What is Reverse Osmosis Water?

How Does Reverse Osmosis Work?

Reverse Osmosis (RO) is a water purification process. The way that reverse osmosis work is by demineralizing or deionizing water.  

Reverse osmosis works by applying pressure to water and forcing the water through a series of filters and, most importantly, a semi-permeable membrane. 

During this process, anywhere from 95% to 99% of salt, minerals, chemicals, and pollutants are removed.

What are the components of a reverse osmosis water system?

Cold Water Line Valve

A residential RO system will use the house’s cold water supply as it’s source of water. The system will use an adapter that fits either a ½” or a ⅜” supply valve. 

It is good to install a feedwater control valve when installing a reverse osmosis system. 

During standard installation,without a feedwater valve, the standard valve from the RO system fits onto the cold water supply line. 

There is a tube in the standard valve that attaches to the RO pre-filter, and this is how the RO system gets its water. 

If you install a feedwater control valve along with the standard valve, it gives you an advantage that might come in handy down the road. 

By installing a feedwater valve in addition to the standard valve, you have the ability to turn off the water to the reverse osmosis system while maintaining cold water flow to the kitchen sink. 

Install a Feedwater Control in the Cold Water Line

It is a simple process that you can do during installation, and even afterward if you choose to add it. 

There is a flex line under the sink through which the cold water runs. The flex line is attached to an angle stop, which is the cold water shut off valve to the faucet. 

Once you remove the flex line from the angle stop, you will attach the feed water supply valve. 

The feedwater supply has two parts, the connector and the valve.  It will go between the flex line and the angle stop. 

Use some Teflon tape on the threads of the supply valve and tighten it down into the feed water connector. 

Replace the flex line onto the feed water connector and tighten it down on the connector’s rubber gasket. 

You will then hook the water supply for the RO membrane to the feed water valve. 

Now, you will be able to shut the water off to your reverse osmosis system and still have cold water running to your kitchen sink. 

This is handy when you are changing filters on the reverse osmosis system or doing any type of maintenance. 

It is a simple step that will be practical and convenient should you need to stop water flowing to the RO system, but keep it flowing to the cold water faucet.

Pre Filters For Your Reverse Osmosis System

Water from the cold water supply will always enter into a filter before being routed into the reverse osmosis membrane. 

While the membrane may be the main component of a reverse osmosis system, an RO system also depends on the pre-filters. 

The pre-filters are an essential part of any RO system, and they protect the membrane and make it possible for it to do its job effectively. 

A reverse osmosis system could have as many as five pre-filters, depending on the system. 

Normal drinking water has materials in it that are not visible to the naked eye. Those materials are solids, like rust, sand, and chlorine. 

Those solids can damage the RO membrane. Pre-filters, like a sediment filter, will reduce particles of rust, dirt, and dust. 

A carbon filter will remove chlorine and other contaminants from the water before it goes to the membrane.

Reverse Osmosis Membrane

The reverse osmosis membrane is the workhorse of the RO system. It is an extremely ultra-fine filter. 

Referred to as a thin-film composite, this semi-permeable membrane will filter out anything from your water that is larger than 0.001 microns. 

Thin-film enables a reverse osmosis system to make more water in less space. 

As much as 98% of dissolved solids, like salts, minerals, and metals, are removed from the water as it passes through the membrane. 

An RO membrane will also filter microorganisms and organic substances from the water.

How does a reverse osmosis membrane work?

The pressure is applied to the semi-permeable thin film membrane in an RO system to force water molecules to pass through the membrane. 

During this process, the dissolved inorganic compounds flush out to the drain. In this way, the membrane separates the water, the pure and the dirty, into two different pathways.

One line will take the pure water to your faucet, and the other line will take the reject water to your drain. 

Post Filters For Your Reverse Osmosis System

This in-line filter is the final stage of filtration that the water goes through in most reverse osmosis systems. 

Often referred to as a “polishing” filter, this filter consists of granular activated carbon. Its job is to remove any remaining tastes or odors from the water. 

The water will flow very slowly through this filter, which means that the time the water is in contact with the filter is higher. 

