Our Top Picks

RO-System.org is your reliable source for the best reverse osmosis system reviews and detailed information about all water filtration systems. Our experts test and rank the best options so you don’t have to!

Our rankings are based on hands-on testing, features, price, total cost of ownership, warranty, and customer reviews.

Best Overall: NU Aqua Tankless 600 RO System
This tankless RO system is super-efficient and comes with a smart faucet, remineralization filter, and 120 day home trial guarantee.
Overview | Full Review

Runner Up: Frizzlife PD-600-TAM3 RO System
An integrated TDS meter and remineralization filter make this very efficient tankless system one of the top choices.
Overview | Full Review

Best Budget: NU Aqua 7 Stage RO System
This system has lots of features including UV and remineralization at a great price.
Overview | Full Review

Best NSF Certified: Waterdrop G3 RO system
The smartest system on our list has an integrated TDS meter, smart faucet, and is the only NSF/ANSI certified high-production option.
Overview | Full Review

Best for Well Water: Home Master TMHP HydroPerfection RO System
With nearly every filter including ultraviolet, this system is the ideal option for well water and can be easily customized.
Overview | Full Review

Best for Ice Makers: Home Master TMAFC-ERP RO System
With a high-pressure storage tank, passive pump, and remineralization, this is a great option if you will connect to your ice maker.
Overview | Full Review

Best Countertop: Frizzlife WA99 Countertop RO System
You only need a power outlet to get a glass of filtered water on demand in any room from this highly-efficient and smart 4-stage system.
Overview | Full Review

Why Do You Need a Water Filter?

You are probably here because you want safe, healthy drinking water for you and your family or want to learn about water filtration systems.

Unfortunately, no tap water is safe anymore, even if you have “high quality” city or well water. Plus, there are no guarantees that bottled water is any better than your tap.

According to the Environmental Working Group’s (EWG) Tap Water Database:

  • 81% of American’s water contains contaminants linked to cancer
  • 77% of Americans are drinking hexavalent chromium (the “Erin Brockovich” chemical)
  • Water samples from all 50 states have been found to contain: arsenic, chromium-6, disinfection byproducts, lead, nitrates from fertilizers, and radiological contaminates.

Also, the EWG recommends reverse osmosis water filters as the “most effective” way to filter your drinking water since it will remove up to 99% of common and dangerous contaminants such as:

  • Lead
  • Arsenic
  • Nitrates
  • Nitrites
  • Chromium
  • Selenium
  • Fluoride
  • PFAS, PFOS, PFOA, and GenX “forever chemicals”
  • Cadmium
  • Phosphate
  • Radium
  • Chlorine
  • Barium
  • Bacteria
  • Cysts
  • Viruses
  • … and many more!

But picking the right options for your first reverse osmosis water filtration system can be overwhelming if you don’t know much about water treatment.

If you decide that a reverse osmosis drinking water filter system is not right for you, check out our whole house water filter reviews for some great alternatives.

Before you start looking at individual RO systems, check out our Reverse Osmosis Buying Guide (below). You will learn how reverse osmosis works, the terms that you will come across in your research, and understand which features matter for your health.

Reverse Osmosis Systems 1-5

Our top 10 reverse osmosis (RO) water filtration systems are “point-of-use” undersink water filter systems that are installed and used at a single location, which is usually under the kitchen sink.

These reverse osmosis water filtration systems include a separate faucet that is installed along side your standard tap water faucet to produce filtered water just for drinking and cooking. With additional parts, you can also run a pure drinking water line to your refrigerator, if it is has a water dispenser and/or ice maker.

If you want to learn more about the newer tankless style options we recommend, check out our review of the top tankless reverse osmosis systems.

ProductTop FeaturesLearn More or Buy
NU Aqua Tankless 600 GPD
NU Aqua Tankless 600 GPD


  • Shortest height

  • Remineralization

  • Super efficient tankless

  • Smart faucet

  • 8 stages and 600 GPD

💲 💲 💲
Exclusive 20% OFF!


Frizzlife PD600-TAM3 RO with Remineralization
Frizzlife PD600-TAM3

  • TDS Meter built in

  • Remineralization

  • Very efficient tankless

  • 7 stages and 600 GPD

💲 💲 💲
Black Friday Sale!


Waterdrop G3 P800 RO System
Waterdrop G3 P800

  • 8 stages and 800 GPD

  • Best 3:1 wastewater ratio

  • Integrated UV filter

  • TDS Meter built in

  • Smart faucet


💲 💲 💲
$299 OFF


Home Master TMAFC-ERP Artesian
Home Master TMAFC-ERP Artesian

  • Most customization options

  • Remineralization

  • Storage tank

  • Non-electric pump

  • 7 stages and 75 GPD

💲 💲 💲
Free Upgrades!


NU Aqua Platinum Series 7-Stage
NU Aqua 7-Stage

  • Ultraviolet (UV) filter

  • Remineralization

  • Storage tank

  • Electric pump

  • 7 stages and 100 GPD

💲 💲 💲
Black Friday Sale!


