Desalination is the process of removing the salt from saline water to make it into drinkable freshwater. Once in the form of fresh water it can be distributed for consumption, agriculture, and other industrial processes. While roughly 70% of the earth’s surface is covered by water, only approximately 0.007% of that water is accessible, drinkable freshwater .
Most of the fresh water on the planet is stored in snowbanks or glaciers making it inaccessible for general use. Additionally, parts of the existing water supply face contamination by industrial processes. As well as hazardous material mismanagement, and plastic pollution . Desalination produces freshwater resources on-demand anywhere there is access to seawater. Some islands and countries with arid climates and limited access to freshwater would continuously need to import freshwater to keep up with demand without the help of desalination plants.
According to the United Nations, by 2025, more than 1.8 billion people will live in regions with limited access to freshwater sources . Advancements in desalination technologies can greatly reduce the risk of water insecurity. And can protect agriculture from harsh droughts. Desalination processes make the ocean, and saline lakes such as the Dead Sea, viable sources of freshwater.
Before understanding how desalination technologies work, it’s important to understand the structure and composition of saline water. Saline water is any water with salinity greater than 0.1% or 1 ppt (parts per thousand) . The world’s oceans have a salinity of about 3.5% on average . The salinity of seawater varies depending on region, depth, and climate.
Saline water forms through a process called ionization. Water is a dipolar molecule meaning its structure is overall neutral. Yet, it has a local positive side and a local negative side . When some ionic compounds such as sodium chloride (table salt) are immersed in water, the charged poles of the water are strong enough to break the ionic bonds of the salt. Thus producing a positively charged sodium ion and negatively charged chlorine ion .
These ions then float freely in the water creating a homogeneous mixture. To be clear, these ions are produced on an atomic scale. They are not little pieces of salt floating in water. This is why the desalination process requires a lot of energy to extract the water .
There are two types of desalination: thermal desalination and reverse osmosis. Thermal desalination is the more straight-forward of the two. Saline water is boiled to produce steam, which rises above the base fluid as pure, gaseous water. This steam is then condensed back into pure, liquid water.
This technology has been used to purify water for a long time and has been used to desalinate water in bulk since the 1960s . Typically electric heaters run by fossil fuels or nuclear power are used to power these systems because it is very energy-intensive to boil large quantities of water.
The second type of desalination, reverse osmosis, is more common today. This process pressurizes saline water and pushes it through a specialized filter. This removes the salt from a portion of the fluid, creating desalinated water.
This process is only 50% efficient. Meaning that processing only converts 50% of the water into freshwater. The other 50% becomes more concentrated saline water. This is because there’s less water containing the same amount of salt ions.
The byproduct, brine, and is expelled from the system after the process . This strategy is less energy-intensive than thermal desalination. However, it is still very energy-intensive, requiring large amounts of energy to pressurize the water to a high enough pressure for it to pass through the filter.
Both desalination processes are very energy-intensive and sometimes require significant energy production. With fossil fuels, this is expensive and produces large amounts of greenhouse gases.
Reverse osmosis desalination produces brine, expelling it back into the ocean. This fluid is still saline water. But its higher salt concentrations make it denser and can make it toxic for the local marine life on the floor of the ocean .
It is also true, as with any infrastructure development, that the start-up costs to build a desalination plant are expensive. Importing freshwater resources is often a cheaper short-term solution. It is widely argued that the benefits of providing at-risk communities with access to freshwater outweigh these negatives, but it important to consider these drawbacks nonetheless.
Desalination is largely used in areas with scarce water supplies and abundant resources such as the Middle-East and North Africa where this technology is a necessity long-term. Overall, desalination is a very promising technology for a more sustainable future, and it will continue to improve as technological advancements continue to be made.
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