Upgrading and Saving
Award-winning water solution offers reciprocal benefits for Iowa Water District
The Clear Lake Sanitary District (CLSD) in northern Iowa received an award of merit at the Water Reuse Symposium in September 2006 for an innovative direct reuse application, the first of its kind in Iowa. Kevin Moler, superintendent of the facility, called it a nice pat on the back, but more than that, he was pleased with the economic and environmental benefits.
“We had just completed a $23 million upgrade,” Moler recalls. “We spent a lot of money on the renovation so we’d have a state-of-the-art facility. Typically, that kind of updating places a financial burden on the taxpayers, but this had a happy ending.”
Upgrading the CLSD Facility
The CLSD was formed in 1950 to address the city’s failing septic systems. In addition to an increasing permanent population, the 3,600-acre lake had become a popular recreational destination, and the city’s population often more than doubles during the summer months, placing additional strain on the septic systems and contaminating the lake. When the CLSD was formed, a network of piping was installed to collect wastewater from the surrounding communities and convey it to a new trickling filter plant.
At the time, the plant achieved effluent objectives of 20 milligrams per liter biological oxygen demand (BOD5) and 30 milligrams per liter total suspended solids (TSS). When other areas were annexed between 1972 and 1986, the trickling filter system struggled to carry the load. Combined with the high amount of inflow and infiltration due to the high water table and additional pipe to convey wastewater around the lake’s perimeter, the system had to be supplemented with bypasses. Nevertheless, permit violations occurred with increasing frequency. In 1988 the Iowa Department of Natural Resources (DNR) issued an administrative order and threatened to initiate a lawsuit.
According to a report penned by Moler and Mark Drake, project engineer for Aqua-Aerobic Systems Inc., the CLSD Board determined that repairing the leaky piping system was too expensive. In order to guarantee reliable treatment without bypasses and to allow for future additional flows, a complete system overhaul was necessary. Basing its decision on the need to handle seasonal variations in organic loads and process a high amount of inflow and infiltration, the CLSD Board chose to upgrade the plant so it could utilize sequencing batch reactor technology with the AquaSBR system created by Aqua-Aerobic Systems Inc.
Work began in 1995 and included construction of a new four-basin AquaSBR secondary treatment system, installation of two storm flow equalization basins providing retention and storage of up to 8 million gallons, upgrading eight lift stations, directional-drilling of a 6,020-foot, 18-inch-diameter force main pipe 30 feet below the bottom of Clear Lake, and 8.5 miles of new force mains.
Pump capacity is 18 million gallons per day, with an additional pumping capacity of 8 million gallons per day to the 3-million-gallon storage retention basin. A 5-million-gallon storm retention basin has a gravity fill rate of 5 million gallons per day. The combined 8-million-gallon storage capacity protects the treatment system from being hydraulically overloaded by allowing diversion of flow. It also provides a total process capacity of 19 million gallons per day.
The four-basin SBR system was retrofitted to the existing trickling filter basins, creating a system that can manage an average daily dry-weather flow capacity of 5.7 million gallons per day during dry weather (11 million gallons per day during wet weather) and a peak flow capacity of 8.2 million gallons per day. Current dry-weather flow averages 2.1 million gallons per day.
Wastewater flows through a screening and grit-removal system prior to entering the SBR system, which operates on a fill-and-draw principle, with two reactors filling simultaneously. At the end of a pre-set duration, flow is transitioned to the other two reactors. All steps, including clarification and supernatant withdrawal, occur within the four reactor basins without need for separate anoxic/anaerobic reactors or clarifiers.
Influent water undergoes two fill phases in the AquaSBR reactor. First, a complete mix of the contents is achieved by a mechanical down-draft mixer without the use of aeration. Wastewater flows from the grit removal system and is mixed to promote an anoxic environment to allow for sludge conditioning and the growth of phosphorus-removing bacteria. This mixed-fill phase helps control filamentous organisms and assists biomass conditioning.
The second phase is known as the react phase. At this point, the flow continues to enter under mixed and aerated conditions for enhanced oxygenation. Aeration is intermittent to provide alternating aerobic and anoxic conditions to achieve complete nitrification and denitrification within each cycle.
