Drought, population growth, sea level rise, and competitive pricing are all converging to prompt a new era of interest in seawater desalination projects. This approach to generating potable water has long held prominence around the world in areas of extreme water constraints, notably the Middle East. However, in the U.S. the number of seawater desalination plants has always been relatively small. The primary use in the U.S. was typically for private industrial use where the cost of operation could be justified.
In light of the new interest in desalination around the country, this article will briefly examine some of the technical challenges this approach faces and discuss some of the innovative approaches that have been employed. The article also addresses some of the primary permitting hurdles that these facilities are subject to.
Technical and Environmental Challenges
From a purely technical standpoint, the use of desalination to generate significant amounts of potable water has always been challenging. The two dominant methods for desalination are thermal based distillation and reverse osmosis (“R/O”). While thermal methods have been prominent in areas where energy costs are low, R/O has become the dominant process for large scale desalination plants in the U.S. Yet, while this method has improved in efficiency, it still is fraught with challenges. R/O essentially removes salts and impurities by forcing the water through a filter or membrane medium that permits the water molecules to pass through, but blocks the passage of the larger salt compounds. This process requires significant energy inputs to generate sufficient pressure to create a utility-scale volume of water. This high energy requirement in turn drives up the price for R/O generated water.
Beyond the cost, energy, and technical demands, desalination plants have significant environmental issues that must be dealt with. The lower the initial salt content, the greater the efficiency of an R/O facility. However sources for brackish water are limited. In some locations, aquifers near the coast that have experienced saltwater intrusion may be a viable source, but drawing down the water table at these locations may exacerbate the saltwater intrusion into inland fresh water sources. Seawater, while voluminous, often creates its own challenges. Higher salinity rates equate to less efficient plant operation, higher costs for pre-treatment and membrane maintenance and greater energy demands. Additionally, environmental advocacy groups often seek permitting assurances that ocean intakes will not siphon in significant sea-life or otherwise disrupt pelagic communities.
The greater environmental challenge is the adequate disposal of the hyper-saline concentrate that is the byproduct of the R/O process. Concentrate and residuals disposal from R/O desalination are the main areas of regulation. Disposal for less salty concentrate from brackish water R/O may include land application for aquifer recharge or irrigation, or for saltier concentrates, deep well injection, surface water discharge, or blending the concentrate with other wastewater treatment plant discharges from permitted facilities. Volume, location and the chemistry of the concentrate determine the available methods.
Despite all of the foregoing obstacles, desalination continues to garner more interest from utilities. Recent droughts in California have reignited the debate over the need to find new sources of water. Despite significant gains in conservation and industrial efficiency, the need for additional water remains. California has seen droughts before, and rain and snow fall in later years have caused hesitation about beginning a new significant investment. An R/O plant was built in Santa Barbara during the serious droughts that occurred during the late 80’s and early 90’s. However after a brief period of operation, sufficient rain patterns increased regional fresh water supplies. Consequently, the cost of running the Santa Barbara plant with older R/O technology became prohibitive and the site was mothballed. With the current drought cycle showing no sign of letting up, and predictions that climate change may make these cycles longer and hotter, Santa Barbara is now looking to dust off the plant and get it up and running again.
California is now once again betting big on desalination. In San Diego County, the $1 billion Carlsbad Desalination Project is nearing the final stages of construction. Slated to be completed by the end of 2015, the plant will be the largest desalination facility in the Western Hemisphere, with the potential of generating 50 million gallons per day (mgd) of potable water from a total intake of 100 mgd. The resulting 50 mgd of water with elevated salinity will be blended with additional seawater in order to sufficiently dilute the concentration down to a level that can be safely discharged to the ocean.
While only time will tell if the Carlsbad Plant will meet its operational goals, the scale of the permitting challenges it has encountered are not unique. Currently, the largest operational desalination facility in North America is located in Tampa Bay, Florida. An examination of the history surrounding the permitting and construction of the Tampa Bay facility provides a useful illustration of obstacles and innovate solutions that are part and parcel of any desalination project.
