What Is A Reverse Osmosis System And How Does It Work?

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What is reverse osmosis and how does it work

A new study suggested that from 1982 to 2015, at least 9 million Americans drank water from a source that violated the Safe Drinking Water Act. It seems like every day we hear about a new self-inflicted drinking water crisis. So it isn’t a surprise if you’re one of the people wondering what is reverse osmosis and how does it work?

RO water filtration is one of the most popular and effective home water treatment solutions. In this guide, we’ll cover everything you need to know including if RO water is good to drink, how reverse osmosis works, the different models available, and the pros and cons.

Reverse Osmosis Filtration Process

Is Reverse Osmosis A Good Water Treatment Option?

RO filtration systems are known for removing up to 99% of water contaminants from unfiltered water including sodium, copper, lead, fluoride, chlorine, magnesium, calcium, and potassium. 

Reverse osmosis is also highly efficient in removing bacteria like Salmonella, E.coli, and Campylobacter, and protozoa like Giardia and Cryptosporidium which cause some of the most common water-borne diseases.

Since reverse osmosis uses a semipermeable membrane with a pore size of up to 0.0001 microns, it can remove chemical contaminants that regular filters can’t. Water also goes through multiple stages of filtration to remove dirt, dust, volatile organic compounds (VOCs), total dissolved solids (TDS), and more.

Does Reverse Osmosis Remove Healthy Minerals?

Because a reverse osmosis membrane is designed to remove up to 99.9% of contaminants in water, it also removes minerals including calcium, magnesium, and potassium. 

Although some people are concerned about the removal of these minerals in drinking water, the World Health Organization (WHO) clarifies that consumption of these minerals in excess can cause health problems and while these minerals are essential to good health, they are primarily consumed through food or dietary sources and not from drinking water. 

“Food is the primary source of calcium and magnesium.”

WHO: “Calcium And Magnesium In Drinking Water”

That said, high-risk individuals who have severe mineral deficiencies can benefit from mineral water. The WHO points out that such individuals should experience positive effects from minerals provided in drinking-water or supplemental mineral intake.

Is Reverse Osmosis Water Healthy To Drink?

Yes, water purified by reverse osmosis is safe to drink. Although reverse osmosis is considered the most effective water filtration method, there is an ongoing debate as to whether drinking water produced from RO systems is safe for everyday consumption because it removes all minerals. On balance, it is better to consume water that is 99% pure of carcinogenic contaminants since your body’s principle mineral intake comes from food or supliments.

Some people assume that since all types of contaminants including minerals are removed from feed water, it’s not healthy for drinking anymore. But again, health experts explain that good health is achieved through a balanced diet and not solely on drinking water. There is also no concrete evidence that proves drinking water from RO filters isn’t healthy.

How Does Reverse Osmosis Work?

The reverse osmosis (RO) process forces water molecules through a semipermeable membrane to remove harmful contaminants from drinking water. During the process, contaminants are separated and drained away, leaving only clean and healthy drinking water.

How Does Reverse Osmosis Work_Reverse Osmosis Membrane Process Diagram

A reverse osmosis membrane is constructed from a flat membrane sheet that is rolled into a cylinder-shaped element around the core tube. Each membrane layer is composed of a polyamide composite layer and an extremely thin barrier layer.  The composite layer helps support the very thin barrier layer. While it is the barrier layer that removes chemicals, heavy metals, minerals, bacteria and viruses from the water. 

The membrane sheet is combined with a feed channel spacer that creates space between the membrane sheets for the feed water.  This allows water to flow evenly across the entire membrane surface, even under high pressure.  Water passes through the membrane surface into the permeate channel.  It flows in a spiral direction and collects in the core tube.  This water is the final purified water product.  

Membranes remove molecules based on size, charge and shape. When molecules larger than water molecules are separated and removed, it creates two streams of water:  

The pure and clean water makes its way through the membrane layers and into the central core tube.  Lastly, purified water exits the membrane tube and is ready for dispensing.  The water that does not permeate the membrane sheets becomes more concentrated with contaminants. This concentrate, or reject water, is flushed clean and drained away so the process is ready to begin again.  

Stages In A Reverse Osmosis System

How Does Reverse Osmosis Work_Reverse Osmosis Stages Diagram
Reverse Osmosis System Stages

RO filtration systems can have many filtration stages depending on the model. For example, a system may include a remineralization filter or UV purification light. The most common RO systems have 3-5 stages, including the following:

  1. Pre-Filtration: It starts with a sediment pre-filter where the larger particles such as sand, and rust are removed.
  2. Activated Carbon Filter: The carbon filter removes chlorine and chloramine, as well as contaminants that cause bad odor and taste in water.
  3. Semipermeable RO Membrane: The reverse osmosis process forces water through the semipermeable membrane where total dissolved particles including those too small to be seen under an electron microscope are filtered out of the water. 
  4. Storage Tank: Once filtration is done, the water is stored in a tank ready for use. During this time, most RO systems have additional filters to continue cleaning the water until it’s supplied on your faucet. 
  5. Dispensing: Water is pushed out from the storage tank to your faucet where it goes through a post-filtration carbon filter to polish your water before you get to use it for drinking, washing, and cooking. 