Non-fill phases include react, when flow is terminated while mixing and aeration continues to complete the nitrification/denitrification process; settle, after mixing and aeration end and solids and liquids separate, with sludge solids settling to the bottom of the tank; decant/sludge waste, when treated effluent is removed by means of subsurface withdrawal; and sludge wasting, when accumulated settled solids are aerobically digested, stored, and applied as fertilizer and soil conditioner in agricultural applications. Moler notes that most reuse water is funneled to agricultural applications such as golf courses and irrigation or is used for industrial purposes. Only a few facilities are using wastewater effluent for cooling purposes.
Once the two phases are complete, the treated water is returned and siphoned into a mile-and-a-half-long private drainage ditch that flows first into Beaver Dam Creek, then the Cedar River, and finally the Mississippi River. Moler stresses that water is never discharged into Clear Lake, due to its small watershed.
Contracting for Mutual Benefit
Alliant Energy, a power company serving much of Iowa and parts of Illinois, Wisconsin, and Minnesota, built a new 560-MW Interstate Power and Light (IPL) natural gas–fired power plant near Clear Lake in 2002. Chosen because it was situated between gas transmission lines, which would enable Alliant to get gas at a good price, the site was not near any major surface water. As Moler explains, IPL requires a lot of clean water for its cooling towers and for generating electricity. “The Jordan Aquifer runs through north-central Iowa and southern Minnesota; that supplies some water, but the Iowa [DNR] code says they can’t use more than 2,000 gallons per minute. IPL’s peak need is about 3,400 gallons per minute, leaving a supplemental need of 1,400 gallons per minute.”
Moler says Alliant contacted the CLSD in 2002 to begin discussions about reusing the effluent from the facility to fulfill its requirements for cooling tower water. It seemed like an easy equation: The average daily effluent flow from the CLSD facility amounted to about 2 million gallons per day, or 1,400 gallons per minute—just what IPL needed. But, despite the recent renovation of the CLSD’s system, it wasn’t sufficient to meet the requirements.
The two parties negotiated a mutually beneficial arrangement. As part of the 25-year deal signed in 2003, Moler says, “We had to build a tertiary system and 6 miles of transmission line to the power plant.” Alliant paid for everything: attorney’s fees to draw up the contract, engineering fees to design the system, and building, operational, and maintenance costs of the system, which includes a 210,000-gallon effluent equalization basin and a new tertiary treatment building that houses three six-disc AquaDisk cloth media filters and an ultraviolet disinfection system. “It’s their plant; they own it and retain exclusive use of it. It just happens to be on our property. They pay us to operate and maintain the equipment in order to get the necessary water quality. They also pay us to receive and discharge the used water.”
Not only is the CLSD prohibited from using the tertiary plant for any other customers, but the facility also granted Alliant first right of refusal for any additional effluent. Alliant has a contractual right to take up to 3 million gallons per day, which Moler says is more than it even produces at this time.
Because IPL has access to the CLSD’s capacity—a certain amount is set aside for IPL’s use, Moler explains —figured into the fee charged to the power plant is an incremental amount of debt reduction to cover the cost of the improvements the treatment plant had already implemented. “There’s definitely an economic impact on the community, between reducing the tax burden and adding jobs. There’s more work involved with IPL; we had to staff up.
“It’s also nice to see the ecological benefits,” he continues. “We have the ability to reuse water and the technology to clean our environment.” The Clean Water Act was passed in 1972 to regulate the wastewater industry in an attempt to improve water quality in lakes and rivers. Moler says advances in technology have combined to enhance the gains made in water quality. “The technology is here. We can treat water to the degree where it’s better than the water taken from the ground.”
The Tertiary System
Because Iowa had no existing reuse water criteria, the CLSD and Alliant worked with the Iowa DNR to establish a protocol that would determine the required additional treatment processes. The newly established criteria required the addition of tertiary filtration and disinfection of the effluent. Because of its record of success, the preferred tertiary filter technology is cloth media, which filters the secondary effluent through a pile cloth media supported on vertical disk segments.
