HO Features - A. 1 People have been pulling freshwater out...

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1 A. 1 People have been pulling freshwater out of the oceans for centuries using technologies that involve evaporation, which leaves the salts and other unwanted constituents behind. 2 Salty source water is treated to speed evaporation, and the evaporated water is then trapped and distilled. 3 This process works well but requires large quantities of heat energy, and costs have been far too high for nearly all but the wealthiest nations, such as Kuwait and Saudi Arabia. (4 One exception is the island of Curacao in the Netherlands Antilles, which has provided continuous municipal supplies using desalination since 1928.) 5 To make the process more affordable, modern distillation plans recycle heat from the evaporation step. 6 A potentially cheaper technology called membrane desalination may expand the role of desalination worldwide, which today accounts for less than 0.2 percent of the water withdrawn from natural sources. 7 Membrane desalination relies on reverse osmosis—a process in which a thin, semipermeable membrane is place between a volume of saltwater and a volume of freshwater. 8 The water on the salty side is highly pressurized to drive water molecules, but not salt and other impurities, to the pure side. 9 In essence, this process pushes freshwater out of saltwater. (Martindale 2001) B. 1 Reverse Osmosis (RO) membrane systems are often used for seawater and brackish water desalination. 2 The systems are typically installed as a network of modules that must be designed to meet the technical, environmental, and economic requirements of the separation process. 3 The complete optimization of an RO network includes the optimal design of both the individual module structure and the network configuration. 4 For a given application, the choice and design of a particular module geometry depends on a number of factors, including ease and cost of module manufacture, energy efficiency, fouling tendency, required recovery, and capital cost of auxiliary equipment. 5 With suitable transport equations to predict the physical performance of the membrane module, it should be possible to obtain an optimal module structure for any given application. (Maskan et al. 2000) Task Five p14 1 For over twenty years now biologists have been alarmed that certain populations of amphibians have been declining. 2 These declines have occurred both in areas populated by humans as well as areas seemingly undisturbed by people. 3. However, offering clear proof of the declining numbers of amphibians has been difficult because in most cases there is no reliable data on past population sizes with which to compare recent numbers. 4 Moreover, it is not entirely clear whether the declines are actually part of a natural fluctuation in populations arising from droughts or a scarcity of food.
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2 5 To address this problem biologists are changing the way that they observe amphibian populations. 6 One good documentation method involves counting species over the course of several years and under a variety of climatic conditions. 7 This
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HO Features - A. 1 People have been pulling freshwater out...

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