As you may remember from Module 1, the majority of Earth’s accessible water (i.e. not including a large amount of water trapped in minerals in the Earth’s interior!) is in the Oceans. In a sense, the Oceans would provide an unlimited supply of water, but of course, they are too salty to drink or use for most purposes. To use seawater for industrial, agricultural, or domestic/municipal supply, therefore, requires the separation of the water from the dissolved ions (mainly Na, Cl, Mg, SO₄, Ca, and CO₃). This can be accomplished in a variety of ways, but most commonly is done via either:

1. Distillation, in which the water is forced to evaporate and then collected, leaving behind a concentrated brine, or
2. Reverse osmosis, in which the water is forced through a semi-permeable membrane under pressure; the membrane physically excludes dissolved ions and other compounds, and only allows H₂O molecules to pass (Figures 1 and 2).

Of these, reverse osmosis (or seawater reverse osmosis, SWRO) has emerged as the more efficient approach, especially when scaled to produce the millions of gallons per day or more needed to meet the demands of even modest population centers.

Of course, removing the salt from seawater requires energy – and money. For that reason, it has been a subject of intense research and engineering efforts, in order to reduce costs through increased scale, improved efficiency, pre-filtration, and improved materials (most importantly, advances in membrane materials that require less pressure to push the water through but still exclude dissolved ions). Early desalination plants were restricted to a relatively small scale, and mainly in desert areas (e.g., the Middle East), or to meet water quality requirements for the CO river treaty of 1944 (e.g., the Yuma desalination plant in Yuma, AZ, brought online in 1997). However, with improving efficiency, increasing demand, and perhaps spurred by drought, desalination is now emerging as one potential viable solution, at least in areas with access to the ocean, and the economic resources to construct and operate the plants.

Figure 1. Photo of a desalination plant. Blue cylindrical coils in the background are reverse osmosis membranes wrapped around pipes that force the water outward under pressure.

Source: James Grellier (Own work) [CC BY-SA 3.0 or GFDL], via Wikimedia Commons

Figure 2 Diagram of a typical SWRO plant process. Click for a text description

Initial Chemical treatment with initial solids removal to traveling screens with filter out shells, wood, and other debris greater than ¼ inch. Then particle settlement: heavier solids are settled and removed from the water, then smaller solids are filtered from the water through sand filters, a diatomaceous earth filter is used next to remove microscopic materials. Any water removed from solids is recycled. Water is then transferred to cartridge filters which are in place to protect reverse osmosis membranes. It then goes through two passes of the reverse osmosis process where the water is put under high pressure and pumped through ¬¬sacks housing reverse osmosis membranes to remove the salt. It then goes to post-treatment and a holding tank.

Source: Tampa Bay Water and appears on NOAA