1. NU Aqua Tankless Reverse Osmosis System

NU Aqua Tankless Reverse Osmosis System
NU Aqua Tankless Reverse Osmosis System

NU Aqua Systems has joined the tankless trend and added the NU Aqua 600GPD Tankless reverse osmosis water filtration system (WU-600GPD-TL) to its already impressive lineup.

What sets this reverse osmosis system apart?

It is tankless

It has an integrated pump and is not slowed down by a water tank so it can process up to 600GPD faster than a conventional under sink reverse osmosis system.

It is super-efficient

There is a 2:1 ratio of pure reverse osmosis water to waste compared to other RO systems that only get 1:1 and many that get 1:2 or 1:3…

If conserving water is important to you, then this is the most efficient undersink reverse osmosis option!

This RO system is smart

The RO faucet has an LED ring that glows blue to let you know everything is working normally or red to let you know that one of your filters needs to be replaced.

The system body also has lights to show filter status and overall system health.

This is also one of the most compact reverse osmosis systems you can put under your sink and is easy to install.

For more details on this system, check out our full NU Aqua Tankless 600GPD Reverse Osmosis Review or click the button below.

Finally, don’t overlook the 5 year warranty and 100% risk-free 120-day home trial!

Exclusive Savings – 20% OFF for RO-System.org visitors

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2. Frizzlife PD600-TAM3 with Alkaline Remineralization

Frizzlife PD600 Reverse Osmosis Water Filter
Frizzlife PD600-TAM3 with Remineralization

The Frizzlife PD-600-TAM3 makes it near the top of the list of reverse osmosis water filtration systems because of its excellent features and competitive price tag.

What makes it so good?

Remineralization

The Frizzlife system comes with an alkaline remineralization filter that adds essential minerals back to the filtered water and raises the pH to more than 7.5.

Smart features

Only one other system on our list has an integrated digital TDS meter built into the system like this Frizzlife. This very helpful feature continuously tests and reports on the quality of your water after it has been filtered.

Tankless design

With the fastest 600 GPD RO membrane and an internal booster pump, this system has extremely high throughput and is very efficient.

A very good 1.5:1 ratio of filtered drinking water to waste water makes it more efficient than any RO system with a tank on this page.

Combine all of the features above with the smaller size and easy maintenance of the tankless design and you have a system that is sure to provide great water in a great package.

For more details on this system, check out our full review of the Frizzlife PD600 or click the button below.

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3. Waterdrop G3 P800 Reverse Osmosis System

Waterdrop G3 P800 Reverse Osmosis System
Waterdrop G3 P800 Tankless Reverse Osmosis System

The Waterdrop G3 P800 is an evolutionary upgrade to the very popular Waterdrop G3 (P400) and offers an array of features that are not available in any other tankless under-sink RO systems.  

This system can produce a whopping 800 gallons of filtered water per day (GPD), which is enough for a very busy household and even small businesses that requires large amounts of filtered water.

In addition, it does this with an exceptional 3:1 waste water ratio, meaning it creates only 1 gallon of waste water for every 3 gallons of pure water.

This is currently the only tankless water filtration system that comes with an integrated UV filtration stage that effectively removes any microorganisms that may still be present in the water.

The system includes a built-in TDS (total dissolved solids) meter to report the quality of the filtered water whenever the system is working, a smart faucet and LEDs on the chassis report the filter life status.

When you consider that the Waterdrop G3 P800 takes up less room, creates more clean water, and provides better feedback than any other system on the market, it should be at the top of your list!

For more details on this system, check out our hands-on Waterdrop G3 P800 reverse osmosis review or click the button below.

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4. Home Master TMAFC-ERP Artesian Reverse Osmosis System

Home Master TMACF-ERP Reverse Osmosis System
Home Master Artesian Reverse Osmosis System

The Home Master TMAFC-ERP water filtration system combines the perfect balance of useful benefits, popular features, and value in one package.

There are two big features that set the TMAFC-ERP apart from other mid-range options and makes it one of the top reverse osmosis systems…

Remineralization stages

First of all, reverse osmosis water can have a bland taste. This is because it is slightly acidic and all of the dissolved minerals that provide flavor have been removed. After the purified water has been purified through the TMAFC-ERP RO membrane, it passes through a remineralization filter TWICE (a unique feature of the Home Master). This adds safe minerals back into the RO water and raises the pH to be more alkaline. The result is a fresher tasting mineral water that many people prefer.

Permeate pump

In addition, it comes standard with a permeate pump, which is a non-electric device that increases the water pressure before it enters the reverse osmosis membrane. This reduces water waste up to 80 percent and increases water production up to 50 percent. Both of these are important if you are pay for city water or care about efficiency. Compared to the average of 2-3 gallons wasted per filtered gallon produced across our reviewed RO systems, the TMAFC-ERP with a permeate pump only wastes 1 gallon.

