What Is Reverse Osmosis? Everything You Need To Know

If you’ve ever heard someone mention “reverse osmosis water,” they’re talking about water treated through a filtration process called reverse osmosis, or RO.

Reverse osmosis filtration uses a semipermeable membrane to trap and flush out a broad range of potential contaminants in drinking water.* It’s a reliable way to ensure that you have cleaner, safer, great-tasting filtered water on demand.

There’s plenty to know about reverse osmosis water, RO filtration and what you can expect from a home RO system. In this complete guide, we’ll cover questions such as:

• What is reverse osmosis?
• How does reverse osmosis work?
• How does a reverse osmosis system work?
• How is reverse osmosis different from filtration?
• What are some common substances removed by reverse osmosis?
• What is the best reverse osmosis system?
• What is the lifespan of a reverse osmosis system?

Read on to learn more about reverse osmosis water filtration and what it can do for your home.

What Is Reverse Osmosis?

Reverse osmosis is just one kind of water treatment. It’s often used in home filtration systems, but it may also be part of city water treatment processes or even a provider’s bottled water delivery services.

Here’s what to know about reverse osmosis and how it works:

What is Osmosis?

To understand reverse osmosis, it is important to first understand what osmosis means. It is a natural process that occurs in all cell-based organisms — plants, animals and even people. During regular osmosis, a diluted solution passes through a semipermeable membrane and flows into a more concentrated solution to equalize the concentration on either side.

One everyday example of this can be found in plant root systems. When roots have a lower concentration of water than the soil around them, they act as a semipermeable membrane and work to balance the water content — which, in turn, keeps the plant alive.

What Is Reverse Osmosis?

As its name implies, RO filtration is the reverse of regular osmosis. It’s a type of water filtration that can reduce more than 99% of a broad range of contaminants in water, including both larger particles and those that can’t be seen. This is typically a multistage process that includes pushing the water through a specialized, semipermeable membrane (as well as other types of filters in pre- and post-filtration stages). The end result is cleaner water for drinking, cooking and other regular uses.

How Does Reverse Osmosis Work?

Unlike osmosis, which is passive and can happen automatically, reverse osmosis requires an external force or pressure. This means that it likely doesn’t exist in nature and is generally only used in reverse osmosis water filtration systems.

Let’s take a closer look at the RO process:

• #1: Water pressure forces a highly concentrated solution through a membrane, turning it into a less concentrated solution.
• #2: The RO membrane allows water to flow through but rejects larger molecules, such as salts, minerals, and metals.
• #3: The reverse osmosis process leaves higher concentrations of dissolved solids on one side of the membrane and filtered water on the other.

How Do Reverse Osmosis Filtration Systems Work?

Reverse osmosis can help reduce unwanted minerals, salts, metals and other impurities in your drinking water. How comprehensive RO water filter systems are varies depending on the brand and design, but in general, they perform four key steps to help improve your water quality:

Step 1: Pre-Filtration

An RO system connects to your tap water line. Remember, RO needs an external force, so these systems use high-pressure pumps to propel water through various levels of filtration. At this stage, tap water is called “feed water.”

The feed water passes through a series of initial filters, such as sediment and carbon filters, before flowing into the reverse osmosis chamber. These early filters can address issues such as:

• Sand
• Rust
• Large particles
• Chlorine tastes and odors

Step 2: Reverse Osmosis

Here, the feed water is pushed through the tightly woven reverse osmosis membrane at pressure to filter out contaminants. In Culligan RO systems, this process uses an advanced membrane to separate contaminants as small as a single atom from the water. These could include heavy metals like lead, mercury and chromium.

Step 3: Specialty Filtration

At this point, some systems also add additional filtration stages that can address other contaminants. These options can include specialty filters like the Culligan Total Defense cartridge, which can deal with contaminants including PFAS chemicals, microplastics, mercury, volatile organic compounds (VOCs) and more, and the Advanced Post-Tank Filter, which addresses viruses, cysts and bacteria. After any specialty filtration, some reverse osmosis systems, like Culligan’s, also give you the option to customize the taste of your water by adding back select minerals.

Step 4: Drainage and Storage

The reverse osmosis system drains out any wastewater. Some RO systems will generate more wastewater than others, which is why it’s best to look for high-efficiency options. For example, Culligan’s Aquasential® Smart Reverse Osmosis Drinking Water Filtration System is the world’s most efficient – it’s 3 to 5 times more efficient than other leading reverse osmosis systems.**

Bonus Step: Polishing

Some systems also provide a final polishing filter after reverse osmosis is complete to make your water as crisp and fresh-tasting as possible. The treated water is then ready for drinking and cooking. It’s stored in the RO tank until it’s used (unless you have a tankless RO system).

What Substances Does Reverse Osmosis Remove?

