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How Does Reverse Osmosis Work

If you’re here, you’re probably wondering “How does reverse osmosis work and what is special about it?” This post walks through what reverse osmosis (RO for short) is and how it works.

Water is an essential natural resource. In its natural state, water may be unusable – dirty or in some cases salty. For these reasons, dirty water has to go through a purification processes such as filtering to be safe to drink.

Reverse Osmosis Water Filter Systems ensure that you have clean and uncontaminated drinking water. For a comparison of different units, see our Best Reverse Osmosis Filters page.

Water is disinfected by most public water distribution authorities. Still, there is room for contamination when the water is in transit to your home. To ensure you are drinking 100% safe water, consider installing an RO system in your home.

To learn how reverse osmosis works, keep reading…

What is reverse osmosis?

Reverse Osmosis is a process of removing contaminants, such as dirt and minerals from water by passing water under pressure through a filter (called a semi-permeable membrane).

Think of the semi-permeable membrane like a piece of skin. Not much can filter through the skin without pressure.

The RO process results in pure water on one side of the membrane and the impurities (minerals, bacteria, etc) on the other side of the membrane.


Yep, that’s a lot of technical talk. Basically we’re forcing water through a filter that has small holes that only the water can pass through. The impurities can’t go through so they’ll stay on the other side of the filter.  These impurities will need to go somewhere so they’re typically flushed down the drain as waste.

The principle of reverse osmosis

The principle of osmosis states that molecules will migrate from regions of low concentration to regions of high concentration through a semi-permeable membrane.

Osmosis explains the absorption of water by plants via the roots.

Um, yeah. But that is osmosis, not reverse osmosis…

Reverse Osmosis or RO is the opposite of osmosis.

In RO the  membrane filter allows water molecules to pass through but prevents the mineral and other solvents molecules from passing through. But this process cannot occur naturally.

The water has to be forced through the reverse osmosis membrane. The pressure of the water has to be higher than the pressure resulting from the naturally occurring osmosis.

How does Reverse Osmosis Work in the Home?

The typical residential Point-of-Use Reverse Osmosis system has four stages. Four stages (or sets of filters) are responsible for the removal of 99% of more than 65 possible contaminants in water.

Some of the contaminants include fluoride, chlorine, lead and other dissolved salts.

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How RO Water Filters Work

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Stage 1 – The Sediment Filter

Made of Melt Blown polypropylene, the sediment filter removes dirt, silt, rust and large sediments of up to 5 microns. This filter is crucial in removing sediments that would otherwise damage or plug up the more delicate filters that follow.

Stage 2 – The Carbon Filter

Chlorine, a common compound in water, will go through the sediment filter. More importantly, chlorine damages the reverse osmosis filter. So we need to filter that out before it gets to the RO membrane. That’s the job of the carbon filter which has slightly smaller holes than the sediment filter.

Chlorine also affects the taste and odor of water.  The carbon filter isolates chlorine, reduces the amount of lead and removes harmful bacteria such as Giardia and Cryptosporidium.

Stage 3 – The Reverse Osmosis Membrane

The Reverse Osmosis membrane handles the isolation of 99% of contaminants. This filter allows water to pass through and stops contaminants as small as 0.0001 microns.

Stage 4 – The Post Carbon Filter

The post carbon filter is the final stage of the water filter. This carbon filter will remove any remaining odor and improves the taste of the final drinking water.

What does Reverse Osmosis remove from water?

As mentioned earlier the process removes contaminants. These contaminants are both organic and inorganic impurities.

Common water contaminants include:

  • Chlorine
  • Chromium
  • Asbestos
  • Arsenic
  • Fluoride
  • Lead
  • Mercury
  • THMs (TriHaloMethanes)
  • Harmful bacteria such as Giardia and Cryptsporidium.

Reverse Osmosis systems also remove odor and bad taste.

Factors that will change the effectiveness of the RO system

  • The pressure of the tap water. Normally, the pressure of tap water is 40-80psi. Pressure lower than that will reduce the effectiveness of the system. Pressure is needed to force the water through the RO filter.
  • The contaminants in the tap water. Some types of impurities reduce the effectiveness of the system. Also, the number of different contaminants determine the effectiveness of the system. Over time the filters will get plugged up and need to be replaced.
  • The quality of the membranes and filters. Always ensure your RO system is using the best filters in the market.
  • The temperature of the water. The temperature is not a big issue if the pressure is okay. However, cold water tends to filter out slower than hot water.

