What Does Reverse Osmosis Do

Reverse osmosis is a separation process used in everything from the food processing industry, to concentrate fruit juices, to water filtration and dialysis, where it is used to remove contaminants. In this article we will discuss reverse osmosis, and osmosis, in order to answer the question, "What does reverse osmosis do," by following the process as it filters pollutants from drinking water.

osmosis filter

To answer the question, "What is reverse osmosis?", we need first to look at osmosis.

OSMOSIS

Let's say you have a container with two compartments, separated by a very fine screen, and there is pure water on one side of the screen and salt water on the other side. The screen is so fine the salt molecules cannot pass through it, but the water molecules can. If normal osmosis takes place, the salt water concentration will slowly become diluted by the fresh water.

In reverse osmosis, pressure is applied to the salt water side, forcing the water out, but leaving the salt molecules behind the filter. Since the process is going against the normal inclination of the water, it is referred to as reverse osmosis.

In the case of reverse osmosis used in filtering water, the screen is called a semi-permeable membrane. The membrane is about as thick as a piece of cellophane with pores so small only particles the size of a water molecule, or smaller, can pass through.

As the water squeezes through, contaminants are left behind to be flushed from the system. To speed up the process, it may be necessary to increase the amount of water pressure by adding a booster pump to the water line. Even with adequate pressure, however, an appreciable amount of water will still not pass through the membrane and will be lost as waste.

Reverse osmosis systems are very slow producers and, in order to have enough clean water for bathing and doing laundry, a storage tank (with a diaphragm to maintain pressure) is needed.

One advantage of these systems is that they do a great job in cleaning up brackish water, or hard water, i.e., water with excessive minerals such as calcium, iron, lead, magnesium, sulfates, etc.

Disadvantages include the fact they remove all minerals, even the trace minerals our bodies need, and water without minerals is slightly acidic and stale or flat tasting.

Also, since cancer seems to thrive in an acidic environment, many doctors advise against drinking de-mineralized water. Anyone using one of these systems might want to look into taking mineral supplements.

Another disadvantage is they do not remove pollutants having a molecular size smaller than water. If there is chlorine in your water, for example, it will not be removed by reverse osmosis because its molecules are too small. All of the carcinogenic by-products of chlorine, and many synthetic organic chemicals, will not be removed either.

The EPA recommends activated charcoal filters to remove such contaminants and one should definitely be used in combination with a reverse osmosis system.

Even if the water is not chlorinated, a carbon pre-filter is recommended to remove any volatile organic chemicals for health reasons and to protect the membrane.

So, there you have it. A short explanation of what reverse osmosis is, how it works, and what it removes.

The Good: As you have seen, reverse osmosis systems will do wonders in cleaning up brackish water and water with excessive mineral content and, when used in combination with a carbon filter, delivers very clean water.

The Bad: The process is also bulky, prone to maintenance issues, is more expensive than other filtering technology and delivers a "flat" tasting product some have deemed unhealthy for long term consumption.

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What Does Reverse Osmosis Do

OSMOSIS