A high-performing, multi-stage RO solution can target many drinking water contaminants, including:

• Lead
• Arsenic
• Chlorine
• Mercury
• Chromium
• TDS (total dissolved solids)
• And many more

The most comprehensive systems can also address emerging contaminants like microplastics and PFAS in drinking water, as well as issues like viruses and bacteria (which may be a concern for well water or if your area is under a boil advisory).

Concerned about potential water quality issues? Learn more about common water problems and solutions with our Water Solutions Finder.

Keep in mind that the reverse osmosis process itself doesn’t necessarily address all these contaminants. When you install an RO solution, you’re actually relying on a whole system of multi-stage filters that work together in addition to the reverse osmosis membrane. If you took just one stage from the process described above, you wouldn’t get the full benefit of a comprehensive RO water filtration system — which is why it’s important to find a comprehensive solution that includes all the right filters, cartridges and steps.

What Does Reverse Osmosis Not Remove?

Even the best RO membrane can’t address every possible drinking water problem. Different types of systems may be required for problems like dissolved gasses such as hydrogen sulfide, a common culprit for that rotten-egg smell often found in well water. Reverse osmosis systems without additional filtration stages also likely won’t be able to address viruses and bacteria, PFAS, pesticides, and volatile organic chemicals (VOCs).

How Are Reverse Osmosis Systems Different From a Water Filter?

With all the different types of filtration options available, it’s helpful to know what RO filtration really means. Remember that reverse osmosis itself isn’t just a filter; it’s a process. The RO membrane is the core part of this, but when someone says “reverse osmosis filtration,” they’re likely referring to the entire multistage process that takes place in most RO systems.

So how do RO systems compare to other types of water filters?’
Many familiar systems like standard pitcher, fridge or faucet filters use a form of activated carbon. Carbon filters can be effective in reducing chlorine taste and odor in drinking water. In their standard versions, they generally focus on aesthetic water issues rather than those that can potentially impact health.

Take a look at how RO compares to other water filters:

Whole-Home Water Filtration

Whole-home water filters treat all the water that flows into your home; they’re meant for water problems that impact not just drinking but also cleaning, bathing and washing. These systems typically use specific types of filtration media designed to address individual problem-water issues and include:

• Chlorine filters
• Sulfur filters
• Iron filters
• Sediment filters
• Arsenic filters
• Nitrate filters
• PFAS filters

Simply put, whole-home filters usually target specific water quality issues, while RO filters can address a broad range of potential problems.

Countertop Filters

A countertop filter is a much simpler, less comprehensive solution to common water quality concerns. It’s often good at targeting unpleasant tastes and odors; it may even handle contaminants such as chlorine and fluoride. However, with fewer filtration stages and no RO component, countertop filters aren’t typically a top choice for water quality issues.

Faucet Filters

Faucet filters are a common purchase, but their main benefit is simplicity. For example, while they can often handle a handful of aesthetic problems like chlorine and sediment, they likely won’t come close to the broad range of contaminants that RO can address.. Meanwhile, a comprehensive RO system does all that and more.

Pitcher Filters

Pitcher filters may be one of the least efficient water quality solutions for many people. That’s not just because these systems are often limited to simple filtration needs; it’s also because they only address small amounts of water at a time, requiring frequent refills and filter changes (not to mention fridge space). With an RO system, you’ll always have fresh, filtered water on demand, right from your kitchen sink.

Fridge Filters

Fridge filters are generally effective in addressing chlorine issues, but many don’t get much further than that. If you want a more robust solution that handles a wider variety of potential contaminants, you need an RO filter system.

Benefits of Choosing a Reverse Osmosis Filtration System

The benefits of reverse osmosis go far beyond its effectiveness and simplicity. Here are just a few examples:

• Volume: With a reverse osmosis system, you can treat as much water as you need without having to wait for slower processes like repeatedly filling pitcher filters.
• Aesthetics: RO doesn’t just address potentially harmful contaminants; it also helps improve the taste, odor and color of your home’s water supply.
• Cost: Reverse osmosis systems tend to cost less over time because a high-quality RO membrane needs to be replaced far less frequently than other systems’ filters. Plus, you can save on single-use bottled water costs.
• Waste reduction: By limiting your reliance on single-use water bottles, you can reduce your environmental footprint. For example, Culligan product use avoided more than 600 million kilograms of single-use plastic in landfills in 2023.
• Reliability: Although there are plenty of smaller-scale solutions out there, RO is often the best choice for water filtration because it addresses a wider range of contaminants and provides a virtually limitless, on-demand supply of treated tap water.

What Are the Best Reverse Osmosis Systems?

With so many options on the market, it’s important to know what you’re shopping for. After all, not all RO systems are designed to be a full, comprehensive solution.

