Tassal Tasmanian Salmon, an Australian salmon farming company, backed away from plans to dump treated wastewater from salmon pens into...
The Mid-Dakota Rural Water System provides treatment services to residents in east-central South Dakota. Construction on the facility began in 1994, and the plant was fully operational three years later. Recent years have seen a steady climb in regional water demands, and in late 2006, plant officials decided to heed community growth and expand operations.
About five years ago, operations at the Mid-Dakota Rural Water System hit a wall. The water treatment plant—located in Miller, S.D.—was fulfilling a record number of service requests, pushing the plant dangerously close to its 9-million-gal-per-day (mgd) capacity.
“[We] still had several hundred connections to install,” Mid-Dakota Water Treatment Plant Manager Bill Sarringar said, noting that treatment demands spiked after continued regional growth. “[Officials] declared a moratorium so any users that were not part of the distribution contracts left to complete [were not added].”
As the plant’s service area expanded, so did the public’s demand for treated water. Plant officials agreed to collaborate with local government agencies to increase capacity. It was quickly decided, though, that complete infrastructure overhauls were not feasible. Soil conditions and the building’s location on drilled piers and void forms made construction of new facilities and basins too expensive.
“[It was decided that] retrofitting the original dual-media filter cells with submerged membranes would give the plant 50% more capacity and keep the original footprint the same,” Sarringar said.
Local officials organized a fund supplied by loans and grants from state and federal agencies to pay for the project. And after a bidding process, GE signed a contract to supply the membrane equipment.
Prior to expansion, the plant’s facilities were in good condition—original operations had started in 1997, making the building relatively new. The challenge of the project was to keep the existing plant working as four filter cells were converted to membrane filtration, according to GE Power & Water General Manager Yuvbir Singh.
“[We] used the ZeeWeed100 immersed ultrafiltration (UF) membranes with an operating mode described as outside-in,” Singh said. “Feedwater surrounds the submerged membranes, and filtered particle-free permeate—filtered water—is drawn through the membrane service via a vacuum.”
Membranes provide a larger filtration surface area within the small footprint of the original filters. Traditionally, the plant used direct filtration technology. But as a result of installing the membrane technology, the plant is now achieving more flow—about 13.5 mgd.
“Membrane filtration has improved the plant’s ability to comply with U.S. Environmental Protection Agency treatment rules and prevent regulated pathogenic organisms from passing into distribution systems,” Singh said.
In addition, upgrades to the intake pumps and high-service motors were conducted. Pumps and motors now function with more horsepower, while a new storage tank—called the Canning Tank—helps move water through the system.
“Also, no piping modifications were needed during the upgrade because [pipes] were [already] hydraulically designed for the new capacity,” Sarringar said.
Lastly, raw water storage—held in tanks bolted with steel and lined with glass—has increased from 408,000 gal to more than 600,000 gal, providing the required space to treat 13.5 mgd.
Location has been integral to maintaining plant operations. Sitting upstream of the Oahe Dam, part of the Missouri River, the plant’s position relative to its water source has been a boon during expansion.
Because of its proximity to the dam, “the plant’s raw water quality is very good,” Sarringar said.
Come November, the Mid-Dakota Water Treatment Plant will be the largest UF membrane facility in the state of South Dakota. The plant spans approximately 7,000 sq miles and serves more than 30,000 people in 14 counties.
Despite the plant’s expansion, Sarringar said, day-to-day employee tasks have gone relatively unchanged.
“There will be more maintenance duties associated with membranes, since they require more equipment,” Sarringar said. “But increasing our capacity has had very little effect on operations.”
Staff currently consists of a plant manager and about two plant operators. For now, employee expansion is not needed, Sarringar said. “But once we run the plant for awhile, we may decide that additional staffing is required,” he added.
Membrane technology has allowed the plant to produce higher-quality effluent to meet the region’s growing demand for treated water. Although future expansions are not planned, “there are more filter cells that could be retrofitted to [increase] capacity,” Singh said.