Home Blog Buyer's Guides What Is a Reverse Osmosis System and How Does It Work

What Is a Reverse Osmosis System and How Does It Work

by Dr. Jonathan Doyle - Updated October 12, 2021

What Are Reverse Osmosis Water Filters?

Reverse osmosis (RO) water filter uses the reverse osmosis process in purifying water. RO filters are capable of removing water pollutants to make water safe and clean for consumption.
Simple appliances like a camper's filter bottle use the RO filter as a semipermeable membrane. RO membranes come with other prefilters as a complete system because the membrane takes time to produce clean water, and they need relatively clean feed water.
Usually, the system is set on sink-top filters, whole-house systems, or under-sink systems.

How Does Reverse Osmosis Work?

RO filters get their name from the reverse osmosis process, and they also take advantage of the process in filtering water. Naturally, osmosis happens when a solvent from a low concentration region moves to a high concentration region through a permeable membrane. Reverse osmosis, on the other hand, is the opposite of osmosis. Hydrostatic pressure forces the solvent to move from a high concentration to a low concentration area during the process.
Precise filtration
In simpler words, reverse osmosis can be explained in the context of water filtration.
Water pressure forces the contaminated water to travel to the other side through the membrane during reverse osmosis. The membrane's pores are usually small (sometimes even smaller than a human red blood cell at 0.0001 microns) such that only pure water molecules can pass through. Therefore, pollutants are rejected and washed away by wastewater.
RO filters are highly efficient at removing water pollutants. The filters can remove more than 90% and up to 99.99% of contaminants, including heavy metals like lead, nickel, and mercury. Furthermore, RO filters can remove algae, bacteria, viruses, hormones, pesticides, and herbicides.
Although reverse osmosis filters do not need electricity for operation, they need a stable feed water pressure for forcing the pure water through the membrane. It is important to note that RO filters won't work with very low water pressure.
Additionally, RO filters require a large amount of water for washing away the contaminants every small amount of pure water makes. The most advanced systems with permeate pumps have pure water to wastewater ratio of 1:1. Also, always check the mileage of your RO system; mileage varies across different systems, and it is best to get highly efficient systems.

Why Do You Need A Water Filter?

You are probably checking out this article for various reasons, including wanting safe, healthy drinking water for you and your loved ones or learning about different water filtration systems.
Unfortunately, tap water is no longer safe even if you have high-quality city or well water. Also, there is no guarantee that bottled water is better than tap water.
The Environmental Working Group's (EWG) tap water database reports that 81% of Americans drinking water contain carcinogenic contaminants. Also, 77% of Americans drink hexavalent chromium - the "Erin Brockovich" chemical. Furthermore, water samples from all 50 states contain arsenic, chromium-6, disinfection byproducts, and lead. The water samples also contain nitrates from fertilizers and radiological contaminants.
The EWG recommends reverse osmosis water filters to filter drinking water effectively. RO filters can remove up to 99% of commonly dangerous water contaminants, including lead, arsenic, nitrites, nitrates, selenium, chromium, and fluoride. The filters can also remove PFAS, PFOS, PFOA, and GenX forever chemicals, cadmium, radium, phosphate, barium, chlorine, barium, bacteria, viruses, cysts, and many more.
However, choosing your first system from the numerous reverse osmosis water filtration systems can be pretty overwhelming, especially if you don't know much about water treatment.
Alternatively, you can check out the whole house water filter reviews if you decide that a RO water filter system is not suitable for you.
Before purchasing individual RO systems, you can check below for our reverse osmosis buying guide.

Tank VS Tankless RO

Reverse Osmosis Water Filter With Tank

The Traditional Reverse Osmosis (RO) Filtration System forces water through a water-permeable membrane and a series of filtration devices. Although it is efficient in water purification, it needs time to push the water molecules through the RO membrane, thus reducing the water quantity coming out of your tap. Filtration managers found a solution to this problem by using a storage tank for filling purified water ready for immediate use.


