The Desalination Process

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The Desalination Process

The Desalination Process

Reverse OsmosisThis process involves seawater being pushed through a membrane separating up to 99% of impurities. It is very commonly used on cruise ships and navy vessels, and is now making its way into mainstream water purification. The process consists of 4 main components:Pretreatment- Solids are removed, the pH is adjusted, and calcium sulphate (CaSO4) is added to reduce scaling.Pressurization- The water pressure is raised to meet the needs of the membrane.Separation- Dissolved salts are separated using a permeable membrane. This changes the salinity of the water from 35,000 ppm to 1,000 ppm.Stabilization- The pH is changed from 5 to as close to 7 as possible to attain a neutral acidity (Meganck).

DistillationDistillation involves heating the saline water, producing a fresh water vapour. There are a variety of different distillation process, including Multistage Flash and Multiple Effect. Both include a goal of reducing the vapour pressure of water to allow boiling at a low temperature.Multistage Flash- This process begins as the seawater passes through heating stages, and then is flash boiled, causing instant evaporation. The condensation is what forms the freshwater. Majority of the leftover brine is mixed in with the next batch of seawater to be further processed.Multiple Effect- In this process, steam is condensed on one side of a tube wall while saline water is evaporated on the opposite side. The saline water is ususally applied to the tubes in thin sections in order to speed up the evaporation process. Again, the condensed vapours are potable (Meganck).

Desalination is the process of removing dissolved salts and other impurities from seawater or brackish water. There are two main types of desalination processes:

Which Process is Best?Over the past ten years reverse osmosis has become the most popular method of desalination. Improvements in efficiency led to less energy consumption, reduced costs, and a more refined product. The costs are estimated to be half as much as distillation due to the high amount of heat required to cause evaporation (Gold Coast Water).

Figure 1: The Reverse Osmosis Desalination Process

Figure 2: The Distillation Desalination Process

Viability of DesalinationThe popularity of desalination is quickly on the rise, with almost 15,000 facilities across the world. It is the only type of water purification that is not dependent on the climate, and technologies are constantly being improved to ensure that there are more pro's than con's to desalination. As only about 1% of the worlds water supply can be used for drinking, and the world's population is growing exponentially, there is going to be a huge increase in demand for water. Therefore, desalination is an option that must be considered (Nellen).

Impact on the EnvironmentImprovements are constantly being made to reduce the amount of negative repercussions on the environment. The two main issues are greenhouse gas emissions and the marine environment. Most facilities are run using fossil fuels, causing environmental concern. However, facilities are beginning to use solar or wind power as an environmentally friendly alternative. As for marine life, the wildlife cannot survive when the brine (50,000 ppm) is released back into the oceans ("Salinity of Water"). In response to this, environmental regulations are soon being implemented (Nellen).

Impact on SocietyAs stated earlier, the population of the world is quickly increasing, and the amount of unpolluted freshwater quickly decreases. This means that the demand for water will become immensely higher. It is said that the demand for desalination facilities is expected to triple in the next 5 years. Desalination will provide access to the huge amounts of undrinkable water that make up 75% of the earth's surface. It is a vital step in sustaining social stability in many dry regions. The expansion of the desalination industry will also create a booming job market (Nellen).

Impact on the EconomyThe development of desalination plants is often questioned to do the high amounts of energy required. It is often more costly than collecting ground water. The cost of energy will determine the cost of the water, so in countries that rely on international energy prices, it can have a huge impact on the industry. Although desalination is the most expensive way to produce drinkable water, increases in demand may sustain its development (Nellen).

Figure 4: An example of an environmentally friendly desalination process run by solar power.

Video 1: A summary of the need for desalination.

Figure 3: A graph outlining the costs of running a desalination facility.



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