How RO Systems Work and What They Do

What is a Reverse Osmosis Water Filter System and How Does it Work?

Reverse Osmosis (RO) water filtration is one of the most popular and cost-effective water filtration methods available. In simple terms, a reverse osmosis water filter works as water is forced across a semi-permeable membrane, leaving contaminants behind that are flushed down the drain. The clean drinking water collects in a holding tank.

Why Millions of American Households Install Reverse Osmosis (RO)

  • Reverse Osmosis produces healthy, fresh, great-tasting water
  • Reverse Osmosis filtration is fully automated
  • Reverse Osmosis water systems are reasonably priced
  • Reverse Osmosis does not use electricity or require a power source
  • Reverse Osmosis saves money over buying bottled water or paying for a delivery service
  • Reverse Osmosis is simple to install

Below you will find diagrams, charts and explanations to answer your questions about Reverse Osmosis drinking water systems. If you have other questions which are not answered here, please visit our main water education page or contact us.

In This Article

Is A Reverse Osmosis Water Filter A Good Filtration Option?

We’ve been in the water filtration business since 2003 and have found Reverse Osmosis filtration to consistently be one of the best ways to remove contaminants from tap water.

Why Choose Reverse Osmosis?

If you're searching for a water filtration system that will give you outstanding water. Here are a few reasons to consider RO:

Improves Taste
RO filtration improves taste, odor and appearance of water by removing contaminants that cause taste and odor problems.

Saves Money
With an RO system, you can cancel your water delivery service and stop purchasing cases of bottled water. Reverse Osmosis filtration provides “better-than-bottled water” quality water for just pennies per gallon.

Simple Maintenance
RO systems have very few moving parts. Replaceable parts make RO systems easy to clean and service.

Removes Impurities
Reverse osmosis water systems can reduce or remove common pollutants from water including nitrates, lead, pesticides, PFOS, sulfates, fluoride, bacteria, pharmaceuticals, Arsenic V, and much more. A Reverse Osmosis systems’ carbon filter will also remove chlorine and chloramines.

Find Out Why Millions of Households Have Reverse Osmosis Drinking Water Systems

How Does Reverse Osmosis Work

Reverse Osmosis water filtration process is simple and straightforward. It is accomplished by water pressure pushing tap water through a semi-permeable membrane to remove contaminants from water. This is a process in which dissolved inorganic solids are removed from a solution. This process differs from standard filtration where impurities are collected within the filter media. The reverse osmosis process pushes water through a series of filters and ultimately the clean water goes to the holding tank, and the contaminants are flushed down the drain.

How Reverse Osmosis Works

How does the Membrane of an RO System Work?

Reverse Osmosis (RO) is a water treatment process that removes contaminants from water by using pressure to force water molecules through a semipermeable membrane. During this process, the contaminants are filtered out and flushed away, leaving clean, delicious drinking water. Reverse osmosis is capable of removing up to 99 percent of 65 different contaminants, including lead, fluoride, PFOS, pesticides, chlorine, dissolved salts, and more.

How a RO System Works Diagram

What Is Reverse Osmosis Water?

When household water pressure pushes water through the RO membrane and additional filters, such as sediment or carbon filters, the impurities are filtered out and subsequently flushed down the drain. What is left is delicious, clean-tasting drinking water. Note that many RO units incorporate a storage tank and a 4 or 5-stage process for optimal water quality.

You may be interested to know that Reverse Osmosis technology is not new. In fact, RO processes began being utilized by municipalities back in 1977. Since then, Reverse Osmosis has become increasingly popular because it is safe, cost-effective, and easy to maintain.

What are the Components of a 4-Stage RO System?

RO Phase 1

This pre-filter stage is designed to strain out sediment, silt, and dirt and is especially important as the sediment filter protects dirt from getting to the delicate RO membranes that can be damaged by sediment. Learn more about sediment filter.

RO Phase 2

The carbon filter is designed to remove chlorine and other contaminants that affect the performance and life of the RO membrane as well as improve the taste and odor of your water. *In a 5-stage RO system, phase 2 and 3 are both carbon filters.

RO Phase 3

The semi-permeable RO membrane in your RO system is designed to allow water through but filter out almost all additional contaminants.

RO Phase 4

In a four-stage RO System, a final post filter (carbon filter) will “polish” off the water to remove any remaining taste and odor in the water. Ensuring you’ll have outstanding drinking water.

Top-Selling RO Filter System

A reverse osmosis water filter system effectively removes dozens of contaminants, for improved taste and odor-free drinking water.

