The USAID Desalination Manual
As the world's population has grown, demand for fresh water has dramatically increased, straining available supplies and impeding development. Desalination, the removal of salts from water, offers an increasingly feasible and necessary solution to this problem. This manual outlines major desalination processes and evaluates their development, performance, potential, problems, and economics to give planners a better understanding of the applicability of desalting in developing countries. Four major desalination processes-distil- lation, freezing, reverse osmosis, and electrodialysis -- are examined. Distillation utilizes a three-step process of vapor production, transportation to a condensor, and condensation to separate dissolved salts from water. Freezing relies on the natural fact that when ice forms in a saline solution, it leaves ice crystals of pure water. Although freezing is currently not a commercial success, there is potential for its development and future use. Reverse osmosis is a membrane separation process in which water from a pressurized saline solution is separated from solutes as it flows through a membrane. Electrodialysis utilizes alternately fixed charged membranes placed in brackish water that has been charged by electrodes to attract and trap ionized salts and leave behind fresh water. The manual details, in regard to each process, theoretical background, commercial development, engineering considerations, state-of-the-art, applications, health aspects, environmental constraints, future prospects, process experience, and applicability for developing countries. The high cost and limited supply of conventional fuels has spurred the use of renewable or alternate energy sources for desalting. The use of solar, wind, wave, and hand energy are briefly considered and examples of existing uses are given. Factors to be considered in gathering data, making potential cost estimates, and selecting processes, particularly in developing countries, are examined. Appendices include a selected list of U.S. desalination equipment manufacturers, worksheets and cost curves to assist in cost-estimating, guidelines for desalting process selection, a glossary of terms, and a 196-item bibliography (1946-80).