The TMAFC-ERP reverse osmosis system has 7 stages and is rated for about 75 gallons of purified reverse osmosis water per day. The included “Fast Flow RO kit” uses larger fittings and tubing to speed up the fill rate at the faucet so you get water as fast as possible.

Home Master makes high quality filters that are modular in design and built for easy filter changes without any tools. Installation of this system is very straight forward and a person with some basic DIY skills should be able to complete the install within about an hour. It comes with both a faucet and pressurized storage tank.

Also, Home Master has an excellent 5-year warranty, which is better than most of the competitors in our list.

For even more details on this system, check out our full Home Master TMAFC-ERP Artesian reverse osmosis review.

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5. NU Aqua 7 Stage Reverse Osmosis System

NU Aqua 7 Stage Reverse Osmosis System
NU Aqua 7 Stage Reverse Osmosis System

The NU Aqua 7 Stage Reverse Osmosis Water Filtration System has all of the same features as many competitors but at a much better price.

Extra features

In addition to having the 5 basic stages of filtration, this system also offers:

  • UV (Ultraviolet light) Filter to destroy bacteria and other mcroorganisms in well water
  • Alkaline Filter to add beneficial minerals back to the RO water
  • Booster Pump to improve efficiency and decrease wastewater

The NU Aqua reverse osmosis system also has a high 100 gallon per day capacity. While most people wouldn’t need this much filtered reverse osmosis drinking and cooking water, higher capacity gives you a higher flow rate.

The other big feature is a 5 year manufacturer warranty and lifetime support. Only one other brand in our recommendations offers a 5 year warranty and it’s a great benefit.

Finally, NU Aqua provides a free digital PPM meter water tester so that you can get a quick measurement of overall reverse osmosis water quality. This is a nice bonus to round out the package.

For more details on this system, check out our full NU Aqua Systems Reverse Osmosis Review or click the button below.

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Don't forget the accessories!
Don’t forget to check out the top accessories that could help you get even more out of your RO system! Read more…

Reverse Osmosis Systems 6-10

The next five recommended reverse osmosis systems all do a great job of removing contaminants from water, but don’t rank quite as high as the first five systems above.

6. Home Master TMHP HydroPerfection Reverse Osmosis System

Home Master HydroPerfection Reverse Osmosis Water Filter System
Home Master TMHP HydroPerfection

If you don’t to want waste time comparing options and just want to be sure you are covered in every situation, then the Home Master TMHP HydroPerfection is the perfect reverse osmosis filtration system for you. It comes complete with every major feature and every possible filter – 9 stages in all!

Permeate pump

The Home Master TMHP includes a permeate pump, which increases the output of the system to 75 gallons per day and reduces the amount of waste water needed to flush the reverse osmosis membrane to just 1 gallon for every gallon of purified drinking water produced.

Remineralization stage

It has a remineralization filter that adds safe calcium and magnesium minerals back into the RO water and raises the pH to be more alkaline. This creates a fresher tasting water that compares to good bottled water.

UV filter

The system also has a UV (ultraviolet) light that will kill 99.8% of microorganisms that may contaminate your water. Municipal water sources are usually low in microorganisms but well water and other natural sources can have higher levels. UV filters are highly recommended in situations with natural water sources such as well water. Keep in mind that a UV filter requires a 110V power outlet.

The TMHP under sink reverse osmosis system is rated for about 75 gallons of purified water per day and in addition to an upgraded faucet, the system now uses a larger 3/8” diameter tubing. This larger bore tubing helps increase the flow rates and the speed at which the purified water reaches the faucet.

The modular filters (which include an iron filter) are very easy to change without tools. Installation of this system is straightforward and a person with some basic DIY skills should be able to complete the install within about an hour. It comes with both a faucet and pressurized storage tank.

In addition, Home Master comes with a 5-year warranty!

For more details on this system, check out our full Home Master TMHP HydroPerfection reverse osmosis review or click the button below.

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7. Waterdrop G2 P600 Reverse Osmosis System

Waterdrop G2 P600 Reverse Osmosis
Waterdrop G2 P600 Reverse Osmosis System

The Waterdrop G2 P600 system has fewer smart features than the higher ranked Waterdrop G3 but more throughput and a cheaper price tag.

In fact, the G2 P600 is the least expensive tankless RO system we recommend even though it still has 6 stages of filtration to provide crystal clear filtered water.

While it’s only smart feature is a filter life indicator on the system housing, it still has a lot of great features worth considering…

With 600 GPD output and an extremely high 2:1 ratio of filtered water to waste water, it is one of the fastest and most efficient systems in our recommendations.

Also consider that it is has a tankless design, which makes it easier to install, MUCH easier to maintain, and it takes up less space under the sink.

When you put it all together, this is a very capable water filter and a great option if you want great performance without paying for bells and whistles.

For even more details on this system, check out our full review of the Waterdrop G2 P600.

