WORLDWIDE SEAWATER DESALINATION CAPABILITIESDesalination has evolved into a viable water supply alternative allowing to tap the largest water reservoir in the world – the ocean. Seawater desalination technology, available for decades, made great strides in many arid areas of the world such as the Middle East, the Mediterranean, and the Caribbean. The figure (1) below shows the location of the existing desalination plants worldwide. Desalination plants operate in more than 120 countries in the world, including Saudi Arabia, Oman, United Arab Emirates, Spain, Cyprus, Malta, Gibraltar, Cape Verde, Portugal, Greece, Italy, India, China, Japan, and Australia. Since early spring of this year the first US large seawater reverse osmosis (RO) desalination plant began operation in Tampa, Florida. Worldwide, desalination plants produce over 3.5 billion gallons of potable water a day. The installed RO desalination plant capacity has increased in an exponential scale over the last 30 years (see Figure 2).
As seen on this figure, major breakthroughs in membrane technology and energy recovery equipment in the early nineties resulted in a significant acceleration of the construction of new RO plants.
Of more than the existing 21,000 desalination plants today almost 50 percent use seawater to produce fresh water and the rest uses brackish water. At present, in the US this ratio is in favor of brackish water desalination. Table 1 presents a list of the largest RO seawater desalination plants constructed in the last 10 years. Table 2 summarizes the new large seawater RO desalination plants (of 10 MGD or higher capacity), which are currently in the stage of design or construction and are expected to be operational in the next several years.
Brackish groundwater desalination has found a widespread use in Florida California and Texas. To date, only a limited number of desalination plants have been build along the West Coast, primarily because the cost of desalination has been higher than the available alternative sources of water supply – groundwater and interstate and out-of-state water transfers. Prolonged drought, dwindling traditional water sources such as Colorado River and Bay Delta water are driving the costs of water up and are triggering the need for introducing alternative local water sources. Water shortages, increasing conventional water treatment costs for compliance with more stringent regulations and dramatic reduction of seawater desalination costs since the first RO plants were build in California, are bringing the desalination back into the limelight. Seawater desalination provides an access to new untapped resources for a sustainable and drought proof water supply is Southern California.
|Plant Name/Location||Capacity (mgd)|
|Tampa Bay Desalination Plant, USA||25.0|
|Point Lisas, Trinidad||28.8|
|Las Palmas – Telde||9.2|
|Muricia, Spain Design-Bid-Build||17.2|
|The Bay of Palma/Palma de Mallorca||16.6|
|Marbella – Malaga, Spain||14.5|
|Plant Name/Location||Capacity Installed/Avg. (mgd)|
|Carboneras – Almeria, Spain||32|
|Ashkelon, Israel||35.4 expandable to 75|
|Cartagena – Mauricia, Spain||17.2|
|Campo de Cartagena – Mauricia, Spain||37|
Typically, a large seawater desalination plant has thousands of membrane elements connected into a highly automated and efficient water treatment system which typically produces 1 gallon of fresh water from approximately 2 gallons of seawater. The productivity, energy use, salt separation efficiency, cost of production and durability of the membrane elements by enlarge determine the cost of the desalinated water. Technological and production improvements in all of these areas in the last two decades are rendering water supply from the ocean affordable. Membrane productivity – i.e. the amount of water that can be produced by one membrane element, has increased two times in the last twenty years (see Figure 4).Introduction of spiral wound membrane elements with a larger number of shorter membrane “leaves” offer increased efficiency vs. older designs. This is because the membrane is more productive when the path water has to travel to reach the exit end of the membrane is shorter. Today’s most efficient elements have more than twice as many membrane leaves compared to older designs. Higher productivity means that the same amount of water can be produced with significantly less membrane elements, which has a profound effect of 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 to 70 times higher than the atmospheric pressure. After the salt/water separation is complete, a great portion of this energy stays with the more concentrated seawater and can be recovered, and reused to minimize the overall energy cost for seawater desalination. Dramatic improvements of the membrane element materials and energy recovery equipment for 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 to reduce the use of power to desalinate seawater from 114 kWh/1,000 gallons in 1979 to 14 kWh/1,000 gallons of produced fresh water today. Taking under consideration that the cost of power is typically 20 to 30 percent of the total cost of desalinated water, these technological innovations contributed greatly to the reduction of the overall cost of seawater desalination. Novel energy recovery systems working of the pressure exchange principle are currently available in the market and use of these technologies is expected to further reduce the desalination power costs with 10 to 15 % (see Figure 5).The pressure exchangers transfer the high pressure of the concentrated seawater directly into the RO feed water, with an efficiency of 95 % or more. Future low-energy RO elements would operate at even lower pressures to continue to improve the RO technology cost effectiveness.
Membrane performance tends to naturally deteriorate over time due to combination of material wear-and-tear and irreversible fouling of the membrane elements. Improvements of membrane element polymer chemistry and production process have made the membranes more durable and have extended their useful life to over 5 years. Seawater pretreatment using ultra and mictrofiltration membrane systems prior to RO desalination is expected to further extend the membrane useful life to over 7 years, thereby reducing the costs for their replacement and the overall cost of water.
Today, the RO membrane technology is highly standardized and commoditized in terms of size, productivity, durability and useful life. There are number of manufacturers of high-quality seawater RO membranes 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 paste no other water technology can compare with. The desalination plant of today is a highly automated water production factory with a number of build-in protection and safety systems allowing reduction of staffing requirements to a minimum and thereby reducing the costs of plant operation.
The recent trend of building large capacity seawater desalination plants 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 small facilities has also contributed to the overall reduction of the cost of desalinated water. Typically, the economy of scale of facilities larger than 20 MGD yields additional cost of water reduction in a range of 5 to 10 %. Use of existing intake and discharge facilities of power plants has also contributed to the desalinated water cost of reduction. Co-location of desalination of power plant in a large scale was first introduced by Poseidon Resources for the 25 MGD Tampa seawater desalination plant, and since than has been considered for numerous plants in the US and worldwide. Today, the construction of large desalination plants is possible mainly due to the availability of large-size off-the-shelf high pressure pumps and large energy recovery systems.
The developments in seawater desalination technology during the past decade combined with transition to construction of large capacity plants, co-llocation with power plant generation facilities and enhanced competition by using Build-Own-Operate-Transfer (BOOT) method of project delivery have resulted in a dramatic decrease of the cost of desalinated water. The figure (6) presented below indicates the trend of desalinated cost of water based on recent seawater RO desalination projects in the US, Israel, Cyprus, Singapore and the Middle East.The advancement of the reverse osmosis desalination technology is closest in dynamics to computer technology. While conventional technologies, such as sedimentation and filtration have seen little advancement since they were first applied for potable water treatment in the 18-th century, new more efficient membranes and membrane technology 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. As seen on the figure below, over the last twenty years, the cost of desalinated water dropped four-fold. Although, no major technology breakthroughs are expected to bring the cost of seawater desalination further down dramatically in the next several years, the steady reduction of desalinated water production costs coupled with increasing costs of water transfer and regulatory driven treatment infrastructure, make the ocean an attractive and competitive water source for Southern California today.
Data provided by Poseidon Water, San Diego, CA.