By Morton Satin
In Samuel Taylor Coleridge’s Rhyme of the Ancient Mariner, the old sailor narrates the woes of his recent travels: “Water, water, everywhere, Nor any drop to drink.” He may have been describing the account of a mythical voyage, but his words reflect one of our future’s greatest challenges—access to quality water. More than 95 percent of the world’s water is sea or brackish water unsuitable for either drinking or agriculture. While the remaining five percent fresh water has remained stable over the eons, the world’s population has increased astronomically, giving us little choice but to make major adjustments to the way we access and manage our water resources.
More than a third of the world’s population lives in regions facing significant water shortages and during the next half-century, the amount of water needed to serve our rapidly expanding needs is likely to double. Since 1950, the amount of agricultural land under irrigation has tripled leading to Asia’s Green Revolution—a technological tour de force that improved the lives of billions. However, a large part of the same region, including India, China, Central and West Asia, have reached the limits of their easily available water supplies.
In common with many other regions, the US also faces severe challenges in meeting its future water needs that cannot be managed by simply relying on conservation or improving water use efficiency. Of course, we have to protect and improve the quality of all our current water resources, but we have little choice but to develop and make available additional water resources to serve the needs of our growing population and the agricultural production needed to sustain it.
Over the next 20 years, it is highly likely that many areas of our country will face dramatic changes in the availability, quality, disposal and regulation of our water supplies. There are no new sources of conventional fresh water. Everything we have is already allocated to specific uses. On the other hand, there are unlimited supplies of sea water, brackish water and impaired groundwater available throughout the country. The haste with which we can access to these new supplies will largely depend upon the political will to accept the reality of a challenging new world and the policies to aggressively support the establishment of cost effective technologies to convert these sources into water of the desired quality.
In a 1988 report, the Congressional Office of Technology Assessment suggested that large scale desalination could find application in treating impaired groundwater, be it runoff from mines, agriculture, landfills or storage tanks. In 2002, Congress authorized the Energy and Water Development Appropriation Bill wherein it recognized, “… that effective desalination cost reduction is the key to wider use of desalination for improving the quality of life in water scarce regions.”
Impaired waters are the result of natural and human processes. In fact, human activity (such as mining, agriculture, or water treatment) accounts for a small proportion of water impairment. Contrary to what many environmental activists believe, as high as 80 percent of the salt and other minerals found in surface and groundwater is the result of simple, natural erosion. However, intensive use of waters for municipal and agricultural purposes does increase the variety of contaminants, such as detergents, found in surface and groundwater and a matter of growing concern.
The technologies that are currently in use for desalination include thermal and membrane processes and, where necessary, combinations of them. The price of treated wastewater using the technologies has been dropping at a rate of about four percent per year. While this may not seem impressive at first glance, it must be remembered that energy is a key component of these technologies and the price of energy in the last few years has been on a roller coaster ride. When this is factored in, the steady drop in desalination costs is, in fact, quite impressive.
One of the areas of greatest concern in desalination technology is the disposition of waste concentrates left over from processing. In fact, disposal of concentrates is beginning to eclipse cost-effectiveness as the primary issue for desalination. This is one of the reasons that thermal and membrane technologies are often combined: so that the waste stream can be minimized. It is the goal of modern desalination to come as close to the ideal technology as possible—where the waste material will be disposed of as a solid while 100 percent of the water will be effectively recaptured for specific uses.
The following table describes the membrane technologies in common use today:
|Microfiltration||Removal of suspended solids, microorganisms|
|Ultrafiltration||Separation of complex organics, volatiles, viruses|
|Nanofiltration||Water softening, sulfate removal|
|Reverse Osmosis||Salt removal in brackish and seawater|
|Electrodialysis||Salt removal in brackish water|
Nanotechnology holds the promise of a bright future for membranes. Lest we forget, membrane technology is one of the original nanotechnologies and the extraordinary interest and investment in this sector will lead to new and more efficient applications.
A key characteristic of most commercial energy-intensive thermal technologies is the optimal capture and re-use of heat . This is a common factor in all desalination technologies. The most widespread thermal method of desalination is the multi-stage flash (MSF) design, which has been in commercial use for more than 30 years. This process involves the use of distillation through several chambers (hence, multi-stage). In the MSF process, each successive stage of the plant operates at progressively lower pressures. The feed water is first heated under high pressure and is led into the first ‘flash chamber’, where the pressure is released, causing the water to boil rapidly resulting in sudden evaporation or ‘flashing’.
This ‘flashing’ of a portion of the feed continues in each successive stage, because the pressure at each stage is lower than in the previous stage. The vapor generated by the flashing is converted into fresh water by being condensed on heat exchanger tubing that run through each stage. The tubes are cooled by the incoming cooler feed water. Generally, only a small percentage of the feed water is converted into vapor and condensed. Multi-stage flash distillation plants have been in regular use since the late 1950s.
