A water treatment system in Mankato, MN, saves the city nearly 700 million gallons of water annuallyand a power plant owner millions.
A three-way public-private partnership recently resulted in the development and utilization of a new water reclamation facility (WRF) in Mankato that treats water from the Minnesota River for reuse in a natural gas-fired turbine at the new Mankato Energy Center—a project that is estimated to save the city 679 million gallons of water, and $1.5 million in potable water costs, a year. Additionally, the city—which otherwise would have needed to build the stage-one facilities to comply with new phosphorus limits within a few years—saved capital costs of about $10 million. The project owner’s savings were an estimated $500,000 per year, through the duration of a 20-year power supply agreement.
The facility—which was realized from a partnership among the City of Mankato; the owner, Calpine Corp., San Jose, CA; and the Minnesota Pollution Control Agency (MPCA)—began operation in April 2006 and was awarded 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 for an environmentally and economically beneficial WRF. Mankato also earned a 2006 Governor’s MnGREAT (Minnesota Government Reaching Environmental Achievements Together) Award for the project.
The new, $20 million, 12-million-gallon WRF is the first of its kind in Minnesota, and one of the first in the nation. It treats effluent from the Mankato Wastewater Treatment Plant (WWTP) to meet specific water reuse criteria before it is conveyed by pipeline to Calpine’s Mankato Energy Center for use in the center’s cooling tower. After the cooling causes 75% of the treated water to evaporate during cooling, the remaining 25% is sent back to the WWTP, where it is mixed with effluent before being discharged back into the river. Without this system, the city would have faced supplying water from its local surface and groundwater supplies to the turbine in order to accommodate the wishes of Calpine—a for-profit power producer—to establish an operation in Mankato, as well as a greater presence in the Midwest.
A tight project deadline and a state mandate to clean up the Minnesota River were challenges that the city, owner, and MPCA faced on this project. The project partners each played a role in overcoming them. The city owns and operates the WRF, Calpine paid for design and construction of the facility, and the MPCA expedited the city’s National Pollutant Discharge Elimination System permit renewal and plan approval. Two other project principals, consulting engineer Black and Veatch, in Kansas City, MO, and water processing solutions provider I Kruger Inc., in Cary, NC, played integral roles in planning and constructing a two-stage tertiary treatment system for the WRF.
Project on a Fast Track
In March 2004, Calpine entered into a 20-year agreement with Minneapolis, MN-based Xcel Energy to provide Xcel with 375 MW of power, with an option to double in size, for a growing customer base in the Upper Midwest. Under the contract, Calpine would build, own, and operate a new electric power plant in Mankato to operate about 60% of the year, producing electricity as demand dictates. Soon afterward, Calpine began planning construction of its Mankato Energy Center and initially considered drawing water directly from the Minnesota River, or from the city’s aquifer.
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| Photo: City of Mankato |
| The city of Mankato received a 2007 Project of the Year Award for this water reclamation facility, which produces high-quality water for reuse in the cooling tower of a natural gas-fired turbine. |
But Mankato was a prime candidate to supply the new power plant with reused water, says Mary Fralish, deputy director of public works environmental for the City of Mankato. “We were in a good position because we had just done an expansion on our treatment plant,” she points out. “We had upgraded and expanded it, and our water quality leaving the plant at that time was excellent.”
State regulations helped to convince Calpine—which sought to get the plant online to begin generating a revenue stream and recoup its capital costs as soon as possible—that water reuse made sense in this case. The state of Minnesota had recently instituted new water quality requirements that limit cities along the Minnesota River, including Mankato, to a stringent 1-milligram-per-liter total phosphorus limit by 2015 to prevent algal blooms and subsequent pollution problems. George Rosati, now-retired director of public works for Mankato, raised the possibility of reusing water from the WWTP during a planning meeting with Calpine.
“They were looking at their water needs for this facility and trying to locate it, and the only options at the time really were to take it out of the Minnesota River or use groundwater,” notes Fralish. “Well, part of our drinking water comes from the groundwater; they would have needed an awful lot of water, and the city would have protested if they would have taken it out of our aquifer. To take it out of the Minnesota River also has some complications with permits and regulations, and it would have changed the temperature a little bit on their discharge. So, they were averse to doing that due to time constraints—they were in a hurry to get this plant online, so they could start to produce electricity and recoup some of their capital.
“I think George [Rosati] had heard little tidbits around the country that people are looking at water reuse,” she continues. “He’s a very innovative person and always liked to try something new. Because we had the good water quality to begin with, it wasn’t as expensive for them to take that water and further treat it in order to satisfy their needs.”
Despite the fact that the city’s WWTP was providing good building blocks for water reuse, the state did not have an actual water reuse standard. The project partners decided that California’s Title 22 Health Laws for recycled water would serve as the water quality standard for effluent to be reused and sent to the power plant’s cooling tower.
“In the state of Minnesota, our controlling agency [the MPCA] didn’t have any regulations on reuse of water, so we turned to California, which has done some work on it,” recalls Fralish. “We could have protested it because there weren’t any regulations, but Calpine said ‘no, go ahead and pay for that portion of it—we want to do it.’ We were really comfortable with that, and it made everyone feel better that there weren’t any questions out there about the quality of the water that we were going to use.”
The city had already responded to the state’s mandate to clean up the Minnesota River. Even before the WRF was constructed, the city’s WWTP was providing a total phosphorus level averaging 1.0–1.5 milligrams per liter. Though turbidity had never been recorded or permitted, the city’s Public Works Department reported that the city’s water typically was unofficially measured at 4–5 nephelometric turbidity units (NTU). The WRF permit would require an NTU threshold of less than 2 NTUs. Biochemical oxygen demand (BOD) in the city’s water averaged about 4–5 milligrams per liter.