The Tampa Bay Example
Florida is a peninsula surrounded by water with a high rate of average annual rainfall, rich groundwater aquifers, and many lakes, estuaries and rivers—hardly a state where water supply would be an expected concern. However, the state has grown in the past one-hundred years to be one of the most populous in the country, primarily through coastal development, with a historically extensive agricultural community that competes with urban areas for water supply.
Over the past 40 years, losses to the State’s springs, wetlands, lakes, river flows and aquifer levels have amply demonstrated that continued reliance on fresh ground and surface water for drinking water and agriculture is not sustainable. Presently, the Florida Department of Environmental Protection (“FDEP”), Office of Water Policy finds that water usage in Florida is approximately 6.4 billion gallons a day, and is expected to increase by the year 2030 to 7.7 billion gallons per day. These volumes of freshwater have not been, and will not be, sustainable for Florida. Diversification and alternative water supplies are needed.
Florida has been a national leader in reverse osmosis desalination for drinking water, primarily through desalination of brackish ground and surface waters. All but one of the potable water desalination plants in Florida rely on R/O technology. Brackish water used for R/O in Florida is typically between 1-10 parts per thousand (ppt) total dissolved solids (“TDS” indicating salinity), with tidally influenced rivers and estuaries fluctuating between 10 and 33 ppt TDS compared to ocean seawater of 35 ppt TDS. Florida presently has over 100 independent brackish desalination water supply facilities.
Tampa Bay Water (“TBW”) is a regional water supply authority in Florida consisting of Hillsborough, Pinellas, and Pasco Counties and the Cities of Tampa, St. Petersburg, and New Port Richey. It has historically supplied drinking water to these locations primarily from well fields and river withdrawals. These sources have become increasingly unsustainable due to population growth, aquifer over pumping causing environmental damage, and regulatory minimum flows and levels for rivers and streams. Based on the need to reduce groundwater pumping and to develop alternative water supplies, TBW looked to seawater desalination.
The Tampa Bay Water Seawater Desalination facility was granted an NPDES permit by FDEP in 2001. Until the Carlsbad desalination plant in California begins operation, the TBW Desalination plant has been the largest seawater desalination facility in North America. Able to produce up to 25 million gallons a day of fresh drinking water, it was permittable due to its unique co-location with the Tampa Electric Company (“TECO”) Big Bend power plant. The location permits the intake of estuarine water with up to 32 ppt TDS, less than ocean seawater, and the estuarine discharge location with extensive flushing and salinity fluctuations.
The TBW Desalination plant was intended to be an all-season water supply source that would be impervious to rainfall and water levels. It was designed as an R/O membrane treatment facility that would produce up to 25 mgd. This presented many permitting challenges: intake water permitting, construction permitting, treatment permitting, assurance of treated drinking water standards, health department permits, and, finally, the discharge of concentrate and residuals. In order to produce 25 mgd of potable water, the plant would need to intake 44 mgd of seawater with a concentrate discharge of 19 mgd.
In its initial search for a plant location, TBW focused on existing area power plants with cooling water intakes for co-location and the use of permitted intake water. The costs and challenges of attempting to address construction and permitting requirements for entrainment and impingement were too great to locate an independent plant. Of the area’s power plants, the TECO plant on Tampa Bay was especially attractive, as it is located in the Tampa Bay Estuary with reduced salinity compared to ocean seawater, which greatly reduced the cost of treatment and provided extensive flushing for the discharge from the TECO’s cooling water, tides, rainfall and river flows. The TECO plant, with all four power units in operation, is permitted to withdraw and discharge up to 1.4 billion gallons a day in cooling water. TECO offered an existing cooling water intake and discharge system that TBW could use to siphon heated cooling water from the plant and return the concentrates to an internal piping system with substantial dilution and mixing prior to discharge. The heated cooling water allows the R/O membrane process to function more efficiently. By the time the facility was permitted by FDEP in 2001, the facility already held twelve permits from other state and local entities that were required for the operation of the plant.