Need help selecting the right RO drinking water system for you?

Take a quick quiz to receive your personalized recommendation.

>> Get Started

How to Find the Correct RO Replacement Filters

Find RO Replacement Filters & Membranes

Considering which replacement filters and membranes you need for your reverse osmosis filtration setup? Here are three steps to guide you in identifying your current RO system, ensuring you select the appropriate replacements.

  1. Identify the brand and model: You can typically find the brand information for your RO system on the manifold, which is either metal or plastic and secures the filters. Look for a model number on a small sticker that might be placed on the back of the manifold. Keep in mind that the labels on the filters themselves are usually not indicative of the brand or model of the RO system.
  2. Select which model resembles your system: Should the brand and model details of your RO system be elusive on the unit itself, choose the image below that best matches your RO setup. By clicking on "learn more," you will gain access to a list of available replacement filters suitable for your RO system.

3. Give us a call: In case the above steps haven't helped you determine the necessary filters, don't hesitate to contact our customer support. We're here to assist you from Monday to Friday, 9 am to 5 pm Central Time at 877-377-9876.

Basic Components of a Reverse Osmosis Water System?

Generally, residential "Point Of Use" (POU) RO systems are installed and stored under the kitchen sink and filtered water dispenses from an RO faucet.

Most RO systems look similar and have the same basic components.
Although most RO systems look and work basically the same way --they differ in the QUALITY of their components.

Diagram of a Reverse Osmosis System with Basic Components

Components Of an RO System

  1. Cold Water Line Valve: Valve that fits onto the cold-water supply line. The valve has a tube that attaches to the inlet side of the RO pre filter. This is the water source for the RO system.  
  2. Pre-Filter(s): Water from the cold-water supply line enters the Reverse Osmosis Pre Filter first. There may be more than one pre-filter used in a Reverse Osmosis system, the most common being sediment and carbon filters. These pre-filters are used to protect the RO membranes by removing sand silt, dirt, and other sediment that could clog the system. Additionally, carbon filters may be used to remove chlorine, which can damage the RO membranes.  
  3. Reverse Osmosis Membrane: The Reverse Osmosis Membrane is the heart of the system. The semipermeable RO membrane is designed to remove a wide variety of both aesthetic and health-related contaminants. After passing through the membrane, the water goes into a pressurized storage tank where treated water is stored.  
  4. Post filter(s): After the water leaves the RO storage tank, but before going to the RO faucet, the treated water goes through a final “post filter”. The post filter is usually a carbon filter. Any remaining tastes or odors are removed from the product water by post filtration “polishing” filter.  
  5. Automatic Shut Off Valve (SOV): To conserve water, the RO system has an automatic shut off valve. When the storage tank is full, the automatic shut off valve closes to stop any more water from entering the membrane and blocks flowing to the drain. Once water is drawn from the RO faucet, the pressure in the tank drops; the shut off valve then opens to send the drinking water through the membrane while the contaminated wastewater is diverted down the drain.  
  6. Check Valve: A check valve is in the outlet end of the RO membrane housing. The check valve prevents the backward flow of treated water from the RO storage tank. A backward flow could rupture the RO membrane.  
  7. Flow Restrictor: Water flowing through the RO membrane is regulated by a flow restrictor. There are many different styles of flow controls, but their common purpose is to maintain the flow rate required to obtain the highest quality drinking water (based on the gallon capacity of the membrane). The flow restrictor also helps maintain pressure on the inlet side of the membrane. Without the additional pressure from the flow control, very little drinking water would be produced because all the incoming water would take the path of least resistance and simply flow down the drain line. The flow control is most often located in the RO drain line tubing.  
  8. Storage Tank: The standard RO storage tank holds from 2 - 4 gallons of water. A bladder inside the tank keeps water pressurized in the tank when it is full. The typical under counter Reverse Osmosis tank is 12 inches in diameter and 15 inches tall.  
  9. Faucet: The RO unit comes with a faucet, which is installed at the kitchen sink. Some areas have plumbing regulations requiring an air gap faucet, but non-air gap models are more common. Designer faucets are also available to complement your kitchen decor.  
  10. Drain line: This line runs from the outlet end of the Reverse Osmosis membrane housing to the drain. The drain line tubing is used to dispose of the wastewater containing the impurities and contaminants that have been filtered out by the reverse osmosis membrane.

What Contaminants do Reverse Osmosis Systems Remove?

