “The plant footprint is the same,” explains Sarringar. “That’s the main reason we went with membrane filtration — we could increase capacity without expanding our treatment plant.”

When finished, the project will convert five of the former media filtration basins to submerged membrane tanks, and the sixth filtration basin will be used as a holding tank when the Mid-Dakota crew does maintenance on the wash water recovery basin.

“With the membranes, we have a lot more ancillary equipment — chemical pumps, permeate pumps, valves, the clean-in-place and integrity testing systems, and numerous other devices,” Sarringar says.

Nonetheless, the transition has gone smoothly. Two of the new membrane systems were put into service during summer 2010. In 2011, the remaining three membrane systems were brought online. When finished sometime in 2012, it will be the state’s largest submerged membrane system for drinking water.

“The supplier has been very helpful,” Sarringar says. “GE had a technician here throughout the system startup. It was pretty remarkable. Within about four hours, we were off the media filters and online with the membranes.”


Simple maintenance

Membrane cleaning is fairly straightforward. Sarringar and his team soak the membranes in a water and chlorine solution for an hour and a half each week to prevent biological growth, and perform a full “recovery clean” every 30 to 45 days, using hot water, chlorine and citric acid to prevent any irreversible biological and mineral fouling. The recovery clean takes two days.

The adoption of membranes has required just a few other changes. “Membranes require pre-filters, or strainers,” explains Sarringar. “We didn’t have enough room at the plant for the strainers, so they were installed in the intake building.” The wash water from the membrane process is directed to a settling basin, and the clarified water is returned to the head of the plant. The chemical clean water passes to a lagoon, where it evaporates.

The new membrane system has its own internal control system, so plugging into the plant’s existing SCADA network caused few difficulties. Besides increasing the capacity of its raw water storage tank, Mid-Dakota is building an elevated 2-million-gallon finished water storage tank east of Pierre. It is also boosting the rating of its intake pumps from 300 hp to 400 hp and its high-service pumps from 400 hp to 500 hp.

“During the 2006 drought, we had one day at a peak of 8.85 million gallons, just 150,000 gallons short of our capacity,” says Sarringar. “Now with the reserve capacity and increased production, we should be in good shape for some time to come.”


Facing challenges

Starting up new technology on the fly isn’t easy, and Sarringar and his resourceful crew have had their hands full. “I think that has been our biggest challenge — producing process water during up-grade, working around shutdowns,” Sarringar says.

The team was able to shut down two of the media filters and convert those to membrane cells capable of producing 8 mgd, while keeping the other four media filters in operation. “When we had two membrane systems in service, then we could switch over to do the demolition work on the four remaining cells,” Sarringar says. The construction work was scheduled to take place during the off-peak seasons.

The weather has so far been cooperative. “We’ve had moderate temperatures during the summer and timely rainfalls,” he says. “That has kept demand at modest levels. You have to take construction in stride — work with the contractor and still produce water.”

Mid-Dakota general manager Kurt Pfeifle gives the treatment plant team high marks for the smooth transition. “It’s a tricky process,” he says. “Replacing one system with another, keeping our customers in water, all while the contractor is running around all over the place.”


Value engineering

An even greater challenge was increasing production capacity at the treatment plant without increasing its size. Sarringar credits that accomplishment to value engineering. With new construction or plant upgrading, he finds it beneficial to do a value engineering study. “When we looked at the plant expansion, we looked at all the options — sedimentation, the original process, and membranes,” he says.

“In our first analysis, membranes were too expensive, and their expected life seven to 10 years. But later, their cost dropped, and the life expectancy increased to 10 to 15 years.

“Plus, membranes put us in a better position to meet not just current regulations, but also new regulations that might come down the road. I think it’s really important to do a value engineering study, look at all your options, and try to put your system a little ahead of the regulation game.”

It’s this kind of resourceful and responsible thinking that has earned the Mid-Dakota team numerous awards. Sarringar, who holds a degree in chemistry from Dakota Wesleyan University, came to Mid-Dakota in 1997 from Aberdeen, where he was a plant operator and plant chemist. He has worked his way up in the water treatment field from plant operator, to plant chemist, and now plant manager. He has been honored by the South Dakota Section of the American Water Works Association, the South Dakota Association of Rural Water Systems, and the South Dakota Water and Wastewater Association.

“Since I started, the regulations have been getting tougher,” he says. “It has made the profession interesting and challenging. I think we’re often taken for granted. A pat on the back is not typical, but I’m glad I chose this profession.”

Ramsey worked with the City of Fort Pierre for eight years, handling wells, wastewater treatment, swimming pools, lift stations, and pumps. He joined Mid-Dakota in 1997 inspecting distribution lines and joined the treatment plant staff in 2000.

He says the current expansion has been challenging: “There’s a high learning curve on some of the new membrane equipment, especially in the maintenance area.” His work has earned him awards from the South Dakota Water and Wastewater Association, and in 2010 he received the Meritorious Service Award from the South Dakota AWWA section.

Bothwell, water treatment plant operator, worked in that capacity and as a distribution operator for the City of Pierre before being hired by Mid-Dakota in 2005.

Al Thomas was hired in 2004 as a plant/distribution operator for the system. His main responsibility is in the distribution system, but he devotes about a third of his time to plant operations. Before joining Mid-Dakota, he was an equipment operator for the City of Pierre.

“I’ve always thought we had the best operators in the state,” concludes general manager Pfeifle. “They’ve handled the upgrade, learned new technology, changed the process with the utmost professionalism. I couldn’t be happier.”


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