RO Systems Revealed: How Does Reverse Osmosis Work?

Reverse Osmosis

A reverse osmosis water filter is the best choice for most homes.

RO systems provide the best quality drinking water free of almost all contaminants, including heavy metal, chemicals, and microorganisms.

RO filters work by reversing the normal osmosis process, which is why they are called reverse osmosis filters.

Instead of water moving naturally from an area of low concentration to one of high concentration, it is forced to move in the opposite direction (high level to low).

In this knowledge guide, we have explained everything you need to know about reverse osmosis (RO) filters, including how they work, what they remove from water, and how they compare to other filtration methods.

For recommendations of the best reverse osmosis filters, read our in-depth RO filters buying guide.

What is Reverse Osmosis?

RO working

Osmosis happens all around us.

Plants rely on osmosis to take up water through their roots. Even your body cells use osmosis to maintain the right concentration of salts within the cells and outside in the plasma. 

During osmosis, water moves from a region of low concentration through a semi-permeable membrane to a region of high concentration.

A semi-permeable membrane is a biological or synthetic membrane that selectively allows some substances through while blocking others.

Reverse osmosis is a reversal of the natural osmosis process.

Water moves from an area of high concentration to low concentration.

Because reverse osmosis doesn’t happen naturally, it requires the application of pressure. The water is forced to cross the semi-permeable membrane to the low concentration region.

In the case of a reverse osmosis water filter, the high concentration region contains a high level of impurities while the low concentration regions have fewer impurities.

The water in the low concentration region is said to be purified. It’s not entirely free of impurities, but the levels are extremely low.

Where is reverse osmosis used?

Other than in residential water filters, reverse osmosis is also used to filter water in industries such as:

Reverse osmosis is also used in maple syrup production, making concentrated fruit juices, making whey protein powders and in hydrogen production.

Another little-known use is window cleaning. Some window cleaners use RO purified water instead of adding detergents to normal water. RO water doesn’t leave spotting or streaking on the glass.

Why Reverse Osmosis is the Best Method of Water Filtration

RCC7AK features

It is effective, it is affordable, and it is easy completely automated.

Let’s start with the first one, effectiveness.

Reverse osmosis removes more impurities than any other filtration system.

By forcing water through a semi-permeable membrane with very tiny pores (as small as 0.0001 microns), an RO filter removes virtually all contaminants from the water.

Even bacteria and viruses are too big to pass through the membrane. Dissolved minerals and heavy metals are also removed.

Carbon pre-filters remove organic chemicals like VOCs, chlorine, and pesticides.

If you want complete peace of mind that your drinking water is safe, a reverse osmosis filter is the best choice.

Standard carbon-based filtration systems mostly remove sediment and chemicals, leaving heavy metals, dissolved salts, and biological untouched.

As for affordability, RO systems have been getting cheaper in the last few years as more and more people learn about them.

The market for RO systems was about $6.6 billion in 2016 and is projected to grow to $11 billion by 2021.

Expect these filters to get even cheaper.

A few years ago, you had to spend over $500 to get a good quality under-sink RO filter. Right now, you can get a high-quality system for under $200.

Then there’s the advantage of automation.

Once you install an RO filter, there’s nothing else you need to do other than occasional filter replacement.

The system works automatically, passing water through multiple filters, into the tank and out through the faucet.

You open the RO faucet, and pure drinking water comes out.

RO systems are also pretty easy to maintain, something we’ll discuss in detail later, and they can last 10-15 years with proper care. 

Components of an RO System

reverse osmosis water filtration system

While the most essential part of an RO system is the semi-permeable membrane, it’s not the only component that filters the water.

A typical RO system will have at least four other filters.

In addition to the filters and membrane, various other components play different roles in the system.

Here’s a quick look at the basic components of all reverse osmosis systems.

Pre-filters: These are the filters that come before the semi-permeable membrane. They usually include a sediment filter and two carbon filters that remove large particles and chemicals that could clog and damage the RO membrane. Pre-filters improve filtration performance while also extending the life of the membrane.

RO membrane: This is the heart of the system. The semi-permeable membrane handles the hardest-to-remove contaminants like heavy metals and microorganisms.

Post-filter: The post-filter is usually the last filter in line before the water comes out of the faucet. It removes any remaining odors and tastes.

Reserve tank: If every time you opened the faucet water had to pass through all the filters, it would take forever to fill a glass. Instead, water sits in a pressurized reserve tank, ready for use. Most RO systems come with a 4-gallon reserve tank with a usable capacity of 3.2 gallons. But you can upgrade to a 14 or 20-gallon tank.

Faucet: You’ll need to set up a dedicated faucet for the RO system. This usually involves drilling a hole in the sink or using an existing hole.

