What is a Reverse Osmosis System and How Does It Work?

Widely considered one of the most effective water filtration methods, reverse osmosis (RO) creates clean, great-tasting water. RO systems are used in various applications, including filtration for whole houses, faucets, restaurants, and aquariums. No matter your water quality concerns, there is likely an RO system that will suit your needs. In this guide, we’ll take a detailed look into reverse osmosis systems, their advantages, and where they’re most beneficial. We’ve also featured some of the best RO systems available so you can find the right water filtration system for your needs.

What is osmosis?

To explain reverse osmosis, let's first look at the osmosis process.

Osmosis is a natural process in which water molecules pass through a semipermeable membrane from a region of lower solute concentration to one of higher concentration. This movement is driven by osmotic pressure, which strengthens as the concentration difference grows. The water molecules continue to migrate until the concentrations on both sides of the membrane are equal, establishing osmotic balance.

A familiar example of osmosis occurs as water ascends from a tree's roots to its highest leaves, moving through cells one at a time. This upward movement is propelled by osmotic pressure, which increases with the solute concentration gradient across the membrane.

What is reverse osmosis?

Reverse osmosis is a filtration process where pressure is applied to the side of a semipermeable membrane with a high concentration of solutes, overcoming the natural osmotic pressure. This pressure forces water molecules to move in the opposite direction—from higher to lower concentration—effectively filtering out impurities. Unlike standard filters that trap contaminants, reverse osmosis separates clean water, which passes through the membrane, while contaminants are flushed away in the waste stream.

How RO works:

High pressure is applied to water containing dissolved substances like salts and minerals, forcing it against the semipermeable membrane.
This pressure drives water molecules through the membrane, which allows only water to pass while larger molecules and impurities are blocked.
Contaminants remain on the pressurized side of the membrane, unable to travel with the water.
The purified water that emerges on the other side of the membrane is collected as clean, drinkable water.
The remaining concentrated solution of impurities, referred to as brine or wastewater, is then flushed away for disposal or further treatment.

What is a Reverse Osmosis System and How Does it Work?

What does reverse osmosis remove?

Reverse osmosis water system filters out chlorine, salt, PFAS, and dissolved solids like arsenic and fluoride through the RO membrane. Additionally, these systems have sediment and carbon filters that enhance purification capabilities. The carbon filters eliminate chlorine, improve taste, and reduce foul odors, while the sediment filter captures dirt and debris, ensuring a comprehensive reduction of impurities.

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 many other contaminants.

However, a reverse osmosis system does not remove most bacteria and viruses. If your water comes from a city treatment plant, then it should already be microbiologically safe. To remove living organisms and viruses, we recommend UV disinfection.

Learn more: How to remove bacteria from your drinking water

How does a reverse osmosis water filtration system work?

Reverse osmosis system stages

RO systems feature three, four, or five stages of filtration. Every reverse osmosis water system has a sediment filter, a carbon filter, and the RO membrane. Depending on whether water passes through them before or after the RO membrane, the filters are called pre-filters or post-filters.

Reverse osmosis system process

1. Pre-treatment stage

The process begins with pre-filters, including sediment filters that remove larger particles like sediment, chlorine, and organic matter, which could damage or clog the RO membrane. This initial step ensures that the water is adequately prepared for the more detailed filtration to come.

2. Reverse osmosis filtration

High pressure is applied to the water at this core stage, pushing it through the semipermeable membrane. This membrane, the critical component of the system, features tiny pores (about 0.0001 microns) that allow only water molecules to pass while effectively blocking and removing a significant amount of dissolved salts, chemicals, and microorganisms. The purified water, termed the permeate, is then collected in a storage tank for later use.

3. Wastewater disposal

The concentrated contaminants, which do not pass through the semipermeable membrane, are expelled as wastewater or "brine." This step involves a flow restrictor and drain system that manages water flow through the membrane, maintaining efficiency and preventing the membrane from becoming overwhelmed by pressure. Typically, this process generates about 3-4 gallons of wastewater for every gallon of purified water, which is then properly drained away.

4. Storage and delivery

The processed water is stored in the storage tank until needed. When there is a demand for water, it is delivered through a dedicated faucet, providing clean, safe drinking water on demand. This system ensures that purified water is available instantly whenever required by the user.

