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One of my cousins recently bought a water purifier with the usual UV as well as reverse osmosis features. Ultra Violet rays kill harmful disease causing microbes. But why does anyone want to buy Reverse Osmosis (RO)  purifier when your water does not contain many salts by spending more money? When I asked her the same Q she replied,"I don't know anything about it. It is a new technology everybody around goes for. So I too bought the purifier that contains the tech!"

Hmmm... The person who is selling the water purifiers usually doesn't tell you why you need RO tech in your purifier. So there is a need for understanding the process of RO and why you require it. In order to do this, first we have to understand what naturally occurring process of Osmosis is.

Osmosis is a naturally occurring phenomenon and one of the most important processes in nature. It is a process where a weaker saline (salt) solution will tend to migrate to a strong saline solution. Natural examples of osmosis are when plant roots absorb water from the soil ( as the plant cells contain more salt) and our kidneys absorb water from our blood.

You will get a better idea if you see the diagram below which shows how osmosis works.

Osmosis Diagram

A solution that is less concentrated will have a natural tendency to migrate to a solution with a higher concentration. For instance, if you had a container full of water with a low salt concentration and another container full of water with a high salt concentration and they were separated by a semi-permeable membrane (a membrane that will allow some atoms or molecules to pass but not others), then the water with the lower salt concentration would begin to migrate towards the water container with the higher salt concentration.

Reverse Osmosis is the process of Osmosis in reverse. While Osmosis occurs naturally without energy required, to reverse the process of osmosis you need to apply energy to the more saline solution. A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not the majority of dissolved salts, organics, bacteria and pyrogens (substances, typically produced by bacteria, which produce fever when introduced or released into the blood by affecting the central neurons).  However, you need to 'push' the water through the reverse osmosis membrane by applying pressure that is greater than the naturally occurring osmotic pressure in order to desalinate (demineralize or deionize) water in the process, allowing pure water through while holding back a majority of contaminants.

The diagram below explains it in a better way.


Reverse Osmosis Diagram

Reverse Osmosis works by using a high pressure pump to increase the pressure on the salt side of the RO and force the water across the semi-permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved salts behind. The amount of pressure required depends on the salt concentration of the water that is fed into the system. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure.

The desalinated water that is demineralized or deionized, is called permeate (or product) water that you collect from the purifier and drink. The water stream that carries the concentrated contaminants that did not pass through the RO membrane is called the reject (or concentrate) stream.

Reverse Osmosis is capable of removing up to 99%+ of the dissolved salts (ions), particles, colloids, organics, bacteria and pyrogens from the feed water (although a RO system should not be relied upon to remove 100% of bacteria and viruses and UV rays should be used for this purpose in a water purifier). A RO membrane rejects contaminants based on their size and charge. Any contaminant that has a molecular weight greater than 200 is likely rejected by a properly running RO system (for comparison a water molecule has a MW of 18). Likewise, the greater the ionic charge of the contaminant, the more likely it will be unable to pass through the RO membrane. For example, a sodium ion has only one charge (monovalent) and is not rejected by the RO membrane as well as calcium for example, which has two charges. This is also why an RO system does not remove gases such as carbon dioxide (CO2) very well because they are not highly ionized (charged) while in solution and have a very low molecular weight. Because an RO system does not remove gases, the permeate water can have a slightly lower (acidic) than normal pH level depending on CO2 levels in the feed water as the CO2 is converted to carbonic acid.

Reverse Osmosis is very effective in treating brackish, surface and ground water for both large and small flows applications.

If your water tastes normal or 'sweet' and does not contain many salts ( Get tested if required and if you are not sure. If the total dissolved salts in your water supply exceeds about 60 to 80 PPM) then it is advisable to install a RO-based water purifier. If not, then you are better off buying a standard UV-based water purifier., you need not buy a water purifier that contains this technology.  You can use the purifier with just UV tech in this case. Only when you are using salt water for drinking or cooking purposes you need RO mechanism to remove unwanted or harmful chemicals.

You have to look for these things too while buying water purifiers:

RO membrane pore size: This is the most important factor in a RO water purifier. Manufacturers indicate this indirectly by specifying the size of impurities that the purifier can block. 

Number of purifying stages: Based on your needs, you can either go for a RO-only purifier or a RO+UV purifier, with additional stages as necessary. Some purifiers also have stages to enhance the taste of the filtered water by making it sweeter.

Storage tank capacity: The capacity of the built-in storage tank should be adequate for your daily needs. This is because, unlike UV-based purification, RO-based purification is a slow process and you need a large storage tank  that can provide adequate supply if there are several members in your family.

Flow rate: As RO-based purification is slow, therefore, it is also necessary to check the rate at which the purifier can deliver filtered water. The manufacturer in units of litres per hour usually indicates this.

Life span of RO membrane and UV lamp: Knowing the life span of these critical components will help you calculate your maintenance costs.

What do these things denote while dealing with water purifiers?

UF= Ultra filtration using a medium fine enough to retain colloidal particles, viruses, or large molecules.

TDS= Total dissolved salts ( Soft water has a lower TDS value while hard water has higher TDS value)

Carbon pre-filter: This is added to remove unpleasant odors from water, as well as chemicals like Chlorine added to the water.

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A new technique to purify water using shock waves:

http://www.sciguru.org/newsitem/19257/desalination-shocking-new-way...

What do these things denote in water purifiers?

UF= Ultra filtration using a medium fine enough to retain colloidal particles, viruses, or large molecules.

TDS= Total dissolved salts ( Soft water has a lower TDS value while hard water has higher TDS value)

Carbon pre-filter: This is added to remove unpleasant odors from water, as well as chemicals like Chlorine added by water suppliers.

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