A New Kind of Hydropower

Power producer Calpine Corp. had approached the city of Mankato, Minn., about building a natural gas-fired power plant and acquiring supporting water sources; however, permitting the Minnesota River-area site posed environmental challenges and taking water from the city aquifer would have upset the local drinking water supply.

Following a Mankato Wastewater Treatment Plant (WWTP) upgrade in 2002, retired city Public Works Director George Rosati proposed that the plant’s high-quality water be reused to support operations at Calpine’s Mankato Energy Center, 1.5 miles away. Following approximately one year of planning and one and a half years of construction, in April 2006, operations began at Mankato WWTP’s new water reclamation facility (WRF)—the result of a three-way public-private partnership between the city of Mankato, Calpine and the Minnesota Pollution Control Agency (MPCA).

Calpine funded the WRF’s design and construction, and the MPCA expedited the city’s National Pollutant Discharge Elimination System permit renewal and plan approval. The city of Mankato owns and operates the facility.

The Power of Reuse

The dual-purpose, $20-million WRF, designed to handle 12 million gal per day, is surrounded on three sides by the Mankato WWTP and on the fourth, the Minnesota River. Extra space around the facility will allow for a virtual duplicate to be built in the future, according to Mary Fralish, the city’s deputy director of public works and environmental services. Internal spacing and piping, too, will allow for capacity increases.

The WRF provides for the city’s treated effluent two stages of tertiary treatment: the first removes phosphorus for all current and future WWTP needs, and the second provides additional filtration and chlorination to meet MPCA reuse requirements based on California’s Title 22 Health Laws for recycled water.

Clean water is sent via pipeline to Mankato Energy Center for use in cooling processes. The power facility operates about 60% of the time, producing electricity as demand dictates. The energy center evaporates roughly 75% of the reuse water in its operations and sends the remainder back to the WWTP, where it is mixed in with effluent and discharged into the Minnesota River. At this rate, an estimated 679 million gal of water will be saved through reuse annually.

Reaping Benefits

The city of Mankato is paying operations and maintenance costs on the WRF until its portion is satisfied or for 20 years, whichever comes first. But having the power plant—fueled partly by reuse water—within city limits has resulted in a solid tax base, reduced surface and groundwater demands, phosphorus removal significantly beyond compliance minimums, improved water quality and cost savings.

“This is a great example of an innovative and sustainable infrastructure solution that is successful on many fronts,” Rosati said. “The project yielded multiple economic and environmental benefits for the partners and the people of Mankato.”

Besides up-sizing the project for future growth and expansion needs, Calpine paid for phosphorous removal technology that Mankato would have needed and been financially responsible for otherwise. MPCA regulations require that the riverside community meet a 1-mg/L phosphorus limit by 2015 in order to prevent algal blooms and subsequent pollution problems. Since the WRF went online, total phosphorus levels have dropped to 0.35 mg/L. Similarly, biochemical oxygen demand has decreased from 1.6 mg/L to undetectable levels.

“The water leaving the reclamation facility looks like drinking water,” Fralish said. “I bring tours out there and have them guess which is drinking water and which is effluent water, and they can’t guess.”

As for finances, the city saved approximately $10 million because it did not have to build stage-one facilities for phosphorus compliance. Calpine will save on operations and maintenance, plus more than $1.5 million annually in potable water costs.

Award-Winning Results

The electricity-generating WRF is the first of its kind in Minnesota and one of the first in the U.S.

The Minnesota chapter of the American Public Works Association recognized the city of Mankato, Calpine and project engineering, consulting and construction provider Black & Veatch for the environmentally and economically beneficial facility. Their work earned the association’s 2007 Project of the Year Award in the environment category for projects greater than $10 million.

“The partners in this unique effort deserve acknowledgement for their exemplary collaboration on this project,” said Dan McCarthy, president and chief executive officer of Black & Veatch’s global water business. “They have demonstrated initiative and flexibility in pursuing a mutually advantageous and environmentally sound solution.”

Also for its WRF efforts, the city of Mankato received the 2006 Governor’s Minnesota Government Reaching Environmental Achievements Together Award.

Caitlin Cunningham is associate editor for Water & Wastes Digest. Cunningham can be reached at 847.391.1025 or by e-mail at ccunningham@sgcmail.com.

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