5. Post-treatment stage

On demand, water from the tank passes through a post filter on the way to the faucet, providing a fresh pass through carbon to remove any residual taste.

Learn more: Stages of reverse osmosis systems

How a Reverse Osmosis System Works

Is reverse osmosis water good for you?

Reverse osmosis systems are highly effective at removing dissolved contaminants that are invisible to the naked eye but potentially harmful if consumed. By filtering water before it reaches the body, reverse osmosis supports the kidneys' natural function and ensures you consume cleaner water.

While it's true that reverse osmosis removes some beneficial minerals like calcium and magnesium, the impact on your health is minimal. The human body is composed of 70-80% water, which primarily hydrates, lubricates joints, and supports organ function—processes that do not rely on mineral content from water. The essential nutrients your body needs are typically absorbed through food rather than water; thus, a balanced diet rich in fruits and vegetables will adequately provide these minerals.

The Environmental Protection Agency (EPA) recommends that total dissolved solids (TDS) not exceed 500 parts per million to ensure safety and taste. Many areas in the U.S. exceed these TDS levels, making reverse osmosis an advantageous option for reducing excess minerals and other contaminants, thereby enhancing the quality and safety of drinking water.

Advantages of reverse osmosis water

Reverse osmosis water offers numerous advantages, particularly for health, household uses, and environmental impact. Here’s a detailed look.

Health benefits

High contaminant removal: RO systems remove up to 99% of harmful substances, including arsenic, chlorine, PFAS, and other hazardous compounds.
Improved drinking water quality: They ensure safer drinking water by eliminating many contaminants, supporting better overall health.

Environmental benefits

Decreases plastic waste: Using an RO system at home reduces reliance on bottled water, cutting down on plastic consumption and waste.
Maximizes water efficiency: Modern RO systems are designed to minimize wastewater, promoting a more sustainable approach to water purification.

Cost benefits

Saves you money: Although the initial investment might be significant, an RO system is more cost-effective than buying bottled water over time.
Lowers maintenance costs: RO systems protect refrigerators and coffee makers from damage caused by mineral deposits.

Does a reverse osmosis system waste water?

Reverse osmosis systems are sometimes criticized for their water use. Most RO systems waste about four gallons of water for every gallon produced. Unlike other filters that trap contaminants, a reverse osmosis system divides water into two streams: one stream delivers purified water to a faucet, and another carries the filtered contaminants to the drain as wastewater, called brine.

However, the brine water is used for a purpose, so it is not exactly wasted. The wastewater in an RO system helps clean the water, just like a dishwasher uses water to wash dishes or a washing machine uses water to clean clothes. However, it is our job to care for the environment, minimize the amount of water sent to the drain, and increase the efficiency of the RO system.

How to improve the efficiency of your RO system

Add a permeate pump. Installing a permeate pump to a reverse osmosis system is the best way to increase efficiency. Permeate pumps reduce the wastewater from an RO system by 75 to 80%. Not every RO system can accommodate a permeate pump, so make sure the system you choose can.
Use the RO reject water for landscaping or artificial lakes. Drain water has higher total dissolved solids (TDS) levels, but the water has been filtered, making it perfect for your lawn or garden.

Do you need a reverse osmosis water filter?

Whether you need a reverse osmosis (RO) system depends on the quality of your local water supply and your preferences. An RO system isn't strictly necessary in areas where tap water is already clean and safe. However, it can still offer benefits such as improved taste, removal of any residual contaminants, and peace of mind regarding water purity.

RO is especially important for houses with water softeners. Softeners use an ion exchange process where hardness minerals are exchanged with sodium so ROs become advantages to reduce almost all the sodium the softener adds. If you’re on a sodium-restricted diet, your softened water needs an RO system.

However, an RO system becomes more necessary for those living in areas with known water quality issues or high contaminants like heavy metals or chlorine. It effectively removes these impurities, providing a critical layer of protection and ensuring safer drinking water.

Beyond health and practicality, an RO system can reduce your environmental impact by lowering your reliance on bottled water. This decreases plastic waste and offers long-term cost savings. Overall, while not everyone may need an RO system, it is always beneficial for ensuring consistently high-quality water in your home.