Advantages of the reverse osmosis water filter with tank include its affordability. The system is more affordable than tankless options. Also, the system is suitable for storing purified water, especially for emergencies. Furthermore, the system does not require a power supply except for the ones with a pump or UV filter


A large system will require more setup space, and the water may taste off when stored for too long.

Tankless Reverse Osmosis Water Filter

The latest iteration of the RO water filter is the Tankless Reverse Osmosis (RO) Filtration System. This system is more compact, easier to manage, and produces fresher water. Furthermore, this system is designed to provide the exact water purification level as the traditional RO system. However, the tankless RO system takes up less storage space, and it purifies water when necessary instead of saving water in a storage tank.


The Tankless RO System uses a significantly small space compared to a Traditional RO filter. Also, the water delivered is fresher because it is not stored in a tank. The system reduces your water bill because it produces less wastewater than Traditional RO Systems.


Tankless RO Systems require a higher upfront investment, and the system needs electric power before it can work. Also, this system is unreliable during emergencies because there is no storage.

Reverse Osmosis System Buying Guide

8 Things of Purchasing an RO System

1. Water Source

Water delivered to your home feeds the Reverse Osmosis System. Although city or municipal water is free of chemicals and bacteria, it may contain inorganic dissolved solids. Fortunately, RO systems are designed to remove such solids. On the other hand, private well water may contain harmful pathogens and chemicals and dissolved solids.
Reverse Osmosis systems are only efficient in treating city and well water as long as they are pretreated. The pretreatment should remove bacteria and particles that could clog the membrane. It is important to note that if your well water contains heavy metal contaminants like arsenic, you need a UV system to disinfect the water once it leaves the storage tank.

2. Water Demand

It is best to consider you and your family's water consumption. According to nutritionists, the ideal daily water intake should be half a person's weight in ounces. Therefore, if your weight is 160 pounds, you should drink 80 ounces of water daily. It would be best to consider water used in ice machines, refrigerators, and other appliances when considering your water demand. A Reverse Osmosis System provides the quantity of water produced daily in GPD (gallons per day)

3. Water Pressure

With water pressure, reverse osmosis can force water through its membrane. For house water pressures below 40psi, increasing the water pressure is necessary for reverse osmosis. You can use an RO booster pump for this. Also, if you use a well water supply, ensure the tanks water pressure is high enough for the RO system to work. Water containing a high level of dissolved solids can also increase the pressure necessary for the RO system to produce clean water.
Additionally, residential reverse osmosis systems are point-of-use systems because you can install them right where you need the filtered water. You can directly install most RO Systems under your sink. Because Whole house reverse osmosis systems require a large storage tank and booster pump before getting adequate pressure to deliver water to every point of your house, they are used less often.

4. Plumbing Requirements

It is advisable to consider few plumbing factors before purchasing a reverse osmosis system. The first step is to check out the space under your kitchen sink; a reverse osmosis system takes up a lot of space due to its multiple filters and large storage tank. Therefore, items stored under your sink have to go. In addition, if you have a garbage disposal, fitting the reverse osmosis system below the sink will be quite challenging.

5. Tank Size

RO tanks have sizes between 3 and 14 gallons. Nevertheless, reverse osmosis system tank size can be quite deceiving. Usually, the actual tank capacity is less than the rated tank capacity because RO storage tanks have a metal bladder and a bubble of air that creates enough pressure for pushing the water through the faucet when the tap is open. For instance, a 4.5-gallon tank holds about 2 to 3 gallons of water. It is advisable always to consider tank sizes when shopping for a RO tank system, ensuring it will meet your family's daily water intake demand.

6. Wastewater

A significant criticism of the reverse osmosis water filtration system is its wastewater creation. Standard water filters remove solid contaminants when water passes through the filter. In contrast, RO systems remove contaminants in liquid form as wastewater, also known as brine. Conventional RO Systems produce a considerable quantity of brine, about 3 to 25 gallons per gallon of purified water; keeping this info in mind, you should only purchase RO systems with low wastewater to purified water ratio.