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8. NU Aqua 5 Stage Reverse Osmosis System

NU Aqua 5 Stage Reverse Osmosis System
NU Aqua 5 Stage Reverse Osmosis System

The NU Aqua 5 stage reverse osmosis water filtration system is the least expensive system in our top 10 but still comes with all the standard filters you need AND a booster pump for greater efficiency.

If you are looking for a really nice budget system that has everything you need without any extras at a great price, then this is the one to get.

A few things about the NU Aqua stand out and make it better than other basic 5 stage reverse osmosis water systems.

First, the NU Aqua has a high 100 gallon per day capacity. While most people wouldn’t filter this much water, higher capacity gives you a higher flow rate. Despite this, the system still gets an average waste water ratio of 3 gallons wasted for every one gallon of purified reverse osmosis water.

The other big feature is a 5 year manufacturer warranty and lifetime support. Only one other brand in our reverse osmosis recommendations offers a 5 year warranty and it’s a great benefit.

Finally, NU Aqua provides a free digital PPM meter water tester so that you can get a quick measurement of overall water quality. This is a nice bonus to round out the package.

This under sink reverse osmosis system comes with everything that you need and is designed to be installed in about 1 hour by someone with basic DIY skills. It comes with both a faucet and pressurized storage tank.

For even more details on this system, check out our full review of NU Aqua Systems.

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9. Pentair-Pelican PRO-RO Reverse Osmosis System

Pentair-Pelican PRO-RO Reverse Osmosis System

If you are looking for a smaller, easy to manage system and have low water volume needs, then consider the Pentair-Pelican PRO-RO undersink reverse osmosis water filtration system.

The PRO-RO combines the system chassis, filter cartridges, and 2-gallon pressurized pure water storage tank into one compact unit.

It’s the only system we recommend that comes with an air-gapped faucet, which is required in some building code.

The PRO-RO does waste an average of about 2.5 gallons of water to produce 1 gallon of purified reverse osmosis water.

While the output of this system is low compared to the competition, it makes up for it with:

  • Simple maintenance
  • An air gap faucet in three different finishes
  • NSF/ANSI Standard 58 certification by WQA
  • Takes up half the space of most competing RO systems
  • It’s made in the USA

For more details on this system, check out our full Pentair-Pelican PRO-RO Reverse Osmosis System review.

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10. iSpring RCC7AK-UV Reverse Osmosis System

The iSpring RCC7AK-UV is a popular and highly-rated basic system and is built for well water with the integrated Ultraviolet (UV light) filter.

UV light sterilizes the water and kills the small percentage of viruses, bacteria, and other microorganisms that could potentially remain in the pure water after the RO process. Most municipal water systems treat for microorganisms but well water and other natural sources can contain higher levels. Keep in mind that a UV filter requires a 110V power outlet.

It is one of the higher capacity RO systems that we have reviewed since it produces 75 gallons of purified reverse osmosis water per day through 7 stages of filtration.

The iSpring RCC7AK-UV wastes 3 gallons to produce a gallon of purified water, although this amount will vary depending on your feed water pressure. This is average compared to the other systems with a tank we recommend.

The system comes with everything that you need and is designed to be installed in about 1 hour by someone with basic DIY skills. It comes with both a faucet and pressurized storage tank.

For more details on this system, check out our full iSpring RCC7AK-UV RO reverse osmosis review or on Amazon below.

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Compare the Top 10 RO Systems