Start by looking for signs of high-quality design and user-friendliness, including:

• Multiple stages of filtration: Reverse osmosis alone isn’t enough to tackle every water problem. For more robust filtration, consider a system with multiple filtration stages, including options for advanced filtration.
• Certifications: Third-party certifications guarantee that a filtration system can efficiently and effectively address certain contaminants. The best RO systems are tested and certified to NSF/ANSI 58, NSF/ANSI 42, NSF/ ANSI 53, NSF/ANSI 401, NSF/ANSI/CAN 372, CSA B483.1 and NSF Protocol P231.
• Service and support: Buy from a provider who will be available through the life of your system to provide service and support. Additionally, they should answer your questions, offer professional installation, recommend maintenance schedules and help simplify your water filtration experience.

The best way to choose an RO system is to have an in-home water test and consultation. Test results are generally available in under 30 minutes, and their goal is to reveal your biggest water quality issues. Your local water expert can interpret the results and help you choose the best reverse osmosis filtration system for your needs.

Spotlight: Smart RO Systems

A smart reverse osmosis system comes with additional benefits thanks to a variety of specialized features. For example, some help you track water usage, contaminant reduction, money saved, filter replacement needs and more. The best systems even have a smartphone app for real-time monitoring. Ultimately, the goal is to get more out of your investment — and smart systems deliver.

Best RO Systems

Culligan’s reverse osmosis systems are designed to make it easy for you to enjoy safer, cleaner water on tap at home. Top systems include:

• Aquasential® Smart Reverse Drinking Water Filtration System: The Aquasential Smart RO reduces over 90 contaminants with up to seven filtration stages and 14 filter options. Plus, smart technology allows you to track water consumption and contaminants reduced.
• Aquasential® Reverse Osmosis Drinking Water System: Get an unlimited supply of cleaner, safer water with a system designed for easy setup and maintenance. This system offers the same protection as the Smart RO, minus the smart features.
• Aquasential® Tankless Reverse Osmosis System: This unique tankless design delivers the quality water your home depends on without taking up extra space under your sink. It includes four stages of filtration and additional options for pH-balanced water.

Reverse Osmosis FAQs

Want to learn even more about reverse osmosis systems and RO water? Check out these frequently asked questions:

What Is the Lifespan of a Reverse Osmosis System?

The lifespan of an RO filtration system depends on two key factors:

• Maintenance: In Culligan RO systems, filters can last about one or two years before needing replacement (the process is simple), depending on factors like the type you select and how much water you use. The RO membrane can last up to five years before replacement is required.
• Manufacturing: Selecting an RO system from a reliable manufacturer and service partner is important to help keep your system running efficiently and effectively. Culligan designs and manufactures its own systems to maximize lifespan, performance and reliability. Many core parts of Culligan RO systems, including the faucet, tank and other parts of the system, are covered by warranty for 10 years.

Is RO Water Pure Water?

Reverse osmosis is considered a type of purified water, but the definition of “pure” water is actually different. Pure water comes via a distillation process in which all dissolved substances mixed in water have been removed by evaporation. Most home filtration systems do not provide pure water according to this definition.

Can You Buy RO Water?

Some brands of single-serve bottled water use reverse osmosis and other treatment techniques. Additionally, if you choose to subscribe to a bottled water delivery service, you may be able to have RO water delivered straight to your home or office (depending on which options are available in your area).

Can You Have RO and a Water Softener?

Reverse osmosis systems and water softeners are two different water solutions. RO handles contaminants that may impact the taste, odor, appearance or quality of your water; meanwhile, a water softener removes hardness minerals such as calcium and magnesium that cause buildup on your faucets, stiff laundry, dry skin and hair and more.

The good news is that you don’t have to choose between filtration and softening. These systems can work together to address common contaminants and eliminate hard water problems at the same time.

Can You Use RO on All Water?

Many people choose reverse osmosis systems regardless of where their water comes from.

For municipal water users, this is often due to the presence of chlorine. Water treatment plants use chlorine to decontaminate the city water supply, and while this is good news for health and safety, it can have an unpleasant odor or taste. It’s also important to note that home RO systems can address certain contaminants that most city systems aren’t yet treating, like PFAS.

For well water users, the choice is even easier. Because private wells aren’t treated or managed by the city, there’s no centralized treatment step to address common contaminants — which means an RO system is a good way to go.

Enjoy Your Own Reverse Osmosis Water

A reverse osmosis system is a reliable, effective way to address many common contaminants, including those that may impact the taste or smell of your tap water as well as your health. Just remember that the best RO solutions aren’t just about the reverse osmosis process itself; they’re a combination of robust filtration steps, long-lasting designs, and quality customer service.

Ready to enjoy filtered water on demand? Schedule your free, in-home water test and consultation to get started.

*Contaminants may not be present in your water.
**Efficiency is based on 3rd party testing to NSF standards.