Osmosis vs Reverse Osmosis – What’s the Difference?

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You may have heard of osmosis if you were paying attention during science class, but most of us are a bit rusty about the details.

Reverse osmosis is closely tied to osmosis, so it’s important to get an understanding of the naturally occurring process of osmosis first.

Let’s take a look at the differences and similarities between osmosis and reverse osmosis, which should give you a solid grasp of the two processes.

Key Takeaways

  • Osmosis refers to the naturally occurring process where water from a weaker saline solution will migrate toward water with a stronger saline solution. This occurs until the concentrations are equal.
  • Reverse osmosis is essentially osmosis in reverse, with pressure used to force water from a stronger saline solution toward a weaker saline solution through a semi-permeable membrane.
  • The main difference between osmosis and reverse osmosis is that osmosis is naturally occurring whereas reverse osmosis is not.

What Is Osmosis?

Osmosis refers to the naturally occurring process whereby water from a weaker saline solution will migrate towards water with a stronger saline solution through a semi-permeable membrane. This migration will continue until the two solutions reach an equal concentration.

This process is naturally occurring and happens without any energy expenditure. It is constantly occurring in all living things and is the primary process allowing water to pass through cell membranes.

What Is Reverse Osmosis?

Reverse osmosis is essentially the naturally occurring process of osmosis but in reverse. In other words, water is forced through a semi-permeable membrane from the stronger saline solution side to the weaker saline solution side.

Because this is the opposite of what would occur naturally, force is required to push the water through the membrane. In reverse osmosis, this force is water pressure.

The reversal of the osmosis process allows for contaminant collection via the membrane. The membrane separates the contaminant-filled water (the high saline solution water) from the clean filtered water (the low saline solution water). At this point, the contaminant-filled wastewater is flushed down the drain, while the clean water is stored or sent onto the next filtration stage.

blue reverse osmosis membrane

Osmosis vs Reverse Osmosis – What’s the Difference?

What is the difference between osmosis and reverse osmosis at a glance?

Water Movement

The biggest difference between osmosis vs reverse osmosis is the direction of water flow. Osmosis occurs through a potential gradient, while RO occurs against one such gradient.

Natural vs Artificial Process

Furthermore, in osmosis, natural osmotic pressure drives the process, while in reverse osmosis an external pressure greater than the naturally occurring osmotic pressure must be applied for the process to work.

This means osmosis will naturally occur without the help of external force, while reverse osmosis requires external pressure for it to work. This is also the reason that reverse osmosis will not work when water pressure is very low, and will work sub-optimally when the pressure is less than ideal.


As mentioned previously, osmosis is a naturally occurring process and is constantly occurring in nature.

A common example would be how a tree’s roots draw water and minerals up from the soil. In this case, the soil is the low-concentrate solution, while the root is the high-concentrate solution. Another example would be how a raisin placed in a glass of water will slowly absorb water and swell up until it reaches osmotic equilibrium.

As far as the industrial use of osmosis, think of the rehydration of dried products. Lettuce soaking up water mist in a grocery store is a common situation we’ve all seen before.

As reverse osmosis is not a naturally occurring process, it doesn’t occur outside of specific industrial, commercial, or home uses. The most common use of reverse osmosis is for water filtration. This includes large-scale filtration and desalination as well as small-scale home filtration.

RO is also used in several food production processes including concentrating egg whites and fruit juices, removing alcohol from wine and beer, and concentrating milk before shipping.

What Are the Similarities Between Osmosis and Reverse Osmosis?

Osmosis vs reverse osmosis – what are the similarities?


Both osmosis and reverse osmosis occur through a semi-permeable membrane. Reverse osmosis uses this membrane to trap and separate dissolved solids, while natural osmosis simply occurs through a membrane.


In both processes, it is primarily water molecules crossing the membrane.

Osmotic Pressure

Both processes involve osmotic pressure. In osmosis, this force pushes water from the low concentrate side to the high concentrate side. In RO, a force greater than the naturally occurring osmotic pressure is needed to push water through the membrane.

Applications of Osmosis

As osmosis is naturally occurring, it is constantly taking place both inside and outside the body. Inside the body, osmosis is used to extract nutrients from food and filter waste products out of blood through the kidneys.

Outside the body, osmosis is used for the preservation of both fruit and meat. In fruit preservation, osmosis is used to dehydrate it, while in meat preservation osmosis draws in salt – curing the meat and preventing bacteria from finding their way in.