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NU Aqua Tankless 600 GPD
Frizzlife PD600-TAM3 RO with Remineralization
Waterdrop G3 P800 RO System
BONUSES!
Home Master TMAFC-ERP Artesian
NU Aqua Platinum Series 7-Stage
Our Rating
Installation
DIY Undersink
DIY Undersink
DIY Undersink
DIY Undersink
DIY Undersink
Stages
8
7
7
7
7
Output
600 GPD
600 GPD
800 GPD
75 GPD
100 GPD
Wasted Water
1 Gal : 2 Gal Filtered
1 Gal : 1.5 Gal Filtered
1 Gal : 3 Gal Filtered
1 Gal : 1 Gal Filtered
2 Gal : 1 Gal Filtered
Tank Size
Tankless
Tankless
Tankless
3.2 Gallons
3.2 Gallons
Smart Features
Pump Included
Remineralization
Optional Add-on
UV Light
Relative Price
Free Shipping
SAVE 20% NOW
NU Aqua Tankless 600 GPD
Our Rating
Installation
DIY Undersink
Stages
8
Output
600 GPD
Wasted Water
1 Gal : 2 Gal Filtered
Tank Size
Tankless
Smart Features
Pump Included
Remineralization
UV Light
Relative Price
Our Review
Free Shipping
Learn More/Buy
Frizzlife PD600-TAM3 RO with Remineralization
Our Rating
Installation
DIY Undersink
Stages
7
Output
600 GPD
Wasted Water
1 Gal : 1.5 Gal Filtered
Tank Size
Tankless
Smart Features
Pump Included
Remineralization
UV Light
Relative Price
Our Review
Free Shipping
Learn More/Buy
Waterdrop G3 P800 RO System
Our Rating
Installation
DIY Undersink
Stages
7
Output
800 GPD
Wasted Water
1 Gal : 3 Gal Filtered
Tank Size
Tankless
Smart Features
Pump Included
Remineralization
Optional Add-on
UV Light
Relative Price
Our Review
Free Shipping
Learn More/Buy
BONUSES!
Home Master TMAFC-ERP Artesian
Our Rating
Installation
DIY Undersink
Stages
7
Output
75 GPD
Wasted Water
1 Gal : 1 Gal Filtered
Tank Size
3.2 Gallons
Smart Features
Pump Included
Remineralization
UV Light
Relative Price
Our Review
Free Shipping
Learn More/Buy
NU Aqua Platinum Series 7-Stage
Our Rating
Installation
DIY Undersink
Stages
7
Output
100 GPD
Wasted Water
2 Gal : 1 Gal Filtered
Tank Size
3.2 Gallons
Smart Features
Pump Included
Remineralization
UV Light
Relative Price
Our Review
Free Shipping
Learn More/Buy
BONUSES!
Home Master TMHP HydroPerfection
Waterdrop G2 P600 RO System
NU Aqua 5 Stage Reverse Osmosis System
Waterdrop G3 RO System
iSpring RCC7AK-UV, NSF Certified, 75GPD 7-Stage Under Sink Reverse Osmosis RO Drinking Water Filtration System with Alkaline Remineralization Filter and UV Ultraviolet Filter
Our Rating
Installation
DIY Undersink
DIY Undersink
DIY Undersink
DIY Undersink
DIY Undersink
Stages
9
6
5
6
7
Output
75 GPD
600 GPD
100 GPD
18 GPD
75 GPD
Wasted Water
1 Gal : 1 Gal Filtered
1 Gal : 2 Gal Filtered
2 Gal : 1 Gal Filtered
2.5 Gal : 1 Gal Filtered
3 Gal : 1 Gal Filtered
Tank Size
3.2 Gallons
Tankless
3.2 Gallons
2 Gallons
3.2 Gallons
Smart Features
Pump Included
Remineralization
Optional Add-on
UV Light
Relative Price
Free Shipping
BONUSES!
Home Master TMHP HydroPerfection
Our Rating
Installation
DIY Undersink
Stages
9
Output
75 GPD
Wasted Water
1 Gal : 1 Gal Filtered
Tank Size
3.2 Gallons
Smart Features
Pump Included
Remineralization
UV Light
Relative Price
Our Review
Free Shipping
Learn More
Waterdrop G2 P600 RO System
Our Rating
Installation
DIY Undersink
Stages
6
Output
600 GPD
Wasted Water
1 Gal : 2 Gal Filtered
Tank Size
Tankless
Smart Features
Pump Included
Remineralization
Optional Add-on
UV Light
Relative Price
Our Review
Free Shipping
Learn More
NU Aqua 5 Stage Reverse Osmosis System
Our Rating
Installation
DIY Undersink
Stages
5
Output
100 GPD
Wasted Water
2 Gal : 1 Gal Filtered
Tank Size
3.2 Gallons
Smart Features
Pump Included
Remineralization
UV Light
Relative Price
Our Review
Free Shipping
Learn More
Waterdrop G3 RO System
Our Rating
Installation
DIY Undersink
Stages
6
Output
18 GPD
Wasted Water
2.5 Gal : 1 Gal Filtered
Tank Size
2 Gallons
Smart Features
Pump Included
Remineralization
UV Light
Relative Price
Our Review
Free Shipping
Learn More
iSpring RCC7AK-UV, NSF Certified, 75GPD 7-Stage Under Sink Reverse Osmosis RO Drinking Water Filtration System with Alkaline Remineralization Filter and UV Ultraviolet Filter
Our Rating
Installation
DIY Undersink
Stages
7
Output
75 GPD
Wasted Water
3 Gal : 1 Gal Filtered
Tank Size
3.2 Gallons
Smart Features
Pump Included
Remineralization
UV Light
Relative Price
Our Review
Free Shipping
Learn More

Whole House Reverse Osmosis Water Filter Systems

The under sink RO water filter systems listed above are point-of-use and will only provide filtered water at the location they are installed.

If you decide you want the clean water everywhere in your house, or you have very high water volume needs, then a whole house reverse osmosis system would be the most complete water treatment option for your home or business.

With a whole house system, you get the same excellent water filtering benefits as the systems described on this page, but at every sink, shower, and appliance in your house.

Whole house RO is not without it’s disadvantages, but you can learn more in our whole house RO review and guide.

There is also the option to install whole house water filters without reverse osmosis that will do a great job of removing common contaminants from city water or well water.

Reverse Osmosis System Buying Guide

What is Reverse Osmosis?

