Project: Water reclamation

Jurisdiction: Mankato, Minn.

Agency: Mankato Public Works, Minnesota Pollution Control Agency

Companies involved: San Jose, Calif.-based Calpine Corp.; Kansas City, Mo.-based Black & Veatch

Date completed: Summer 2006

Cost: $20 million

Mankato, Minn., recently constructed a water reclamation facility (WRF) to treat effluent from its wastewater treatment plant (WWTP) for use at a nearby privately owned electric power generation plant. The project is the first in the state to treat municipal wastewater for industrial use, and it is helping improve the water quality in the Lower Minnesota River as well as preserve the city's drinking water supply.

Mankato officials began investigating additional wastewater treatment methods when the state began calling for tighter restrictions on phosphorus in wastewater effluent discharges to the Minnesota River a few years ago. The city had upgraded its 7-million-gallon-per-day (mgd) plant in 2000 to improve effluent quality, but the new restrictions would require further adjustments.

Soon after, Calpine Corp., an independent power producer that focuses on clean natural gas and geothermal electricity generation, began planning construction of the Mankato Energy Center (MEC), which initially would produce approximately 300 megawatts of electricity with an option to double in size. The MEC needed 3.1 mgd of water in the first phase and 6.2 mgd at full capacity to meet its cooling needs. At first, company officials considered drawing water from the Minnesota River or the Mankato aquifer, but then, the city offered its WWTP effluent as a water source. Calpine agreed to build a WRF that would turn the effluent into water suitable for use in the power plant and meet the state's impending phosphorus restrictions.

The company paid for the design and construction of the $20 million, two-stage tertiary treatment facility, employing Black and Veatch for the design, plans and specifications at the city's request. The first stage removes phosphorus to a concentration well below the state requirements, and the second stage filters and chlorinates the water to a level suitable for cooling and process use at the MEC. The WRF's capacity is enough for the MEC supply needs of up to 6.2 mgd, and the phosphorus removal system was built for the city's current and future needs up to 12 mgd.

The reclaimed water is cycled up to four times through the MEC's cooling tower before it is sent back to the WWTP, where it is combined with the city's effluent and discharged in the Minnesota River. As a result, Mankato has reduced the amount of phosphorus discharged into the Minnesota River, and nearly 679 million gallons of potable water per year will not be withdrawn from the aquifer, saving it for municipal needs. If Calpine builds out to full expansion, the amount of water saved will double — greater than 25 billion gallons of water.

Mankato will own, operate and maintain the WRF, charging MEC for the water when the city's portion of the facility is paid off or in 20 years. By transferring ownership of the facility, Calpine saves an anticipated $500,000 per year in operations and maintenance costs. It also does not have to pay an estimated $1.5 million per year for drinking water. The project earned the 2007 Project of the Year Award in the Environment category (for projects greater than $10 million) from the Minnesota chapter of the American Public Works Association.