New outfall pipe carries treated wastewater to Mississippi
After three years of construction through Dakota County farmland, the edge of a college campus, and rugged and steep river bluffs, a new 12-mile outfall pipe is now carrying treated wastewater from the Metropolitan Council’s Empire Plant to the Mississippi River at Rosemount.
The outfall was part of a $130 million project that doubled the size of the plant and in March 2008 discontinued the discharge of treated wastewater – also called effluent – to the scenic and environmentally sensitive Vermillion River. The Vermillion, a protected waterway that is clean and cool enough to support trout, empties into the Mississippi River at Hastings.
Effluent from the Empire Wastewater Treatment Plant in central Dakota County will no longer flow into the Vermillion River, a waterway known for its good trout fishing.
Population in service area expect to double
The Empire Plant, located in Empire Township just east of Farmington, treats about nine million gallons of wastewater per day from more than 100,000 residents in Apple Valley, Lakeville, and Farmington, and portions of Rosemount and Empire Township. The population of this service area is expected to surpass 200,000 by 2030, thus the increase in the plant capacity from 12 million gallons per day to 24 million gallons per day. The plant expansion also added a phosphorus removal process, which is important because phosphorus is a nutrient that contributes to excessive plant growth in waterways.
“The expanded Empire Plant will be able to provide service to this growing area for decades to come,” said Bill Moore, general manager of Metropolitan Council Environmental Services (MCES). “We look forward to the environmental benefits that will come from discontinuing the effluent discharge into the Vermillion River. The Vermillion was a suitable receiving water for nearly 30 years, but the time was right to move on to a better alternative in the Mississippi River.”
Increased effluent had potential to erode river banks
The decision to change the effluent discharge location resulted from the Council’s public outreach effort with local residents, government representatives and environmental experts. After reviewing the technical data, this group agreed that the long-term increases in effluent flows would be more than the Vermillion River could handle.
Empire Plant operators Jim Crocco and Eloyce Shaw check equipment in the effluent pumping building.
“The Vermillion River is one of the best rivers in the state for trophy-sized trout fishing,” said Brian Nerbonne, a stream habitat specialist at the Minnesota Department of Natural Resources. “The river is especially important given its location in the Twin Cities metro area, where trout resources are limited to only a few streams.
“If the expanded Empire Plant continued to discharge its effluent to the Vermillion, it may have threatened the trout population because the eventual doubling of flow from the plant could cause erosion of the already unstable stream channel,” Nerbonne explained. “Discharge from the expanded plant at peak capacity would have doubled the normal flow of the river.”
New pipe will allow Council to close Rosemount Plant
The outfall consists of pipes ranging in size from 54 to 66 inches, some of which were installed in tunnels as deep as 100 feet. Large pumps push the effluent through the outfall to higher ground north of the plant, then the effluent flows by gravity to the Mississippi.
The outfall also will benefit the city of Rosemount, which can use some of the reserve capacity in the pipe to periodically convey treated stormwater to the Mississippi. And to streamline MCES’s operations, construction of the outfall is allowing the agency to phase out its Rosemount Plant.
The Rosemount Plant is the smallest of eight plants in the regional wastewater system and serves most of the city of Rosemount. The city’s wastewater will still be collected at the plant site, but it will be pumped to the Empire Plant through a new interceptor sewer that was installed in the outfall pipe trench and will flow in the opposite direction. A portion of the city’s wastewater recently was rerouted where local sewer pipes cross the new interceptor. The rest of the rerouting will be done later this year when two new MCES pumping stations are complete.