When you might not need an RO system

Good water quality: If your local tap water meets health and safety standards, you may not need an RO system. You can check water quality reports from your municipality or have your water tested.
Budget considerations: If you’re on a tight budget, you might consider other filtration options (e.g., activated carbon filters, pitcher filters) that can improve water quality without the higher initial cost of an RO system.

Where to use a reverse osmosis system

Point of use

Reverse osmosis is most commonly installed at the point of use (POU), like under a kitchen or bathroom sink. A point-of-use RO system could also be mounted in a cabinet or remotely in the garage or basement. Likewise, you can connect a reverse osmosis system to your refrigerator. RO systems remove minerals from water, making your ice clear and beverages more refreshing.

A reverse osmosis water filter is also beneficial for aquariums. Seawater aquariums commonly use RO in conjunction with deionization (DI) to meticulously control the balance of salt and nutrients. This combination first strips minerals from the water using RO and DI, after which the precise amount of sodium is added back to create the ideal environment for fish.

Point of entry

Reverse osmosis can treat water for the whole house. However, an RO system may be overkill unless your water has a specific contaminant that requires it. An RO system solves particular problems, such as saltwater intrusion in a well or high levels of silica or arsenic in the water.

An RO system will not provide the flow rate needed to pressurize an entire house. In the rare case where a whole house requires RO water, a large booster pump, like a Grundfos or Davey, provides adequate water pressure. In addition to a large water pump and storage tank, a UV system is needed to disinfect the water once it leaves the tank.

Homeowners have many considerations when purchasing an RO system for the whole house. If your water quality is dire enough to warrant whole house reverse osmosis, you likely have other water quality issues that must be addressed before the water reaches the RO membrane. High water hardness levels will cause scale build-up on the membrane, reducing its performance and causing it to fail prematurely. Contaminants like iron can also foul the membrane, and they must be eliminated from the water before being treated by the reverse osmosis water filter.

If you believe your water quality may require whole house reverse osmosis treatment, check out our in-depth guide to these systems.

Learn more: How water booster pumps work

Industrial uses

Commercial or industrial reverse osmosis systems are common because commercial units, where possible, can recirculate drain water back to the feed supply. They reduce scaling and corrosion in equipment, extending operational life and minimizing maintenance costs. RO systems support regulatory compliance and improve efficiency in pharmaceuticals, food and beverage production, and power generation industries.

How much does a reverse osmosis system cost?

The cost of a reverse osmosis system can vary depending on factors such as its capacity, features, brand, and installation requirements.

Basic under-sink RO systems

$150 to $500

These systems are designed for residential use and are installed under the kitchen or bathroom sink. Depending on the water quality, they usually consist of 3 to 5 stages of filtration.

Whole-house RO systems

$1,000 to $5,000+

These larger systems are designed to filter all water entering a home, providing purified water to every faucet. As a result, they’re a bit more expensive than under-sink options. Reverse osmosis is just one element of the entire whole-house system–the costs mentioned refer specifically to the RO component.

Portable or countertop RO systems

$100 to $300

Portable RO systems sit on your countertop and either connect to your faucet or operate independently. They’re ideal for apartments or small spaces and have a more affordable price point than more permanent solutions.

Commercial/Industrial RO Systems

$5,000 to $20,000+

These large-scale systems are designed for businesses that process food or manufacture or for large office buildings. Because of their increased capacity, they are often robust and expensive.

Maintenance & other factors affecting cost

Filters and membranes: Expect to replace filters every 6-12 months ($30 to $100), and the RO membrane every 2-3 years ($150 to $300).
Total cost over time: The annual cost of maintaining an RO system, including filter replacements and occasional system sanitization, can range from $100 to $400.

Water quality: Higher-quality systems may be needed for water with high levels of contaminants.
Capacity: Larger systems with higher output capacity generally cost more.
Brand and features: Premium systems with advanced features (e.g., UV filtration, alkaline filters) tend to cost more.

