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WHAT IS REVERSE OSMOSIS & HOW DOES IT WORK?

Scott Winfield
Written by Scott Winfield
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If you’re looking for incredibly effective water filtration that removes up to 99% of contaminants, then reverse osmosis is for you. It’s a powerhouse filtration system for any type of water that guarantees your home enjoys crystal-clear, purified water.

WHAT IS REVERSE OSMOSIS & HOW DOES IT WORK?

So, what is reverse osmosis and how does it filter water so effectively?

We created this guide to lay out all the details. Don’t be surprised if, by the end, you start to consider installing a reverse osmosis filter in your own home.

Reverse Osmosis Overview:

What is Reverse Osmosis?

Reverse osmosis, commonly referred to as RO, is a water purification process that uses a semi-permeable membrane to remove ions, unwanted molecules, sediments, and contaminants like chlorine, fluoride, and arsenic from drinking water.

Reverse Osmosis
Reverse Osmosis System

RO systems are mainly used to filter municipal water. However, when paired with a sediment pre-filter, they can also be used for well water.

The RO process involves putting water under pressure to force it through an RO membrane. The membrane itself only allows water particles through, meaning that all the impurities (such as salt) are stuck on the other side.

The main downside of RO systems is that they also remove beneficial minerals, like calcium and magnesium, alongside contaminants. That being said, there are RO systems that can reintroduce these minerals back into the water.

The most common application for reverse osmosis is in the removal of salt from seawater through a process called desalination. RO systems are also commonly used in households as well as in the food industry to filter out selective particles or de-alcoholize beverages.

Who Invented Reverse Osmosis?

Reverse osmosis was first discovered in 1748 by French physicist and researcher Jean-Antoine Nollet. Nollet was able to reproduce the effect of reverse osmosis by filtering water through a pig’s bladder. The bladder, in this case, worked as a semi-permeable membrane. However, his research didn’t gain much interest until the 1940s.

At this time, many researchers from various American universities were interested in using reverse osmosis for desalination purposes. The American researchers engineered an RO membrane that allowed freshwater particles to pass through while trapping salt particles.

This was a vast improvement from Nollet’s experiment using pig bladders. They also opted to force water through the membrane using pressure as opposed to Nollet’s passive approach.

The first commercial desalination plant started as a pilot program in 1965 in Coalinga, California. Soon after this project got off the ground, researchers worldwide began to work on their own desalination plants using reverse osmosis as the method of filtration.

In short, reverse osmosis was a means to provide clean drinking water to areas with very little natural freshwater but an abundance of saltwater.

RO Process: How Does a Reverse Osmosis System Work?

A reverse osmosis system works by forcing water through a semi-permeable membrane to remove dissolved solids and impurities. Generally, there will be a source of water piped into the filtration system, such as from a holding tank or a source of saltwater. This water feeds through a pump, forcing it through a membrane. The membrane stops dissolved solids and impurities from passing through, which results in clean water.

The Reverse Osmosis Process
The Reverse Osmosis Process

The reverse osmosis process removes up to 99% of minerals, contaminants and impurities from water. The only reason reverse osmosis can’t filter out 100% of all impurities is that some bacteria and viruses are too small to be caught by the membrane.

The impurities are caught by the reverse osmosis membrane filter off into the reject stream, where they can be disposed of. The water that passes through the RO membrane is now clean and can be collected for drinking.

The difference between reverse osmosis and standard filtered water is that a standard filter will simply trap impurities within the filter medium itself. With this kind of system, the filter needs to be changed regularly to ensure that it does not get clogged. With reverse osmosis, the impurities are free to flow off of the membrane into the reject stream.

The Different Applications of Reverse Osmosis Treatment

Reverse osmosis has many applications, though, as mentioned, it’s most commonly used for desalination. Desalination is the process of removing salt from saltwater. It’s highly beneficial in areas that don’t have good access to fresh water but are near saltwater. It can also be used in water purification, selective separation, and concentration.

Seawater Desalination System in Tampa Bay Florida
Seawater Desalination System in Tampa Bay Florida

Even in areas that don’t need desalination, reverse osmosis removes impurities from water. This is a common application in areas near peat bogs, where reverse osmosis can be used to remove organic matter from the water.

Reverse osmosis is also used in food preparation. For example, reverse osmosis separates milk and whey, or it can be used to make fruit concentrates or essential oils. Lastly, reverse osmosis can be used for de-alcoholization of alcoholic beverages.

