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New outfall pipe accompanies Empire Plant expansion

Piece by piece, 12-mile pipe to the Mississippi is coming together

A deer hopscotches over ruts left by a front-end loader. A rabbit scampers under an extended-cab pickup truck on its way back into the brush. A bald eagle soars overhead, surveying first the sewer construction area, then the afternoon snack prospects in the open waters of the Mississippi River.

Such is the scene these days in the rugged, steep riverbluffs and adjacent riverbottoms in northeast Rosemount. This is where contractors for the Metropolitan Council have been working on the first stretch of a new, 12-mile outfall pipe that will carry treated wastewater from the Empire Wastewater Treatment Plant to the Mississippi River. After 16 months and more than 90 percent of the work completed, wildlife is returning to the site after a temporary stay in nearby ravines and riverflats.

Drop structures carry water down the bluffs

In this phase of the project, all of the 54- and 66-inch-diameter pipe segments have been installed, both in tunnels drilled through the bluffs and in open-cut trenches dug four feet into the riverbed to extend the pipe through backwater and into the main river channel. Remaining work centers on two drop structures — one 30 feet deep, the other 65 feet. Treated wastewater — also called effluent — will flow by gravity through most of the outfall, constructed with a gradual downward slope. The two large drops are the only way to negotiate through the steep bluffs and down to the river’s elevation.

The drop structures, however, are not merely waterfalls. They are shaped to induce a swirling motion for the effluent, reducing turbulence as it falls. This also decreases the amount of air getting pulled along with the falling water, leaving the optimum amount of space in the pipe for the water. A separate vertical conduit in the concrete structures releases whatever air does get drawn in. The general contractor for this phase is Lametti & Sons.

A second phase of the outfall, stretching roughly from east of Highway 55 west to Highway 52, is about 20 percent complete. It includes nearly equal amounts of open-cut and tunneling work, including a tunnel as deep as 100 feet along the edge of the SKB Landfill. Work here, under general contractor Ames Construction, should be completed early in 2007.

Final phase expected to be done in 2007

Ames also is building a phase of the project roughly along County Road 42 from Highway 52 on the east to Biscayne Avenue on the west. A contract for the final phase, stretching south to the Empire Plant, is scheduled to be awarded in March 2006. Both of these phases should be done by the end of 2007.

The new outfall is part of a larger project to expand the Empire Plant to treat up to 24 million gallons of wastewater daily, double its previous capacity. The outfall will allow the plant to stop discharging effluent into the environmentally sensitive Vermillion River.

Construction of the outfall will have two additional benefits. Under an agreement between the Metropolitan Council and the city of Rosemount, the city will be able to use some of the reserve capacity in the outfall to periodically convey treated stormwater to the Mississippi River. The construction will also allow the Council to close its Rosemount Treatment Plant. That plant is the smallest of eight in the regional system and would reach its capacity in the next couple of years. Wastewater will still be collected at the old plant site, but it will be pumped to the Empire Plant through a new interceptor sewer that will flow in the opposite direction of the outfall and will be installed in the same pipe trench.