7. RO Style

Reverse Osmosis Systems are available in three cartridge styles - quick-change, drop-in, and standard. Choosing a filter style should depend on the cost of maintenance, how easy it is to maintain, or the level of environmental impact.
Quick-change cartridges are the simplest to maintain. However, they are the most expensive and least eco-friendly. Furthermore, you can easily replace quick change filters. All you need do is twist the old filter and pull it out. Next, put the new filter in the old filter's place and turn it in the opposite direction. Nevertheless, this easy maintenance comes at a high price. Since the filter cartridges are from plastic, they get wasted every time an old filter is discarded.
Drop-in and standard cartridges, on the other hand, are more difficult to replace but are less expensive and more eco-friendly
When replacing a drop-in or standard filter, unscrew the housing and throw the filter cartridge away. Next, place the new cartridge in and screw the housing back in place. Although you can replace standard filters with compatible cartridges, you need exact replacements for drop-in systems.
Furthermore, reverse osmosis systems are available in multiple stages - three, four, or five, depending on the number of prefilters and post-filters in the system. Usually, the RO membrane does most of the work by removing dissolved solids. However, the additional prefilters and post-filters can remove sediment, add chlorine, and polish and remineralize the water. Eventually, when the membrane wears out, the prefilters can extend the membrane's life by giving an initial pass-through filter media.

8. Budget

Before budgeting for a reverse osmosis system, you should consider several factors, including the system's cost, the installation cost, especially if you hire someone, and the ongoing maintenance cost. Considering these factors will affect your choice of an RO system if your budget permits you to use one.

Pros and Cons of RO Systems

Using an RO system in your home has its pros and cons. The section below discusses the significant points you should know to determine if the system is right for you.

Pros of an RO System

The System Produces Almost Pure Water
When Reverse osmosis systems filter water, the water is close to completely pure water. RO systems are excellent for cleaning up water, and they are reliable for getting rid of dissolved solids.
You can refer to the contaminants discussed above to understand how comprehensive reverse osmosis systems are when it comes to purifying water
The System Does Not Get in The Way
Although getting a new water filtration system may be thrilling at first, you realize that it is often in your way with time. Then, you start second-guessing your decision of purchasing the system in the first place.
Many RO systems can conveniently fit under your kitchen sink, and this is a significant advantage. You can use your RO system continuously without them hindering your work in the kitchen.
There Are No Installation Problems
RO systems are pretty easy to install compared to other water filtration systems. You can easily install this system even if you are not quite experienced with working with household appliances.

Cons Of an RO System

Minerals Removal
Because of its highly efficient filtration, the RO system may filter out essential minerals; This is the major drawback of the RO system. Considering your daily water demand, the filtered-out minerals most likely accounts for a considerable part of your mineral intake.
Fortunately, remineralizing your water can minimize the impact of this drawback.

The Components Of The Reverse Osmosis System

If you are in the market for a Reverse Osmosis System, you will discover the biggest difference between this system is the quality of its manufacturing and the individual parts.


Pre-filters are essential for all reverse osmosis systems. This component filters the water by removing large grains of sediment, like dirt, which can easily obstruct other parts of the system preventing it from working properly.

Semipermeable Membrane

This membrane allows certain elements to pass through but not others. This is one of the most important components of the reverse osmosis system. It traps unwanted elements from water including harmful elements such as pesticides.

Post Filtering

Post filtering happens when the water leaves the pressurized tank but before it goes into your tap for drinking. It gives the water one more treatment which removes any remaining carbon to give water the best taste and appearance possible.

The Shut-Off Valve

The RO system would have a hard time operating efficiently without the shut-off valve. The shut-off valve will turn off the system automatically when it senses the water tank is full.

