Seawater desalination produces fresh, low-salinity potable water from seawater via membrane separation or evaporation. The mineral/salt content of the water is usually measured by the water quality parameter total dissolved solids (TDS), in milligrams per liter (mg/L) or parts per thousand (ppt).
The World Health Organization and the U.S. EPA, under the Safe Drinking Water Act, have established a maximum TDS concentration of 500 mg/L as a potable water standard. This TDS level can be used as a classification limit to define potable water.
Typically, water of TDS concentration higher than 500 mg/L and low or equal to 15,000 mg/L is classified as brackish. Natural water sources such as sea, bay or ocean waters usually have TDS concentration higher than 15,000 mg/L and are generally classified as seawater.
For example, Pacific Ocean seawater along the U.S. West Coast has a TDS concentration of 33,500 mg/L of which approximately 75% is sodium chloride.
Approximately 97.5% of the water on the planet is located in the oceans and therefore is classified as seawater. Of the 2.5% of the planet’s fresh water, approximately 70% is in the form of polar ice and snow, and 30% is groundwater, river and lake water, and air moisture. Even though the volume of the earth’s water is vast, less than 10 million of the 1.4 billion cubic meters of the water on the planet are of low salinity and suitable for use after applying conventional water treatment only. Desalination provides means for tapping the world’s main water resource—the ocean.
Seawater desalination is gaining popularity for production of potable water worldwide as many municipalities and utilities face increasing population growth pressures, shortage of suitable local water resources and more stringent water quality regulations. Over the last 30 years, seawater desalination made great strides in many arid regions of the world such as the Middle East and the Mediterranean. Today, over 15,000 desalination facilities operate in more than 120 countries worldwide and some desert states, such as Spain, Saudi Arabia and the United Arab Emirates, rely on desalinated water for more than 70% of their water supply.
Worldwide, seawater desalination plants produce over 3.5 billion gallons of potable water a day. The installed reverse osmosis (RO) desalination plant capacity has increased dramatically over the past 30 years. This is due to major breakthroughs in membrane technology and energy recovery equipment in the early 90’s resulting in a significant acceleration in the construction of new desalination facilities.
Typically, seawater is desalinated using two general types of water treatment technologies—thermal evaporation and membrane separation. Currently, approximately 43.5% of the world’s desalination systems use thermal evaporation technologies. This percentage has been decreasing steadily over the past 10 years due to the increasing popularity of membrane desalination, which is driven by remarkable advances in the membrane separation and energy recovery technologies, and associated reduction of the overall water production costs.
Most of the large seawater desalination facilities built in the last 10 years, or these currently undergoing construction, are delivered under public-private partnership arrangements using build-own-operate-transfer (BOOT) methods of project implementation. The BOOT project delivery method is preferred by municipalities and public utilities worldwide because it allows cost-effective transfers to the private sector involving the risks associated with the number of variables affecting the cost of desalinated water. These include intake water quality and the difficulty in predicting its effect on plant performance; permitting challenges; startup and commissioning difficulties; fast-changing membrane technology and equipment market; and limited public sector experience with the operation of large seawater desalination facilities.
Currently, seawater desalination plants are gaining popularity in the U.S. Since the early spring of 2003, the first large seawater RO desalination plant began operation in Tampa, Fla. This facility has the capacity to produce 25 mgd of high-quality fresh water from seawater originating from the Tampa Bay. The desalination plant is co-located with the Tampa Electrical Co.’s Big Bend power plant and uses the power plant outfall for seawater intake and desalination byproduct discharge.
After a rocky start in the spring of 2003, the Tampa desalination plant produced and delivered over 3.5 billion gal of potable water during that year. In 2004, the current owner of the plant, Tampa Bay Water, retained a team of American Water Works and Pridesa to optimize plant performance and address changes in intake water quality that have a measurable effect on plant pretreatment system efficiency. The plant improvements are planned to be completed in 2006. Meanwhile, plant operation will continue at reduced production capacity.