Features to enhance your RO system's efficiency

Water filtration and purification are never a one-size-fits-all solution. When purchasing a reverse osmosis system for your needs, consider additional features that will make your RO system work to its full potential. Here are a few considerations.

RO storage tank

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, taking up to 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, you would have to wait at least 5 minutes for it to fill. With a storage tank, your glass fills instantly. Tanks come in various sizes, so choosing the output of the RO and the size of the tank can be very beneficial in ensuring you always have enough water ready to go.

Learn more: How a water storage tank works

Pretreatment

Pretreatment can significantly 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.

Permeate pump

Permeate pumps enhance the efficiency of reverse osmosis water filters, though not all RO systems are equipped to accommodate them. If your system can integrate a permeate pump, it can boost its efficiency by up to 80%. These pumps save water by harnessing the energy from the reject water to move purified water into the storage tank, allowing the system to use up to 80% less water compared to systems without a permeate pump.

Typically, reverse osmosis systems use about four gallons of water for every gallon of purified water produced. However, a permeate pump can dramatically improve the wastewater-to-clean water ratio, resulting in significant water savings. If your RO system supports a permeate pump, installing one could substantially reduce the amount of water wasted annually.

Learn more: How a permeate pump works with a reverse osmosis system

Booster pump

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 your water pressure is low, your RO system will not produce water as efficiently or effectively as it otherwise could.

Learn more: What is a reverse osmosis booster pump and how does it work?

What to consider when purchasing an RO system

Water source

The water that enters your home feeds the reverse osmosis system. City water or municipally treated water is relatively free of chemicals and bacteria but may contain inorganic dissolved solids, which an RO system is designed to reduce. Private well water could contain harmful pathogens and chemicals in addition to dissolved solids.

Reverse osmosis systems can treat both city and well water, as long as the well water has been pretreated for bacteria and particles that could clog the membrane. If your home's well water is contaminated with heavy metals like arsenic, then you may need a UV system to disinfect the water once it leaves the storage tank.

Water demand

Consider how much water you and your family are going to use. Every person, according to nutritionists, should drink half their weight in ounces every day. For example, if you weigh 160 pounds, then you should be drinking 80 ounces of water per day. You also must take water used in ice machines, refrigerators, and all your other appliances into account. Each reverse osmosis system provides the amount of water produced each day, measured in GPD (gallons per day).

Water pressure

Reverse osmosis uses water pressure to force clean water through the membrane. If your household water pressure is below 40 psi, then you need to increase the pressure for reverse osmosis with an RO booster pump. If you are on well water supply, make sure your tank’s water pressure is set high enough for an RO system to work. A high level of dissolved solids in water also increases the amount of pressure needed for the RO system to produce clean water.

A residential reverse osmosis system is point-of-use, which means you install it right where you need the RO water. Most RO systems install directly under your sink. Whole house reverse osmosis systems are used less often because they require a large storage tank and booster pump for adequate pressure to supply water to every point in your house.

RO style

Reverse osmosis systems come three different cartridge styles: quick-change, drop-in, and standard. Your choice of filter style depends on whether you prefer easy maintenance, low maintenance cost, or the least environmental impact.

Quick-change cartridges: simplest to maintain, but also the most expensive and the least environmentally friendly.

Quick-change filters are easy to replace. Simply twist the filter and pull it out. Then, take the new filter and turn it in the opposite direction to replace the old one. However, the ease of maintenance comes with a high price tag, and the filter cartridges are made of plastic that gets wasted every time an old filter is discarded.

Drop-in and standard cartridges: More difficult to replace, but also the least expensive and most environmentally friendly.

To replace a drop-in or standard filter, unscrew the housing, throw the filter cartridge away, put a new cartridge in, and put the housing back in its place. Standard filters can be replaced with compatible cartridges, but drop-in systems require exact replacements.

Reverse osmosis systems also come in different stages, usually three, four, or five, based on the number of prefilters and postfilters included. The RO membrane does most of the work removing dissolved solids, but additional prefilters and postfilters can polish, re-mineralize, and provide additional chlorine and sediment removal. Eventually, the membrane wears out, but prefilters give an initial pass-through filter media to help extend the life of the membrane.

