Setting an Example

Low-impact development features, including a green roof, put a Twin Cites wastewater treatment plant on the forefront of earth-friendly landscaping

It started with the trout — the big trout, living in the Vermillion River, a Mississippi River tributary that flows through several fast-growing suburbs of Minneapolis, Minn. It’s the same river to which the Empire Wastewater Treatment Plant used to discharge treated wastewater effluent.

The brown trout were a key reason the Empire Plant is now home to a variety of low-impact development features: pervious pavements, rain gardens, infiltration basins fed by vegetated swales and — perhaps the greatest source of pride — a green roof on one of the plant buildings.

Secret no longer

But it all began with the fish. “The trout were kind of a best-kept secret until the Minnesota Department of Natural Resources (DNR) did a stream survey about 15 years ago,” says Karen Jensen, environmental engineer with Metropolitan Council Environmental Services (MCES). “There were not only some very large trout, but they were reproducing naturally and doing very well.”

Staff at MCES, which operates a wastewater collection system and seven treatment plants serving communities in the seven-county Twin Cities metro area, weren’t the only ones surprised by the survey. But plant manager Pat Oates, Jensen and their colleagues realized those trout presented a call to action.

Besides discharging into the trophy trout water, the Empire Plant faced the challenges of treating an increasing volume of wastewater for four rapidly expanding communities. “At the time of the stream survey, our plant was contributing about 20 percent of the flow volume to the Vermillion, depending on the season,” Jensen notes.

The plant’s service area was projected to grow to 200,000 residents by 2030, and regulators and downstream citizens were concerned about the impact that increased stormwater would have on the water volume, temperature and quality of the river. A series of public meetings and hearings was held to gather input, and that helped in crafting a long-term solution for the plant and its increasing effluent discharge.

“After the meetings, we decided that a smaller, nearby treatment plant would be shut down and the Empire facility would be expanded,” says Cammy Johnson, an assistant business unit manager for the Empire Plant.

“We decided that discharging to the Vermillion River was no longer a suitable option, so construction began on a 12-mile outfall pipe that would carry effluent to the Mississippi, where it would be more readily absorbed.”

The outfall system consists of large pumps that push effluent to higher ground north of the plant. From there, the effluent flows by gravity — through pipes ranging from 54 to 66 inches, some in tunnels as deep as 100 feet — toward Old Man River.

Managing stormwater

The Vermillion got help in other ways, too. “The citizens group Friends of the Mississippi River helped develop a management plan that included stream restoration and habitat work on the Vermillion,” Jensen notes. “They did some bank improvement projects that included installing ‘lunker’ structures that create trout habitat.

“They also helped us restore a 50-acre wetland on our property. The Empire Plant site encompasses 460 acres. A portion of the plant area is former marginal farmland that is now left wild, while much of the upland acreage outside of the actual physical treatment plant site is spread with the plant’s biosolids and leased to local farmers.”

In addition, MCES incorporated innovative stormwater management features into the plant’s design for expansion. “These features all fall under a Low Impact Development (LID) philosophy that incorporates techniques where most of the stormwater is allowed to soak into the ground where it falls,” Jensen says.

“Traditional practices that involve piping stormwater as quickly as possible to settling ponds may cause downstream flooding, do little to improve water quality, and actually cause warming of the storm-water, which is detrimental to the trout. LID methods are a cost-effective, environmentally friendly alternative, and they have worked well for us. They involve a little more effort to construct and establish, but they are more environmentally effective in the long run.”

Keeping it green

There are four major LID improvements on the Empire Plant site. Three rain gardens (also known as infiltration basins) allow stormwater to soak slowly into the rich loam beneath them, filtering out pollutants and reducing runoff. The gardens, ranging in size from 0.3 to 1.2 acres, are depressed basins planted with native grasses.

“They’re in the ‘baby stage’ right now,” Jensen says. “They consist of a loose layer of fluffy soil, not compacted by heavy equipment, followed by a thin layer of compost, organic soil and sand that allows a good root base for the native grasses.

“This layer filters out potential pollutants like metals and nutrients, and the native plants take up the water. Even better, these plants require no fertilizer or pesticides to maintain, and they create wildlife habitat.”

In addition, a series of 10 vegetated swales (grassy ditches) convey stormwater toward the rain gardens. “Like the rain gardens, these ditches help absorb water and reduce flow, while collecting potential pollutants,” Jensen explains. “We let nature treat as much of the water as possible.”

Jensen’s pride and joy is a green roof of sedums, native grasses and other plants growing on a specially engineered roof on the plant’s Reverse Activated Sludge (RAS) Pump Building. The 2,400-square-foot roof does more than capture stormwater that would otherwise drain away — it naturally cools the building and reduces air conditioning costs. Additionally, the green roof should last twice as long as a typical hot-applied membrane roof.

“This roof is a thing of beauty in the summer,” Jensen says. “The grasses and blooms are visited by birds and butterflies, and it’s wonderful to look at. We wanted to install it as an example of what can be done to reduce stormwater flows in future suburban and urban construction projects.

“Our green roof is one of the most innovative and interesting features of the plant. And because the building is below grade, the roof is easily viewed from ground level. During drought periods we do water the roof, but we use treated effluent rather than groundwater.”

Pervious parking

Another innovation at Empire is a parking lot constructed with pervious pavers — paving bricks separated by gravel-filled spaces that allow rain to soak in and reduce runoff. Several parking areas on the site use these pavers, which Jensen says are surprisingly tough and easy to install and maintain.

Jensen and Johnson both point with pride to the Empire plant’s earth-friendly features. “We’ve given several tours of the plant since adding these features, and people are excited to see these practices in use,” Johnson says. “We can experience a 2-inch rain event and hardly a drop of water leaves this facility. It’s all absorbed.

“I’m really proud that we took the leap to install these features, and I hope that they’ll inspire other organizations, private citizens and builders to consider them as an environmentally friendly way to build and expand.”


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