Desalination is the process of removing soluble salts from water to render it suitable for drinking, irrigation, or industrial uses. In distillation, salt water is heated in one container to make the water evaporate, leaving the salt behind. Other desalination processes include:
♦ Electrodialysis (ED) is the the use of porous membranes to filter out negatively and positively charged salt ions. This process filters water using an applied electric potential difference.
♦ Freezing is done based on the principle that water excludes salt when it crystallizes to ice. When the salt water is reduced to a specific, critical temperature, ice crystals composed of fresh water are formed. It is then possible to mechanically separate the ice crystals from the solution and remelt them to get fresh water
♦ Reverse Osmosis, (RO) is based on pressure, generated by the presence of salt in the water, forces water through a membrane permeable only by pure water. The solute, in this case, salt and other impurities, is retained on one side of the membrane while the solvent, pure water is filtered through the other side.
Considerations
♦ Energy: One of the first things to note about desalination is that, whichever method is used, a lot of energy is needed. It is both expensive and the use of energy can create undesirable carbon emissions. It is very important to note that the amount of energy for distillation is fixed for a given volume of water, but the energy for reverse osmosis depends on how salty the water is to start with. For this reason, it is much more attractive to desalinate slightly salty water or treated sewage effluent than it is to deal with seawater, which has roughly 35 grams of salt in every liter of water. For reverse osmosis, the energy also depends on the temperature of the water, less energy being needed for warmer water.
♦ Cost: A significant part of the cost equation for desalination is owing to the high energy consumption, but desalination plants are sophisticated pieces of equipment with high capital costs and quite significant maintenance requirements too. Desalination plants do not last as long as traditional water treatment plants which sets a problem when thinking about future sustainability. The actual cost for a given plant is very site specificand also dependent on the size of the plant.
♦ Environmental Impacts: It is not possible to use desalination as a water source and to assume that it has no environmental impact. As mentioned earlier, the high energy consumption leads to greenhouse gas production and the salt that has been extracted has to go somewhere. Depending on the process, the salt concentration in the waste stream could be anywhere from double that of the source water, up to a solid salt product (although that would seldom happen in practice).
♦ Management and operation: A desalination plant is complex and has serious challenges in terms of corrosion and fouling, regardless of the specific process being used. While it is possible to design a small reverse osmosis plant, say, for rugged conditions, it is not feasible to expect a large desalination plant of any sort to operate reliably without expert operational and maintenance support.
International Desalination Association – Desalination for a Better World
References
Video: “Desalination for a Better World,” International Desalination Association. Web. http://www.idadesal.org/news.aspx?ArticleID=ee04e16d-077e-44df-8c3a-95fda80ab1cb
“Desalination Process,” Sea Water Applications. Web. http://www.arvanitakis.com/en/sw/desalination_process.htm
Image 1: “Desalination,” Australian Water Association. Web. http://www.awa.asn.au/AM/PrinterTemplate.cfm?Section=Desalination
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Table of Contents:
o CSO in NYC
• Combined Sewer System
• Eliminating CSO
o Drinking Water in NYC
• Desalination
• Buried Streams
o Holland