The Flow Restrictor

The reverse osmosis filter feeds water through the filters very quickly which will affect how much purified water passes into the storage tank after being filtered. The flow restrictor will increase pressure to the water and push it through the membrane.

The Remineralization Filters

RO system removes everything from water so it's important to add healthy minerals back in, such as calcium, zinc, magnesium, and iron. Even though you can get these minerals from food, it's nifty to know you can also get them from your water. This is the purpose of the remineralization filter. This filter brings minerals back into the water before it reaches the tank for storage.
Remineralization Filters

The Faucet

If you decide to get a reverse osmosis system, you will probably have to change out your faucet for a faucet that comes with the system. The entire process is conveniently done in your kitchen sink.

The Electric Pump

This pump works very much like the permeate pump in the system, it's just designed differently. The pump uses a pressure switch to monitor the water pressure in the water storage tank. It will turn on the pump to increase water pressure when needed.
Not all RO systems need an electric pump. However, if your water pressure falls below 40 PSI, you don't want water waste but be more efficient, an electric pump will come in very handy.

The Best Reverse Osmosis Systems

Waterdrop G3 Reverse Osmosis Water Filter System

This system is very powerful under sink tankless RO water filtration system that offers many excellent features. It comes with a sleek compact design that gives you plenty of pure, clean water for you and your family by using a 7 stage filtration. The system has been tested and certified against NSF/ANSI standard 58 so more than 90% of TDS can be reduced after completely flushing.
One really great feature when it's in use, an LED ring on your faucet will glow with different colors to let you the status of the filters. In real-time, the panel on the system will deliver a message regarding the condition of the filter and the TDS level of the filtered water. TDS is total dissolved solids.

The APEC Essence Series Water Filter System

The APEC is another under sink reverse osmosis water filter system. It will produce 75 gallons of pure water each day. This is almost three times the amount of water that goes to waste as is purified. This is attributed to the system's automatic shutoff valve and flow restrictor.
The water goes through 6 stages of filtration and provides remineralization by adding minerals back into the water. Filtered water can taste really bland but remineralization brings back the flavor of the water.

The Express Water RO5DX Reverse Osmosis Filtration

The Express Water System has been tested independently by the third-party ANAB that uses the established NSF/ANSI Standards 42,58,401, and EPA 524.2 for the reduction of TDS, the taste of chlorine, and odors. It's also shoulders above dealing with any arising compounds or secondary contaminants, or other issues.
This system can be customized with additional filtering attachments. Another feature includes a leak detection system. The Express Water RO system is very easy to install, especially for those who believe in “Do It Yourself”. You will also get excellent support from experts who will walk you through the entire installation process.

The NU Aqua Platinum Series 100GPD RO System

This system has been through rigorous testing in their lab to make sure it offers consistent taste, clean and healthy water on demand. It features professional-grade high-quality fast connection fittings, an elegant NSF 100% lead-free designed faucet, a double o-ring sealed filter housing, and will make your countertop look fabulous while giving you a leak-free experience.
The NU Aqua Platinum Series 100GPD RO System is backed by a 5-year manufacturer warranty and lifetime support. This system is considered to have the best support in the industry along with guaranteed customer satisfaction.

The Home Master TMHP HydroPerfection RO System

If you want a system that is completely covered, you should consider this model. It is loaded with all the best features found in a water filter. It offers a 5-stage reverse osmosis filtration system that includes a Modular Sediment filter, Iron Filter With catalytic carbon, Ultra Violet Sterilizer filter, RO Membrane, and a Remineralization filter.
Home Master claims this system is capable of removing up to 99% dissolved solids, sediments, organic chemicals, heavy metals, and VOC. The UV light will kill any bacteria, viruses, or living microorganisms found in well water.

