When it comes to water treatment, two words commonly heard are purification and desalination. Like divisions of any niche topic, both terms are similar, yet distinctly different. What does each of these mean and what are the key points of difference between the two? To better find out and explain the key points of difference between the two I divided up this article by the different types of impurities and the relevant effects of each process.
Desalination is specifically the removal of salt from water whereas purification is a broad term encompassing all things that increase water quality, including desalination. Purification methods can include physical separation, biological processes, chemical treatments and electromagnetic radiation.
Biological impurities
Depending on what kind of desalination method, microorganisms and bacteria can potentially make it through to the final product. Distillation as a method of desalination is synonymous with purification as the water is reduced back to its pure form, free of bacteria and all other impurities INCLUDING salt whereas Reverse Osmosis alone can let some microorganisms make it through to the final product.
Although reverse osmosis does remove the majority of bacteria, it is not designed for this purpose so if the water is to be treated as a mixture of impurities other than just salt (seawater), RO will be combined with other water purification methods like filtration, chemical treatments or even electromagnetic radiation like UV.
Filtration
Perhaps the most simple form of water purification, filtration is excellent at removing some impurities from water which can be separated by physical means alone like sand, larger organic materials and even some bacteria. Two things general filtration is not good at removing are salt and microscopic organisms, which is where desalination methods and more intense purification techniques get involved. Filtration is however a key part of the overall desalination process, RO desalination systems require the water to be filtered prior to treatment to remove grit and other particles that could damage the membranes. So in summary filtration is almost always used in the process of desalination as a purification method to improve the efficiency of desalination however filtration itself does not remove salt from water.
If you are interested in finding out the various filtration stages which form a part of the reverse-osmosis desalination process, I encourage you to have a look at my article How Many Stages Are There In Reverse Osmosis? It Depends!
Petroleum waste
Petroleum waste is a tricky impurity than has the potential to clog up and even destroy most desalination systems which are inherently poor at removing hydrocarbons. Although desalination processes will remove a portion of hydrocarbons when salt is removed from the water the process cannot completely remove hydrocarbons from the water. Purification methods that can remove all hydrocarbons from water include chemical treatment and certain types of intense filtration. Multi-stage filtration systems are adept at removing hydrocarbons and other dissolved hydrocarbons from water, generally through a fine filter before passing through some sort of hydrophobic cotton or organic (carbon) filter. A new chemical method of purifying water that is heavily polluted with hydrocarbons is through ozone treatment, typically a reactor will produce ozone in situ and directly pump it into the water passing through a system, this ozone actively breaks up the hydrocarbons allowing them to be removed with simple filtration. This new technology is relatively nascent however it is being adopted on a larger scale by municipalities and industrial water users. If trying to extract drinking water from hydrocarbon-contaminated seawater either intense filtration or ozone treatment would need to be in conjunction with desalination due to neither of the aforementioned techniques having the ability to remove salt from the solution.
Chemical treatment
Although technically possible, chemical desalination is not practical. Chemical treatment involves using additives to improve the water quality, be it something to alter the acidity levels, kill bacteria or remove certain metallic ions from the water. Although not a feasible method of removing salt from water by itself, chemical treatment is often utilized as a part of the pre-treatment of the water prior to the liquid undergoing desalination. The main purpose of the chemical pre-treatment of the water prior to desalination is to minimize fouling and other wear and tear on the desalination membranes, the chemicals used in these processes are called “online” as they are used during production.
Online chemicals include: coagulants, flocculants, chlorination and de-chlorination agents and biocides, these are utilized to remove bacteria, sediments and other chemical properties to ensure efficient production takes place and clean water is produced. Some more hazardous chemicals are also used in desalination known as offline chemicals, these chemicals like acids are used when the plant is offline to clean membranes and other residues from the system to ensure optimal production when the plant is online. Chemical treatment is used in the purification of water in many ways including water softening where magnesium and calcium are removed from water and chlorination where bacteria are eradicated.
Electrolysis and Radiation
Direct exposure to electricity and/or radiation is a method that is utilized in both purification and more specifically desalination. Electrolysis is a method of purification in that it returns water to near 100% purity like distillation, this removes all salts, bacteria and other impurities. The interesting thing with electrolysis is that it chemically separates hydrogen and oxygen and then when it is burnt or ‘recombined’ with oxygen it forms pure water and also releases energy in the form of heat, much like gasoline. So, the entire concept of hydrogen as a fuel source is also technically a method of desalination! This process is not incredibly energy efficient when compared with techniques such as Reverse-Osmosis and Multiple Stage Flash Distillation however it does have a unique advantage in that it is incredibly scalable. The scalability of electrolysis desalination means that technically a system can be created with a battery and electrodes. Electrolysis was recently combined with a niche technique of partial desalination called ion concentration polarization by MIT to create portable desalination that weighs under 10kg and can be run off a small solar panel!
Although electrolysis technically desalinates and completely purifies the water it is not the most economic method on a large scale. Ultraviolet radiation, however, is a treatment technique that destroys bacteria and any other microbes from water in an incredibly efficient manner. Ultraviolet treatment can be performed at a huge industrial scale right down a pen-sized light that fits in your water bottle and for this reason, it has had rapid adoption with municipalities, remote households and even hikers!
https://www.waterworld.com/home/article/16202099/desalination-chemical-use-in-water-treatment
https://news.mit.edu/2022/portable-desalination-drinking-water-0428