What Is Reverse Osmosis?

What Is Reverse Osmosis - Definition
Reverse osmosis or RO is a process that purifies water by pressing it through a semi-permeable membrane.

Reverse osmosis or RO is a water purification process that presses water through a semi-permeable membrane. Particles, molecules, and ions that are larger than water molecules remain on one side of the barrier, while nearly pure water exits across the membrane. Here is a look at how reverse osmosis works, its advantages and disadvantages, and its uses.

How Reverse Osmosis Works

As its name suggests, reverse osmosis is the opposite of normal osmosis. In osmosis, water moves across a semipermeable membrane, but from higher to lower solute concentration. It’s like diffusion, except with a membrane, and it is thermodynamically favorable.

Reverse osmosis, on the other hand, requires pressure that forces water across the membrane. The pressure overcomes the osmotic pressure opposing the water molecule movement. The pores in the membrane are semi-permeable, meaning they are large enough that water molecules slip through, but too small to allow larger particles.

How Reverse Osmosis Differs From Filtration

At a glance, you may think reverse osmosis is just a special kind of mechanical filtration. However, there are differences.

  • Filtration has pore sizes that are 0.01 micrometers or larger. In contrast, reverse osmosis is either nonporous or else has pores only about 0.001 micrometers in diameter.
  • Filtration strains materials solely based on particle size. Theoretically, all of the solvent passes through a filter, without any dependence on the concentration of solutes or pressure. Size, solubility, and diffusivity determine what passes through a reverse osmosis membrane. There is always some water left on the unfiltered side of the membrane.

What Reverse Osmosis Removes (and Doesn’t Remove)

Reverse osmosis greatly purifies water, but it does not remove all compounds besides water.

Here is a list of common contaminants reverse osmosis does remove:

  • Insoluble metal compounds
  • Some ions, such as nitrates, nitrites, and sulfates
  • Some soluble metal ions, including sodium, lead, chromium, calcium, copper, magnesium, and manganese
  • Arsenic
  • Radioactive fall-out particles
  • Some bacteria, fungi, algae, protozoa, and viruses
  • Most oils
  • Most drugs and hormones

*Note that sodium is a small ion, so RO systems make leak some sodium through the membrane.

On the other hand, reverse osmosis, on its own, does not remove the following substances:

  • Alcohol
  • Most organic compounds (VOCs, pesticides, herbicides, etc.)
  • Dissolved gases, including radon, methane, or carbon dioxide
  • Chlorine
  • Chloramine and other chlorine by-products
  • Fluoride
  • A few microorganisms, including some bacteria and viruses, especially those that grow on the membrane

However, reverse osmosis filters often include an activated charcoal filter, which removes many organic contaminants, chlorine, and fluoride. Some systems include an ultraviolet light that kills any microorganisms that make it through the RO membrane. Another disinfection process is ozonation, which also breaks apart some undesirable chemicals.

So, you’ll read conflicting reports on what reverse osmosis does and does not remove because it depends on other system components. Combining RO with other methods leaves water clean of pretty much everything besides dissolved gases and a few trace contaminants.

Advantages and Disadvantages

Reverse osmosis offers both advantages and disadvantages for water purification.

The main advantage is that the water from a good system is very, very pure. It is safe for drinking, release into the environment, and use in commercial processes.

There are also disadvantages:

  • There is the expense of buying the system and the energy cost of running it.
  • Because of low back-pressure, most home systems are very inefficient. Anywhere from 5% to 55% of the water exits the filter. The remainder gets lost as waste.
  • For drinking water, reverse osmosis requires additional treatment methods, such as charcoal filtration and UV lights.
  • RO removes minerals from water that aid human health. In commercial drinking water operations, companies often add some of these minerals back. Otherwise, drinking only demineralized water carries some health risks.
  • The membrane requires cleaning or replacement.
  • Not all of the input water gets recovered.
Distilled water is free of minerals.

5 Ways to Distill Water

Distillation is a popular alternative to osmosis. Here’s how it works in water purification.

Reverse Osmosis Uses

The largest use of reverse osmosis is for drinking water purification and desalination. Commercial systems include several steps, so they remove more contaminants than reverse osmosis on its own. Home systems assume the source water is already potable, so they don’t always remove additional contaminants.

Another common use is waste water purification. This water removes effluents, freeing the water for a return into a system, release into the environment, or further purification for drinking.

In the food industry, reverse osmosis concentrates juices and syrups with the use of heat. The dairy industry applies this method for concentrating milk and making whey powder. Reverse osmosis also produces low-alcohol beer.

Industries use RO as a means of removing minerals for water so it won’t form deposits on machinery. For example, RO water protects electrodes in hydrogen production.

RO water finds use when making artificial sea water for aquariums. It is also a good water choice for fresh water aquarists because it is very soft water.

Window cleaning is easier using reverse osmosis water than tap water. It does not leave deposits or spots.


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