If you are looking for a water filter, you have probably seen the term “reverse osmosis” more than once. There are numerous reverse osmosis filters to choose from, made by some of the leading names in filtration.
Reverse osmosis is currently the most popular filtration option and with good reason, as it will remove the vast majority of contaminants from water. Despite being used in millions of homes, you might not be familiar with what it means.
In this article, we will explain how reverse osmosis works, the different stages of the filtration process, and the contaminants it can remove from your water. We will also answer some of the most popular questions people have about reverse osmosis filters.
- Reverse osmosis filters effectively remove contaminants, providing cleaner and better-tasting water.
- These systems are automated, requiring minimal effort to use and maintain.
- Using a reverse osmosis system can reduce plastic waste from single-use water bottles.
- There are various options available, from under-sink models to whole-house systems, to suit different needs.
What Is Reverse Osmosis?
Regular osmosis is a process that filters liquids — most commonly water — by having them flow through a semipermeable membrane.
This membrane functions like a mosquito net — it only allows certain materials through it. For example, air can pass through the mesh of a mosquito net, but mosquitoes and other insects can’t.
In the case of an osmosis filter, water molecules can flow through but solid contaminants and dissolved materials won’t be able to.
The issue with this type of osmosis is that it can’t keep the filtered water and contaminated water separate after filtration. The clean water will naturally be drawn towards the dirty water, taking in some of the contaminants and leaving both sides dirty (1). This makes it ineffective as a method of purifying your drinking water.
Regular vs. Reverse Osmosis
In reverse osmosis, the semipermeable membrane has pressure applied to it from the higher-concentration side, where the contaminants are (2).
If enough pressure is applied, osmosis will occur in reverse. The purified water can’t be drawn back towards the higher-concentration side. Instead, it is forced away from the membrane after passing through it, leaving the unwanted compounds behind.
How Does Reverse Osmosis Work?
Reverse osmosis units are complex systems. They contain everything from the mechanisms that encourage the water through the system to the various phases the water passes through to ensure it is as clean as possible.
To have a deeper understanding of how reverse osmosis systems work, there are some important parts you should be aware of:
Components of a Reverse Osmosis System
A reverse osmosis filter isn’t a single piece. There are many distinct parts that work together to purify your water.
Some of these components are essential, so every RO filter should have them. However, some other parts will differ depending on which model you buy. These are the mechanisms that make up reverse osmosis systems: (3)
The Difference Between Stages and Passes
You will probably see two terms used frequently when reading about reverse osmosis systems — stages and passes. These terms sound similar but they have important differences in this context.
If you see an RO system described as being a four, five, or six-stage system, this usually refers to the number of filters it has. Generally speaking, the more stages, the better.
How Does an RO System Work?
Now that you are familiar with the basics of reverse osmosis, we can take a closer look at how these filtration units operate. This is the process your water will undergo before it reaches your glass:
- Entry: Water flows through your pipe into the RO system.
- Pre-filtration: One or more pre-filters clear the water of physical debris and, depending on your unit, some chemicals such as chlorine. These filters ease the burden on the RO membranes.
- Reverse osmosis: The water is forced through the reverse osmosis membrane or membranes at sufficient pressure to ensure it keeps moving forward. The concentrate or wastewater is either fed back into the system for a second pass or drained away, depending on your system.
- Different paths: At this point, the filtered water will either continue to your faucet or make a second pass through your RO system. In single-stage and single-pass units, the water will be ready for use but a double-pass model will complete the process twice.
- Post-filtration: The water may pass through some extra filters to remove any micro contaminants still left in the water. This usually only happens in high-end systems.
- Exit: With the water filtered, it can now exit your system via an outlet, often a dedicated RO system faucet.
What Does Reverse Osmosis Remove?
The most important aspect of any filtration system is how effective it is at purifying your water. Most residential water filters can remove larger debris, such as sediment and rust particles.
However, if you are considering a reverse osmosis system, you are probably looking for more thorough filtration. This can include removing unseen hazards such as viruses and bacteria.
The legal definition of a contaminant is anything in your water other than H2O. A very low percentage of contaminants are dangerous, but some of these can be very harmful (8).
The Centers for Disease Control and Prevention (CDC) rates reverse osmosis as highly effective at removing or reducing the following (9):
- Common chemical contaminants such as metal ions and aqueous salts
- Certain biological contaminants such as bacteria, protozoa, and viruses
Reverse Osmosis Filter Specifications
Not all reverse osmosis units are created equal. NSF International, formerly the National Sanitation Foundation, sets the industry safety standards for water filtration devices.
The NSF codes describe what an RO filter can protect against. Here are the common certification numbers and what they mean (17):
- NSF 41: The filter can treat taste and odor. This means your water should emerge without an unpleasant smell or taste.
