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The Desalination and Water Purification Research & Development Program Newsletter - No. 20 - Summer 2001

PROGRAM REPORTS

The Desalination and Water Purification Research & Development (DWPR) Program publishes a final report for each project that the program funds. Two reports were issued in May 2001: Initial Evaluation of the Subfloor Water Intake Structure System (SWISS) vs. Conventional Multimedia Pretreatment Techniques (Report No. 66) and VARI-ROTM Desalting Pilot Plant Advancement Project Testing and Evaluation (Report No. 62). For more information about the program and a list of available reports, please contact Bill Boegli at (303) 445-2248 or visit the program web site: http://www.usbr.gov/pmts/water/research/DWPR/.

IMPROVING DESALINATION PLANT EFFICIENCY

The world will face a looming potable water crisis in a few decades if alternative water supplies are not found soon. Desalination can help eliminate this shortage, remove and concentrate contaminants in industrial and municipal wastewater, and reduce the salinity of irrigation water to increase crop yields. At present, the price of desalted water is very competitive - especially when the high cost of transporting natural fresh water comes into the picture.

Major desalination processes include multi-stage flash distillation, multiple effect distillation, vapor compression, and reverse osmosis. About 12,500 land-based desalting units worldwide have a total combined capacity of over 6 billion gallons per day.

The thermodynamic, or second-law, efficiency of many plants is just 10 to 20 percent, which is very low for any engineering system. Doubling these second-law efficiencies will result in reducing current energy uses by half.

The second-law analysis of thermodynamics has been used extensively in recent years to identify the sites of greatest losses, and to improve the performance of power plants. As a result, DWPR awarded the study, Improving the Thermodynamic and Economic Efficiencies of Desalination Plants (Agreement No 99-FC-81-0183). A draft final report is currently being reviewed. The University of Nevada analyzed the thermodynamics of reverse osmosis, distillation, and freeze desalination. To put various techniques in perspective, the study analyzed desalination processes as well as actual plant operational data, with particular attention to the minimum separation work requirement and the flow exergy.

The minimum work of general complete separation processes was first investigated by considering reversible processes for which entropy generation and exergy destruction were zero. Minimum work relations for complete separation of mixtures were obtained and presented in various convenient forms. These relations are later used to develop the minimum separation work for incomplete separation of saline water solutions encountered in desalination plants. The minimum work input was determined for various salinities of incoming saline water and outgoing brine and product water. The results were tabulated and plotted. The minimum work values show that a lower and an upper limit for the minimum work exist at corresponding recovery ratios of zero and one hundred percent. The plots of the minimum work versus recovery ratios at various salinities of the incoming saline water also show that there is an optimum value of the recovery ratio which decreases with increasing salinity.

The analysis of the minimum separation work was performed for the reverse osmosis, distillation, and the freeze desalination processes. It showed that the minimum separation work is independent of any hardware or process and thus is the same for all three processes. Next, the exergy analysis of typical ideal and actual reverse osmosis desalination processes was conducted together with the discussion of the minimum separation work requirement. The energy changes of major components were calculated and illustrated using exergy flow diagrams.

Draft study conclusions included:

The minimum work is a strong function of salinity: it increases linearly with salinity at concentrations encountered in practice. Therefore, the minimum work needed to separate saline water with 2 percent salinity is almost twice as much as that of saline water with 1 percent salinity.

The minimum work input requirement increases with decreasing salinity of the freshwater produced. Therefore, it takes more work to produce water with less salinity.

The minimum work input requirement remains fairly constant for recovery ratios of up to about 80 percent. This is especially true at low salinities. Therefore, operating actual desalination plants at low recovery ratios to minimize energy consumption is not necessarily a good idea.

The minimum work increases drastically at high recovery ratios. Therefore, recovery ratios above about 80 percent should be avoided. This is especially the case for saline waters with high salinities.

There is an optimum value of recovery ratio to minimize the power consumption of actual desalination plants. The optimum recovery ratio decreases with increasing salinity of the incoming saline water. Therefore, particular attention must be paid to the recovery ratios when desalination plants are being designed.

Hardware or processes analytically carry out the minimum work input requirement for reverse osmosis, distillation, and freezing processes. The study proposed and described in sufficient detail a typical ideal desalination process.

DEVELOPING RESEARCH ROADMAPS

DWPR and the American Water Works Association Research Foundation (AWWARF) co-sponsored a workshop in Golden Colorado to develop a membrane research roadmap by:

  • Developing a strategy to improve effectiveness and applicability of membrane technologies
  • Developing databases, lessons learned, decision tools and other ways to capture the knowledge gained from existing facilities
  • Developing design information for large systems
  • Finding cost-effective and permittable disposal options
  • Standardizing methods and permitting criteria

The workshop also identified priority projects. To further these priorities, DWPR and AWWARF are jointly funding the three top priority projects. In 2001, we are working on two projects: integrating membrane filtration into existing water treatment systems and developing a low-pressure membrane knowledge base. DWPR is providing $60,000 and AWWARF is providing $190,000 for each project. We are planning to jointly sponsor a third project in FY02.

DWPR's cost share has shown how important these projects are and has helped accelerated private funding of membrane research. Our support has also helped expand interests to include desalination membranes. The report from the workshop can be viewed at http://www.awwarf.com/ research/membrane.htm or at our reports web site www.usbr.gov/water/publications/reports.html

Twenty-nine people from six nations participated in the 2000 AWWARF/Reclamation Membrane Workshop to develop a strategy to improve applicability of membrane technologies by drinking water utilities.

PROGRAM

The DWPR program focuses on:

(1) Research and studies on desalination technologies and related issues that pushes the state of the art forward so costs can be reduced.

(2) Development and demonstration activities to test technological advancements, confirm economics, and gain public acceptance.


CONTACT US

Water from Water is published by Reclamation's Water Treatment Engineering and Research Group - Susan Martella, Editor. For more information about the DWPR program, contact Kevin Price at: Bureau of Reclamation, 86-69000, PO Box 25007, Denver CO 80225; phone (303) 445-2260; or e-mail a message to MPrice@usbr.gov.