Public water suppliers work hard to provide clean water for their customers. The problem is that there are many contaminants, especially those that cause taste and odor issues, which are simply not EPA (Environmental Protection Agency) regulated. These contaminants can easily penetrate aquifers, streams and rivers, bringing impurities straight to your water lines.

That is where Reverse Osmosis comes in. With an RO water filtration system, you can filter out impurities and produce outstanding drinking water for your home or business.


Can an RO System Be Connected to a Fridge or Ice Machine?

How Much of a Contaminant Can a Reverse Osmosis System Remove?

          • Fluoride (85-92%)
          • Lead (95-98%)
          • Chlorine (98%)
          • Pesticides (up to 99%)
          • Nitrates (60-75%)
          • Sulfate (96-98%)
          • Calcium (94-98%)
          • Phosphate (96-98%)
          • Arsenic (92-96%)
          • Nickel (96-98%)
          • Mercury (95-98%)
          • Sodium (85-94%)
          • Barium (95-98%)

More details on RO contamination available here.

Will Reverse Osmosis Remove Minerals that Are Healthy for My Body?

RO drinking water is healthy and delicious

Tap water varies with each city and water source. Some tap water is heavy in mineral content which can affect the taste and smell of water. Filtering water with a reverse osmosis system will remove most minerals in water and greatly improve the aesthetics of drinking water.

Because minerals in water are in an inorganic state that our bodies cannot digest, removing the minerals does not make RO-filtered water unhealthy. Keep in mind that while minerals are essential for proper health, food is the primary source of the minerals our bodies need to be healthy--and in a form our bodies can digest.

Explore Popular Reverse Osmosis Filter Systems

At ESP Water Products, we’ve been selling water filtration equipment for 18+ years. This longevity in the industry has made us very selective in what we sell and endorse. We carry only Reverse Osmosis systems that have won our confidence--those that are manufactured with the highest-quality components, offer outstanding warranties, and have the highest ratings for customer satisfaction.

Top-Selling Reverse Osmosis Drinking Water Systems

Are All Reverse Osmosis Drinking Water Systems and Filters the Same?

Reverse Osmosis (RO) Systems Differ in Three Basic Ways:

• Number of Stages or filters

• Quality of their components

• Amount of water that can be filtered or produced each day (this amount is in "gpd" or gallons per day)

Quality Components Are the Key to an Optimal Functioning RO System

Since 2003, we’ve found that the quality of components is truly the key differentiation between reverse osmosis systems. This is because RO units manufactured using excellent craftsmanship, as well as superior parts and components are what ultimately produce the best intended results--top-quality water for the home and business.

As you compare Reverse Osmosis Water filter systems to find the product that is right for you, be sure to ask yourself these 5 questions:

  1. Does this system have good reviews?

  2. Does it use quality parts from trusted manufacturers, and are the materials FDA or NSF (National Sanitation Foundation) approved?

  3. Is the technology up to date?

  4. Some systems have higher contaminant rejection abilities. How does this one compare to others like it?

  5. Read up on the company. Is it reputable?

When is Reverse Osmosis Filtration Used?

Reverse Osmosis water filters are the perfect solution if you’re looking to improve the taste, odor and appearance of your water. Here are some of the most common applications for RO technology:

Installed at the kitchen sink: A small RO filtration system can be installed at the kitchen tap to provide top-quality drinking water. This is called a “point of use” (POU) water filtration system and can also connect to your fridge and ice machine. Water pressure is a consideration with some refrigerators, so be sure to refer to your RO system’s owner’s manual as the pressure from the Reverse Osmosis unit is about two-thirds of the incoming line pressure. Related content: How to Install a Reverse Osmosis System

Commercial Application: Reverse Osmosis water filtration is a perfect fit for many businesses, as certain commercial applications require reverse osmosis filtration to remove a specific contaminant from water. Reverse osmosis commercial water treatment may require prefiltration, a demand pump, a large holding tank, and more. But RO filtration might still be the right solution when a high level of a specific contaminant is most economically removed by the RO process. At ESPWaterProducts.com, we carry a wide variety of large RO systems designed for many different commercial applications. If you're interested in commercial reverse osmosis, please give us a call to talk through specifics: 877-377-9876.

Common Commercial Reverse Osmosis (RO) System Applications

• Food & Beverage Industry
• Agriculture
• Boiler Feed
• Disaster Relief
• Environmental
• Hospital
• Hotel
• Marine
• Military
• Mining
• Pharmaceutical

Expert Tip

Some name brand Reverse Osmosis systems such as Culligan and Rainsoft require expensive proprietary filter replacements. An RO "retrofit" kit allows you to keep your existing RO tank and faucet, but install a new manifold so that going forward, you can have delicious, clean filtered drinking water, without the excessive cost of name-brand filters. Our high-quality compatible filters are guaranteed to meet the performance of proprietary filter.