Things to consider when purchasing a residential RO system

Reverse Osmosis systems differ so when buying such a system consider these factors:

  • Number of filters – most systems have four filters (discussed above). Some systems have five filters. The extra stage is usually a carbon pre-filter that comes before the water passes through the reverse osmosis filter (so there are 2 carbon filters before the RO membrane).
  • Quality of the filters and other components – a little research will help here. Check the reviews of the RO water filter system. Also, the costlier a system the more likely it has quality components.
  • Input Water Quality – If the input water is very contaminated, the RO filter will not last as long. Also, hard water tends to plug up RO filters. Consider softening the water before reverse osmosis filter.  When considering water softeners, our Best Water Softeners Reviews page is helpful.
  • Gallons per day – RO systems have a specified amount of water they can produce each day. A huge household uses many gallons per day, so choose carefully.

Summary of RO system benefits

This article answers the question, how does Reverse Osmosis work? Now that you understand the inner workings behind these filters, consider going to our reverse osmosis systems reviews page to see RO Water filter comparisons and reviews.

Drinking contaminant-free water reduces the risk of water-borne diseases.  Water purified through an RO system is safe for whatever purpose, be it drinking, cooking or washing.

Priced at less than $1000 and in many cases below $500, RO systems are affordable and worthwhile investments.

Types of Water Filters

Coffee Filter

Why you should replace Water Bottles with Water Filters

In 2015, US families bought over 50 billion water bottles because most people don’t think about using water filters. Many people spend money on water bottles instead of using water filters. Although water bottles seem cheap, in the long run using one of these types of water filters is more affordable. Using water filters also reduces the amount of plastics in our landfills.

Research has shown that some water bottles can still contain foreign contaminants (such as BPA) which can have a negative health effects.

Even more alarming – a staggering 25% of bottled water produced may not go through the full filtration process.

In this case, it is no better than tap water.

Most people have no clue where to start when buying water filters – there are so many different categories.

So we are going to discuss the best water filters. We start with the 5 types of Water Filtration that are available on the market and why you should stop using water bottles.

1 – Water Pitcher Filters

Water filter pitchers work by filling a small tank at the top of the pitcher with tap water. The water slowly drips into the bottom tank through a filter. Typical filters last around 2 months.

*Please note – no Water Pitcher Filters can remove fluoride. It is better to choose a reverse osmosis water filter if you are worried about fluoride.

We have an entire post with a detailed look at choices for the best water filter pitcher comparison.

Mavea - Best Water Filter Pitcher

The MAVEA 1005722 is one of the leading brands when it comes to Water Pitcher Filters. This water filter uses silver to prevent any foreign bacteria from entering the water. It comes with a “smart meter” which can measure water volume and notify you when the filter needs to be replaced.


  • Very Affordable
  • No installation – no plumbing
  • Uses BPA-free plastic
  • Filters can last for up to 6 months
  • Easy to maintain
  • Fresh tasting water

Customer Ratings

Many customers have compared MAVEA to “Brita Everyday”.  MAVEA is certified to remove a lot more impurities than Brita.  It is easy to refill and for the price is definitely worth the investment. 65% of customers have rated this product 5-stars.


Priced below $50, the MAVEA is affordable and the filters are made to last 6 months. This is a bargain with its 9-glass water capacity

Check the Price On Amazon

2 – Faucet Filters

Faucet water filters attach to the water tap in the kitchen. They normally have a way to select whether the water coming out of the faucet is filtered or not. These filters last about 2 months as well.

*Please note – no Faucet Filters can remove fluoride. It is better to choose a reverse osmosis filter if you are worried about fluoride.

We have an entire post with a detailed look at choices for the best faucet water filter comparison.

Pur Water Faucet Filter

PUR Faucet filter is the best-rated filter on Amazon. It is made to remove 70+ foreign contaminants from the water we drink every day.

It comes with an electronic filter life reader so you will be able to check when the filter needs to be replaced. PUR is a combination faucet filter and carbon filter.


  • Affordable
  • Easy Installation
  • Filters can last for up to 2-3 months
  • 360 swivel to fit in most sinks

Customer Ratings

40% of customers have rated this product 5-star. This is because of the filter life in most cases. If you don’t mind upgrading your filter every 2-3 months then a PUR filter can save a lot of money.


It is certified to remove 70+ contaminants (including more than 95% of lead and mercury) which is much better than bottled water.

3 – Carbon Filters

Carbon filters are generally found on the water line going to the ice maker in typical refrigerators. They are the best water filter if you are looking for small size and the ability to tuck it out-of-sight. Carbon filters are also the second stage of  an RO water filter assembly.

In Line Water Filter

Nahla Pure is the best carbon filter to purchase. It filters the kitchen sink water. It can be easily installed under the kitchen sink with normal household tools.

This filter is installed directly to the standard 3/8″ cold water line.


  • Affordable
  • Smaller in size than RO Filters.
  • Filters can last up to 6 months or more
  • Fresh tasting water

Customer Ratings

Again, the rating for this product is 81% 5-star on Amazon. Customers have argued that the carbon filter is the game-changer for this filter. It removes the chlorine taste better than any other filter. There are a number of complaints about the filter having the wrong parts.


