What is Reverse Osmosis?

Reverse osmosis uses pressure to force water through a semi-permeable membrane. This separates a stream of treated water from a concentrated stream of contaminants like dissolved salts, volatile organic compounds, bacteria, and viruses.


To illustrate this process, we’ve set up an experiment that anyone can do at home. First, prepare freshwater and a concentrated aqueous solution on opposite sides of the membrane.

What is Reverse Osmosis?

Reverse Osmosis (RO) is one of the most effective water filtration methods to produce clean, great-tasting drinking water. It removes 98% of total dissolved solids (TDS), making it safer and healthier than tap water.

An RO system uses a semi-permeable membrane to separate water molecules from contaminants. By applying pressure to reverse the osmotic force that favors even distribution of solute concentrations across a membrane, pure solvent is forced through the membrane to the side with lower solute concentration. This is called the “reverse osmosis effect”.

During reverse osmosis, large molecule groups are rejected from the membrane and flushed down the drain. To avoid wasting precious drinking water, the best RO systems include a storage tank to hold the rejected water until it is needed.

RO systems are available for point-of-use (POU) installation under kitchen or bathroom sinks, or for whole house filtration with a booster pump and larger storage tank. There are also several different cartridge styles to choose from depending on your preferences for ease of maintenance, cost and environmental impact.

Some people worry that reverse osmosis will strip vital minerals from the water, but our bodies get the minerals we need from food, not water. Most tap water is already mineral-rich and contains many beneficial minerals. However, some homes may have high TDS levels and require a more extensive filtering method like RO to remove them.

How Does Reverse Osmosis Work?

Reverse osmosis is a process that relies on osmosis to separate water molecules from dissolved salts and minerals. It does this by increasing the pressure of a stronger solution, forcing it across a semi-permeable membrane to a less concentrated side. This forces a majority of the dissolved contaminants away, leaving behind pure water.

The reverse osmosis membrane is made of a synthetic plastic material that allows only water molecules through and rejects contaminants such as sodium, chlorine, calcium and more. A pre-filter and carbon filter are also often part of a standard RO system to prevent these materials from clogging the membrane.

As the pressure is applied to force water through the membrane, some of the rejected water off-shoot can be used to power the pumps that create this high level of pressure, making this a self-sustaining system in terms of energy efficiency. This is referred to as ‘re-harvesting’ energy.

Reverse osmosis is effective on a municipal, commercial and even military scale, with many desalination plants using it to produce fresh drinking water. It is also a popular choice for homeowners who want to ensure their family is only drinking safe and healthy water, especially those with sensitive stomachs or skin issues. Those raising fish or coral in home aquariums may also choose to use reverse osmosis to provide their aquatic pets with the best, cleanest water.

How Does Reverse Osmosis Remove Contaminants?

Reverse Osmosis is a filtration process that uses semi-permeable membranes to remove contaminants from drinking water. The first stage of a typical reverse osmosis system includes a sediment filter that removes the largest particles like dirt, sand and rust to prevent them from clogging the next filters in the system. The pre-carbon block removes organic molecules and ions, and the reverse osmosis membrane removes molecules larger than water and dissolved salts, as well as heavy metals like lead. A post-carbon filter polishes the water and adds back essential minerals that were removed in the osmosis process.

During normal (forward) osmosis, solvent molecules naturally move from an area of lower concentration to an area of higher concentration across a semi-permeable membrane. This movement is driven by the reduction of Gibbs free energy caused by the difference in solute concentration on either side of the membrane. Applying pressure to the solution forcefully reverses this process and allows pure water molecules to pass through while rejecting contaminants that are heavier than water.

Reverse osmosis systems are point-of-use (POU) devices that treat drinking water at the point of use, such as at the faucet. Most RO systems are equipped with a separate faucet that provides the option of using treated water or drain water. To prevent the dirty drain water from flowing back into the clean drinking water, most RO systems are installed with an air gap faucet that creates a physical separation between the water line and the waste/drain line. Some systems can also be fitted with a permeate pump that increases the efficiency of the RO system by reducing the amount of wastewater that is discharged.

What Are the Benefits of Reverse Osmosis?

In addition to removing unwanted minerals and contaminants like chlorine, fluoride, heavy metals, and dissolved solids, reverse osmosis water filters improve the odor and taste of your home’s drinking water. This helps make your home’s drinking water safer to drink and can even help cut down on your family’s consumption of bottled beverages which contribute to a mountain of plastic waste.

While your local tap water may be award-worthy clean when it leaves the municipal plant, over the miles it travels to your house it can pick up a number of contaminants from its journey. Those contaminants along with any naturally occurring minerals can cause serious health problems for you and your family, such as arsenic, lead, cysts, parasites, bacteria and more.

In addition to helping to eliminate the harmful contaminants mentioned above, a reverse osmosis system can reduce the amount of hardness in your water. Hardness causes scale build-up on the RO membrane which can negatively impact your system’s performance. In addition, hardness can also affect the amount of water your household is able to use since a high amount of pressure is required to push water through an RO membrane. You can avoid this problem by installing an RO booster pump to increase the water pressure in your household. This will reduce energy costs and avoid wasting a lot of water.