Health Effect of Reverse Osmosis

Healthy humans can drink reverse osmosis (low tds) water with no ill effects as it’s simply highly purified water with no minerals, salts, or impurities.

However, removing all of the salts and impurities from water lowers its pH. For some medical conditions, such as stomach ulcers or severe acid reflux, low-pH water may worsen the symptoms. For people with conditions like these, drinking higher-pH water, such as mineral water, is recommended.

Still, it’s possible to find RO water where the minerals and salts are reintroduced after filtering to raise the pH.

How RO Compares to Bottled Water

A reverse osmosis system can be installed in residential buildings, allowing homeowners to access fresh, clean water in their own homes. On the other hand, bottled water has to be collected from its source, whether icebergs, springs, or simply the tap. They then need to be bottled, shipped, and bought before being hauled home.

Unfortunately, this is bad for the environment as used plastic bottles often end up in the oceans. Plus, shipping and hauling them creates greenhouse gas emissions. By using an RO filter instead, you can avoid buying plastic bottles and reduce your home’s carbon footprint.

Bottled Water
Bottled Water

How RO Compares to Distilled Water

Distilled water is made by boiling water, catching the condensation, and turning it back into a liquid. In contrast, reverse osmosis uses a membrane to filter impurities from water.

Distilled water is often used in systems like humidifiers and CPAP machines. While both methods remove impurities, the distillation process doesn’t remove volatile compounds such as chlorine.

The issue with distilled water is that it’s very time-consuming to produce, which means it’s a fairly impractical way to purify water for your household. Generally, distilled water doesn’t have many uses, and if you need it, you can buy it in both small and large amounts from pharmacies and stores.

Reverse osmosis, on the other hand, only requires a filtration system setup. This not only removes more contaminants but also takes far less time. So, for general home use, reverse osmosis is the superior option.

Environmental Effect of Reverse Osmosis

Reverse osmosis has both upsides and downsides when it comes to environmental impact.

On the plus side, drinking reverse osmosis water helps to reduce single-use plastics such as water bottles. A single reverse osmosis system can replace thousands of water bottles that otherwise would have wound up in a recycling plant or a landfill, taking hundreds of years to decompose.

Plastic Bottle Waste
Reverse Osmosis Reduces Plastic Bottle Waste

Additionally, reverse osmosis creates wastewater that isn’t nearly as harmful as the wastewater created by typical water treatment plants. This is because reverse osmosis doesn’t require any harmful chemicals to clean the water and only uses a semipermeable membrane.

Reverse osmosis does have some negative environmental effects associated with it as well. The most notable of these is the runoff from the filtration process.

This runoff is very high in salt and other impurities and is often dumped back into the environment. If this wastewater isn’t properly treated before being disposed of, it can cause hypersalinity in seawater. This creates pockets of super salty water that disrupt the delicate balance of the marine ecosystem.

Large-scale reverse osmosis plants can also be quite noisy and cause air pollution.

What Contaminants Do Reverse Osmosis Systems Filter From Water?

Reverse osmosis systems filter up to 99% of metal ions and contaminants such as chlorine, fluoride, lead, arsenic, mercury, phosphate, sodium, nitrates, amoeba, pharmaceuticals, and bacteria. As RO systems are so good at removing contaminants, they’re the ideal choice for homes with all types of water supplies. So, whether your home uses well water or municipal water, an RO system will be able to provide you with purified water.

Reverse osmosis is also commonly used in areas with a high concentration of organic matter in their water, such as areas near peat bogs.

What is the TDS for RO?

TDS (or total dissolved solids) measures particles that remain in water after filtration. Reverse osmosis systems can lower TDS as far as 50 ppm. For reference, typical tap water can be anywhere between 100 and 500 ppm. Groundwater sources can have an even higher TDS ppm.

The recommended TDS for clean drinking water that tastes great is around 150 ppm. After filtering, some reverse osmosis systems will reintroduce minerals to reach this desired mineral TDS.

Pros and Cons of Reverse Osmosis Water Treatment

Reverse osmosis has many benefits. However, like all water filtration methods, it also has its downsides that need to be taken into consideration. So, let’s take a look at both the pros and cons.