4 – Reverse Osmosis Water Filters

Reverse Osmosis (or RO filters) are extremely good at removing most impurities. RO systems usually are 3 or 4 stages with each stage a filter with smaller holes for catching impurities. The last stage is a RO filter. Since there are a number of stages, RO water filter systems are more expensive as a result.

We have an entire post with a detailed look at choices for the best reverse osmosis system comparison.

If you’d like to learn more about RO water filters, try looking at our “how does reverse osmosis work​” webpage.

APEC is the leading brand for RO filters. They use a 4-stage filtration process.

Stage 1: Sediment Filter: Entry water is filtered through a polypropylene filter to remove any dust or rust. This stage filters out the larger particles from the water.

Stage 2: Carbon Filter. A carbon filter block removes harmful chemicals in tap water and removes the chlorine which can damage the R.O. filter.

Stage 3: Reverse Osmosis. The reverse osmosis membrane removes 99% of the foreign bacteria and other impurities such as arsenic, lead and fluoride.

Stage 4: Carbon Filter. In the last stage, the coconut carbon filter removes any remaining bad odor and taste.


  • Reverse Osmosis filters remove 99% of foreign contaminants in the water we drink.
  • Filters up to 50 gallons per day
  • “Ultra-fresh taste” from purified water
  • Filters can last for up to 12 months
  • Easy to maintain

Customer Ratings

On Amazon, 92% of people have given APEC 5 stars, which is rare on any product. This definitely shows that this product has a large customer following.  Customer reviews show that this filter can last a year if well cleaned and looked after.


The filter lasts for a full 12 months – which is incredible in comparison to some filters that breakdown quickly.

It is also easy to maintain and still works out a lot cheaper than water bottles – an average family spends $346 per year on bottled water.

5 – Distilled Water

Water distillers work by boiling the water, capturing the pure steam, and cooling and condensing it. The impurities are left behind while the steam condenses into purified water – like rain.

MegaHome Water DistillerThe best water distiller to buy would be “MegaHome Countertop Water Distiller” as shown by the large number of positive reviews.

It distills 1 gallon every 6 hours which is why this isn’t a more popular choice for water purification.


  • Stainless steel and glass lined
  • Easy to maintain
  • 1 Gallon
  • Removes 99.6% of Total Dissolved Solids (TDS) with its distillation process

Customer Ratings

There is a 78% 5-star rating on Amazon, which is pretty good. There are a lot of happy customers.

Most complaints are about the taste. Weird tasting water is a common complaint about distilled water. Adding in “good” minerals may help add good taste but that defeats the purpose.


Water Distillers are more expensive than Water Pitchers and Faucet Filters but they claim to remove more contaminants (TDS).

Our Recommendation

Out of all the types of water filters, we would recommend Reverse Osmosis water filter systems as the best water filter to invest in.

Not only are nearly 100% of customers happy but it also is a long-term investment. The product lasts a lot longer than other filters and is easy to maintain.

  • It can be installed underneath the sink.
  • Can filter a large amount of water per day (50 to 90 gallons per day)
  • RO Filtering removes the worst contaminants.

Reverse Osmosis Filters are popular due to the incredible filtering qualities. They have been proven to remove a lot more contaminants than any other water filter.

APEC’s purer, tastier filtered water will definitely save you money in the long run when compared to water bottles.

It is worth scrapping water bottles and investing in water filters.

Types of Water Contaminants

Is tap water really safe for drinking? Well, that is a controversial question. The truth is that municipalities do their best to ensure that they provide clean water. Unfortunately, the water they deliver is not clean enough.  With a Reverse Osmosis (RO) system in place you can be certain that the water you drink is free of most types of contaminants.

The process of reverse osmosis involves water being filtered through a semi-permeable membrane (see our page for additional information about how a RO filter works). A typical RO system has four filters (the sediment filter, carbon filter, the reverse osmosis filter, and the post carbon filter). Each filter is responsible for removing certain types of contaminants.

Types of water contaminants

Generally, RO systems get rid of contaminants such as viruses, bacteria, protozoa and chemicals. The list below shows the types of chemicals removed and the percentage of effectiveness of the system in their removal.