Three six-disk AquaDisk cloth media filters provide a total peak flow capacity of 9.4 million gallons per day. As Moler reports, to equalize the batch discharges from the SBR system, a new 210,000-gallon post-equalization basin was installed ahead of the tertiary filter system. Exercising forethought about potential future disinfection requirements and to minimize chlorine requirements at IPL, the CLSD implemented ultraviolet (UV) disinfection to follow the filtration step. The system operates with automatic UV bulb intensity adjustment based on UV transmittance measurement, allowing the CLSD to minimize energy consumption without compromising the level of disinfection. The cloth media improves the effectiveness and reliability of the UV disinfection system by maintaining high-quality TSS effluent with minimized particle sizes.
The tertiary system went online in January 2004 and began supplying cooling water to IPL soon after.
Disposal
In addition to supplying water, the contract calls for the CLSD to dispose of the reject water from the cooling towers. Moler estimates that 50% to 80% of the supply water to the cooling towers is evaporated in the process, but that still leaves 680 gallons per minute to be disposed of. Because so much of the water evaporates, the concentration of minerals in what remains is doubled. IPL adds chlorine to prevent bio-growth within the cooling towers, so the reject water is also high in chlorine, as well as total dissolved solids and turbidity.
Moler explains that Alliant would have had to secure federal permits to discharge the water itself, but since the CLSD already had everything in order, it decided to send the water back for disposal. Therefore, a second pipe was installed. Fortunately, the right of way to connect the two major natural gas pipelines to the power plant ran within less than a mile from the CLSD facility, so running the reuse water piping the 6 miles between the treatment facility and power plant didn’t pose an issue.
Although Moler says the returned water is of “direct discharge quality” and that no treatment is required (although sodium bisulfate is added for dechlorination), it still requires inline monitoring. Inline probes monitor the cooling water supply, cooling water return, and plant effluent for chlorine, conductivity, turbidity, pH, temperature, and line pressure. If values exceed permit limits, the system automatically diverts the return flow to the storm flow equalization basin.
To assist with monitoring, Moler uses Supervisory, Control, and Acquisition Data Analysis (SCADA) software systems to monitor and control plant treatment and pump station equipment. “It allows us to monitor and control remotely from small computer systems—even laptops. We can make adjustments if the concentration gets too high, closing or opening valves so we don’t discharge and violate our permits.” The system also monitors the anaerobic process, turning on or shutting off an air pump according to oxygen meter readings.
With an alarm system alerting on-call personnel of an emergency situation, SCADA cuts labor costs, allowing CLSD employees to work a 7:30 to 4:30 Monday through Friday and half-day Saturday schedule. Other facilities may require around-the-clock personnel, but “we can program, reconfigure and reset controls from home,” Moler claims.
Challenges and Benefits
According to Moler, the arrangement has been mutually beneficial and has run without incident. “IPL was built to be a peaking plant. It wasn’t supposed to run all the time. But they’re finding, because of natural gas prices, it offers more benefits, so they run more—although still not all the time.”
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Despite running smoothly, a challenge arose in 2005. The water in the Jordan Aquifer contains high amounts of dissolved iron; when the water taken from it for the cooling towers evaporates, the iron concentration increases. Because travel velocities along the 6-mile piping are sometimes low, significant amounts began to accumulate, leading to high TSS levels in the cooling water return when velocities exceeded 3 feet per second. Rather than rerouting and retreating the water, the CLSD opted to modify the piping and valving options for the cooling water return and filter influent. An option of completely segregating one of the six-disk filters to directly filter only the cooling water return was added. This removes the iron-related TSS without recycling the cooling water return through the SBR reactors. It means the two remaining filters are loaded near their peak flow capacity, but Moler isn’t unduly concerned.
He reports that the AquaSBR and AquaDisk filter system consistently achieves BOD5 and TSS levels of less than 5.0 milligrams per liter and effluent ammonia-nitrogen and nitrate-nitrogen values of less than 0.1 and 3.0 milligrams per liter, respectively. With a contract extending until 2028, Moler expects the reuse water volume available to IPL to increase as communities continue to grow and the strain on the Jordan Aquifer is reduced. While it was “business as usual without the power plant,” Moler is pleased with the situation, noting that “it’s a good price for them and it helps us.” A happy ending for all.