Pros
  • Reverse osmosis systems filter out up to 99% of all contaminants and impurities from water.
  • It doesn’t use any harsh chemicals or cleaners to treat the water.
  • The associated runoff isn’t nearly as toxic as traditional water treatment plants.
  • Reduces the use of bottles and single-use plastics.
  • A reasonably low energy system that can be used in both commercial settings and residential.
  • It can provide clean, fresh drinking water to areas that may not have had access to large amounts of water in the past, such as near oceans or peat bogs.
  • The reverse osmosis membrane filters off impurities rather than simply trapping them. The membrane doesn’t need to be changed as often as in typical filtration systems.
  • You can choose a system that suits your needs. This can be from a small countertop system to an extensive whole house filtration system.
Cons
  • Runoff from the filtration system can still be high in salts and other dissolved particles. As this is usually re-added back to the environment, it can negatively impact the local aquatic environment.
  • Commercial plants can be noisy and are not visually appealing.

The Different Reverse Osmosis Water Filter Stages

Different reverse osmosis systems have a different number of stages they use to filter water. We’ll use a five-stage system for this particular example, but other systems can have less or more.

Stage 1 – Sediment Prefilter

This stage catches small particles such as sand and silt. A sediment filter can catch anything that is up to 15 times smaller than a grain of sand.

Stage 2 – Carbon Prefilter

The carbon pre-filter contains activated carbon that removes impurities that can affect the taste and smell of water. This step removes compounds such as chlorine.

Stage 3 – Second Carbon Prefilter

This stage runs the water through a second carbon filter that contains more activated carbon.

Stage 4 – RO Membrane

At this stage, water passes through a semi-porous membrane, and any remaining impurities are filtered out of the water. The impurities and runoff are removed from the system, and the clean water moves on to the next stage.

Stage 5 – Polishing Filter

The polishing filter is yet another activated carbon filter that will remove any lingering unpleasant tastes or odors that may still be in the water. After this stage, water will be crystal clear and ready for drinking.

Other systems will typically add or remove additional filters aside from the reverse osmosis membrane. For example, a three-stage system will have a carbon pre-filter, reverse osmosis membrane, and carbon post-filter. This option forgoes the large particle filter and additional carbon filter altogether.

Components of a Reverse Osmosis System

Many components make up an RO system, but the most important part is the filters. Different systems have a different number of filters. Still, generally, you will have a sediment filter for larger particulates, a carbon pre-filter, the RO membrane, and a carbon post-filter.

Some systems don’t have the particulate filter, and some have more or fewer carbon filters. The feed water valve connects the cold water line to the reverse osmosis system, and this is where water is piped in.

The system will also contain an automatic valve shut-off. This is placed after the filters and automatically stops the water flow once the pure water holding tank is full. The drain valve is responsible for draining off the wastewater and particulates that are caught by the RO membrane.

The bladder storage tank holds the filtered water until it’s needed for consumption. As RO can sometimes be a slow process, it’s impractical to expect water on demand with this system.

Whenever water is removed from the tank, the valve will open again and allow filtered water to fill the tank.

The drinking water faucet is a tiny faucet on a sink that feeds directly from the RO storage tank. RO systems require plenty of tubing, usually to get the water from filter to filter, then to the tank and up into the faucet.

Finally, quick-connect fittings are used to connect the tubing and components.

While plenty of reverse osmosis systems are currently on the market, not all are created equal.

Below, we’ve created a list of some of the most popular brands. It should be noted that we focused on residential systems rather than commercial ones for this list.

  • APEC Water
  • Waterdrop
  • Home Master
  • AquaTru
  • iSpring Water Systems
  • PureDrop

We focused on curating the widest variety of RO systems for this list. This means that every one of the filters we pick is the best for its own reasons. To find the right one for you, you should consider the specific requirements of your home.

For a more comprehensive list, refer to our separate article on the best reverse osmosis systems.

APEC ROES-50

This system is considered one of the best overall RO systems for household use. It uses a five-stage filter system that makes your water crystal clear. The only downside to this system is that there is no remineralization stage, so the water will lack beneficial minerals and may taste flat compared to other options.

The APEC system is installed under a sink and includes a faucet and a 4-gallon storage tank. It can filter up to 50 gallons of water per day, making it perfect for up to a four-person home. Larger families may find that the storage tank doesn’t have enough capacity for their needs.

AquaTru Countertop Filter

This is an excellent option for those who either don’t want to install an extensive bulky system under the sink or are in a rental situation where they can’t do that. It’s also a good option for those who don’t need a high-capacity storage tank.

This system uses three filters: a sediment filter, a carbon filter, and an RO filter. Instead of connecting directly to the waterline, this RO system has a 1-gallon tank that can be filled with tap water. It can produce up to 3 liters of filtered water in just a few minutes. This water can then be kept in the machine or transferred into a jug and placed in the fridge.