This high contact time leads to higher absorptions of those tastes and odors by the filter and makes the filter highly effective in doing so.

Automatic Shutoff Valve For Your Reverse Osmosis System

A feature of a reverse osmosis system is the automatic shutoff valve on the RO membrane. 

This valve will use a small amount of the pressure created by the float valve to shut off the water supply coming into your RO membrane when the storage tank is full. 

The shutoff system monitors the pressure in the storage tank, and when the tank pressure reaches about ⅔ of the pressure of the incoming tap water, the water shuts off.

Check Valve For Your Reverse Osmosis System

It is important to know that a check valve (a one-way valve) needs installing in the permeate tube between the membrane and the shut off valve. 

Every reverse osmosis system should have a check valve in the permeate line, which is the line that leaves the RO membrane housing.  

Without the check valve, which maintains the tank’s back pressure, the unit will not shut off when it is full. 

Clean water will go down the drain, wasting it, and it will eventually ruin the membrane. 

The check valve can be one of two types; it can be a small valve mounted inside the elbow fitting that takes the clean water from the membrane. 

It can also be an in-line valve installed in the line that goes between the membrane’s permeate port and one of the tank ports on the auto-shutoff valve. 

While it is not standard on a RO system, you can also install a check valve in your reverse osmosis system’s drain line. 

This check valve will prevent backflow into your RO system in the event that your drain gets clogged.

Flow Restrictor For Your Reverse Osmosis System

This device restricts reverse osmosis flow and determines the amount of water that flows to the drain. 

In doing so, a flow restrictor maintains high pressure on the inside of the membrane. The flow restrictor keeps all the water that enters the membrane from going down the drain. 

When the membrane on your reverse osmosis system needs replacing, change the flow restrictor at the same time. 

Scale can build up inside the restrictor and clog up the workings. The flow restrictor should match the capacity of your reverse osmosis membrane.

A reverse osmosis membrane capacity is given in GPD, or gallons per day. The flow rate on a restrictor is given in mL, milliliters per minute. 

Generally, the flow restrictor should be about four times the capacity of the membrane. 

Consult the chart below to find the flow restrictor that would be the right one for the capacity of your reverse osmosis membrane.

Membrane Size (in GPD) Flow Restrictor (mL per minute)
18 200
25 250
36 250 or 360
50 420 or 525
75 550 to 800
80 800
100 800

Storage Tank For Your Reverse Osmosis System

In a reverse osmosis system, a storage tank catches the clean water that has come through your RO process and holds it until you are ready to use it. 

Because a reverse osmosis system works slowly, purifying water one drop at a time, the storage tank allows the system to collect the clean water in order to provide you water on demand. 

It may seem like this is an excessively slow process, but it is the contact time that the water has with the membrane that eliminates the contaminants from the water.  

The storage tank gives the pure water a place to accumulate until you access it. Systems like reverse osmosis use pressure tanks for storage of clean water. 

These tanks use compressed air to create water pressure within the tank itself. The storage tank will have an air chamber (or bladder), and the tank will come pre-charged. 

As the tank fills with clean water, the water’s weight will expand the bladder and further compress the casing’s air. 

When you open the designated faucet, it causes the air that is under pressure to push the water out.

Faucet For Your Reverse Osmosis System

Most reverse osmosis systems will come with a designated faucet for the sink that disperses the clean water from the RO system. 

The faucet style will be according to what comes with the system, and, in general, reverse osmosis systems come with an air gap faucet. 

An air gap faucet has an air gap built in the base of the faucet. This design prevents backflow into the RO system. 

In the event of clogging, a small amount of air, rather than dirty water, would go back into the RO system. 

This provides protection for the system. An air gap faucet will have three lines, rather than just one. 

One line will be for the upward drain water, one for a second downward drain water, and the third for the water you drink. 

Most plumbing codes require an air gap faucet with an RO system, so it is a good thing to check for before you purchase a system. It is also good to know that an air gap faucet will not work with a system other than RO.

There are also non-air gap faucets. The main difference between the two faucets is that non-air gap faucets have one water line that goes up to the faucet. 

With this type of faucet, if the drain backs up, damage could occur to your RO system and the membrane contaminated. 