How Much Service and Maintenance Does an RO System Require?

The guidelines outlined below are very general. Specific information about how often your membrane or filter should be replaced can be found in the products’ “Specs” or your product owner’s manual. If you’ve purchased a system or filter from us here at ESP Water Products, we will send reminder emails when it comes time to change your filter.

As a general rule, here are three things you can do to maintain (and increase the lifespan) of your reverse osmosis system:

1. Change Filter

RO membranes and filters will become clogged and foul with time. When this happens, the RO system will begin to produce less and less drinking water. How often you should change the filters will vary based on water usage and local water conditions.

RO replacement filters should be installed based on the following schedule:

• Pre-filters (sediment/carbon filter): Every 6-12 months

• RO Membrane: Every 24 months

• Post filter (GAC polishing filter): Every 12 months

Not sure how often your parts need replacement? Replacement recommendations are listed under each product's "SPECS"

This guideline is very general. Specific information about how often your RO membrane and filters should be replaced can be found in the product owner's manual for each system. If you’ve purchased your RO drinking water system or replacement filters from us here at ESP Water Products, we will send reminder emails when it comes time to change your filter.

We highly recommend that you purchase top-quality replacement filters that are designed for optimal performance and reliability. To help our customers, we send reminder emails so thye'll know exactly when it's time to change out the filters or membrane.

2. Sanitize System

A reverse osmosis drinking water system should be cleaned, sanitized and recharged annually. To simplify things we recommend sanitizing your system when you change out your filters. Read detailed instructions on how to sanitize an RO system. You can hire a local water treatment professional to clean and sanitize your system or you can perform the annual sanitation and filter change yourself. Learn more about how to replace your RO water filters.

3. Replace Parts

The various parts on a reverse osmosis system (such as the storage tank or faucet) will wear out with time. We provide an RO troubleshooting guide to help you diagnose the issue and determine which part needs to be replaced. Also, RO part replacement recommendations may be listed in your RO system's owner's manual.

Reverse Osmosis (RO) Frequently Asked Questions

Can an RO System Be Connected to a Fridge or Ice Machine?

Under-the-counter reverse osmosis systems are plumbed to the incoming coldwater line, drain, and an independent faucet. But it is possible for a water treatment professional to plumb an RO unit to your fridge and/or ice machine, as well.

The water professional will most likely run a quarter-inch tube from the under-sink RO unit to your fridge. Water pressure is a consideration with some refrigerators, so be sure to refer to your RO system’s owner’s manual as the pressure from the reverse osmosis unit is about two-thirds of the incoming line pressure.

How Much Water Can an RO System Produce Each Day?

The typical under-counter reverse osmosis tank is 12 inches in diameter and 15 inches tall. The reverse osmosis system will operate under water pressure ranging from 40-100 PSI and generate 10-75 gallons (that is 45-340 Liters) per day.

Is a Reverse Osmosis System Noisy?

Generally, reverse osmosis systems are very quiet-- though you may hear a “gurgling” sound as the “concentrate” or waste water flows from the membrane to the drain. Reverse osmosis is designed so that “crossflow” water flushes the contaminated concentrate away from the system. This flushing keeps the unit from fouling.

If you hear other sounds, such as a “hissing” noise, immediately call your local water treatment professional as there may be a problem with water pressure, the air gap or a leak.

Non-air gap faucets tend to be quieter than air-gap faucets. Non-air gap faucets make no gurgling sound since the drain line from the RO membrane flows directly into the sink drain line. Learn more about the difference between “Air Gap” and “Non-Air Gap” faucets.

How Does RO Water Compare to Bottled Water?

An under sink RO drinking water filtration system uses the same technology that major water bottling plants use to filter water.

Reverse Osmosis incorporates carbon filtration which removes chlorine, chloramines, forever chemicals like PFOS, Arsenic V, pesticides, herbicides and more. The removal of these contaminants not only makes for safer drinking water, but also greatly improves the taste.

The RO process leaves trace amounts of minerals and salts which makes for an overall better taste rather than the “flat” taste of distilled or bottled water. Best of all, RO-filtered water saves you the expense of bottled water and is better for the environment since you’re not adding water bottles to local landfill. This eco-friendly solution means you can have healthy, naturally-fresh drinking water available right at your kitchen tap.

Is An RO System Worth It?