Applications of Reverse Osmosis in Water Treatment and Outside

As mentioned previously, the primary use of reverse osmosis is water filtration. This spans several different applications including desalination of seawater, desalination of brackish water, production of ultrapure water for industry or drinking, as well as for wastewater treatment. This also includes home water filtration via small-scale RO systems.

Under Sink Reverse Osmosis System

As far as industrial applications go, RO is used in the power, oil and gas, petrochemical, mining, food and beverage, and paper production industries.

In industrial applications, RO has multiple uses beyond simply producing high-quality purified water. It’s used for concentrating materials found in feedwater, recovering valuable resources from reject and wastewater, reducing wastewater volumes, and for solvent concentration.

What Does Reverse Osmosis Purification Remove from Water?

Reverse osmosis is a highly effective purification method and removes nearly all contaminants from water down to just .0001 microns. As a result, RO will remove pesticides, salts, heavy metals, sediments, industrial solvents, etc., as well as most microbial contaminants like bacteria, viruses, and cysts. It’s also capable of removing radioactive contaminants like radium and uranium.

While it’s highly effective, reverse osmosis alone won’t remove everything from feed water. It has trouble with certain contaminants like chlorine, which can damage or destroy the RO membrane. This is why most RO systems for home use come with carbon pre-filters. Sediment pre-filters are used to prevent membrane clogging.

In addition to removing harmful contaminants, RO membranes will remove healthy minerals like magnesium, calcium, and potassium. This is an unavoidable part of the filtration process but can be mitigated using a remineralization filter stage to add back essential minerals.

Pros and Cons of RO Water Treatment

Let’s take a look at some of the advantages and disadvantages to reverse osmosis water treatment.


Advanced Filtration

Reverse osmosis provides one of the most thorough water filtration types available. It will remove a broad range of organic and non-organic contaminants.

Water Taste and Aesthetics

After feed water has passed through the reverse osmosis process, the resulting product is exceptionally clean and neutral-tasting water. The process not only removes harmful contaminants but also eliminates the vast majority of chemicals that can impart an off taste to water.

Healthy Drinking Water

As RO strips nearly everything from water except for pure H2O, you’re left with exceptionally healthy drinking water. Nearly all harmful contaminants are removed so any health concerns you might have should be relieved.

Reduced Bottled Water Use

If you’re drinking bottled water regularly, then switching to RO water will not only save you money but it will reduce your usage of plastics. Plastic from bottled water is a significant environmental concern, so cutting down on this is a nice bonus.


While RO water is exceptionally pure, it’s not all that expensive to produce. A typical under sink home RO system can be purchased for just a couple hundred dollars, and ongoing maintenance costs are fairly minimal.


Waste Water

Reverse osmosis produces waste water as a result of the filtration process. This is unavoidable, and shouldn’t be seen as a major drawback but rather as a by-product of the system working properly.

A standard ratio in RO filtration is 4 parts waste water to one part filtered water (permeate). This can be reduced by as much as 90% by installing a permeate or pressure pump. Additionally, waste water can be collected and recycled by using it for gardening, washing clothes, cleaning patios and driveways, etc.

Mineral Loss

Another drawback of such an effective water filtration method is that beneficial minerals are also stripped away by the membrane. This means that healthy minerals like calcium, magnesium, and potassium are removed alongside harmful contaminants.

Luckily, drinking water is not a significant source of mineral intake in the human diet. Only around 5% of our daily intake comes from water.

woman sitting in front of water glass

Additionally, if you wish to add these essential minerals back to your water, you can do so fairly easily by adding a remineralization stage to your RO system.

Space Requirement

An under sink RO unit typically takes up about 2’ x 2’ of space when you include the storage tank. This means you’ll need enough space under your sink or in an adjacent cabinet to accommodate the system.

Flat Taste

Some people find RO water to have a bit of a flat taste, especially if they’re used to drinking tap water. This typically passes with time, and most people quickly get used to the taste of RO water.

If you have any questions about reverse osmosis vs osmosis please don’t hesitate to leave a comment below!

About the Author Alexandra Uta

Alex is a content writer with an affinity for research and a methodical attention to detail. Since 2020, she has fully immersed herself into the home water treatment industry only to become an expert herself. Alex has been using water filters and similar products for years which has gained her lots of hands-on experience.
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