To date, only a few small-size seawater desalination plants have been built along the West Coast of the U.S. primarily because the cost of desalination has been higher than that of available alternative sources of water supply—groundwater and water transfers. Prolonged drought, dwindling traditional water sources such as the Colorado River and Bay Delta water, and new more stringent regulatory requirements are driving the costs of conventional water supplies up and are bringing seawater back into the limelight in California.
Currently, there are five large projects in various stages of development in southern California. Two of these projects are being developed in a public-private partnership between Poseidon Resources and local municipalities and utilities.
These desalination plants are planned to be located at existing coastal electrical power generation stations. The Huntington Beach and Carlsbad desalination plants are both projected to have an ultimate product water capacity of 50 mgd and may be developed in one or more phases. Currently, two projects are in process of environmental feasibility review and permitting, and are planned to begin construction by the end of 2005.
On Sept. 8, 2004, the city of Carlsbad reached an agreement with Poseidon Resources for purchasing 25 mgd of the desalination plant’s fresh product water at a cost of $861/ft. The rest of the plant production will be procured by other municipalities and utilities in the vicinity of the desalination plant.
The Metropolitan Water District of Southern California has been very supportive of the development of new local drought-proof potable water resources and has plans to subsidize the cost of water produced at most of the area’s desalination facilities with a $250/acre-ft credit. With this credit, the cost of desalinated water will become comparable to the cost of water imported from northern California and the Colorado River. However, compared to the existing water sources, the desalinated water will be of lower salinity, and will have better overall water quality. More importantly, the desalination plant will be a reliable, local drought-proof water resource.
Most of the projects in southern California are expected to be operational by 2010, and to cumulatively provide over 150 mgd of fresh water to the area. Although this amount is significant, it would be adequate to satisfy only a small portion of California’s commitment to reduce its use of Colorado River water and accommodate ever-growing water pressures.
In addition to seawater desalination, other alternative water sources, which would be used to achieve this significant water reduction, are increased reliance on water reuse, conservation and development of new groundwater resources.
Desalination in Texas
Desalination is in Texas’ future as well. In December 2004, the Texas Water Development Board (TWDB) submitted to Gov. Rick Perry the 2004 Biennial Report on Seawater Desalination. The report statutorily mandated after passage of House Bill 1370 of the 78th Texas Legislature. HB 1370 directs TWDB to “undertake or participate in research, feasibility and facility planning studies, investigations, and surveys as it considers necessary to further the development of cost-effective water supplies from seawater desalination in the state.”
The 2004 Biennial Report underlines, that among the many sources of water Texans will rely upon in the future for human consumption—rivers, streams, rain, groundwater and seawater—only seawater has the unique potential to provide an uninterruptible and limitless water supply during times of drought.
In 2003, the Texas Legislature directed the TWDB to allocate $1.5 million for feasibility and regional facility planning studies to determine the technical and economic viability of proposed seawater desalination projects. This resulted in the TWDB funding studies for development of large-scale desalination plants at Lower Rio Grande Valley-Brownsville, Corpus Christi and Freeport. As a next step on the road to exploring the feasibility of seawater desalination in Texas, the TWDB recommended the implementation of pilot plant studies to prove the desalination technology feasibility and to formulate state policy regarding the mechanisms for providing financial assistance needed for future development of seawater desalination.
The TWDB is currently finalizing research related to the development of a permitting model for desalination projects in Texas; the feasibility of desalinated produced water from natural resource extraction activities; the potential for capacitive deionization technology; and the development of an inventory and database of desalination facilities in Texas. Also, the U.S. Bureau of Reclamation awarded the TWDB a federal grant to assess the potential for using oil field-related deep-well injection sites for desalination brine disposal.