Absolutely. Looking at cost alone, RO systems generally cost less than 30-cents per day to own and operate for a family of four. From a cost-savings perspective, since most Americans spend $100/year on bottled water translating to $400/year for the household, an RO system would save nearly $300/year as it operates for roughly $100/year. And the number crunching doesn’t reveal anything about the health benefits of RO.

RO systems can remove harmful contaminants such as pesticides, PFAS, herbicides, chlorine, chloramine, and Arsenic V---certainly making an RO system “worth it.”

Should I Install a Whole-House Reverse Osmosis System?

A whole-house Reverse Osmosis system (also called a POE “point of entry” system) allows RO filtered water at all sinks, showers and appliances throughout the house. A POE system ties into the main water line coming into your home and is generally installed in the garage.

In theory, whole house reverse osmosis systems sound like a good idea. But here are the reasons we don't typically recommend whole house RO water filters:

1. Whole-house reverse osmosis water filters can be expensive
2. Whole-house RO water filter systems require a lot of maintenance.
3. Whole-house reverse osmosis consumes a lot of water
4. Whole-house RO flushes a lot of filtered water down the drain.
5. Additional pre-treatment and post treatment will probably be needed.
6. A lot of storage space is required for the large whole-house RO system.

For more information about whole-house reverse osmosis and what whole house water filters we recommend, please read this article.

Is there general "Performance data" available for a POU RO drinking water)?

General performance data is available below for under-counter RO systems. Manufacturers often provide specific information on their system to help consumers understand the capabilities and effectiveness of their reverse osmosis system.

Water Supply Conditions

Incoming Water Requirements to RO System with Typical TFC/TFMn Membrane:

Community/Private: Non­-Chlorinated
System Pressure: 30 – 100 PSI
Temperature: 40° – 100° F (4° – 38° C)
pH Range: 3.0 – 11.0
Maximum Supply TDS (total dissolved solids) Level: 2000 mg/L
Turbidity: < 1.0 Net Turbidity (NTU)
Hardness (CaCO3: < 350 mg/L (< 01 mg/L
Iron (Fe): < 0.05 mg/L
Manganese (Mn): 0.00 mg/L
Hydrogen Sulfide (H2S): 0.00 mg/L
Chlorine (Cl2): N/A

Incoming Water Requirements to RO System with CTA Membrane:

Community/Private: Chlorinated
System Pressure: 30 – 100 PSI
Temperature: 40° – 90° F (4°– 32° C)
pH Range: 4.0 – 8.0
Maximum Supply TDS Level: 1500 mg/L
Turbidity: < 1.0 Net Turbidity (NTU)
Hardness (CaCO3: < 350 mg/L (< 0.1 mg/L)
Iron (Fe): < 0.05 mg/L
Manganese (Mn): 0.00 mg/L
Hydrogen Sulfide (H2S): 0.1 – 10.0 mg/L
Chlorine (Cl2): N/A

Performance Data

Typical Rejection Characteristics of Reverse Osmosis Membranes:

Arsenic V: 92-96%
Barium: 95-98%
Cadmium: 95-98%
Calcium: 94-98%
Chloride: 85-92%
Cyanide: 84-92%
Fluoride: 85-92%
Iron: 94-98%
Lead: 95-98%
Manganese: 94-98%
Magnesium: 94-98%
Mercury: 95-98%
Nickel: 96-98%
Nitrate: 60-75%
Phosphate: 96-98%
Potassium: 85-95%
Selenium: 94-96%
Sodium: 85-94%
Sulfate: 96-98%
Zinc: 96-98%

Definitions of Abbreviations:

CTA - cellulose tri-acetate - membrane is chlorine tolerant
TFC - thin film composite - not chlorine tolerant but has a higher production capacity
psi - pounds per square inch
TDS - total dissolved solids
mg/L - milligrams per liter
gpg - grains per gallon
NTU - nephelometric turbidity units

Answers To Your Most Common Questions

Air Gap vs. Non-Air Gap RO Faucet

What's the difference between an “Air Gap” faucet and a “Non-Air Gap” faucet? Which one should you use with your reverse osmosis filter system?
Read More>

Advantages & Disadvantages of RO

Is reverse osmosis filtered water worth it? Are RO replacement filters expensive? Let's explore the pros and cons of Reverse Osmosis filtration.
Read More>

Benefits of UV Water Disinfection

Treating water with Ultraviolet (UV) light is one of the MOST effective ways to treat waterborne microorganisms, such as bacteria and viruses...
Read More>

Explore Other System Types