Reverse Osmosis (also referred to as “RO”) is a process where you remove impurities from water by pushing it through a permeable membrane (reverse osmosis water filter). It is the opposite of osmosis, which is one of the most important processes in nature. In osmosis, water that is more pure naturally flows towards water that is less pure even it if has to pass through a membrane. Reverse osmosis uses pressure to force less pure water through a membrane that blocks impurities so that you end up with very pure water on the other side.

RO filters perform this same process to clean water in a reverse osmosis water filtration system.

Here is an informational resource that explains reverse osmosis in greater detail and the US Centers for Disease Control and Prevention (CDC) has prepared a fact sheet about the effectiveness of different water filtration systems for purifying water.

What Does a RO Filtration System Remove?

Only reverse osmosis filters can remove or drastically reduce all of the following harmful contaminants:

  • Aluminum
  • Ammonium
  • Arsenic 5
  • Bacteria
  • Barium
  • Cadmium
  • Calcium
  • Chloride
  • Chlorine
  • Chloramines
  • Chromate
  • Chromium
  • Copper
  • Cyanide
  • Cysts
  • Fluoride
  • Iron
  • Lead
  • Magnesium
  • Manganese
  • Mercury
  • Nitrate
  • Nitrite
  • Pesticides
  • Phosphate
  • Potassium
  • Protozoa such as Cryptosporidium and Giardia
  • Radium
  • Radon
  • Selenium
  • Silver
  • Sodium
  • Strontium
  • Sulfate
  • Sulfite
  • Turbidity
  • Viruses
  • Zinc
  • … and many more!

How Does Reverse Osmosis Work?

How Does Reverse Osmosis Systems Works
Diagram of an under sink reverse osmosis system.

Although all the systems that we have reviewed are referred to as “reverse osmosis systems,” the RO filter is only one stage of the whole water treatment system. The RO membrane is just a very fine filter.

The actual setup varies slightly from system to system and between manufacturers but the layout is basically the same as shown in the image below.

Water passes through one or more pre-filters (1) that remove large particles, then through the RO filter (2), which removes almost all other smaller particles. Finally, it will pass through one or more post-filters (3 and 4) that remove off-tastes or gasses that affect the odor of the water.

While some RO systems will have additional components such as UV lights, permeate pumps to increase the pressure of the in-flowing water, and remineralization functions, the main process does not vary much.

Pre-Membrane Filters

In unfiltered drinking water, there are materials that, although invisible to the naked eye, are large enough to damage the reverse osmosis membrane. Typically, this sediment filter (or filters) will remove all solids such as rust and sand, chlorine, etc. with a 5-micron filter at the start of the filtration process.

RO Membrane

The RO membrane (also known as reverse osmosis filter) is an ultra-fine filter that removes everything larger than 0.001 microns. This means that virtually the only thing that can pass through the permeable membrane is pure water.

Reverse osmosis filters remove up to 99 percent of dissolved solids from the water. With the pores of this RO membrane being so fine, it is understandable that water pressure would affect how quickly the water could pass through. Higher pressure is ideal for efficient reverse osmosis water filtering.

Post Filters

Most RO systems pass water through an additional carbon filter at the final stage to remove any lingering tastes and odors from the purified water. The purified reverse osmosis water is then delivered to a storage tank at the end of the filtration process until required. There can be other stages that are considered a post filter as well.

Hard Water

Reverse osmosis filters do remove minerals from purified water but is not meant to replace a water softener if you have hard water.

Water softeners work specifically to remove the “hard” minerals of calcium and magnesium from water and make it “soft.” Hard water is not unhealthy but can cause many problems for your appliances, plumbing, and for the people that bathe in it. Be sure to check out our reviews of water softeners to find out more.

85% of U.S. households have hard water and installing a water softener will both improve the water at all points of use and protect reverse osmosis systems from dealing with hard water.

Optional Accessories

Be sure to check out our guide to the most popular water filter accessories for even more options. This can include refrigerator connection kits, alternative faucets and water tank sizes, as well as water quality testing tools.

Remineralization

Standard on some systems or available as an optional extra, a remineralization stage adds beneficial minerals such as calcium and magnesium, and sometimes potassium back to the reverse osmosis water. Not only does this enhance the taste of drinking water but it also raises its pH value by making it less acidic and more alkaline.

Permeate Pump

If you have low water pressure or even just want to increase the output of your RO filter system and waste less water, you should add a pump. It is possible to buy an electric pump but a permeate pump is recommended and is included as standard on some of our recommended systems, such the Home Master TMHP.

A permeate pump is a clever non-electric device that circulates the water to increase the pressure entering the system. Not only does this improve the amount of water output by the system by 50% it also greatly reduces the water needed to flush the reverse osmosis membrane. Instead of using 4 gallons of wastewater to produce a gallon of purified water, systems with a permeate pump waste 1 gallon to produce a gallon of purified reverse osmosis drinking water. You can add a permeate pump to any system after installation.