March-April 2007
Upgrading and Saving
Award-winning water solution offers reciprocal benefits for Iowa Water District
The Clear Lake Sanitary District (CLSD) in northern Iowa received an award of merit at the Water Reuse Symposium in September 2006 for an innovative direct reuse application, the first of its kind in Iowa. Kevin Moler, superintendent of the facility, called it a nice pat on the back, but more than that, he was pleased with the economic and environmental benefits.
“We had just completed a $23 million upgrade,” Moler recalls. “We spent a lot of money on the renovation so we’d have a state-of-the-art facility. Typically, that kind of updating places a financial burden on the taxpayers, but this had a happy ending.”
Upgrading the CLSD Facility
The CLSD was formed in 1950 to address the city’s failing septic systems. In addition to an increasing permanent population, the 3,600-acre lake had become a popular recreational destination, and the city’s population often more than doubles during the summer months, placing additional strain on the septic systems and contaminating the lake. When the CLSD was formed, a network of piping was installed to collect wastewater from the surrounding communities and convey it to a new trickling filter plant.
At the time, the plant achieved effluent objectives of 20 milligrams per liter biological oxygen demand (BOD5) and 30 milligrams per liter total suspended solids (TSS). When other areas were annexed between 1972 and 1986, the trickling filter system struggled to carry the load. Combined with the high amount of inflow and infiltration due to the high water table and additional pipe to convey wastewater around the lake’s perimeter, the system had to be supplemented with bypasses. Nevertheless, permit violations occurred with increasing frequency. In 1988 the Iowa Department of Natural Resources (DNR) issued an administrative order and threatened to initiate a lawsuit.
According to a report penned by Moler and Mark Drake, project engineer for Aqua-Aerobic Systems Inc., the CLSD Board determined that repairing the leaky piping system was too expensive. In order to guarantee reliable treatment without bypasses and to allow for future additional flows, a complete system overhaul was necessary. Basing its decision on the need to handle seasonal variations in organic loads and process a high amount of inflow and infiltration, the CLSD Board chose to upgrade the plant so it could utilize sequencing batch reactor technology with the AquaSBR system created by Aqua-Aerobic Systems Inc.
Work began in 1995 and included construction of a new four-basin AquaSBR secondary treatment system, installation of two storm flow equalization basins providing retention and storage of up to 8 million gallons, upgrading eight lift stations, directional-drilling of a 6,020-foot, 18-inch-diameter force main pipe 30 feet below the bottom of Clear Lake, and 8.5 miles of new force mains.
Pump capacity is 18 million gallons per day, with an additional pumping capacity of 8 million gallons per day to the 3-million-gallon storage retention basin. A 5-million-gallon storm retention basin has a gravity fill rate of 5 million gallons per day. The combined 8-million-gallon storage capacity protects the treatment system from being hydraulically overloaded by allowing diversion of flow. It also provides a total process capacity of 19 million gallons per day.
The four-basin SBR system was retrofitted to the existing trickling filter basins, creating a system that can manage an average daily dry-weather flow capacity of 5.7 million gallons per day during dry weather (11 million gallons per day during wet weather) and a peak flow capacity of 8.2 million gallons per day. Current dry-weather flow averages 2.1 million gallons per day.
Wastewater flows through a screening and grit-removal system prior to entering the SBR system, which operates on a fill-and-draw principle, with two reactors filling simultaneously. At the end of a pre-set duration, flow is transitioned to the other two reactors. All steps, including clarification and supernatant withdrawal, occur within the four reactor basins without need for separate anoxic/anaerobic reactors or clarifiers.
Influent water undergoes two fill phases in the AquaSBR reactor. First, a complete mix of the contents is achieved by a mechanical down-draft mixer without the use of aeration. Wastewater flows from the grit removal system and is mixed to promote an anoxic environment to allow for sludge conditioning and the growth of phosphorus-removing bacteria. This mixed-fill phase helps control filamentous organisms and assists biomass conditioning.
The second phase is known as the react phase. At this point, the flow continues to enter under mixed and aerated conditions for enhanced oxygenation. Aeration is intermittent to provide alternating aerobic and anoxic conditions to achieve complete nitrification and denitrification within each cycle.