- NSF 53: This code refers to cyst reduction, specifically spores or oocysts parasites such as giardia, that exist in water.
- NSF 58: If you are choosing a reverse osmosis filter, you want one that is NSF 58 compliant. This means it meets the NSF’s reverse osmosis safety standards.
Why Use Reverse Osmosis Filters?
Replacing your current filtration system or leaving behind bottled water can be a difficult decision. However, there are many benefits of reverse osmosis systems, including:
Cleaner, Better-Tasting Water
As long as you buy a quality reverse osmosis system from a reputable manufacturer, you can rest assured that your drinking water will be much cleaner.
If the water from your faucet tastes bitter or metallic, a reverse osmosis system should remove the contaminants causing the issue (18). Your unit can also remove unpleasant odors and colors from your water.
A pitcher-style filter might seem cheap and convenient, but you will need to refill it multiple times per day. In contrast, a reverse osmosis system is largely automated.
Once the unit is installed, it will automatically filter the water from your supply. You will need to replace its filters, but far less often than the smaller filters in pitchers.
Reduce Your Plastic Use
Plastic is a major source of marine debris. It is also hazardous to wildlife on land. Although recycling habits are improving, in the United States alone, millions of tons of plastic still end up in landfill every year.
One of the main contributors to this is single-use plastic water bottles. A reverse osmosis system will bring your home’s drinking water up to a similar standard to bottled water, allowing you to use reusable bottles instead. This makes an RO system an eco-friendly choice if you previously found your water supply to be undrinkable.
Lots of Options
Reverse osmosis filters are available in all sizes and styles. You can opt for a small unit that fits under your sink. Or, if you want purified water throughout your home, you can install a whole-house system.
Look for a model that meets your specific demands. If you know your water is safe but it tastes unpleasant, look for a system that eliminates bad odors and tastes.
Maintaining a Reverse Osmosis System
Although reverse osmosis systems don’t require any effort to use, they do need maintenance from time to time. Specifically, this means sanitizing and replacing filters and membranes when they get dirty, as they won’t perform optimally when they are covered in contaminants.
Specific maintenance guidelines will differ based on the size and type of your unit.
How to Clean a Reverse Osmosis System
Any water treatment system will require disinfecting periodically. You should sanitize your unit at least once a year or however often the manufacturer recommends.
We recommend cleaning it at the same time as you install new filters. This means you won’t have to perform your maintenance in two sessions. You should always follow the manufacturer’s instructions but here is a general guide on how to clean your reverse osmosis system:
- Switch off the water: Shut off the water supply to your unit. If you have any other appliances connected to your RO system, such as an ice maker, disconnect them.
- Depressurize: Open the water faucet attached to your RO tank and run it until your storage tank drains completely.
- Remove all membranes and filters: Take out all of the RO membranes and filters in your unit. If you are unsure how many there are and how to remove them, consult the manual.
- Apply sanitation solution: Apply bleach or a product such as Sani-System, which is approved by the EPA and NSF. Pour the solution into the part of your unit that water travels through first, which is usually the filter housing marked “sediment”.
- Replace housing and tubes: Put the filter housings back in place and reconnect any lines. Don’t put the new filters in yet.
- Reconnect the system: Set the system up again, connecting it to your water supply line.
- Switch water on: Turn the water back on. Your chosen cleaning agent will be distributed throughout the system.
- Wait: Leave the solution to work for at least 10 minutes.
- First flush: Open the faucet and allow the water to run for another 10 minutes or so. This time may vary depending on your specific model.
- Let the system rest: Shut off your faucet and leave the RO system as it is. Take a break for 10 minutes.
- Second flush: Repeat the same process, letting the water run from your faucet for 10 minutes.
- Shut off the water supply: Repeat the initial steps to shut off the water supply. Then, depressurize by running water from your faucet.
- Install filters and membranes: Place your new membranes and filters in their correct housings.
- Set up system: You can start using your reverse osmosis system, safe in the knowledge that it will stay hygienic and provide safe water.
Expected Lifespan of an RO System
The expected lifespan of a reverse osmosis system depends on its quality, maintenance, and how contaminated your water supply is. A properly maintained RO system from a reputable company should serve you for 10 to 15 years.
- Maintenance: Poor maintenance will shorten the lifespan of any technology. Replace the filters and membranes when necessary and don’t forget to perform a full-system sanitization when required. If a part such as tubing or filter housing breaks, replace it quickly. Leaving a damaged part in place can lead to a wider issue with the system.
- Water conditions: If your water supply is of particularly low quality, your filters and membranes will likely wear out faster (19). If your system has to process more severe pollutants, you will probably need to replace the membranes and sanitize the system more frequently.