For an RO system, this type of faucet can be considered non-compliant by city/county plumbing codes.

Drain Line For Your Reverse Osmosis System

An essential component in a reverse osmosis system is the drain line that carries away the contaminants and impurities in the rejected water. 

The pre-filters hold those contaminants and impurities, the reverse osmosis membrane screens out the impurities and then rinses them down the drain. 

The drain water leaves the membrane housing via a small elbow fitting. The rejected water passes through a flow restrictor that we mentioned earlier in the article. 

The drain water then passes through a backflow prevention device (sometimes an air gap faucet like mentioned above) or a check valve, through the drain line and then into the sink drain pipe. 

This water contains the contaminants like lead, nitrates, sodium, and other pollutants filtered from your drinking water.

As this water is full of contaminants, it is not recommended that you retain it for any other purpose. 

What contaminants does Reverse Osmosis (RO) remove?

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


Although there are no definite, confirming studies that show a direct connection to aluminum and diseases, there are studies that suspect that ingestion of aluminum to neurodegenerative diseases. Alzheimer’s, Parkinson’s, bone disorder, and breast cancer appear to be a risk factor with the infestation or absorption of aluminum.


This is the waste product of the metabolism of animals. In fish and aquatic invertebrates, it is secreted directly into the water.

Arsenic 5

A naturally occurring element, arsenic exposure at high levels can cause nausea, diarrhea, and abdominal pain.


Contamination from bacteria can come from improperly designed, failing, or overloaded wastewater treatment systems. Floodwaters can have high levels of bacteria. If there are bacteria in the water, it can result in diarrhea, cramps, nausea, headaches, and long-term health effects. They pose a particular health risk to infants, children, the elderly, or someone with severely compromised immune systems.


The potential hazardous effects of barium in drinking water are difficulties in breathing, increased blood pressure, heart rhythm changes, muscle weakness, and possible damage to the liver, kidney, and heart.


Corrosion of galvanized pipes, runoff from waste batteries, and discharge from metal refineries can add cadmium to drinking water. This can cause kidney damage when ingested.


Chromium is naturally occurring, and it is a likely carcinogen. The safe level of chromium in water is unknown. It can potentially cause nausea, stomach and skin ulcers, allergic reactions, kidney and liver damage, and lung and nasal cancer.


While copper is essential to human health, too little is unhealthy, and too much can lead to copper poisoning. As copper is a common plumbing material, corrosion in copper pipes can lead to copper in the water. Copper poisoning can cause nausea, vomiting, diarrhea, and abdominal and muscle pain. Severe cases can lead to anemia and kidney failure.


A natural trace element, fluoride is added to public drinking water to reduce tooth decay. The sources of fluoride can be natural deposits or malfunctioning or poorly monitored equipment at the municipal level. Too much fluoride (from long-term consumption) can result in skeletal fluorosis, a severe bone disorder.


The majority of lead in water comes after the water has left a municipal treatment plant and is the result of corrosion, leaching from service connections, solder, and brass fixtures. Children are at the most risk for damage to the brain, kidneys, and bone marrow. Damage to the nervous system and red blood cells can also occur.


The primary source of mercury is from the natural degassing of the earth’s crust. Mercury has the potential to cause damage to the brain and kidneys. Changes in vision, hearing, and memory problems can also occur.


Most often caused by fertilizers, animal waste, and septic tanks, nitrate in drinking water can be responsible for a temporary blood disorder in infants called “blue baby syndrome”. Most health effects occur in infants under six months old.


This substance forms through decay in the environment. Radium in drinking water may cause cancer, kidney damage, and congenital disabilities.


This metal exists in natural deposits as ores containing other elements. It can cause fatigue and irritability, hair and fingernail changes, and damage to the peripheral nervous system.


Uranium is a normal part of rocks, soil, air, and water. It leaches into water from soil and rocks. It can have a toxic effect on kidneys, causing inflammation.

