What is a Reverse Osmosis System?

How reverse osmosis systems work – and when homeowners should use them.

Reverse osmosis (often shortened to RO) is a popular water filtration method, used in both home and industrial settings, which removes contaminants from water by forcing water molecules through a semipermeable membrane, using water pressure.

Almost all common water contaminants, including most bacteria, chemicals, metals, and minerals are caught by the membrane, while purified drinking water flows through.

Most reverse osmosis systems contain additional filtration stages to catch remaining contaminants that are resistant to osmosis treatment. Activated carbon is a common type of filtration media included in RO systems, which can effectively adsorb and remove organic chemicals such as chlorine and pesticides.

A complete reverse osmosis system is also likely to include a sediment screening stage, which is designed to capture small undissolved particles of sand, silt, dirt, and other materials. These particles can damage the osmosis membrane if left unfiltered.

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How does a reverse osmosis system work?

Inside a reverse osmosis filter are semi-permeable membranes – screens made of porous material with tiny, microscopic holes that water can travel through in a single direction.

In many RO systems, the pores in the membrane are so small that they’re measured in fractions of a micron and aren’t much bigger than water molecules themselves.

The term “osmosis” describes the natural action of contaminated water when in a semi-permeable membrane. As water flows through the openings of the membrane, it will seek to equally distribute contaminants on either side of the opening. The result is a steady movement of contaminants across the membrane, from a high-to-low concentration.

Reverse osmosis occurs when water is exposed to a semi-permeable membrane under pressure. When pressure is applied to the water supply, water is forced through the membrane and leaves all of the contaminants behind.

What does a reverse osmosis system do?

A reverse osmosis system protects water from the vast majority of commonly-occurring contaminants. That means RO systems can be used to protect the quality and safety of drinking water supply for homes on pre-treated municipal city water and untreated well water.

In a RO system, the RO membrane is used to get rid of dissolved solids like arsenic and fluoride. If you want to get rid of a wide range of things, a RO system also has sediment and carbon filters. RO systems use carbon filters to get rid of chlorine and bad tastes and odors, and a sediment filter to get rid of dirt and other debris.

What shouldn’t a reverse osmosis system be used for?

When it comes to purifying water, reverse osmosis is one of the most versatile methods available. But like any type of filtration, RO systems are better at removing some classes of contaminants than others. Contaminants with especially small molecular structures are often able to make it past the membrane’s filtering process. These include:

Hydrogen sulfide

Hydrogen sulfide gas is a common well water contaminant, causing nasty odors to emanate from faucets. Due to the molecular structure of sulfur gas, it can often pass through reverse osmosis membranes.

A better way of tackling sulfur is to use an oxidizing filter, which converts sulfide gas into a solid so that it can be mechanically screened from the water supply.

Some organic chemicals

Chlorine can be removed from water by reverse osmosis at certain concentrations, although a typical home RO filter may not be able to completely eradicate it. The RO membranes themselves can also be destroyed quickly by chlorine exposure, reducing their lifespan significantly.

This is why most RO systems contain an activated carbon filtration stage, to remove chlorine before the water reaches the membrane.

Bacteria and viruses

Reverse osmosis systems are filtration products – not disinfectant products. That means that they’re not ideal for removing microorganisms from drinking water, or for guaranteeing that unsafe water is purified for potable uses.

Despite this, reverse osmosis technology is effective at filtering out most forms of bacteria. The RO filter needs to be replaced regularly and should be used in conjunction with a UV purifier or chlorination if the source water isn’t pre-treated.

What are the different types of reverse osmosis systems?

There are three main types of RO systems that are used in homes around the country:

  1. Whole house RO systems. In order to provide enough water for the entire household, these filters use large RO membranes, high-powered pumps, and full-size water storage tanks.
  2. Combined RO systems. In a combined system, smaller RO units and tanks are used in conjunction with a carbon filter and/or softener on the mainline.
  3. Under-sink RO systems. This is a typical RO system, designed to be installed under any sink and used to produce drinking water for a single faucet.

1. Whole house RO systems

A whole-house reverse osmosis system processes water for an entire home using powerful reverse osmosis membranes, big pumps, and 100-plus gallon storage tanks.

In most cases, these systems are positioned near the point where the water supply enters the house. Whole house systems are usually available in several sizes and power options so that they’re capable of providing as much filtered water as a home needs.

2. Combined RO systems

A combined reverse osmosis system is a good option for maximizing performance and value for money. These systems use a non-RO filtration system on the main water line and a smaller reverse osmosis filter at every outlet where drinking water is required.

Using a non-RO system, such as a carbon filter or a water softener, for processing non-drinking water has the advantage of being faster than an RO device, and not requiring a large tank to store up water reserves.

The extra filtering power of RO filters can then be saved for where it’s really needed – at drinking water faucets.

3. Under-sink RO systems

A standard RO filter can be put under any sink for drinking and cooking water filtration. Thanks to their affordability, effectiveness, and ease of installation, these products are among the most popular home filtration devices.

Reverse osmosis water vs Distilled water

It’s common for people to confuse reverse osmosis and distillation since they both produce extremely pure water that is cleaned of nearly all pollutants.

There are, however, fundamental differences between the two approaches. Distilled water is made by boiling water to a gaseous state and then re-condensing it. Water is cleaned of dissolved particles and microbes with this method, but compounds with a boiling point similar to water may not be removed.

In reverse osmosis filtration, water remains in a liquid state and is subject to intense filtration.

What is the best reverse osmosis system?

The best reverse osmosis systems process water efficiently to produce a fast flow rate with minimal water wastage. They also contain long-lasting carbon and sediment filtration stages to ensure that water gets comprehensive treatment.

Head over to our 2022 review page to see the results of our survey of the best whole house reverse osmosis systems.