Ease of installation

Most under-sink reverse osmosis systems are easy to install. However, installing an under-sink RO system requires a dedicated faucet, which means you may have to drill a hole in your countertop. If you're not comfortable doing this job yourself, then you may need assistance from a plumber or a handy friend or family member.

Reverse osmosis systems must have a dedicated faucet and a drain connection to prevent the dirty drain water from flowing back into the clean drinking water. An RO system cross-connects the drain line and the water line. Plumbing code requires that cross-connections have an air gap faucet that allows drain water to pass through without returning to the clean water.

Learn more: How to fix an air gap leak.

Consult a water specialist before installing a whole house reverse osmosis system to make sure there is not a simpler, equally effective solution and to make sure you apply proper pretreatment based on your water conditions.

Remineralization

Reverse osmosis removes dissolved solids from water and reduces alkalinity or lowers water's pH in the process. The lower pH makes the water slightly acidic. Acidic water is corrosive, but adding an acid-neutralizing filter increases the pH adds mineral flavor.

water pH scale

RO system maintenance tips

Maintaining a reverse osmosis system is essential to ensure optimal performance and longevity. Here are some key maintenance tips for keeping your RO system in top shape.

Replace filters regularly

Pre- and post-filters (usually sediment and carbon filters) need to be replaced every 6-12 months, depending on your water quality and usage. The membrane typically lasts 2-3 years but can wear out faster if not maintained properly. Have the system checked annually to ensure everything is working efficiently.

Monitor the RO membrane

It is important to check the membrane performance with a TDS meter periodically. The TDS level from the system will start to increase as the membrane closes in on replacement time. They should be replaced when RO water TDS reaches about 3035% of tap water. Typically, an RO membrane needs replacing every 2–3 years.

Check the RO storage tank

The RO storage tank requires a certain amount of air pressure (usually 7-8 psi) to function correctly. If the tank is under-pressurized, it can affect the water flow rate. Inspect the tank for leaks or signs of corrosion that could compromise water quality.

Monitor water flow and quality

If the water flow has slowed down or the system produces less filtered water, it may be time to replace the filters or replace the membrane. Periodically test the water's TDS to check the efficiency of the system. High TDS levels may indicate that the membrane is not functioning properly.

Protect from freezing

If your RO system is installed in an area prone to freezing temperatures, ensure the pipes and components are insulated or relocated to avoid freezing, which can damage the system.

The best reverse osmosis systems

Need help finding the right RO system for your needs? Check out our top recommendations.

1. Neo-Pure Residential 4-Stage Full Solution RO System

The Pro 4 Reverse Osmosis system from NeoPure is our top recommendation for a variety of applications. Perfect for homes or offices, it combines top-tier performance with unmatched reliability. It delivers clean, refreshing water with a permeate pump for better efficiency. This system also features a permeate pump to improve system efficiency and deliver refreshing RO water. While not certified for PFAS removal, its carbon and membrane design significantly reduces PFAS levels.

We recommend: 25 GPD RO System

For a family of 4 or more, we recommend: 50 GPD RO System

For light commercial use, we recommend: 75 GPD RO System

2. Pentair FreshPoint GRO-350B 3-Stage Reverse Osmosis System

Pentair also offers an impressive three-stage RO solution. This 50-gallon RO system includes a lead-free faucet, storage tank, and three stages of filtration to deliver fresh, clean drinking water daily. The Pentair RO system is designed to fit snugly under your sinks, and its high-performance GRO technology means less wasted water than a traditional reverse osmosis membrane.

Pentair FreshPoint GRO-350B 3-Stage Reverse Osmosis System

Invest in fresh, pure drinking water for your home

Reverse osmosis systems are a powerful solution for improving water quality in various settings, from homes and businesses to industrial applications. Removing impurities and enhancing water purity contribute to better health, efficient operations, and environmental sustainability. Investing in an RO system ensures cleaner, safer, and more reliable water for all your needs.

Do you have more questions about whether a reverse osmosis system is right for your home or business? Reach out to the knowledgeable professionals at Fresh Water Systems. With decades of experience in water filtration, we’re happy to answer your questions and set you up with the best system for any application.

Contact Fresh Water Systems today →

What Is a Reverse Osmosis System and How Does It Work?