Burbank Water and Power’s Magnolia Power Project saves up to two million gallons of potable water a day with its zero liquid discharge and advanced water treatment systems.
By Teresa Hansen, Associate Editor
Water conservation and environmental stewardship are nothing new for Burbank Water and Power (BWP), one of six owners of the Magnolia Power Project, as well as its operator. In 1967 BWP became one of the first utilities in the nation to use reclaimed water in place of potable water in its cooling towers. This approach expanded in 2001 from just cooling tower water to also include boiler water.
BWP extended its commitment to water conservation in summer 2005 when it dedicated the Magnolia Power Project. By using reclaimed water and recycling water through a zero liquid discharge (ZLD) system, Magnolia saves up to two million gallons of potable water each day that otherwise would be used to operate the steam and gas turbines and the cooling-water system.
BWP conceived the Magnolia Power Project in early 2000 after deciding it needed to repower or add generation. Because BWP’s need for 50 to 75 MW would be most economical if provided by a larger unit, BWP approached fellow members of the Southern California Public Power Authority with a proposal to build a larger project that would provide benefits of lower initial cost and higher operating efficiency. In addition to Burbank, five other cities-Anaheim, Cerritos, Colton, Glendale and Pasadena-own a share of the plant, ranging from 4 percent to 38 percent.
Magnolia Power Project is a 310-MW combined-cycle natural gas-fired plant that BWP claims is twice as efficient and 90 percent cleaner for the environment than the units it replaced. The project’s core is a GE 7FA gas turbine and a GE A14 HEAT (high efficiency advanced technology) steam turbine. Magnolia uses an Alstom triple pressure heat recovery steam generator.
Fred Fletcher, BWP Assistant General Manager, said BWP decided early on that the Magnolia Project would use only reclaimed water. In fact, Fletcher said the investigation into using reclaimed water for Magnolia’s boiler applications led to BWP’s 2001 decision to use reclaimed water for the boilers in the older plants on the same site with Magnolia.
The initial plan for Magnolia was to discharge cooling tower water into the local drainage channel that flows into the Los Angeles River and then into Santa Monica Bay. Clean water standards for such discharge, however, were (and still are) under review and were in flux during Magnolia’s permitting stage.
“It was not possible for the Regional Water Quality Control Board to define what standards we would have to meet in order to discharge our cooling tower blow down until such standards were defined,” Fletcher said. “Unfortunately they didn’t expect the standards to be defined for several years. Because the standards for such an NPDES (national pollutant discharge elimination system) water discharge permit were not known, the California Energy Commission could not determine if the project would be capable of meeting those standards, and the Commission therefore could not approve the project until the standards were known.”
Magnolia Power Project is a 310-MW combined-cycle natural gas-fired plant located in the heart of Burbank, Calif. Photo courtesy of BWP.
Without a discharge permit, BWP had to find another way to dispose of the cooling tower blow down or wait months or years to begin plant construction. Because the older Burbank plants no longer discharge to the river but to the Burbank sewer instead, all discharges to the river were eliminated for Burbank power plants, removing that as an option. In addition, Magnolia was unable to discharge its blow down to the Burbank sewers because the system is not large enough to handle the volume. Therefore, to keep the project on schedule, BWP management decided to equip Magnolia with a zero liquid discharge (ZLD) system.
Magnolia’s ZLD System
The major source of waste water from any power plant is the blow down from the cooling towers. The Magnolia Plant is designed to process 400 gallons per minute (gpm) of blow down water, so without the ZLD system either part or all of that amount, approximately 0.58 million gallons a day, would be wasted.
Magnolia’s ZLD system is designed to recover this waste and recycle it for use as makeup to the cooling tower itself or, if needed, after further polishing, even as makeup to the boiler. The ZLD system, provided by Aquatech International Corp., Canonsburg, Pa., processes the effluent and the cooling tower blow down waste, which is reclaimed water concentrated at least five times in the cooling tower.
The amount of blow down from a cooling tower depends on several factors, including the makeup water’s quality and the condenser units’ metallurgy; in general, the poorer the water quality, the higher the blow down rate. Because Magnolia uses reclaimed water as feed water, its ZLD system faces additional challenges, which are magnified when the water is cycled through the cooling tower. Magnolia’s blow down has high hardness, turbidity (caused by suspended solids) and high total dissolved solids including silica.
To clean the blow down, Magnolia’s ZLD requires three treatment “stages”-pretreatment, HERO (High-Efficiency Reverse Osmosis) and the Forced Circulation (FC) Crystallizer. The system’s pretreatment stage includes a lime-soda ash softening clarifier where the major job is to reduce hardness to about 100 parts per million (ppm). It also reduces the suspended solids. The media filters further reduce the suspended solids before the water is processed through the weak acid cation (WAC) units. The WAC units reduce hardness to less than 0.5 ppm.
The next stage, the HERO, is a membrane process that operates at a high concentration of dissolved solids, including silica. It is designed to operate effectively with silica levels in excess of 1,600 ppm in the reject water as compared to conventional membrane process, which is limited to approximately 150 ppm silica concentration in the reject. The HERO process has the potential to recover more than 90 percent of the cooling tower blow down because it is limited only by osmotic pressure. The process is also resistant to organic and biological fouling.