Non-fill phases include react, when flow is terminated while mixing and aeration continues to complete the nitrification/denitrification process; settle, after mixing and aeration end and solids and liquids separate, with sludge solids settling to the bottom of the tank; decant/sludge waste, when treated effluent is removed by means of subsurface withdrawal; and sludge wasting, when accumulated settled solids are aerobically digested, stored, and applied as fertilizer and soil conditioner in agricultural applications. Moler notes that most reuse water is funneled to agricultural applications such as golf courses and irrigation or is used for industrial purposes. Only a few facilities are using wastewater effluent for cooling purposes.
Once the two phases are complete, the treated water is returned and siphoned into a mile-and-a-half-long private drainage ditch that flows first into Beaver Dam Creek, then the Cedar River, and finally the Mississippi River. Moler stresses that water is never discharged into Clear Lake, due to its small watershed.
Contracting for Mutual Benefit
Alliant Energy, a power company serving much of Iowa and parts of Illinois, Wisconsin, and Minnesota, built a new 560-MW Interstate Power and Light (IPL) natural gas–fired power plant near Clear Lake in 2002. Chosen because it was situated between gas transmission lines, which would enable Alliant to get gas at a good price, the site was not near any major surface water. As Moler explains, IPL requires a lot of clean water for its cooling towers and for generating electricity. “The Jordan Aquifer runs through north-central Iowa and southern Minnesota; that supplies some water, but the Iowa [DNR] code says they can’t use more than 2,000 gallons per minute. IPL’s peak need is about 3,400 gallons per minute, leaving a supplemental need of 1,400 gallons per minute.”
Moler says Alliant contacted the CLSD in 2002 to begin discussions about reusing the effluent from the facility to fulfill its requirements for cooling tower water. It seemed like an easy equation: The average daily effluent flow from the CLSD facility amounted to about 2 million gallons per day, or 1,400 gallons per minute—just what IPL needed. But, despite the recent renovation of the CLSD’s system, it wasn’t sufficient to meet the requirements.
The two parties negotiated a mutually beneficial arrangement. As part of the 25-year deal signed in 2003, Moler says, “We had to build a tertiary system and 6 miles of transmission line to the power plant.” Alliant paid for everything: attorney’s fees to draw up the contract, engineering fees to design the system, and building, operational, and maintenance costs of the system, which includes a 210,000-gallon effluent equalization basin and a new tertiary treatment building that houses three six-disc AquaDisk cloth media filters and an ultraviolet disinfection system. “It’s their plant; they own it and retain exclusive use of it. It just happens to be on our property. They pay us to operate and maintain the equipment in order to get the necessary water quality. They also pay us to receive and discharge the used water.”
Not only is the CLSD prohibited from using the tertiary plant for any other customers, but the facility also granted Alliant first right of refusal for any additional effluent. Alliant has a contractual right to take up to 3 million gallons per day, which Moler says is more than it even produces at this time.
Because IPL has access to the CLSD’s capacity—a certain amount is set aside for IPL’s use, Moler explains —figured into the fee charged to the power plant is an incremental amount of debt reduction to cover the cost of the improvements the treatment plant had already implemented. “There’s definitely an economic impact on the community, between reducing the tax burden and adding jobs. There’s more work involved with IPL; we had to staff up.
“It’s also nice to see the ecological benefits,” he continues. “We have the ability to reuse water and the technology to clean our environment.” The Clean Water Act was passed in 1972 to regulate the wastewater industry in an attempt to improve water quality in lakes and rivers. Moler says advances in technology have combined to enhance the gains made in water quality. “The technology is here. We can treat water to the degree where it’s better than the water taken from the ground.”
The Tertiary System
Because Iowa had no existing reuse water criteria, the CLSD and Alliant worked with the Iowa DNR to establish a protocol that would determine the required additional treatment processes. The newly established criteria required the addition of tertiary filtration and disinfection of the effluent. Because of its record of success, the preferred tertiary filter technology is cloth media, which filters the secondary effluent through a pile cloth media supported on vertical disk segments.
