Launch Slideshow

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From Trash to Treasure

From Trash to Treasure

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    Feed points within the chlorine contact tank reduce chemical consumption; this photo shows the initial pass of the chlorine contact basin. Photos: City of Mankato

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    Mankato's water reclamation facility includes a treated-effluent transfer station, phosphorus-removal equipment, a filtration tank and chlorine contact basin, a final pump station and pipeline to supply a local electricity plant with high-quality cooling water, and a return pipeline.

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    Phosphorus is removed using a ballasted flocculation process. From there, effluent weir and collection flumes convey the water to filtration and disinfection.

That's James Bruender, wastewater superintendent for the city of Mankato in Minnesota, referring to his treatment plant's 2-year-old reclamation facility. Instead of discharging directly into the Minnesota River, the effluent is pumped about 2,700 yards to an electricity plant, where it cools a natural gas-fired turbine before being returned to the plant for discharge into the river.

The first of its kind in the state and one of the first nationwide, the facility earned a Project of the Year award from the Minnesota Chapter of the American Public Works Association last year. “Stakeholders with differing goals but coinciding needs pulled together to explore and implement an unusual approach that benefited everyone,” Bruender says.

In this case, three stakeholders — two public and one private — joined forces. The city needed to renew its National Pollutant Discharge Elimination System (NPDES) permit, a California-based energy provider needed a reliable source of cooling water for a natural gas-powered turbine, and the Minnesota Pollution Control Agency needed a solution that would satisfy pollutant-reduction goals.

SEEDS OF MUTUAL REWARD

A recently passed “Lower Minnesota River Dissolved Oxygen Total Maximum Daily Load” (TMDL) identifies cities along the Minnesota River as a source of phosphorus during low river flow periods, causing algal blooms and low dissolved oxygen when the algae die off. The TMDL requires that these cities, including Mankato, meet a 1 mg/l total phosphorus limit by the year 2015.

Mankato's wastewater treatment plant serves North Mankato, Eagle Lake, South Bend Township, Skyline Village, and the Lake Washington Sanitary District. In 2000, the plant expanded its maximum month treatment capacity to 11.25 mgd with the addition of new aeration basins and secondary clarifiers to increase treatment capacity and provide ammonia removal. The $22 million expansion also included a new influent pumping station and screening facilities, new grit removal facilities, a new digester, and new solids processing and dewatered cake storage facilities.

With the improvements, effluent was about 3 mg/l in total suspended solids and biochemical oxygen demand; and phosphorus discharge over the four years before the reclamation facility was built averaged 1.6 mg/l.

In anticipation of new permit limits, the city did a preliminary study of possible technologies, including biological and chemical phosphorus removal. The lowest cost option to meet the 1 mg/L phosphorus limit was an estimated $10 million to implement.

Meanwhile, Calpine Corp., an independent power producer that focuses on natural gas and geothermal electricity generation, wanted to build a “peaking plant” — i.e., one that produces electricity as demand dictates — that would initially produce 300 megawatts but could double in size. The plant would require 3.1 mgd of cooling water in the first phase and 6.2 mgd in the final build-out.

Calpine's potential cooling water sources included the Minnesota River or groundwater, but both presented permitting challenges. The bigger challenge was discharging back to the Minnesota River and meeting the zero phosphorus discharge requirements for all new discharges on the river.

Because of the year-2000 upgrade, the quality of the treatment plant's effluent was extremely good, and Calpine accepted the city's offer to use it for cooling water. But after looking into designing and building the treatment facilities its plant would need, the company decided it didn't want to be in the water business. It proposed building the reclamation facility and giving it to the city to own, operate, and maintain in return for the treated water.