Widely considered one of the most effective water filtration methods, reverse osmosis (RO) creates clean, great-tasting water. RO systems are used in a variety of applications, including filtration for whole houses, faucets, aquariums, and restaurants. No matter what kind of water you start out with, there is likely an RO system that will suit your needs. Below you will find what reverse osmosis systems are, how they are beneficial, and what they are used for. You can also find a list of the best reverse osmosis systems on the market.

Reverse osmosis is a multi-stage water treatment process that removes contaminants from unfiltered water, or feed water, when pressure forces it through a semipermeable membrane. Residential reverse osmosis systems utilize at least three stages of treatment to reduce levels of virtually all contaminant types. In the final stage, the RO membrane, water flows from the more concentrated side (more contaminants) of the membrane to the less concentrated side (fewer contaminants) to provide clean drinking water. The fresh water produced is called the permeate. The concentrated water left over is called the waste or brine.

Reverse osmosis utilizes a semipermeable membrane with very small pores that traps contaminants while water is pushed through. In osmosis, water becomes more concentrated as it passes through the membrane to obtain equilibrium on both sides. Reverse osmosis, however, blocks contaminants from entering the less concentrated side of the membrane. For example, when pressure is applied to a volume of saltwater during reverse osmosis, the salt is left behind and only clean water flows through.

A  reverse osmosis system removes sediment and chlorine from water with a prefilter before it forces water through a semipermeable membrane to remove dissolved solids. After water exits the RO membrane, it passes through a postfilter to polish the drinking water before it enters a dedicated faucet. Reverse osmosis systems have various stages depending on their number of prefilters and postfilters. 

The RO membrane is the focal point of a reverse osmosis system, but an RO system also includes other types of filtration. RO systems are made up of 3, 4, or 5 stages of filtration. 

Every reverse osmosis water system contains a sediment filter and a carbon filter in addition to the RO membrane. The filters are called either prefilters or postfilters depending on whether water passes through them before or after it passes through the membrane.

Each type of system contains one or more of the following filters:

• Sediment filter: Reduces particles like dirt, dust, and rust
• Carbon filter: Reduces volatile organic compounds (VOCs), chlorine, and other contaminants that give water a bad taste or odor
• Semipermeable membrane: Removes up to 98% of total dissolved solids (TDS)

Learn more: Stages of reverse osmosis systems

1. When water first enters an RO system, it goes through prefiltration. Prefiltration typically includes a carbon filter and a sediment filter to remove sediment and chlorine that could clog or damage the RO membrane.
2. Next, water goes through the reverse osmosis membrane where dissolved particles, even too small to be seen with an electron microscope, are removed.
3. After filtration, water flows to the storage tank, where it is held until needed. A reverse osmosis system continues to filter water until the storage tank is full and then shuts off.
4. Once you turn on your drinking water faucet, water comes out of the storage tank through another postfilter to polish drinking water before it gets to your faucet.

An RO storage tank holds reverse osmosis water so you have plenty to use when you need it. A reverse osmosis system outputs water slowly. It takes one minute to produce two to three ounces of RO water. If you were to turn on your faucet for a glass of water at the actual membrane production rate, then you would have to wait at least 5 minutes for it to fill. With a storage tank, your glass fills instantly.

Learn more: How a water storage tank works

Reverse osmosis removes chlorine, salt, PFAS, and dissolved solids like arsenic and fluoride through the RO membrane. RO systems also include sediment and carbon filtration for a broad spectrum of reduction. The carbon filters in an RO system remove chlorine, bad taste, and foul odors, and the sediment filter removes dirt and debris.

Reverse osmosis removes

• Fluoride
• Salt
• Sediment
• Chlorine
• Arsenic
• VOCs
• PFAS
• Microplastics
• Herbicides and pesticides
• Heavy metals
• Many other contaminants

The contaminants listed are some of the most popular ones treated with an RO system, but the system also removes a slew of other contaminants.

Reverse osmosis systems do not remove some bacteria and viruses. If your water comes from a city treatment plant, then it should already be microbiologically safe. Reverse osmosis may remove some bacteria, but bacteria could grow on the membrane and potentially enter your water supply. To remove living organisms and viruses, we recommend UV disinfection.

Learn more: How to remove bacteria from your drinking water

Pretreatment can greatly extend the life of a reverse osmosis membrane when water quality is poor. When certain minerals, bacteria, chemicals, or other substances contaminate water as it flows through an RO membrane, they can absorb or form deposits on the membrane. The buildup of contaminants in an RO membrane is known as membrane fouling, and the forming of inorganic deposits is known as membrane scaling. To prevent membrane fouling and scaling, you must remove the contaminants before they can harm your RO membrane. Different contaminants require different pretreatment solutions, so you must know the makeup of your water before deciding on an appropriate pretreatment system. Well water, for example, will need more thorough prefiltration than city-treated water.

Biofouling is the result of the buildup of biological contamination on a reverse osmosis membrane. City-treated water should be free of microbiological contaminants unless a boil water advisory is issued, so homeowners with well water should be the ones most concerned with biofouling. To get rid of biofouling, you must get rid of the microorganisms that make their way to the membrane. The most practical method for biofouling prevention for reverse osmosis is ultraviolet disinfection.

Ultraviolet (UV) water disinfection systems deactivate microorganisms with UV light, preventing bacteria, viruses, parasites, and other microorganisms from reproducing. When using a UV disinfection system, ensure that you install a sediment filter before the UV system. If not paired with a sediment filter, dirt, debris, and sediment particles can hinder the effectiveness of the UV system. If your home uses well water, incorporating a UV system into your home well water system is essential in keeping you and your family safe from waterborne illness.

Learn more: What is a UV water purifier and how does it work?

Reverse osmosis membrane scaling can be resolved by a few pretreatment solutions. Two common residential pretreatment solutions for dealing with membrane scaling are water softeners and scale inhibitors.

Water softeners exchange sodium or potassium ions from a brine with calcium and magnesium ions in hard water. Because of how they operate, water softeners are commonly called ion exchange systems. Hard water can be treated by a reverse osmosis membrane, but it will greatly shorten its lifespan. As a result, water softening is the most efficient way to receive high-quality soft water from a reverse osmosis system while elongating the lifespan of the membrane. Water softeners are also a practical method for preventing scale buildup in residential RO systems.

Learn more: What is a water softener and how does it work?

Scale inhibitors are chemicals that are added to feed water delivered to an RO system. These chemicals increase the amount of inorganic compounds that water can dissolve, allowing the system to operate at a higher production rate. Scale inhibitors are excellent for dealing with the two most common inorganic salts in water, calcium carbonate and calcium sulphate. They are also effective across a wide pH range, safe to handle and use. Feed water should contain low levels of iron, a common well water contaminant, when using a scale inhibitor to prevent iron from reducing the efficacy of the inhibitor.

Learn more: How to recognize membrane foulants

Reverse osmosis booster pumps are essential if your home’s water pressure is below 40 psi. If your water pressure is between 40 and 50 psi and your water’s TDS levels are over 500 ppm, you should also use a booster pump. If you do not use an RO booster pump and you have low water pressure, your RO system will not produce water as efficiently or effectively as it is can. Homes that use city-treated water rarely need an RO booster pump, but homes with well water are more likely to experience low water pressure that necessitates a booster pump.