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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.
The dual-purpose facility provides two stages of tertiary treatment of the city's effluent.
The first stage provides phosphorus removal for all of the treatment plant's current and future needs — up to 12 mgd — to a concentration below the 1 mg/l requirement. To account for the reduced amount of flow the river will receive and the trace amounts of phosphorus that Calpine adds, the internal compliance point was set at 0.9 mg/l to ensure the impact to the river would be 1 mg/l or less.
The second stage provides additional filtration and chlorination to meet reuse requirements established by the Minnesota Pollution Control Agency based on Title 22 of the California Code of Regulations.
California's recycled water law was passed in 2001 to protect public health while providing for future water requirements. The highest level of treatment is “disinfected tertiary recycled water,” which is used for food crops where the water comes in contact with the edible portion of the crop; parks and playgrounds; school yards; residential landscaping; and unrestricted golf courses. The same level of treatment is required where the recycled water is used in industrial or commercial cooling systems that involve a cooling tower.
In Mankato, the reclaimed water is cycled up to four times through Calpine's cooling tower before being discharged back to the waste water treatment plant. After dechlorination, the combined effluent of the city and the electricity plant is discharged to the river through the city's outfall structure.
The city's discharge monitoring point for phosphorus is downstream of the phosphorus removal system and upstream of the second stage of the tertiary treatment system. This monitoring point was selected to ensure control of the total quantity of phosphorus being discharged to the river.
LEADERS IN POLLUTION PREVENTION
Using effluent rather than ground-water to cool Calpine's turbine keeps an estimated 7 million gallons/year of groundwater from being used for industrial purposes. The amount of water saved over the agreement's 20-year term is expected to be almost 14 billion gallons. If Calpine doubles its plant's capacity to 600 megawatts, the amount of water saved will be more than 25 billion gallons.
For new entities discharging into the Minnesota River, the current phosphorus discharge limit is zero. Because Mankato now has the capability to remove phosphorus to levels below its current limit, the Minnesota Pollution Control Agency is referring new dischargers to the city. Nutrient trading enables new dischargers to pay Mankato for the right to discharge phosphorus for which the city has a discharge permit but no longer releases into the river.
Although there are no regulations on normal effluent for turbidity, the plant normally discharged at a level of 4 to 5 NTUs. The current discharge is below the required 2 NTUs for water reuse, generally 0.6 to 1 NTU. The decrease in turbidity increases water transparency and provides a better aquatic habitat.
Treatment plant employees are completing a two-year process to become members of the National Bio-solids Partnership, a voluntary program sponsored by the EPA, the Water Environment Federation, and the National Association of Clean Water Agencies. Similar to an ISO certification, the program requires an external audit and documented citizen involvement.
They're also studying ways to further treat biosolids to a Class A product that can be used for soil amendment on parks, playgrounds, and golf courses as well as the current agricultural land application program.
“We are a river city and take pride in doing our part to clean up the Minnesota River,” says retired Public Works Director George Rosati, who spearheaded the water reclamation project. “Our overall commitment is to produce the best quality effluent possible in the most cost-effective manner achievable.”
— Derek Cambridge is a project manager in the Kansas City, Mo., office of Black & Veatch Corp.; Mary Fralish is deputy director of Public Works, Environmental Services for Mankato; and Chad Hill served as project director in the Minneapolis office of Black & Veatch.
Resource swap
Each party in the 20-year cooperative agreement between Mankato, Minn., and Calpine Corp. benefits from the other's contribution to meeting an overall goal.
Contributions
Calpine Corp.: $20 million to design and build a water reclamation facility at Mankato's existing wastewater treatment plant
Mankato: Operations & maintenance for 20 years at $500,000/year for a total of $20 million. (Costs will be calculated each year; when the total amounts equal $10 million the city will consider its portion paid and begin charging Calpine for water. If the city's portion is outstanding after 20 years, the bill will be considered paid and the city will begin charging Calpine.)
Minnesota Pollution Control Agency: Expedited Mankato's NPDES discharge elimination permit
Rewards
Calpine Corp.: An estimated $1.5 million/year savings on cooling water
Mankato: Elimination of $10 million in capital improvements to meet a 1 mg/l total phosphorus limit by the year 2015. Biochemical oxygen demand has dropped from 4.9 mg/l to below detection levels and total phosphorus levels have dropped to 0.35 mg/l.
Minnesota Pollution Control Agency: A framework for permitting other water reclamation facilities; phosphorus reduction of 52,000 pounds over four years.