Contaminant % of removal Contaminant % of removal
Aluminum 97-98% Nickel 97-99%
Ammonium 85-95% Nitrate 93-96%
Arsenic 94-96% Phosphate 99+%
Bacteria 99+% Polyphosphate 98%-99%
Bicarbonate 95-96% Potassium 92%
Boron 50-70% Pyrogen 99+%
Bromide 93-96% Radioactivity 95-98%
Cadmium 96-98% Radium 97%
Calcium 96-98% Selenium 97%
Chloride 94-95% Silica 85-90%
Chromate 90-98% Silicate 95-97%
Chromium 96-98% Silver 95-97%
Copper 97-99% Sodium 92-98%
Cyanide 90-95% Sulphate 96-98%
Ferro cyanide 98-99% Sulphite 96-98%
Iron 98-99% Zinc 98-99%
Lead 96-98% Insecticides 97%
Magnesium 96-98% Detergents 97%
Manganese 96-98% Herbicides 97%
Mercury 96-98% Virus 99+%
Total Dissolved Solids TDS 95-99% Hardness 93-97%

The percentages indicated above are generally accepted in the industry. However, an RO system could perform better or worse than the figures noted above because other factors could affect the filtration process. Such factors include the PH, temperature, pressure and chemicals in the water.  The above chart is handy for comparing the Consumer Confidence Report that details what’s in your water to potential pollutants.

The filtration process

Reverse Osmosis is primarily based on the physical process of filtration. Filtration occurs when matter (gas, liquid or solid), passes through the pores of a medium or an absorbent material. Where water contaminants are concerned, filtration is dependent on the size of contaminant, and where applicable, its charge. To increase the effectiveness of the RO filters, pretreatment of drinking water may be necessary. The pretreatment might involve adjusting the pH, adding coagulants in the water or altering the chlorine levels.

There are three types of filtration described by the CDC and understanding them will give you a clearer idea of how a good water filtration system works.

  • Water first goes through the microfiltration to remove large particles that would otherwise destroy delicate filters such as the RO membrane.
  • The 2nd stage is ultrafiltration to trap larger chemicals, bacteria and protozoa. In many systems, there may be 2 of these filters.
  • The last filtration stage is nanofiltration or Reverse Osmosis (of these 2 choices, reverse osmosis is better) which gets rid of most chemicals to produce safe drinking water.

1. Microfiltration

Microfiltration filters have a pore size of about 0.1 microns. Some microfiltration filters have a pore size as small as 0.05 microns while others have a pore as big as 5 microns. Due to the pore size, microfiltration is effective in removing some contaminates as show below:

  • Yes – Filter is effective in removing protozoa such as Giardia and Cryptosporidium
  • Maybe – Filter is moderately effective at removing bacteria such as E. Coli and Salmonella
  • No – Filter is not effective at removing viruses such as Rotavirus or Hepatitus A.
  • No – Filter is not effective at removing chemicals

2. Ultrafiltration

Ultrafiltration filters have a pore size of about 0.003 microns with a range from 0.003 on the lower scale to 0.05 on the upper scale. The Molecular Weight Cut Off (MWCO) on these filters is between 13,000 and 200,000 Daltons. In the RO system, the ultrafiltration filter is the carbon filter.

  • Yes – Filter is effective in removing protozoa such as Giardia and Cryptosporidium
  • Yes – Filter is effective in removing bacteria such as E. Coli, Salmonella, and Shigella.
  • Maybe – Filter is moderately effective at removing viruses such as Rotavirus and Hepatitus A.
  • No – Filter is not effective at removing chemicals

3. Nanofiltration

Nanofiltration filters have even smaller pores with pore sizes between 0.001 microns and 0.01 micron. The MWCO of these filters is between 200 and 2,000 Daltons. Like the ultrafiltration filters, the effectiveness of nanofilters is affected by the size of particles, the weight, and the charge. Nanofilters aren’t typically used in a RO filter because the even smaller pores of the Reverse Osmosis membrane are used.

  • Yes – Filter is effective in removing protozoa such as Giardia and Cryptosporidium
  • Yes – Filter is effective in removing bacteria such as E. Coli, Salmonella, and Shigella.
  • Yes – Filter is effective at removing viruses such as Rotavirus and Hepatitus A.
  • Maybe – Filter is moderately effective at removing chemicals such as metal ions of copper, chromium, or lead.

4. Reverse Osmosis Membrane

Reverse Osmosis membrane filters have the smallest pores at about 0.0001 micron or 0.1nm. These filters are best at filtering out ions and molecules that the other filters can’t.  In the RO system, the RO membrane filter is the last stage and filters out the smallest particles.

  • Yes – Filter is effective in removing protozoa such as Giardia and Cryptosporidium
  • Yes – Filter is effective in removing bacteria such as E. Coli, Salmonella, and Shigella.
  • Yes – Filter is effective at removing viruses such as Rotavirus and Hepatitus A.
  • Yes – Filter is effective at removing chemicals such as chromium, copper, sodium, and lead.


RO systems are cheap and easy to install. Also, these systems do not require lots of maintenance but they do require filter changes are regular intervals – typically every 6 months to a year depending upon the amount of contaminants in the water.

Take a look at our Reverse Osmosis Filter Reviews page to see our top picks for RO Filter systems.​