In some cases, water isn’t stored in a tank. Instead, it goes straight to your faucet if you’re using a tankless reverse osmosis system.

What Contaminants Does Reverse Osmosis Filtration Remove

The reverse osmosis process removes 95-99% of contaminants, especially TDS and VOCs including:

Where Are Reverse Osmosis Systems Installed?

Hydroviv Vs Reverse Osmosis_Types Of RO Systems_Under Sink RO_Countertop RO_Tankless RO

When buying a RO system, you usually have three options depending on how you want the filter to be installed, the space you have and of course, your budget:

Point Of Use (Under Sink)

This type of RO filtration system is installed directly under your kitchen sink where it’s connected directly to your plumbing system. A point-of-use RO system is also fitted with a storage tank where water is stored after it is filtered and ready for use. Since there are many options to choose from, these can be the most affordable option to choose from and generally cost between $300 – $900.

An under-sink water filtration system usually takes just 15 seconds to fill a 1-liter bottle, which is why it can guarantee a more consistent water supply, although it can only filter water where it’s installed like your kitchen.


From its name, a countertop reverse osmosis system can be used on the counter of your kitchen, RV, or bathroom. This system offers the same filtration that your under-sink filter does, the only difference being that it’s not installed directly under your sink and drain system. Because of their portability, RO systems are ideal for use in rental homes where permanent modifications to the plumbing system aren’t possible, apartments, dorms, RVs, and vacation homes.

Point Of Entry (Whole House)

A whole house RO filtration system is essentially designed to filter water that’s supplied to your entire house. Being a point-of-entry system, it is installed where water enters your home, so you may need to hire professionals for the modifications that need to be made to your plumbing system to accommodate the RO system. 

If you want to make sure that all your faucets and showers will have filtered water, this is definitely a good investment, although it’s more expensive than other types of RO systems.

Tankless vs Tank Reverse Osmosis Systems

Point of use RO systems come in two types, tankless and tank based. Tankless RO systems are a newer offering, however, both options have their pros and cons.

Tankless Reverse Osmosis System

A tankless RO system is designed without a storage tank making it more compact and portable while still providing you with the same levels of filtration that you need. Since you don’t need a tank to store water, a tankless RO system takes up less space making it ideal if you have a small home or if you live in a rental. 

Water may also taste fresher since it’s filtered on demand as opposed to being stored in a tank. Tankless RO systems also produce less wastewater to help you cut down on your water bill.

The biggest disadvantage of this type of system is that it’s expensive and you also need electricity to make it work. And if there are emergencies such as power outages, you won’t have access to clean water since it doesn’t have a storage tank.

Tank Reverse Osmosis System

A tank RO system is a more traditional option where you have a filtration device connected to your plumbing and a storage tank where water is kept ready for use. This means that water is filtered continuously and stored, so you have access to clean water all the time. 

Traditional tank RO systems tend to create more waste water. These days you can get efficiency tank-based units, however, that conserve more water.

A tank RO system can be less expensive than a tankless unit. Additionally, you’ll have water supply even if the power goes out because water is stored in the tank. 

Tank RO systems also don’t need a power supply to filter water, except for variants that contain a UV filter and pump. A disadvantage to using this type of RO system is that it requires more space for the storage tank.

How To Maintain An RO System

Like all home filtration options, RO systems require regular maintenance. If properly maintained, your RO filter system will last for up tot 10 years. Here are some important tips to keep your RO drinking water system in good shape:

Change Your Filters Regularly

RO membranes are designed to last for up to 12 – 24 months. This will depend on the type of water contamination being fed through the filters. To ensure your filtration system runs efficiently, it’s best to replace it every 12 months or if your water begins to have bad taste or odors. This is especially true if you have a well water source that is heavy in sediment or iron levels. Heavy contamination can lead to early fouling of the filters and RO membrane.

Clean According To The Manual

Most RO filtration systems are designed to last for up to 15 years, but only if you maintain them well. Aside from replacing filters regularly, it’s also very important to clean your filter system following the manufacturer’s manual. If you can’t handle it yourself, you can always hire a professional to do it for you.