The only downside of this system is that it will take up some counter space (unless you want to store it away after each use). Unlike other systems, this one also needs to be plugged into a wall outlet to operate.

Waterdrop D6 RO Filter

Although RO systems are usually installed under your sink, not everyone has the space for a large tank and all those filters. Not to mention that changing the filters of an under-the-sink system can be quite tricky.

This RO system from Waterdrop eliminates the hassle of changing filters and removes the tank all at once. This particular system doesn’t use a water storage tank as it can filter water fast enough to use on-demand. The procedure itself uses five filters housed in a single condensing unit.

All you need to do is replace the whole filtration unit once a year. To make things even easier, the included faucet has an intelligent sensor to tell you the TDS of the water as you are dispensing it, so you’ll know exactly when your filters need to be replaced.

iSpring RCC7AK

All of the previous options don’t include a remineralization stage. This means the water will lack certain beneficial minerals that are removed during the RO process.

This system, however, is the best option for reintroducing beneficial minerals to the water once it’s been filtered. It features a six-stage filtration process rather than the standard five filters, with the final filter reintroducing alkaline minerals. This creates better-tasting and slightly less acidic water.

Yet, there are a couple of downsides to this system. The main one is that it’s bigger than the average reverse osmosis system and will take up more space under your sink. This system also has a much higher ratio of wastewater to pure water at 3:1.

Home Master TMHP HydroPerfection

Finally, we have the Home Master option. This system is perfect for those who use well water and is the only system on our list that includes a UV light filter for sterilization. It consists of a five-stage filter process before finishing under UV light. The UV light is able to neutralize any remaining microorganisms that manage to pass through the RO membrane.

One downside is that it’s more expensive than some of the other options on our list. It also doesn’t have housing for the separate filters, so the setup can look a little unappealing. However, the lack of housing makes it very easy to swap out filters as needed.

RO System Cost and Maintenance Requirements

Once installed, reverse osmosis systems are relatively easy to maintain. The main requirement is replacing the filters regularly. The schedule for replacing the filters differs from system to system, but generally, the filters should be changed about once a year.

Depending on your water supply, some filters may need to be swapped out more frequently. For example, if you live in an area with lots of particulates in the water, the first-stage filter may need to be replaced every six months rather than annually. For this reason, many systems include a transparent first-stage filter so that you can see when it’s time to change it.

RO System Cost and Maintenance Requirements

Beyond that, the primary maintenance comes from simply keeping an eye out for leaks and ensuring that the entire system is stored safely under the sink where nothing can harm it.

How Much Does An RO System Cost?

The cost of a reverse osmosis system varies widely based on the type, features, and capacity.

On the least expensive end, you may expect to spend a little under $200 for a basic reverse osmosis system. It’s important to note that the more budget-friendly options may not have large storage capacities, and the filters may not last as long as the more premium options. The cheaper systems are also unlikely to include things like remineralization stages or UV filters.

More advanced RO systems cost upwards of $500. For example, the Home Master system we included in our list costs almost $500 because it includes an advanced UV light filtration stage that neutralizes microorganisms.

Then, there are whole-home systems that filter the water for the entire house. These systems start at $500 at the low end and can go up to $15,000 at the higher end. The cost mainly depends on the amount of water needed, the complexity of the setup, and the filters used.

How Much Does the Membrane Cost?

Most reverse osmosis membranes cost around $50. Some may cost a little more, some a little less, but most residential varieties will be around this price.

In most cases, you will be replacing the membrane once a year. If you’re using the system a lot, you may need to change it more frequently, as soon as you notice the signs. So, if your home has high water demands, replacing the filters might become quite costly over time.

How Much Does It Cost to Install?

Again, the answer to this question is – it depends.

Under-sink systems may only cost a couple hundred dollars to install. These systems are relatively easy to install, and it might take a professional about an hour to complete. A homeowner with enough plumbing know-how could also do it themselves, but it might take longer.

The cost is a bit higher for whole-house RO systems. These systems are often more complex and require a plumber to hook them up to the main water line coming into the house. For these systems, a full installation may cost up to $400.

Again, this job should only take an hour or two to complete for a professional plumber. For whole-home installations, it’s strongly recommended to hire a professional.

How Much Does It Cost to Maintain?

Once the system is installed, there are meager costs to maintain it. The bulk of the maintenance fees come from replacing the filters. To help manage the cost of filter replacements, you can often find them in sets for around $100 for all the required filters. Again, that’s just $100 to cover you for the entire year.