The TBW Desalination plant required NPDES industrial water discharge permits from FDEP for both the plant itself and for TECO’s modified discharge with the mixed concentrate. Both permits were then challenged by an association formed by nearby residents living on a series of canals located not far from TECO’s cooling water discharge canal in Tampa Bay. The cases were consolidated for hearing and a full evidentiary hearing was held before the Florida Division of Administrative Hearings: Save Our Bays, Air and Canals v. Tampa Bay Desalination and Department of Environmental Protection, Case No. 01-1949, and Save Our Bays, Air and Canals v. Tampa Electric Company, Case No. 01-2720. The Administrative Law Judge’s Recommended Order to the agency reviewed the evidence in great detail relating to permitting standards for water quality including dissolved oxygen, nutrients, salinity, toxicity, metals, bioavailability of metals, synergistic effects, and ph. Additionally, the order reviewed effects of the discharge on seagrasses, fish, manatees and phytoplankton.
A primary feature of the facility is the dilution of up to 19 mgd of concentrate in the maximum cooling water discharge from TECO of 1.4 bgd. At these rates, the maximum concentrate discharged would be diluted under best case scenarios at 70:1 prior to entering the Bay. The judge imposed permit limits on the minimum amount and timing of dilution based on the number of power units operating at TECO at any time. The Order further noted that fish and wildlife species living in the estuary are accustomed to wide fluctuations in salinity based on tides, river flows and rainfall, and have a wide range of salinity tolerance. Manatees, which frequent TECO’s cooling water discharge canal in the cold winter months, tolerate seawater salinity levels up to 40 ppt.
A key analysis related to the flushing in Tampa Bay, supported by three-dimensional far field and near field hydrographic models provided by TECO and TBW. The petitioner had alleged that salinity levels in their nearby canals would accumulate and become elevated, reducing dissolved oxygen and affecting algal growth and water quality. The judge found that the three-dimensional modelling efforts supported the conclusion that this buildup of salinity would not occur.
TBW began its quest for the desalination plant in 1997 with an RFP for consulting firms. It was meant to be a design, build, own and operate facility. The plant was built and tested in 2003, but was unable to properly operate at capacity due to membrane fouling and expense issues. Over the next several years, other contractors took over the plant, resolved design and construction deficiencies, modified the plant for effective pre-treatment and membrane treatment, and the plant finally became fully operational in 2007, meeting its capacity goals of 25 mgd drinking water for months at a time in 2010. When the plant is operating, the drinking water produced is blended with other sources of treated water at an offsite plant for distribution. By utilizing TBW’s various local surface and ground water supply sources and blending these lower cost systems, the cost to consumers for the treated seawater is greatly reduced. Due to the higher cost of producing water from the facility, it is only used by TBW when other sources are insufficient.
As discussed above, the Clean Water Act (“CWA”) provides for National Pollution Discharge Elimination System (“NPDES”) permitting. NPDES permits are typically required for the disposal of the R/O concentrate waste products. The NPDES requirements are administered by the U.S. Environmental Protection Agency, which in turn may delegate state agencies with compliant CWA programs. The EPA retains ultimate oversight responsibilities to the program. Similarly, the CWA provides for the U.S. Army Corps of Engineers to permit the disposal of dredge or fill materials to navigable waters, which may be implicated by the construction of a desalination facility.
As discussed above in Tampa Bay, co-location with a power plant relying on cooling water is critical for permitted intake of large quantities of water without facing federal and state permitting concerns for entrainment and impingement for biota, fish and wildlife. The California Carlsbad project is similarly co-located.
Other federal regulations that may be implicated in the construction or operational plan of a desalination plant include: Safe Drinking Water Act, Resource Conservation and Recovery Act, Superfund Amendments and Reauthorization Act, and the Endangered Species Act.
State permitting requirements may vary widely. Zoning and land use restrictions are often controlling factors in locating major utilities, complicating the already challenging task of finding a site that will provide a desalination facility with the resources it needs for both intake and disposal. Furthermore, states are free to set their own environmental regulations that exceed Federal requirements.
As water supplies become increasingly constrained the interest in seawater desalination facilities will continue to increase. However, the technical and environmental challenges these facilities face make their adoption a difficult choice for governments and public water utilities. Innovative siting, funding, and careful planning of concentrate disposal will be needed for future implementation.