Cost spurs membrane use
Historically, the key concern related to the use of seawater desalination on a large scale has been the high cost of water production. A number of cost-saving innovations in seawater desalination technology are transforming this once expensive option of last resort into a fiscally viable water supply alternative.
A typical RO membrane desalination plant includes the following key components: source water intake system; pretreatment facilities; high-pressure feed pumps; RO membrane trains, and a desalinated water conditioning system. The source water intake system could be an open surface water intake or series of seawater beach wells or brackish groundwater wells. Depending on the source water quality, the pretreatment system may include one or more of the following processes: screening, chemical conditioning, sedimentation and filtration.
In a typical configuration of a seawater RO membrane system, the filtered water produced by the plant’s pretreatment system is conveyed by transfer pumps from a filtrate water storage tank through cartridge filters and into the suction pipe of the high pressure RO feed pumps. The cartridge filters are designed to retain particles of 1–20 microns, which have remained in the source water after pretreatment. The main purpose of the cartridge filter is to protect the RO membranes from damage. High pressure feed pumps deliver the source water to the RO membranes and the pressure required for membrane separation of the fresh water from the salts.
The “engine” of every desalination plant that turns seawater into fresh potable water is the RO membrane element.
The most widely used type of RO membrane elements consist of two membrane sheets glued together and spirally wound around a perforated central tube through which the desalinated water exits the membrane element. The first membrane sheet, which actually retains the source water minerals on one side of the membrane surface, is typically made of thin-film composite polyamide material and has microscopic pores that can retain compounds of a size smaller than 200 Daltons. This sheet, however, is usually less than 0.2 microns thin. In order to withstand the high pressure required for salt separation, it is supported by a second thicker membrane sheet, which is typically made of higher-porosity polysulfone material that has several orders of magnitude larger pore openings.
A large seawater desalination plant usually has thousands of membrane elements connected into a highly automated and efficient water treatment system, which typically produces 1 gal of fresh water from approximately 2 gal of seawater. The membrane productivity, energy use, salt separation efficiency, cost of production and durability of the membrane elements by large determine the cost of the desalinated water. Technological and production improvements in all of these areas in the last two decades are now rendering water supply from the ocean affordable. Membrane productivity has increased over two times in the last 20 years.
Recent introduction of spiral wound membrane elements with a larger number of membrane “leaves” and denser packing offer increased efficiency versus old designs. Today’s most efficient elements have more than twice as many membrane leaves compared to older designs. Higher productivity means the same amount of water can be produced with significantly less membrane elements, which has a profound effect on the size of the membrane equipment, treatment plant buildings and the footprint of the desalination facility—all of which ultimately reduce the cost of water production.
In seawater desalination plants, salts are separated from the fresh water applying pressure to the seawater, which is 60–70 times higher than the atmospheric pressure (typically in a range of 800–1,000 psi). After the salt/water separation is complete, a great portion of this energy stays with the more concentrated seawater and can be removed, and reused to minimize the overall energy cost for seawater desalination. Dramatic improvements of the membrane element materials and energy recovery equipment over the last 20 years coupled with enhancements in the efficiency of RO feed pumps, and reduction of the pressure losses through the membrane elements have allowed for a reduction of power to desalinate seawater to less than 14 kWh/1,000 gal of produced fresh water.
Taking into consideration the cost of power is typically 20–30% of the total cost of desalinated water, these technological innovations contribute greatly to the reduction of the overall cost of seawater desalination.
Novel energy recovery systems working on the pressure exchange principle (pressure exchangers) are currently available in the market and use of these systems is expected to further reduce the desalination power costs approximately 10–15%.
The pressure exchangers transfer the high pressure of the concentrated seawater directly into the RO feed water with an efficiency exceeding 95%.
Future lower-energy RO membrane elements are expected to operate at even lower pressures and to continue to yield further reduction in cost of desalinated water.