Some systems contain a powered booster pump that will ensure proper water pressure inside of the RO system for optimal efficiency.

UV Light

A few of our top undersink RO systems come with an Ultraviolet (UV) light, although more should include one.

The purpose of the UV light is to kill 99.9% of all microorganisms that may be in the water including bacteria and viruses. The RO filter often blocks most microorganisms before they can reach the UV filter, but you want to be sure if you have well water or a questionable water source.

Municipal (city) water is often treated to kill microorganisms already although old or dirty pipes can still re-introduce more. Well water and other natural sources can contain a much higher level of microorganisms and a UV filter is recommended if your water could be contaminated. Testing your water is the only way to know for sure and you can add a UV light to any system after installation.

Keep in mind that you will need an outlet near the RO system so you can plug in the UV light.

Reverse Osmosis Terms

When learning about reverse osmosis filtration systems, there are various terms that manufacturers use that may be new to you. Here are a few of the more common terms and what they may mean.

Stages

Every RO system has a distinct number of stages that water will pass through in the filtration process. Each stage performs a different function to either filter or process the water in some way. The number of stages is not the only way to compare how good systems are but more stages usually means more is done to purify and guarantee high quality water.

Storage Tank

All of our top reverse osmosis systems that install under the sink come with a pressurized storage tank that will hold the purified reverse osmosis water. By pre-filtering enough purified water to fill the tank, the system ensures that you will get a quick response from your purified water faucet.

Without the storage tank, you can only have filtered water as fast as the system can produce it in real time. You can replace the standard tank with a larger one if you often need more pure water on demand. Alternatively, you can look at one of the tankless reverse osmosis systems that don’t use a tank and produce reverse osmosis water faster.

Water Output

A measure of how much purified water a system can typically process in a day.

All RO systems that we have reviewed use the same standard test for documenting their gallons per day (GPD) output figures. These tests are based on the water coming in at 77 degrees Fahrenheit (25 Celsius) and with a water pressure of 60 psi.

The truth is that in real life installations, very few places have these ideal conditions. This means that quite often the output from a RO system is less (in some cases considerably less) than suggested in the datasheet. It is quite complicated to calculate the exact figures that you can expect to get from any system since lower pressure, lower temperature, and high TDS levels also lead to a reduction in output.

If your system suffers from the low output, you can probably blame lower water pressure and increase it by adding a pump to the feed water side. A permeate pump can increase output by 50% per day. Additionally, if your water supply is delivered at a very low temperature, introducing some coiled water pipe onto the feed side of the filters will allow the water to warm up before it enters the RO membrane.

Waste Water

One of the common complaints about RO systems is that they waste a great deal of water when flushing the RO membrane.

With undersink RO systems, the range is 0.5-4 gallons of water being wasted to produce a gallon of purified water. The amount of water used is highly dependent on the pressure of the inflowing water – higher pressure means less water is needed to flush the reverse osmosis filters.

The average across the systems we reviewed is about 2 gallons wasted per 1 gallon filtered. A permeate pump reduces the water waste to 1 gallon for every gallon of filtered reverse osmosis water produced and a powered pump can lower waste water to 0.5 gallons.

If waste water is a concern, make sure to look for a system with a lower waste to filtered water ratio. If you want to reclaim the “wasted” water, the waste pipe from the system can be fed into the normal cold faucet or even diverted for watering the garden.

pH / Acidic / Alkaline

The pH refers to how acidic (low pH) or alkaline (high pH) the water is.

The RO systems that we have reviewed accept a wide range of pH values in the feed water. Manufacturers that comply with EPA Drinking Water Standards will accept water that has a pH value of between 6.5 and 8.5. Water that passes through RO systems tends to have the pH value lowered when the healthy minerals are filtered out with everything else. This lower pH makes the purified water slightly more acidic, which can taste bland to some people.

If this is a problem for you, you may want to look at systems that remineralize your drinking water. A remineralization stage increases the pH value to a level closer to neutral and creates a taste that is similar to good bottled water.

The scale of pH values

TDS

TDS stands for “total dissolved solids.” Total dissolved solids usually include inorganic salts such as calcium, magnesium, potassium, sodium, bicarbonates, chlorides, and sulfates, along with some small volumes of organic matter that are dissolved in water. The EPA sets a maximum TDS level of 500mg/L in drinking water is its Secondary Drinking Water Standards.

There are two ways that the total dissolved solids can be measured. The first way is in a lab under stringent controlled conditions, which is expensive and time consuming. The second is by measuring electrical conductivity. Although this method is not as accurate as the lab test, the results gained by a fairly inexpensive meter are within 10% of the lab results. An RO system that is functioning well should reduce TDS in purified water by about 90% – but this is variable.

Air Gap Faucet

The best way to know if the filters in your RO systems are due for replacement is to measure both the pre-filtered and post-filtered reverse osmosis water to compare the TDS values.