This was the water that flows into the new WRF, which was designed to meet the power plant’s cooling water needs of 3.1 million gallons per day (mgd) in the first phase and 6.2 mgd at full capacity. There it undergoes further purification via a two-stage tertiary treatment system.
Construction of the WRF was said not to present any out-of-the-ordinary challenges, aside from the fast track nature of the project. The project broke ground in spring 2005, and the WRF went online in about 12 months. “One of the challenges was that [Calpine was] in such a hurry to get everything done, and our regulatory agency, the MPCA, was a partner in this because they saw a real opportunity for reusing wastewater,” says Fralish. “So they did what they could to facilitate this process by speeding up the permitting—they had to change the city’s discharge permit as well.”
Nathen Myers, drinking water product manager for I Kruger Inc.—which provided equipment to the WRF—adds that the project partners showed a high level of cooperation during planning and construction. “Coordination was a big key,” says Myers. “Once the job was awarded to a vendor, it was not over with—there was a certain amount of design that needed to be finalized. There was a lot of coordination that needed to occur between the installation contractor, the design engineer, the vendor, and, also, the city. People were taking time out of busy schedules and making this priority number one. As long as you’ve got a team that is willing to learn and take time out of its schedule in order to address unknowns, you can meet something that has a very tight schedule.”
Two-Stage Process
The two-stage treatment process uses a combination of coagulation, flocculation, and sedimentation in the first stage, and filtration and chlorination in the second stage. The first stage is designed to provide phosphorus removal for all of the WWTP’s current and future needs. The second stage provides additional filtration and chlorination to meet reuse requirements established by MPCA, based on California’s Title 22 Health Laws for recycled water, which mainly focus upon turbidity, total coliform bacteria, and chlorine residual, according to the city.
Stage one utilizes Kruger’s Actiflo process, which uses a high-rate ballasted clarification process often used in communities that have combined sewer-overflow systems and face the resulting water purity challenges to local water supplies, as well as ones whose drinking water requires treatment. The process combines microsand-enhanced flocculation and lamellar tube settling. The microsand serves as a flocculation aid and ballasting agent. The process consists of a series of consecutive steps: coagulation, microsand, and polymer injection; floc maturation; settling; and sand recirculation. “We add microsand, and that microsand is a lot heavier versus the floc that forms,” notes Myers. “The polymer binds the floc to the sand, and then the sand settles extremely fast, so you’re able to remove a majority of all the particles coming in. And because the sand settles so fast, you can actually get the same amount of treatment in a smaller footprint.”
He adds that the lamellar tubes serve as a settling barrier for smaller particles, which flow into the tubes after a weir. “You’ll get fine particles, and they’re really small,” explains Myers. “They had a really hard time getting coagulated, and those tubes are on an incline, so when the water travels up it must go through those tubes before it is sent out in the effluent. Those tubes act like a settling barrier, so any of those fine particles, once they hit the lamellar tubes on an incline, fall back down and settle out.”
Remaining particles are then removed from the effluent via filtration. The effluent flows through a Hydrotech Filter that is approved by the California Department of Health and Services for Title 22 applications and employs polyester as the filter media. The filter’s media is a woven material with absolute pore sizes held within an ABS plastic frame and arranged in a disc configuration. Myers reports that particles 10 microns or larger do not pass the Mankato WRF’s filter.
Myers explains how the entire filtration system works. The effluent flows by gravity into the filter panel segments from a center drum. Filter panels mounted on the two sides of the disc segments separate solids from the water. The filter discs retain the solids, while the clean water flows to the outside of the discs and into the collection tank. The discs remain static until the water level in the inlet channels rises to a set point. Filtered effluent is pumped to a spray header, and nozzles then automatically backwash the filter. The final treatment process is chlorination, which neutralizes remaining biological matter, such as total coliform bacteria and fecal coliform bacteria not removed by the other mechanical and chemical processes.
After treatment at the WRF, the effluent is transported about a mile and a half via pipeline to the Mankato Energy Center. “The water that goes to the power plant has gone through the entire wastewater plant, then through the water reclamation facility and to the power plant, and they evaporate about 75% as part of their process,” which may consist of the cooling water cycling through the cooling tower up to four times, says Fralish. “Then, the remaining 25%—which is extremely clean water—comes back and is discharged with our final effluent. When it comes back, it doesn’t get treated again; it just goes out our outfall mixed with the rest of our effluent.”
Fralish reiterates that the reused water is very clean industrial water. So clean, in fact, that she says those attending public tours of the WRF cannot tell the difference between the reused water and
the city’s drinking water.
Testing bears out the effectiveness of the WRF. Since it began operation in April 2006, BOD has dropped from 4–5 milligrams per liter to undetectable levels, and total phosphorus levels have dropped from 1–1.5 milligrams per liter to 0.35 milligrams per liter. The WRF-treated water is also providing about 0.6 NTU of turbidity, compared with the limit of less than 2 NTUs.
Fralish says that the WRF has allowed the city to sell phosphorus credits to seven other cities and two industries in its watershed district. “The state has been assessing all of the water bodies for impairments, then they’re setting the total maximum daily load of phosphorus, and then it divides that up among point sources, nonpoint sources and whoever’s contributing to the daily load,” she says. “They did this one for the lower Minnesota, took all of the cities that were contributing, and figured it had to be down to one milligram per liter.”
Fralish adds that the awards that the project received have generated interest among other communities in the state. “It was nice to get the recognition,” she says. “A lot of wastewater facilities in the state were constructed in the 1970s. I think they all saw that they have a commodity they could be selling or taking advantage of.”