The only maintenance cost that may pop up is if you need to replace a faulty bit of tubing or a fitting. Fortunately, these parts are usually fairly inexpensive.

Most Common RO System Problems:

As with all appliances, RO systems are likely to experience specific issues over the course of their lifespan. But don’t worry – we’ve created comprehensive guides on how to resolve all of the most common RO system problems:

Common RO Questions

If you still have some unanswered questions about RO systems, you’ll find the answers you’re looking for below.

How Much Water can an RO System Produce Each Day?

The amount of water an RO system can produce a day depends largely on the system itself. For example, a smaller under-the-sink system may only be able to make around 50 gallons a day.

This is usually adequate for a small family. A whole-home installation, on the other hand, can filter up to 1200 gallons in a single day.

When looking for a RO system, it is important to pay attention to the output that it is capable of in a single day to ensure that it is enough for you and your family.

Is a Reverse Osmosis System Noisy?

A reverse osmosis system doesn’t make a lot of noise. At most, you may hear a soft humming from the system when it filters water. When the tank is full again, that sound will stop. Occasionally, you may hear a faint gurgling as well.
 
If the reverse osmosis system starts to get truly noisy, then it’s a sign that there is a problem within the setup. For example, a hissing noise is a sign that air has entered the system. This is a common problem after replacing the filters. Luckily, the hissing noise shouldn’t take long to go away.
 
Most small noises should go away within a couple of days. If you hear something that concerns you, or if the noise has been going on for a long time, reach out to a plumber or water treatment expert to take a look at your RO system for any issues.
 
This doesn’t apply to huge commercial RO plants, as they do make a lot of noise.

Are There Reasons Not to Use Reverse Osmosis?

There are only a couple of reasons why reverse osmosis water may bad for you. The first, and the most important of the two, is if you have a health condition that requires high pH water.

Some medical conditions, such as ulcers and severe acid reflux, can be worsened by low pH water. Reverse osmosis can lower the pH of water as it removes all the minerals and contaminants.

Another case where you shouldn’t use reverse osmosis water is in medical equipment. Equipment such as c-pap machines and humidifiers often specify using only distilled water. If your medical device determines that it requires distilled water, then use that over reverse osmosis.

Finally, reverse osmosis can make water taste a bit flat, thanks to most minerals being removed. If you really enjoy great-tasting water, reverse osmosis may not be for you.

However, you can get some reverse osmosis systems that will add minerals back into the water after passing through the RO membrane, fixing these issues.

If you still aren’t convinced about the RO process, there are several healthy reverse osmosis alternatives you can use in place of an RO system.

Why Does Reverse Osmosis Water Taste Funny?

RO water can taste funny or look cloudy because all the minerals in water are removed during the filtration process. When drinking tap or mineral water, much of the taste comes from the minerals that are in the water. In a reverse osmosis system, those minerals are removed by the semi-permeable membrane. The end result is water that can taste a bit flat to the average person.

However, it should be noted that some reverse osmosis systems include a filter that adds minerals back into the water, which improves the taste.

On the other hand, if your reverse osmosis system is used to provide great-tasting water, the change in taste may be a sign that it is time to change your filters. It is usually the activated carbon filters that help remove foul tastes and smells from the water. So if you notice a taste that wasn’t there before, look into changing out those carbon filters and see if there is any improvement.

Can You Hook Up Reverse Osmosis to the Refrigerator?

Yes, you can connect reverse osmosis to refrigerator. Most under sink systems include hooking into the cold water line, filtering it, and holding it in a storage tank. This water is then usually piped up through a separate faucet.

Someone experienced in plumbing could easily run a line from the system storage tank into the fridge. The fridge would then be able to use the RO water through its dispenser and to make ice cubes.

Will Reverse Osmosis Make Clear Ice Cubes?

Yes reverse osmosis can make ice cubes. The cloudiness in ice is generally caused by minerals and impurities in the water. If you make ice cubes with RO water, the ice cubes will come out incredibly clear.

Also, ice cubes made with RO water will have almost no taste, meaning they won’t add a taste to your drink.

If you notice your ice cubes getting cloudy over time, it may be a sign that it is time to change out the filters of your reverse osmosis system.

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Scott Winfield
Scott Winfield
My name is Scott Winfield and researching and writing about water filters and other strategies to purify water has become my full time passion in recent years. I'm glad that you found our site and you can look forward to authoritative and well researched content here to help you get the best in water.
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