Magnolia’s effluent from the WAC units is sent to the HERO system via a decarbonator where carbon dioxide is removed from the water. Caustic is added at the HERO system inlet to elevate the pH. This is an important part of the process because at higher pH levels silica will tend to stay in solution rather than drop out as a solid, said Fancis D’Sa, Aquatech’s representative for the Magnolia ZLD Project. Because the HERO operates at more than 90 percent recovery (meaning that 90 percent of the feed water is purified and 10 percent is rejected from the system) the HERO’s reject water is 10 times more concentrated than the feed water. Hence, it is important to ensure that none of the feed water constituents-including silica-are allowed to precipitate out at the concentrated water stage. The purified effluent (permeate) from the HERO is recycled back to the cooling tower.
Magnolia’s ZLD system is designed to recover cooling tower blow down and recycle it for use as makeup to the cooling tower itself or, if needed, after further polishing, even as makeup to the boiler. Photo courtesy of BWP.
The reject from the HERO is collected and processed in the crystallizer where it is evaporated. The FC Crystallizer unit is a fully integrated automated system incorporating a mechanical vapor compressor. The system is designed to handle the bulk precipitation of salts from the brine (HERO reject) as evaporation occurs. The system concentrates the waste stream to slurry of salt crystals and mother liquor containing about 50 percent solids. The brine slurry from the flash tank is further dewatered in a belt filter press unit to deliver a dewatered salt cake with about 10 percent to 20 percent moisture content. This salt cake can be disposed of at a landfill, while the filtrate from the belt filter press is recycled back to the FC evaporator process.
According to D’Sa, the HERO system’s 90 percent recovery rate is one of its most important features because it allows the ZLD system to operate efficiently with a small crystallizer. “A conventional membrane system operates at about 60 percent to 70 percent recovery,” he said. “This means that 30 percent to 40 percent of the water is rejected to the crystallizer. The crystallizer would have to be much bigger to accommodate this volume of reject, resulting in at least a three to four time higher auxiliary power requirement.”
Work in Progress
The ZLD system has not been problem free. “The zero liquid discharge system is a work in progress,” Fletcher said. “We chose the Aquatech system because we believed, and still believe, it is the best system for our needs, but it still isn’t always sufficient as it is.”
D’Sa agrees that the ZLD system can complicate plant operations. “The ZLD system is another unit to be operated in order to run the power plant” he said. “Most power plant operators are familiar with conventional treatment processes, however, with recent environmental restrictions, the ZLD system is a relatively new addition to power plants. The system requires more skilled operators and close monitoring to ensure consistent performance.”
D’Sa also said that every issue associated with operating a ZLD cannot be foreseen at the design stage and there are always post-commissioning issues to be worked out. Accordingly it takes longer to start up and stabilize the ZLD system, he said.
In addition, due to the high alloy metallurgy requirements for the crystallizer unit, the cost of installing redundancy is high. Therefore, with no redundant system to relay on during process upsets or equipment breakdown, the only option is to haul the waste brine off-site, D’Sa said.
Currently, Magnolia’s waste brine is stored in an on-site storage tank and the waste is periodically trucked off-site to a disposal facility. When the ZLD is operating as designed, it costs BWP about $8,000 to $10,000 a month to dispose of the waste in a hazardous material landfill. The disposal cost has been as high as $80,000 a month when BWP has experienced problems with the ZLD system. Because both the sewer system and the power system are operated by the City of Burbank, BWP and the Burbank Public Works are working to increase the sewer plant’s capability so that it can accept the liquid that is currently being discharged into the storage tanks. One option is to install a similar ZLD system at the sewer plant to treat its sludge more cost effectively.
Aquatech currently has five ZLD systems, with similar configuration, installed in power plants; hence operation and maintenance experience is limited. Recently, however, Magnolia personnel were instrumental in establishing a quasi “users’ group” so that Aquatech customers can begin to share their experiences.
In addition to the ZLD system, Magnolia has a water treatment system, Puretec’s Reclaimed Water Demineralizer System, that cleans and purifies reclaimed water from Burbank’s wastewater treatment plant to be used in the plant’s boiler.
Fletcher said other plants have used reclaimed water as the feedwater for a boiler makeup system, however, they typically use reverse osmosis to clean the water. This process leaves impurities that can affect the system, Fletcher said. BWP uses a more advanced system for its boiler make up.
“The treatment system uses the same process used by chip manufacturers in computer chip manufacturing. The resulting quality of the water is very high,” he said. “It cleans to non-detect levels, which lengthen the life of the boiler and boiler tubes substantially. We have been watching water chemistry for decades and believe the extra effort and expense that goes into keeping the water clean is worth it.”
Magnolia management doesn’t know yet what effects the ultra pure water will have on the heat recovery steam generators, but Fletcher said he believes that in a year or two, when they do know, the results will be favorable.
The reclamation system is an above ground integrated system that uses microfiltration, chemical treatment, reverse osmosis (hyperfiltration) and polish demineralization (ion exchange) to achieve the ultra high purity demanded by the turbines and boilers. The system operates automatically requiring little or no human intervention. It is capable of producing approximately 220,000 gallons of ultra pure demineralized water daily.
By using reclaimed water for power generation, the City of Burbank is in a unique position of treating its water as a renewable resource. The city buys its water from various sources and sells it to its residents and businesses. The water is used and discharged by those users, captured through the piping network and then processed at the city’s sewage treatment plant. Once treated, the water is used a second and third time to generate electricity, which is sold, thus creating a revenue source that can be used many times. BWP estimates that its water solution tripled its reclaimed water sales.