Membrane performance tends to naturally deteriorate over time due to combination of material wear-and-tear and irreversible fouling of the membrane elements. Typically, membrane elements have to be replaced every five years to maintain their performance in terms of water quality and power demand for salt separation. Improvements of membrane element polymer chemistry and production process have made the membranes more durable and extended their useful life. Use of elaborate conventional media pretreatment technologies and ultra- and micro-filtration membrane pretreatment systems prior to RO desalination is expected to extend the membrane useful life to seven years and beyond, thereby reducing the cost of their replacement and the overall cost of water.
Today, the overall RO membrane technology and elements are highly standardized and commoditized in terms of size, productivity, durability and useful life. There are a number of manufacturers of high-quality seawater RO membrane elements, which provide interchangeable products of excellent quality, proven track record and performance. All of the leading membrane manufacturers are dedicated to supporting the water desalination market and advancing membrane technology, and science at a pace no other water technology can compare with. The desalination plant of today is a highly automated water production factory with a number of built-in protection and safety systems allowing reduction of staffing requirements to a minimum and thereby reducing the costs of plant operation.
Further cost reductions
The recent trend of building large capacity seawater desalination facilities is driven by the cost benefits offered by the advantage of size and centralization. The economy of scale related to building fewer large capacity RO plants rather than a large amount of smaller facilities is a recent trend that has also contributed to the overall reduction of the cost of desalinated water. Typically, the economy of scale of desalination facilities larger than 10 mgd yields additional cost of water reduction in a range of 5-15%. For example, the cost of water savings produced by building one 40 mgd plant instead of four 10 mgd plants is at least 10%.
Today, the construction if large desalination plants is possible mainly due to the availability of large-size off-the-shelf high pressure pumps, large energy recovery systems and other auxiliary equipment with proven performance.
Co-location of desalination plants with large power generation stations can also yield significant cost-savings and further reduce the cost of desalinated water. Co-location with a power station in a large scale was first introduced by Poseidon Resources for the Tampa Bay Seawater Desalination Project and since has been considered for numerous plants in the U.S. and worldwide.
The key feature of the co-location concept is the direct connection of the desalination plant intake and/or discharge outfalls to an adjacently located coastal power plant. This approach allows using the power plant cooling water both as source water for the seawater desalination plant and as blending water to reduce the salinity of the desalination plant concentrate prior to discharge to the ocean.
Co-location of desalination plants and power generation stations has a number of advantages, such as using the existing power plant intake and discharge facilities which reduces the construction costs of the desalination facility; reducing the overall desalination facility power demand by using warmer ocean water; and minimizing the environmental impact of both the thermal discharge of the power plant and the high-salinity desalination facility discharge by their blending.
The developments in seawater desalination technology during the past two decades combined with transition to construction of large capacity plants, co-location with power generation facilities, and enhanced competition by using the BOOT method of project delivery have resulted in a dramatic decrease of the cost of desalinated water. A trend of decreasing cost of water produced by seawater desalination is based on recent large seawater RO desalination projects in the U.S., Israel, Cyprus, Singapore and the Middle East.
The advance of RO desalination technology is closest in dynamics to that of computer technology. While conventional technologies, such as sedimentation and filtration have seen modest advancement since their initial use for potable water treatment several centuries ago, new more efficient seawater desalination membranes and membrane technologies and equipment improvements are released every several years. Similar to computers, the RO membranes of today are many times smaller, more productive and cheaper than the first working prototypes.
Over the last 10 years, the cost of desalinated water dropped more than two-fold. Although, no major technology breakthroughs are expected to bring to cost of seawater desalination dramatically down in the next several years, the steady reduction of desalinated water production costs coupled with increasing costs of water treatment driven by more stringent regulatory requirements, are expected to accelerate the current trend of increased reliance on the ocean as an attractive and competitive water source. This trend is forecasted to continue in the future and to further establish ocean water desalination as a reliable drought-proof alternative for many coastal communities in the U.S. and worldwide.
Falling costs spur membrane use