Air Gap Faucet

What is an air gap faucet? The air gap provides a siphon break for under sink RO systems. This means that if the sink should get backed up, there is no way that the dirty water can be siphoned back into your RO system. In some localities, it is part of the plumbing code that all RO systems should have an air gap faucet, but it is rare.

A few RO systems come with an air gap faucet, although most offer only a standard faucet. You can always upgrade your faucet in any system if you prefer or local plumbing code requires it.

While there are advantages to air gap faucets, a standard faucet is usually preferred. This is because air gap faucets are more complicated to install and usually need a larger, non-standard counter hole. Air gap faucets need three pipes to connect instead of just one and, in some cases, they can be noisy.

Installation and Maintenance of RO Systems

All of the top reverse osmosis systems that we have reviewed here require some degree of installation before they will operate.

Reverse osmosis water filtration systems are designed to be permanently installed with connections into the cold water feed pipes and waste pipes under a sink. Installation for any of these systems is similar and can be easily carried out by anyone with basic DIY skills and tools. The only difficult part is drilling a mounting hole in the sink/counter for the filtered water faucet, if a hole does not already exist.

However, if these systems require too much installation or don’t work for your situation, check out our reviews of the countertop water filters including countertop reverse osmosis filters. With three different categories to choose from, it’s possible to have filtered water anywhere!

Faucet Types

There are two types of pure water faucets available with an under sink reverse osmosis system: a standard faucet and an air gap faucet. Note, in some municipalities, it is mandatory to install an air gap faucet with an RO system, although this is widely ignored. Some sinks will have covered hole, spray hose opening, or integrated soap dispenser that can be used for your RO faucet.

A standard RO faucet usually requires a ½” to 5/8” hole. The air gap faucet, however, usually requires a 1 ½” opening and this can be beyond the scope of DIY skills, especially if a hole has to be drilled through the counter top. This requirement may affect your choice of RO system. Once the hole for the faucet has been solved, the reaming installation is very straightforward.

Installation Process

The top under sink reverse osmosis systems that we recommend use press fit quick connectors for the water tubes, which provides a good joint without the need for any special tools or knowledge.

Although there are slight differences from system to system, installation is generally as follows:

  • Turn off and disconnect the cold water feed pipe from the wall to the faucet with the shut off valve.
  • Attach the supplied ‘T’ connector to the cold water feed outlet.
  • Reconnect the cold water feed to the ‘T’ connector (and if necessary, the push fit connector – in some cases, it may already be attached).
  • Drill a hole in the waste pipe for the filter waste connector. In our experience, we found that having the drain connector on a horizontal section of waste pipe reduced the noise when the system is flushing.
  • Using the drill bit as a guide, attach the RO drain connector to the drain pipe.
  • Attach the RO system filter package to the side wall of the cupboard or back wall. Make sure that the is enough room in all directions to remove the filters and perform maintenance. Some systems required a large, plastic wrench to loosen filter housings.
  • Place the reservoir tank on the base of the cupboard.
  • Install the filter faucet on the top of the sink.
  • Attach all of the water tubes. In most cases, the pipes are color-coded to ensure that the correct connections are made.
  • If a UV light system is present in the system, plug it into the electrical outlet. You may need an electrician if you don’t already have power available.
  • Turn the source cold water on at the shut off valve and check for leaks throughout the water path.
  • Fill and empty the reservoir 3 to 4 times (the exact amount of times depends on the individual system).

The system is now producing filtered reverse osmosis water.

After initial installation, it is worth checking the system every day or so for leaks. If the system remains leak free for a while, then check every month to make sure that no leaks occur.

Maintenance

All RO systems will, from time to time, need periodic maintenance and the intervals are dependent on the type of system that you have and on the quality of the water that enters the filters.

Pre and post filters will need replacing every 6-12 months or by your usage volume. Manufacturers have recommended schedules but the most effective way to know for sure is to test the water before and after it is processed by your system. You can do this yourself with a simple TDS meter.

All RO membranes have a life span of between 1 and 5 years. Again, the better the quality of the feed water the longer the membrane will last. One area that will help extend the life to the membrane is to make sure to always change the pre-filters membranes when needed.

The process varies by manufacturer although most RO systems have easy to change filters. In some cases, all that is needed is to lift the filter slightly out at the base, press a button on the holder and the filter comes away. Other filters pull out and the replacement just pushes in. Finally, there are the types of filters that are contained in housings. These housings are unscrewed so that the filter cartridge can be replaced and then the housing is screwed back into place.

Apart from spending a few minutes every few months replacing old filters, the rest of an under sink reverse osmosis system is maintenance free.

Need More Info?

We have covered a wide range of reverse osmosis system reviews and hope that our many hours of research, exclusive ranking algorithm, and detailed comparisons have helped you to find the right reverse osmosis systems for your pure water.

If you are still unsure, please feel free to contact us and let us know what other questions you have. Finally, you can learn more about RO-System.org or read our in-depth reviews of the top systems and accessories.