Another technique for commercial desalination is Multi-Effect Distillation (MED) and was widely used before the MSF method. This process occurs in a series of vessels (effects) and uses the same principles of evaporation and condensation at reduced ambient pressure. In MED, a series of evaporator effects produce water at progressively lower pressures. Water boils at lower temperatures as pressure decreases, so the water vapor of the first vessel or effect serves as the heating medium for the second, and so on. The more vessels or effects there are, the higher the performance ratio. Depending upon the arrangement of the heat exchanger tubing, MED units could be classified as horizontal or vertical tube operations as shown in the next diagram.
A new low-energy thermal process that has been designed to use excess heat recovered from operations such as electric utility plants is Diffusion Driven Desalination (DDD) developed at the University of Florida. The water supply is drawn in and then pumped through a regenerative heat exchanger which is then sprayed into the top of a diffusion tower. The diffusion tower is filled with specific packing materials to enhance the water/air surface area. Air is blown through the bottom of the tower and becomes humidified. The humidified air goes to a direct-contact condenser where the fresh water is condensed.
Nothing exemplifies the importance of recovering waste heat as a critical factor in future technologies better than the concept of nuclear desalination. Nuclear desalination is generically defined as the production of water in a facility where a nuclear reactor is used as the source of energy (electrical and/or thermal) for the desalination process. The facility may be dedicated solely to the production of water, or may be used for the generation of electricity and the production of quality water, in which case only a portion of the total energy output of the reactor is used for water production. In either case, the notion of nuclear desalination is taken to mean an integrated facility in which both the reactor and the desalination system are located on a common site and energy is produced on-site for use in the desalination system.
The feasibility of integrated nuclear desalination plants has been proven with over 150 reactor-years of experience in Kazakhstan, India and Japan. A reactor at Aktau, Kazakhstan, successfully produced up to 135 MWe of electricity and 80,000 m3/day of potable water over 27 years. In Japan, 10 desalination facilities linked to pressurized water reactors operating for electricity production have yielded 1000-3000 m3/day of potable water each., Over 100 reactor-years of experience have accrued, originally with MSF, but now converted to a combination of but MED and RO. In 2002, India set up a demonstration plant coupled to twin 170 MWe nuclear power reactors at their Madras Atomic Power Station, in southeast India. This project is a hybrid reverse osmosis/multi-stage flash plant, the RO with 1800 m3/day capacity and the higher-quality MSF 4500 m3/day.
The long-overdue recognition that our dependence on foreign oil is the greatest threat to our security has shaken many policy makers out of their politically-correct reverie and forced them to consider meaningful strategies for our energy-intensive future. At least for the short term, nuclear energy holds the greatest promise to answer our energy needs. While there is no doubt that initial capital costs are high, the operating costs of nuclear energy make the technology very competitive and fully free from foreign domination—and we are leaders in this technology. The Chernobyl disaster notwithstanding (the result of reckless management rather than poor engineering), nuclear energy has an extraordinary safety record and is ideally suited for highly decentralized electricity production – a characteristic which also coincides with a good deal of our future desalination needs.
While coastal areas can easily access sea water for desalination, the rest of the country will have to access impaired groundwater and brackish water resources. This will require a decentralized effort much in the same way as nuclear energy. Therefore, it makes sense to consider commercial scale desalination of impaired groundwater and nuclear energy together. They are both imperatives we cannot ignore and the coincident alliance of these technologies is serendipitous.
The Rhyme of the Ancient Mariner was a long poem, yet we always seem to quote only that one stanza about the lack of water. Perhaps we would be better off quoting the very last line that speaks of a more optimistic future, “A sadder and a wiser man, He rose the morrow morn.” It’s time to stop complaining about the unfair hand we have been dealt and use our will and our wits to tackle the challenges we face with some élan. Then, we’ll have “…every drop to drink!”
About the author
Mort Satin is Director for Technical and Regulatory Affairs at the Salt Institute, 700 N. Fairfax Street, Suite 600, Alexandria, VA 22314. Trained as a molecular biologist, Satin is a well-known executive and author with international experience in research, marketing and management. For 16 years, he directed the global Agro-Industry program at the Food and Agriculture Organization of the United Nations (FAO) in Rome, where he received international recognition for his work on the impact of technology on economic development. He developed several new food processes and received the only patents ever awarded to the United Nations System. Satin also served as the FAO liaison to the IAEA Chernobyl Committee. In 1989, while at FAO, he was nominated for the World Food Prize by the World Food Council. Satin retuned to North America as Executive Director of the International Agribusiness Management Association (a group of academics and CEO’s representing > $700 billion in global sales).
Satin has published over 250 articles on a wide range of food science, health, technology and agribusiness topics including a Scenario on the Future of Food Science for the Science Council of Canada. For the Asian Productivity Organization, Satin has spoken extensively on food safety, labeling, international certification, and other issues related to food technology and marketing. He has authored four text books in English and Spanish on the subjects of food safety, food history and irradiation and has a fifth book coming out this year (Death in the Pot: The Impact of Food Poisoning on History).