Water Reuse

Mankato, Minnesota

Mankato, Minnesota, 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 discahrges to the Minnesota River.  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 contruction 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 the city offered its WWTP 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.  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.

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 into the Minnesota River.  As a result, Mankato has reduced the amount of phosphorus discharged into the river and saved nearly 679 million gallons of potable water per year. 

Source:
American City & County. Cool Use for Wastewater.
http://americancityandcounty.com/water/treatment/cool_wastewater_wwtp_wrf/