The above are a few of the contaminants that reverse osmosis can remove from your drinking water. Others that the RO process can remove are:

  • Calcium
  • Chloride
  • Chlorine
  • Chloramines
  • Chromate
  • Chromium
  • Iron
  • Cyanide
  • Cysts
  • Magnesium
  • Manganese
  • Pesticides
  • Phosphate
  • Potassium
  • Protozoa such as Cryptosporidium and Giardia
  • Radon
  • Silver
  • Sodium
  • Strontium
  • Sulfate
  • Sulfite
  • Turbidity
  • Viruses
  • Zinc

5 Reasons Why Reverse Osmosis Is A Good Water Filtration Option

Reverse Osmosis Improves The Taste Of The Water

Reverse osmosis reduces the amount of chlorine, dissolved solids, and organic and inorganic substances present in the water you get from your tap.

The water will have less taste than untreated water because of the elimination of sodium, magnesium, and calcium in the RO process.

Some users will choose to add some minerals, in the form of drops, back into the RO water. 

However, if you get the necessary minerals from the foods you eat, this is strictly an option on your part.

RO water will also have removed odors and cloudiness from your water.

Reverse Osmosis Saves You Money

The importance of pure, clean water has been made clear as the detrimental impact of pollutants in our drinking water has become evident in recent years.

Many people have turned to bottled water as a clean water source; however, some bottled water comes from the same municipal sources that service our homes.

Bottled water in the United States has reached $34.6 billion per year in 2019. The average person spends over $100 a year on bottled water, each. 

If you have several members of your household, it adds up fast. 

That is a considerable amount of money spent per capita on a beverage that you could generate from your home with a far purer result.

Having your RO system at home gives you clean water right from your tap and saves you the expense of purchasing bottled water.

With your own clean water, available from your tap, you will not have to purchase, lug home and store single use bottles.

You will also have clean water for cooking, making ice cubes, and watering plants. 

Reverse Osmosis is Convenient to Maintain

Just like your car will need an oil change or your furnace filter needs replacing, the pre-filters and membrane on your RO system will need to be changed periodically. 

Those filters are easy to swap out and will take you just a few minutes to do.

The membrane on an RO system needs to be changed about every three years. The process of changing a membrane is much like that of changing a filter and will take you a matter of minutes to do.

When you change a membrane, you will need to discard two water tanks, which consists of letting the tap run for 10 to 15 minutes and allowing the system to refill itself, which takes about two to four hours.

That process is automatic, and so the routine maintenance on an RO system takes very little time. 

Reverse osmosis systems are user friendly, and the homeowner can easily do the care and maintenance of an RO system.

Reverse Osmosis is Energy Efficient and Environment Friendly, Reducing Plastic Waste

In 2019, Americans consumed 14.4 billion gallons of bottled water, most of that in single-use containers. Sadly, 80% of those plastic water bottles end up in landfills.

It can take over 1,000 years for one of those bottles to bio-degrade. If incinerated, the bottles produce toxic fumes.

If you have your own reverse osmosis water bottle and fill it from home, you have helped put a dent in reducing plastic waste and protecting the environment.

Reverse Osmosis Can Protect Your Health

The Flint water crisis started in 2014 and is still ongoing today. 

This crisis was so severe that it saw criminal charges filed against government employees who were supposed to have overseen the water system’s safety and public health and failed to do so.

Flint was not the only crisis, Water Contamination Disasters in Woburn, Massachusetts, back in 1979 led to incidences of childhood leukemia. Residences showed higher levels of liver, kidney, and urinary cancer.

We can see from these two incidences, decades apart, that decisions made by officials in charge of protecting the safety of your drinking water can adversely affect your water.

Duties of care have not improved over time but still exist.

The only real way you can protect your drinking and cooking water from contaminants that can impact and endanger the health of you and your loved ones is by having a reverse osmosis system in your home.

Removing contaminants and pollutants will give you and your family the peace of mind that comes from having clean, pure water to ingest.

In Conclusion

Water is life, and clean water is essential to the good health of you and your loved ones. It has been shown over time that relying on municipalities to provide clean water is not a safe and sure opton.

Using a reverse osmosis system in your home gives you the protection of clean water and safeguards your loved ones. 

If that was not enough, you are saving yourself money and helping to keep the environment free from additional waste and pollution. 

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