The Maxblue Stainless Steel Tankless RO System

The Maxblue Stainless Steel RO System is setting new standards for RO systems. It comes with all the standard filters you could possibly need and a tough housing for greater effectiveness. If you want a finished and polished system that has everything you need without all the extras at a great cost, then this is for you.
The Maxblue offers food-grade materials and a beautiful stainless steel finish. It comes with an attractive look that will complement your area. It has been upgraded with a tougher and more durable housing so it will not crack or leak. Its lifespan is 5 times longer than plastic systems.
This reverse osmosis water filter has a 1:1 drain ratio and will save 300% of the water, due to its water-saving advanced technology.

Frequently Asked Questions

Why Should I Install an RO System Rather Than Using Mineral Water?

People prefer to drink mineral water because it is convenient and provides them with the assurance that they are drinking safe water. The major problem with depending so much on mineral water is that it generates a tremendous amount of plastic trash. The world has already been suffering as a result of the amount of plastic in our environment. It would help if you made an effort to prevent contributing to the situation.
RO systems provide the same water quality as traditional systems but without the associated plastic trash.

Is It Worth It to Invest In A Reverse Osmosis System?

Getting a Reverse Osmosis (RO) unit is a very sensible move that a homeowner may make for all purposes and goals. It not only provides healthier and best-tasting water, but it also significantly reduces expenses. For example, A four-person family may save up to 30 cents per day on water expenditures by using the most acceptable water filtration unit.
The typical American spends $100 per person on mineral water ($400 = four people). The yearly operating expenses of an RO water filter device are around $100, so you may save up to $300 on water bills. These are in addition to the numerous health benefits provided by reverse osmosis (RO) water. Drinking clean, better-tasting, and filtered water is highly beneficial to your body.
For all these factors, and (RO) water filtration system is an excellent purchase.

How Long Will A RO System Work?

A RO system is typically expected to last 10 to 15 years. However, it is crucial to note that the system, like other devices, has consumables that must be changed regularly. Filters and membranes are examples of this.
Filters should be replaced once or either twice a year in most cases. The membrane, on the other side, has a long lifetime of 2 to 4 years. The membrane's lifespan is determined by the condition of water under which it is utilized.

Is Electricity Required to Run RO?

They do not operate on electricity; instead, they are powered by water pressure. If you attach an electric pressure booster pump or a U.V. light, you need electricity.

Do I require a UV-Lighted Reverse Osmosis (RO) System?

Some Reverse Osmosis (RO) systems are equipped with UV lamps. Those UV lamps are beneficial improvements. They help to purify water even more since they mainly target viruses and bacteria.

What Exactly Is TDS?

TDS is an abbreviation for dissolved solids in their complete state, present in all surface and groundwater sources. It relates to all negative (anion) and positive (cation) inorganic ions dissolved in the water. Sodium, magnesium, potassium, chloride, sulfates, carbonates, bicarbonates, and nitrates are typical inorganic salts soluble in water.

How Much Sewage Produce Reverse Osmosis Systems?

True, an (RO) reverse osmosis filtration system uses more water than it generates, but it's not that terrible in the perspective of wastage. After utilizing an RO (reverse osmosis) filtering unit for two months, you won't notice any difference in your water bills.

How Noisy Is A RO System?

No, a functioning reverse osmosis (RO) system should not create any noise. You should only hear a "gurgling" noise due to the sewage flowing from membrane to drain. The RO filter unit's design prevents it from becoming disorganized. This is accomplished by draining the sewage from the unit via a bridge.
Your reverse osmosis unit may occasionally produce a hissing sound. This implies an issue with the system, and you should contact a local sewage treatment specialist right once. Water pressure, air gaps, or leakage are possible culprits in this situation.

