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For some small island nations there is no choice but to liberate fresh water from the sea. Larger countries and coastal communities have or are considering augmenting their potable water supply with desalinated water in efforts to meet their populations growing needs and add a component of supply that is truly drought proof.
Ground and surface waters have little dissolved salts or solids and with minimal treatment can yield potable water safe for drinking. Seawater has a considerable amount of dissolved salts/solids, some 70 times the amounts considered safe for human consumption. It takes a large amount of energy and highly specialized corrosion proof equipment to liberate fresh water from saline waters.
The primary difference between brackish water and seawater is in the amount of dissolved salts/solids. Seawater contains higher amounts of dissolved salts/solids (from 15,000 milligrams per liter (mg/l) to over 40,000 mg/l of total dissolved solids). Water that has only 1,000 – 15,000 mg/l dissolved salts/solids is considered to be Brackish. The greater the salt content of the water, the higher the pressure or electric power needed to treat water using membranes, resulting in higher energy costs.
There are principally two main methods of desalting, thermal distillation and membrane separation. Due to the lower energy costs required for membrane separation methods, it is the preferred technology globally. Specifically, Reverse Osmosis membrane separation is the mostly widely used technology used for brackish and seawater desalination. Reverse osmosis is a fluid separation process in which the saline water is pressurized, and fresh water is separated from the saline by passing through a semi-permeable membrane that rejects the salts.
Desalinated water is highly purified, meaning very low in dissolved salts/solids, because of this desalinated water has a high potential to dissolve solid materials, corroding pipes and other distribution fittings/equipment. Typically, desalinated water is “stabilized” by adding naturally occurring minerals, calcium and/or magnesium back into the water prior to it being put into the distribution system.
Obtaining fresh water from the sea is arguably more sustainable than removing water from fresh water sources such as ground water aquifers, lakes and rivers, given the relative size of the World’s oceans compared to the dwindling size and increasingly polluted fresh water sources. Billions of gallons of fresh water are liberated daily from the oceans by the natural evaporative/precipitation water cycle.
Environmental concerns include air pollution and greenhouse gas emissions from the power plants that provide electricity and/or thermal energy to the desalination plants.
Regardless of the method used, there is always a concentrated waste product consisting of everything that was removed from the created fresh water. This is sometimes referred to as brine, often disposed of in the ocean, and careful planning and design needs to ensure that this brine stream is dispersed as quickly as possible.
Marine life entrapment and impingement of the open sea intakes and brine outfalls for these systems is an environmental concern. Design improvements and extensive monitoring of existing systems have reduced the environmental impact of both.