Monday, June 09, 2008

A Sip of The Sea

What could be less thirst-quenching than a mouthful of salty seawater? Perhaps nothing, thanks to desalinization plants that are turning salt water into one of the most widespread sources of drinking water for Cape Verdeans. With 965 kilometers of coastline, and dwindling underground sources, the arid nation has reason to seek solutions to its water problem through desalination. Plants on Sal, Boa Vista, in Praia, on Maio and on Sao Vicente--managed by Electra, the state owned energy company, and Aguas da Ponta Preta, a private enterprise--produce roughly 4,109,229 cubic meters of water annually. That supplies almost 30,000 Cape Verdeans with water and means that ocean-bathers are not the only ones drinking seawater regularly.
While plant technologies vary, the preferred, and most pervasive method in Cape Verde is Reverse Osmosis (RO). In regular osmosis, the solvent (in this case, salt) moves from an area of high concentration to a lower one through a semi-permeable membrane, equalizing the substance’s distribution. In contrast, during Reverse Osmosis, the salt water is pressurized to encourage highly concentrated salt water to separate from clean water, which collects on the opposite side of the semi-permeable membrane. Once this water’s salinity has decreased from about 38,500 mg of salt per liter to 400, it is treated and sent to a holding tank. RO Plants in Cape Verde produce between 1,000 and 5,000 cubic meters of water per day respectively.
RO boasts lower installation costs and is cheaper that Vapor Compression Distillation, and Mult-effect distillation, two other desalinization technologies used in Cape Verde. Still, operation is far more expensive than other water collection methods, like drilling and rainwater collection. Even modern RO plants that recycle energy require between 2 and 3 kwh per cubic meter of water. That is an enormous amount of energy for an island nation that must import most of its energy, especially when a single kilowatt hour costs about $0.30 US dollars. For a 1,000 cubic meter capacity plant, that’s about $600-900 dollars a day.
Moreover, because only 43% of the seawater is converted to potable water, the remainder—a highly saline “brine”-- is dumped back into the ocean. Scientists assert this saline concentrate is very harmful to marine life.
Other forms of desalinization are being tested to address these problems. Two Peace Corps volunteers, Brian Newhouse and Nick Hanson, are working with students at Assomada’s technical school to develop solar stills that use sunlight alone to convert seawater into fresh water. As sun light filters through a glass pane, the salt water heats and evaporates within the hot black box. Fresh water condenses on the glass, dripping down into a catch basin. While construction costs are low and materials readily available, so far the prototype produces only about two liters a day.
Nevertheless, Cape Verde is counting on desalinization for the future. Two new RO plants are under construction in Santiago’s Interior, which will produce a combined total of 12,000 cubic meters of water per day. Public-private partnerships modeled on “Aguas do Porto Novo”, established in 2007 on Santo Antao, will likely sprout up elsewhere, while private golf courses and hotels will continue to run their own private desal plants. If all goes according to planned, underground water sources will be left exclusively to agriculture and Cape Verdeans—ocean-bathers and otherwise--will drink seawater everyday.


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