Automatic shut-off valve: The shut-off valve stops water from flowing through the RO system when the reserve tank fills up or gets to a certain level. This prevents water from going down the drain. Once you draw some of the water from the tank, the pressure drop opens the valve to allow more water through the filters.

Flow restrictor: A flow restrictor maintains high water pressure in the RO membrane. If the water pressure is too low, the water won’t go through the RO membrane, and most of it will go down the drain. The flow restrictor is sized to balance the RO membrane’s daily production capacity.

Drain saddle and drain line: The drain saddle provides a way to connect the drain line from the RO system to the drainpipe. Most but not all RO systems use a drain saddle. Others use a drain adapter that doesn’t require drilling into the drain pipe.

Feed water valve: This is the valve that you connect to your ½” or 3/8” cold water outlet under the sink. You then connect your main cold water line and the RO system’s inlet line to it.

Filter housing wrench: This is a special wrench that is explicitly designed for use with filter housings. You can loosen or tighten the filter using the wrench.

These are the basic components found in just about all RO systems regardless of price or brand.

Of course, premium RO filters come with extra features. Here are a couple of the most common ones.

Remineralization filter: The remineralization or alkaline filter adds back healthy minerals that the RO membrane stripped out of the water. Most alkaline filters add small amounts of calcium or magnesium or both.

Leak detection: To ensure that a small leak doesn’t go unnoticed into a flood, some RO systems come with a leak detection sensor that triggers a shut-off valve when it detects moisture on the floor under the counter.

UV filter: A UV filter or sterilizer adds extra protection against bacteria and viruses. The filter is simply a lamp that directs intensive UV radiation at the water, neutralizing any microorganisms present.

Filter replacement indicator: Some systems have a small battery-powered indicator at the bottom of the faucet that lights up to remind you to replace the filters. But the reminder is time-based so it might tell you to replace the filters too early or too late (filter life depends mostly on water quantity and quality). A better way to monitor filter life is by using a TDS monitor.

How an RO System Works

RO working


Certain filtration stages like the sediment filter and RO membrane are present in virtually all RO filters, but some systems have a slightly different design. 

For example, Home Master RO systems do not use a post-filter, which is present in just about all other RO filters. 

Also, some systems have just one carbon pre-filter, while others have two. 

Generally, basic RO systems have five stages, while more advanced ones have 6 or 7 stages.

First Stage: Sediment Filter

The first filter is always a sediment filter. It catches large particles of dust, sand, and sediment that could easily clog the filters up ahead, reducing their performance and lifespan.

Most RO systems use a 5-micron or 10-micron sediment filter.

Second Stage: Carbon Filter

A second stage carbon filter is also standard for all home RO systems. It can be a CTO carbon block or a GAC (granular activated carbon) filter.

The second stage is where serious cleanup begins. All the stuff you can’t see but can taste or smell is removed by the carbon filter.

This mostly includes chlorine, chloramines, and organic chemicals such as hormones, pharmaceuticals, VOCs, and pesticides.

Filtering these chemicals serves two purposes. One, it makes the water safer to drink and gets rid of any unpleasant smells and tastes.

Two, it protects the RO membrane.

Chlorine can significantly degrade the semi-permeable membrane, making it less effective at filtering out impurities and reducing its lifespan.

Third Stage: Carbon Filter

Most RO systems have dual carbon pre-filters to ensure the freshest water possible and provide the best protection to the RO membrane.

Both carbon filters can be the same (usually CTO blocks) or different (one GAC and the other CTO).

Fourth Stage: Reverse Osmosis Membrane

This is where the most essential filtration takes place.

The pre-filters do a good job removing a wide range of contaminants. But they also let a lot though.

The RO membrane removes almost all the contaminants still left in water.

The RO membrane generally removes two groups of impurities: TDS or total dissolved solids and biologicals.

TDS includes all kinds of stuff that are dissolved in the water including hardness minerals, salts like nitrates, fluoride and heavy metals like chromium and lead.

Biologicals refer to harmful microorganisms that cause waterborne diseases. These include bacteria, viruses, and cysts.

After a while, the membrane accumulates with impurities. To prevent clogging, the RO system automatically flushes the membrane with some of the water coming in.

The wastewater is directed down the drain.

The Reserve Tank

Before going to the next stage, the now-purified water goes into a reserve tank.

The tank ensures you always have quick access to drinking water when you open the faucet.

Most tanks have a 4-gallon capacity, but only 3.2 gallons of those are usable. A rubber bladder takes up the other space.

If you use more RO water, you can upgrade to a larger reserve tank.

An RO tank has to be pressurized to work. The pressure is what pushes the water out by force through the faucet.