Maintain Tank Pressure

Your tank needs to be pressurized every now and then to keep it efficient. A typical RO tank should achieve a pressure of 7 to 8 psi. To check, you can drain the water from the tank and attach a pressure gauge to the tank valve. This is a similar process to checking the pressure on your car tire. If it requires more pressure, you can pump air into the tank to achieve the ideal pressure.

Replace Damaged Parts

One of the easiest ways to keep your filter efficient is to make sure that damaged parts are replaced. Completing repairs early will prevent it from getting worse and from shortening the life of your system.

How To Install A Reverse Osmosis System

How To Install A Reverse Osmosis System

Installing a point-of-use RO system can be a DIY project that takes no more than two hours. Most systems come with all the components required, you only need to make minor plumbing modifications under your sink. If you want to hire a professional, it will cost approximately $200 to install a RO system in your home. For a full walk-through of how to install a reverse osmosis filtration unit, read our RO install tutorial here.

A countertop RO system is the easiest to set up as they require no installation. Simply plug it in and you are ready for fresh purified water.

Whole house reverse osmosis systems require the most installation time. Unlike a point-of-use unit, a whole-house RO system will likely require professional installation. This task will take up to 5 hours and will cost approximately $500 to install, depending on the hourly rate.

Pros And Cons Of Reverse Osmosis

Advantages Of Reverse Osmosis Filtration

  • Removes most harmful contaminants: RO filtration systems are still considered the most efficient home water treatment methods because of how many contaminants they remove with the use of a semipermeable membrane.
  • Removes sodium in water: High consumption of sodium can result in health issues, but a RO filtration system helps to reduce sodium in water to keep it healthy. Homes with an ion exchange water softener can have elevated levels of sodium. Reverse Osmosis systems are frequently paired with these to remove sodium.
  • Purified water on demand: You can get a nearly unlimited supply of purified water from your home’s tap. Compared to distilled water vs reverse osmosis, a water distiller creates very pure water but takes a long time to do it.
  • Eliminates bad odor and taste: RO filters are designed to remove contaminants that cause bad odor and taste in water. This is why users report that their water tastes a lot better after using RO.
  • Flexible to use: You can find a RO system that will fit the space in your house or your budget, so you can invest in one that suits your needs.
  • More eco-friendly: When you have your own RO system at home, you don’t need to use plastic water bottles that create waste. RO can create bottled water quality drinking water right at home.

Drawbacks Of Reverse Osmosis Filtration

  • Requires upfront investment: RO filtration systems are more expensive than regular filters because they’re more effective. That said, you can get a quality reverse osmosis filtration system for less than $500. Especially when compared to purchasing bottled water, RO water is significantly cheaper. You will have a payback period of less than a year.
  • Can take up space: This is especially true for tank RO systems that require space to be installed in your house. A smaller carbon filter may take up less space, but a comprehensive RO system requires a little more space because of its functionality. If you want water that is up to 99.9% free of contaminants, it’s a small price to pay.
  • Requires maintenance: Like any other filtration system, you need to make the effort to maintain it to keep it efficient. Filters need to be replaced every 6 – 12 months, but other than that, they are pretty hands-off. A single carbon filter like Hydroviv vs reverse osmosis only requires one filter to be replaced, but that one filter is more expensive.
  • Wastes water: One of the biggest knocks against RO systems is that they “waste” water. A traditional system has a 4:1 waste water ratio. Meaning it wastes 4 gallons for every purified gallon of water. The good news is that high-efficiency models only use one gallon of water to purify three gallons of water.
  • Removes healthy minerals: Since there’s no way for a RO system to keep healthy minerals, they are also removed in the filtration process and it also reduces your water’s pH levels. As discussed above, the primary source of mineral intake is food. However, if you want to remineralize your RO water, you can do so with remineralization drops.

Is reverse osmosis water safe to drink?

Yes, reverse osmosis water is safe to drink on a regular basis. The RO process removes hundreds of harmful contaminants so you have bottled water quality drinking water. Although healthy minerals are removed, humans only get approximately 5% daily mineral intake from water. The rest is from a balanced diet and food.

Are reverse osmosis systems expensive?

Reverse osmosis systems are reasonably priced as an under-sink unit costs between $300 – $900. A countertop RO system will cost on average between $250 – $600. Furthermore, the amount of savings from not having to purchase bottled water far outweighs the cost.

Do reverse osmosis systems waste water?

Reverse osmosis systems use excess water to flush the semipermeable membrane clean of harmful contaminants. This concentrate water is then drained away. A standard RO system has a 4:1 drain ratio, meaning it uses 4 gallons of water to produce 1 gallon of purified water. A high efficiency system will have a 1:3 ratio where it uses only 1 gallon of water to produce 3 gallons of purified water.
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