Salt Water Distillation
Salt water distillation is the process of separating fresh water from a salt water solution. This process forms the basis of the water cycle, and therefore life on earth as we know it. Distillation takes place in nature every single day: the sun evaporates water from the surface of lakes, streams, and oceans, and the resultant water vapor is then condensed again when it meets with cooler air, forming dew or falling as rain.
We can recreate the distillation process artificially to obtain fresh drinking water from salt water and in many parts of the world this important process provides the majority of fresh water for the local population. Many countries have limited sources of fresh water and when 97.5% of the world’s water supplies are locked in polar ice caps or salt water oceans, desalination plants are an important method of providing safe, clean drinking water.
Desalination plants have been commonly used since the 1950’s due to the increasing demand for fresh water for domestic and industrial usage. They are commonly found in arid regions near large sources of salt water, for example oceans. Many countries in the Middle East, for example United Arab Emirates, rely on desalination plants for the majority of their fresh water supply, as internal aquifers are often remote and inaccessible.
The world’s largest desalination plant is the Jebel Ali Plant in the United Arab Emirates. At peak operating capacity, the plant is capable of producing as much as 300 million cubic meters of water per year. The largest desalination facility in the United States is in Tampa Bay, Florida. This only runs at about 12% of the output of the UAE plant. On a smaller scale, desalination is also practiced on boats and submarines during long ocean voyages.
While there are many variations of industrial desalination processes practiced, the Jebel Ali Desalination Plant uses multi-stage flash distillation—salt water distillation is basically a two part process. In the first step the water must be evaporated into steam. In order to do this, heat must be applied. Eventually, if enough heat is applied, the water begins to boil. More heat must then be added to evaporate the water into steam. This is known as the ‘heat of vaporization of water’ point and is about five times greater than the temperature required to heat water from freezing point to boiling point.
In the second stage, water needs to be condensed again. This involves heat being added and then removed, which if done separately, would be very costly and inefficient. In saline distillation plants, some of the heat energy is reused by transferring it from the steam to the salt water in order to heat up more water. Table salt is sometimes produced as a by product of the desalination process.
Unfortunately, while desalination plants are very effective at producing a fresh water supply from salt water, they are very costly to run in terms of energy requirements when compared to using fresh water from groundwater supplies or rivers. As a result, there is a great deal of interest currently focused on developing methods of providing other, alternative, ways of providing fresh water in regions where supplies are limited.
There are companies that make small salt water distillation units that can be used on boats or during a disaster situation. You can also find directions for building an emergency type distillation unit on the internet. It would also not be a bad idea to keep emergency water pouches on hand for a disaster situation.