Three six-disk AquaDisk cloth media filters provide a total peak flow capacity of 9.4 million gallons per day. As Moler reports, to equalize the batch discharges from the SBR system, a new 210,000-gallon post-equalization basin was installed ahead of the tertiary filter system. Exercising forethought about potential future disinfection requirements and to minimize chlorine requirements at IPL, the CLSD implemented ultraviolet (UV) disinfection to follow the filtration step. The system operates with automatic UV bulb intensity adjustment based on UV transmittance measurement, allowing the CLSD to minimize energy consumption without compromising the level of disinfection. The cloth media improves the effectiveness and reliability of the UV disinfection system by maintaining high-quality TSS effluent with minimized particle sizes.
The tertiary system went online in January 2004 and began supplying cooling water to IPL soon after.
Disposal
In addition to supplying water, the contract calls for the CLSD to dispose of the reject water from the cooling towers. Moler estimates that 50% to 80% of the supply water to the cooling towers is evaporated in the process, but that still leaves 680 gallons per minute to be disposed of. Because so much of the water evaporates, the concentration of minerals in what remains is doubled. IPL adds chlorine to prevent bio-growth within the cooling towers, so the reject water is also high in chlorine, as well as total dissolved solids and turbidity.
Moler explains that Alliant would have had to secure federal permits to discharge the water itself, but since the CLSD already had everything in order, it decided to send the water back for disposal. Therefore, a second pipe was installed. Fortunately, the right of way to connect the two major natural gas pipelines to the power plant ran within less than a mile from the CLSD facility, so running the reuse water piping the 6 miles between the treatment facility and power plant didn’t pose an issue.
Although Moler says the returned water is of “direct discharge quality” and that no treatment is required (although sodium bisulfate is added for dechlorination), it still requires inline monitoring. Inline probes monitor the cooling water supply, cooling water return, and plant effluent for chlorine, conductivity, turbidity, pH, temperature, and line pressure. If values exceed permit limits, the system automatically diverts the return flow to the storm flow equalization basin.
To assist with monitoring, Moler uses Supervisory, Control, and Acquisition Data Analysis (SCADA) software systems to monitor and control plant treatment and pump station equipment. “It allows us to monitor and control remotely from small computer systems—even laptops. We can make adjustments if the concentration gets too high, closing or opening valves so we don’t discharge and violate our permits.” The system also monitors the anaerobic process, turning on or shutting off an air pump according to oxygen meter readings.
With an alarm system alerting on-call personnel of an emergency situation, SCADA cuts labor costs, allowing CLSD employees to work a 7:30 to 4:30 Monday through Friday and half-day Saturday schedule. Other facilities may require around-the-clock personnel, but “we can program, reconfigure and reset controls from home,” Moler claims.
Challenges and Benefits
According to Moler, the arrangement has been mutually beneficial and has run without incident. “IPL was built to be a peaking plant. It wasn’t supposed to run all the time. But they’re finding, because of natural gas prices, it offers more benefits, so they run more—although still not all the time.”
Despite running smoothly, a challenge arose in 2005. The water in the Jordan Aquifer contains high amounts of dissolved iron; when the water taken from it for the cooling towers evaporates, the iron concentration increases. Because travel velocities along the 6-mile piping are sometimes low, significant amounts began to accumulate, leading to high TSS levels in the cooling water return when velocities exceeded 3 feet per second. Rather than rerouting and retreating the water, the CLSD opted to modify the piping and valving options for the cooling water return and filter influent. An option of completely segregating one of the six-disk filters to directly filter only the cooling water return was added. This removes the iron-related TSS without recycling the cooling water return through the SBR reactors. It means the two remaining filters are loaded near their peak flow capacity, but Moler isn’t unduly concerned.
He reports that the AquaSBR and AquaDisk filter system consistently achieves BOD5 and TSS levels of less than 5.0 milligrams per liter and effluent ammonia-nitrogen and nitrate-nitrogen values of less than 0.1 and 3.0 milligrams per liter, respectively. With a contract extending until 2028, Moler expects the reuse water volume available to IPL to increase as communities continue to grow and the strain on the Jordan Aquifer is reduced. While it was “business as usual without the power plant,” Moler is pleased with the situation, noting that “it’s a good price for them and it helps us.” A happy ending for all.