Unfortunately, this pressure also causes increased water wastage by making it harder for water to get into the tank, especially when it is almost full.

The tank will come already pressurized with air.

But you can check the pressure using a pressure gauge (make sure the tank is empty). Look for the air valve at the bottom or side of the tank.

If the pressure is lower than that specified by the manual, use a standard bicycle pump to add pressure. But do not exceed the max pressure rating as that could reduce your tank’s capacity.

The flow of water to the tank is regulated by an automatic shut-off valve.

When the tank fills up or gets to a certain pressure, the valve automatically prevents more water from coming into the system.

Fifth Stage: Post-filter

Apart from Home Master RO systems, most of the other RO brands utilize an in-line post-filter.

The post-filter removes any remaining chemicals that might affect the taste or smell of the water. Some of these chemicals leach into the water when it’s sitting in the tank.

Sixth Stage: Remineralization

The fifth stage is the end of the main filtration process. Filters 1 to 5 are the essential ones on the system.

Some RO filter has one more stage for remineralization.

Remineralization is the addition of specific minerals back into the water. Calcium and magnesium are the minerals most commonly added back.

Why do RO filters need remineralization?

It’s because the system is too effective at its job.

The RO membrane doesn’t discriminate what it filters out. As long as impurities are bigger than the membrane’s pores, they get filtered.

This includes almost all dissolved minerals, even the healthy ones.

That’s why RO purified water is slightly acidic, which gives it a signature flat taste.

Remineralization adds back one or two of the healthy minerals that were stripped out to improve the water’s taste and make it a tiny bit healthier.

Some systems such as the Express Water 10-stage RO filter go as far as to create alkaline water that’s even healthier for you.

Now, water from an RO system without a remineralization filter is still good for you.

Water only contributes a small percentage of your daily mineral intake. Most of your minerals come from food, so drinking mineral-free water won’t cause any health problems in the short term.

Researchers are not sure about long term health effects.

If you have any concerns, talk to your doctor or find a way to add minerals to your drinking water by installing a remineralization filter or buying mineral drops.

What Does an RO System Remove?

Express Water alkaline RO System

Water contains numerous impurities, most of them invisible to the eyes.

The number of type of impurities depends on the source of the water (water from a well or stream has more contaminants) and whether it has been treated or not.

A reverse osmosis system targets almost all impurities in water, which is what makes it so effective in water filtration.

Water generally contains four groups of impurities that a reverse osmosis system can remove.

Total Suspended Solids (Sediment)

Insoluble impurities

Total suspended solids or TSS refers to all impurities that are not dissolved in water. You can usually see them.

They include sand, sediment, dust, and rust.

These impurities often cause water turbidity or cloudiness and can introduce harmful pathogens into the water.

RO systems use a sediment pre-filter to remove TSS. The filter is located at the beginning of the filtration process to ensure the impurities don’t clog other filters.


RO filter

Water picks up chemicals from a wide range of sources.

The most common source is your city’s water treatment facility. Most utilities use chlorine or chloramines to disinfect water.

While these disinfectants are largely safe, most people find their taste and smell unpleasant.

In some cities, chlorine levels in tap water can be particularly high.

Farm runoff, factory waste, sewage, and animal or plant waste can also introduce organic chemicals into the water.

Some of these chemicals are dangerous for your family’s health, while others affect the taste of water.

RO systems use carbon pre-filters to remove chemicals from water.

There are usually two carbon filters that work together to remove pesticides, pharmaceuticals, hormones, chlorine, and chloramines.

Total Dissolved Solids (TDS)

Reverse osmosis water

Total dissolved solids include all the stuff you can’t see because it’s dissolved in the water.

TDS includes inorganic chemicals like arsenic and fluoride, heavy metals like lead and chromium, salts like nitrates and minerals like calcium.

Not all total dissolved solids are harmful.

Some like calcium and magnesium make the water hard. But others like chromium and arsenic can be hazardous even in small amounts.

RO systems rely on the semi-permeable membrane to remove total dissolved solids.

The membrane can typically remove 94%-99% of total dissolved solids.

The individual removal rate for specific solids depends on the quality and performance of a particular RO system. Check the manufacturer specifications to see what percentages of each impurity it removes.



Biologicals include all microorganisms in water. These encompass bacteria such as E. Coli and Salmonella, viruses such as Rotavirus and Hepatitis A and microbial cysts such as Cryptosporidium and Giardia.

Pathogens in water are common causes of waterborne diseases such as typhoid, cholera, and Hepatitis A.

Reverse osmosis systems remove almost all biologicals using a semi-permeable membrane.

Most microbes are too big to pass through the incredibly tiny pores in the membrane.

