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.