Reverse Osmosis

Reverse osmosis (RO) is the separation of solvent from a solution using external pressure to drive the solvent towards a semi-permeable membrane. In water purification treatment, the solution is seawater or untreated water, and the solvent is clean drinking water. By applying pressure on contaminated water, pure water is forced to pass through the membrane, separating it from the solutes or impurities.

In the mid 1900’s, RO was used solely as a means of desalinating seawater. Over time however, the method’s decontaminating capabilities were also discovered which led to the production of commercial reverse osmosis systems designed primarily for household use. Today, it is is recognized as a proven water treatment technology.

Understanding How Reverse Osmosis Works

As implied by its name, reverse osmosis is the inverse of the osmosis process. To have a better understanding of this method therefore, requires some knowledge of osmosis.

Osmosis works on the principle that a solvent always moves from an area of low solute concentration to an area of high solute concentration.

To illustrate this, let’s consider a simple salt solution. The salt dissolved in the water, acts as the solute, while the water is the solvent. If you get a clear, U-shaped container, partition it using a semi-permeable membrane (a thin film of porous material that allows only the water to pass through), and pour the salt solution on one side and plain water on the other, what will happen is that the unsalted water will slowly move over to the side of the salt solution, going through the membrane.

The movement of water will continue until equilibrium is reached; that is, the two sides will be more or less equal in a salt-to-water ratio. This is how normal osmosis takes place, and the natural force moving the water from one side to the other until equilibrium is achieved is known as osmotic pressure.

Reverse osmosis on the other hand, goes against the natural order of things because in this phenomenon, the water moves from the area of greater solute concentration to the less concentrated area. While the porous membrane lets the water freely enter the other side, it blocks the passage of ions and molecules that are larger than water molecules, thus leaving behind the solutes, or in the example given above, the salt molecules.

On its own though, water would not flow in this manner. This is why reverse osmosis only happens when hydrostatic pressure greater than the osmotic pressure is applied on the more concentrated side of the semi-permeable membrane, forcing the water to go against its natural flow and move towards the area of weaker concentration.

For fresh or brackish water, the added pressure applied should be about 2 to 17 bar or 30 to 50 psi, while for seawater which exerts a higher osmotic pressure, the applied pressure should be approximately 40 to 82 bar or 600 to 1200 psi.

Reverse Osmosis in Water Purification

A key factor in the effectiveness of reverse osmosis in water treatment purification is the semi-permeable membrane used. In order to capably separate pure drinking water from the impurities contaminating it, the membrane’s microscopic pores should ideally be highly permeable to water and impenetrable to dissolved solutes.

The membrane should also be able to withstand high operating pressures and wide-ranging temperatures, and resist attacks brought about by bacteria and by chemicals like chlorine. Most semi-permeable membranes utilized for reverse osmosis are generally created from polyamide-based materials, which meet the given characteristics.

In a typical reverse osmosis unit, the product or clean water that is able to pass through the semi-permeable membrane is collected in a storage tank, while the impurities left behind are flushed down the drain or the “reject stream.” Contaminants that are filtered out through this water treatment method include salts, sediments, some organic compounds, heavy metals, nitrates, fluoride, and in general, substances that have larger molecular composition than water.

The efficacy of an RO unit in purifying water is measured by the rejection rate. This refers to the percentage or volume of the contaminants that are prevented passage by the membrane.

According to laboratory tests results published by the North Dakota State University, a properly maintained RO unit can achieve up to 92% rejection rate for nitrates, up to 99% for total dissolved solids, up to 98% for sulfates, and up to 93% for sodium.

Reverse Osmosis: The Pros and Cons

The reverse osmosis technology is very effective in the process of desalinating water. In fact, with over 15,000 RO desalinating plants worldwide, reverse osmosis is one of the most widely-used methods in seawater desalination.

Reverse osmosis has also proven to be useful in the production of mineral-free water utilized in the photo and print industry. This is because, like salt, most naturally-occurring minerals have molecules that are easily larger than water molecules.

But while RO units are less expensive to operate and maintain than water distilling systems, they are not the most efficient when used as standalone treatment systems for purifying ground and surface fresh water.

One reason is that reverse osmosis does not completely remove bacteria and pathogens that carry water-borne diseases. More importantly, majority of volatile organic compounds (VOCs) are smaller in size than water and can easily pass through the semi-permeable membrane. These contaminants therefore, remain in the drinking water if no other purification treatment is applied.

Another significant drawback of reverse osmosis is that it uses up a lot of water in the process. For every gallon of clean drinking water produced by an RO unit, about three gallons of reject water goes down the drain. And compared to other methods, it is one of the slowest in production, with a typical unit managing only to produce a few gallons of clean water a day.

Reverse osmosis systems also require regular maintenance and failure to do so can result in damaged membranes that may stop working altogether. This is particularly crucial because most users are unable to determine if a unit is still functioning safely and properly.

Consumers who want to be certain of the quality of their drinking water use reverse osmosis systems in combination with other water purification methods such as pre-treatment filtration, UV treatment and carbon filters.
 
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