Some RO systems also have a UV sterilizer that provides additional protection against the microbes.

The UV lamp neutralizes the pathogens by damaging their DNA.

Frequently Asked Questions

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Can I Install an RO System Myself?

Most residential reverse osmosis systems are designed for DIY installation.

The plumbing involved is very basic.

Most systems use push-to-connect fittings that don’t need any tools or nuts. The fittings will also be color-coded to make it easier to know how to connect them.

The most challenging part for most people is drilling a hole in the sink for the faucet.

If you are uncomfortable doing it or don’t have the right tools, you can ask a pro to drill it for you. You’ll also need to drill another hole in the drainpipe for the drain line.

The rest of the installation process is pretty simple. It mostly involves connecting tubes, mounting the filters, and tightening all the valves.

Even though you can install an RO filter yourself, it may not be possible depending on where you live.

Some property owners and homeowner associations may have rules in place, preventing residents from doing DIY plumbing.

Check whether you are under any restrictions before you go ahead.

What Kind of Maintenance Does an RO System Require?

Reverse osmosis systems are generally easy to maintain. The most important part of maintenance is to replace the filters on time.

Check the manufacturer specifications for the replacement period of each filter.

For most RO systems, the pre-filters and post-filter last 6-12 months while the RO membrane can last 2-3 years.

In addition to replacing the filters, regularly check the system for leaks around valves and connectors.

If you find a leak, it could be that the connector is not tight enough, a part has worn down, or there is a broken O-ring.

Open the leaking part to check.

RO Water vs. Distilled Water: Which is Better?

RO water is generally better quality than distilled water. Distilled water may still contain some chlorine and other chemicals that cannot be removed by distillation.

As for the machines themselves, an RO system is easier to operate and maintain compared to a distillation system.

Reverse osmosis doesn’t require electricity and wastes less water compared to distillation.

If you are considering between the two options, we recommend reverse osmosis.

What is a Permeate Pump and Do I Need One?

A permeate pump is a non-electric pump that increases the pressure of feed water in the RO system. This has two big benefits.

One, it reduces the amount of water that goes down the drain and forces more of the water into the storage tank.

Adding a permeate pump can increase water efficiency by as much as 85%.

RO systems with an integrated permeate pump have wastewater to pure water ratio of 1:1. Those without have a ratio of 3:1 – that’s three gallons wasted for every gallon of pure water.

Two, it fills up the tank faster and increases the flow rate out of the tap.

Not all RO systems come with a permeate pump. But you can easily add one later.

You can also opt for a booster pump instead. A booster pump is electric and has the same effects as a permeate pump.

A booster pump is especially useful for homes with low water pressure.

Can I Connect an RO System to my Fridge or Ice Maker?

An RO system typically connects to a dedicated faucet. But you can set up multiple outlets, with one of them connecting to the icemaker or refrigerator.

You can even connect to a coffee maker or a second faucet.

You’ll need to buy an ice maker kit separately if you want to supply RO water to your fridge or ice maker.

How Much Water can an RO System Filter Each Day?

This depends on the system’s daily capacity. The manufacturer will usually mention it somewhere in the product description or specifications.

The daily capacity is given in GPD or gallons per day. It ranges from 50 GPD for budget RO systems (though some cheaper RO systems can go as low as 18 GPD) to 100 GPD for high-capacity RO filters.

However, the figure provided by the manufacturer does not always hold in a real-life scenario. It only applies to optimal water pressure and temperature conditions.

If your home water pressure is low, the system will produce less water.

If you want the most out of your RO system, make sure you achieve the minimum pressure and temperature requirements specified.

You may need to use a booster pump to bring your water pressure up.

Do You Need to Connect an RO System to Electricity?

A basic RO filter doesn’t need electricity to work. It’s designed to use the water’s pressure to push it through the filters and membrane.

A flow restrictor is used to maintain the right water pressure.

However, if the RO system includes a UV filter, you’ll need to plug it into power the UV lamp.

You’ll also need to have an electrical outlet nearby if you install an electric booster pump. If you want something non-electric, get a permeate pump instead.

Is RO Water Healthy?

Because it is demineralized, most people are concerned about the effects of RO water on their health.

There is no clear answer. Researchers are not sure about the long term health effects of drinking demineralized RO water.

Some studies indicate that RO water could significantly reduce the mineral content in food when used in cooking. So it might be a good idea to use it for drinking only.

If you want mineral water, look for an RO system with a remineralization filter, which adds back healthy minerals into the water.

Alternatively, add the minerals yourself using mineral drops.

How Long will an RO System Last?

With proper care and maintenance, most RO systems can last 10-15 years.

Of course, durability depends on the quality of components as well as the brand. Cheap RO systems may only last five years or less.