RO System Installation & Maintenance

Every one of the best reverse osmosis setups we've looked at so far needs some level of setup before they can work. RO water filtering systems are intended to be securely installed, having links to chilly water supply & waste pipes beneath a sink. Setup is identical for all these systems but could be completed by anybody with minimal DIY abilities plus tools. Assuming a mounting hole for the purified water tap doesn't exist, the only problematic step is drilling one in the counter or sink. The installation of a whole home RO system or maybe any of our suggested whole house filtration systems is a little more complicated and might even need the services of a plumber.
Look at our evaluations of countertop filtration systems, particularly countertop RO filters, should these systems need excessive installation and therefore don't fit your circumstance. It's feasible to just have purified water everywhere with three distinct categories to select from!

Types of Faucets

A conventional faucet or an air gap one is the different sorts of purified water faucets accessible using an under-sink RO system. It's worth noting that installing an air gap tap using a reverse Osmosis system is required in certain localities, even though it's often overlooked. Most sinks include a covered opening, an opening for spray hose, or even a built-in soap dispenser which might be utilized with your reverse Osmosis faucet. A ½" to 5/8" space is generally required for a basic RO faucet. Nevertheless, typical air gap faucets often demand a 1 ½" hole outside the reach of most DIY abilities, mainly when a hole must be drilled via countertop.
This need may influence your selection of a Reverse osmosis system. The rest of the setup is straightforward after the opening for said faucet has gotten addressed.

Setup Procedure

1. Select an installation place - The first step is to choose a suitable site for putting your reverse Osmosis water system. For most situations, the system will fit well beneath your sink. However, you could opt to place it somewhere else if that is more convenient for you.
2. turn off the water — Installing a filtering system before turning off the cold water valve is wrong. This is often located beneath the sink but is most likely linked to copper pipe.
3. Attach your system with your cold-water supply — just before you begin, turn on the cold faucet so all line pressure can be released. After that, you may start attaching the RO filter with the water supply.
4. Attach your drain connection — During this point, assuming there is visible rust in the drain connection, you should replace it before proceeding with the setup. If the drain connection is in excellent condition, you're ready to move forward. Drill a hole inside the pipe's exterior wall, then secure the drainage clamp. Adjust this clamp, taking care not to overtighten it. If the system possesses one, then also add a shut-off switch at about this point.
5. Setup the faucet - It's typically an excellent method to put your faucet relatively near where your prior faucet had been as reasonable while installing it. Suppose you don't have one already, drill a hole in the sink. Place the faucet stem within the hole, then ensure the base remains centered above the sink. Adjust the faucet, ensuring it's pointing in the spot you want to utilize it. Then, just at the bottom of a stem, connect the necessary tubing.
6. Install your unit — Now is the time to install your unit beneath the sink within the appropriate position. If you don't have a shelf to put the system on, you may have to hang it on the wall.


All Reverse osmosis systems will require routine maintenance at some point. The periods will vary depending on the kind of system and the water quality that reaches the filters.
Pre & post-filters should be replaced every six to twelve months or as needed based on utilization volume. Companies propose timetables. However, the most efficient method is to monitor the water quality beforehand, and afterward, the system treats it. It is possible to do it oneself if you have a TDS meter. All RO membranes possess a lifespan of one to five years. Remember, the higher the input water condition, the more the membrane would survive. One thing you can do to prolong the lifespan of the membrane would be to replace the pre-filter membranes as required.
The procedure varies depending on the company; however, most reverse Osmosis systems feature quickly replaceable filters. In certain situations, all required is to pull the filter out towards the foundation lightly, hit a button present on the holder, and then the filter will come free. Some filters come out, whereas the substitute goes in. Lastly, there are many sorts of filters found in housing units. Such housings typically are unscrewed such that the filtering cartridge may be changed, then fitted back into position. Except for a couple of minutes after few months to replace old filters, an under-sink RO system requires no maintenance.


Contaminants Detected in  Fruitland Water Special Service District

30  Total Contaminants in Your Water

Water Provider

Fruitland Water Special Service District

Population Affected


Water Source

Ground water
Exceeds Guidelines

Others Detected

See What's in Your Tap Water
Choose Your Water Solutions