What Does Reverse Osmosis Remove from Water? (Contaminant List)

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Reverse osmosis is widely regarded as the most effective home-based water filtration method due to its ability to remove such a broad range of contaminants.

That being said, it isn’t 100% effective against every contaminant type, so it makes sense to take a closer look at which water contaminants it will remove.

This article will break down exactly which contaminants reverse osmosis removes, which ones it can’t remove, and how additional pre and post-filters fit into contaminant removal.

What Does Reverse Osmosis Remove from Water? (Contaminant List)

So, what does reverse osmosis remove from water? Well, here is a long list of dissolved ions to begin with:

  • 98% of aluminum
  • 98% of ammonium
  • 96% of arsenic
  • 96% of barium
  • 96% of cadmium
  • 96% of calcium
  • 88% of chloride
  • 94% of chromium 6
  • 97% of copper
  • 88% of cyanide
  • 88% of fluoride
  • 96% of iron
  • 96% of lead
  • 96% of manganese
  • 96% of magnesium
  • 97% of mercury
  • 97% of nickel
  • 68% of nitrates
  • 97% of phosphate
  • 90% of potassium
  • 95% of selenium
  • 90% of sodium
  • 97% of sulfate
  • 97% of sulfite
  • 97% of zinc

As you can see reduction rates are pretty high.

Aside from dissolved ions, reverse osmosis can also remove radionuclides such as radium (80%) and uranium. Certain pesticides aren’t safe either, including lindane and endrin. Physical (particulate) contaminants that can be removed using reverse osmosis water purification are bacteria, viruses, cysts, and asbestos among others.

Reverse Osmosis Can Remove Almost Anything

Due to its ultra-fine membrane, reverse osmosis filtration is extraordinarily broad-based – and can remove almost any contaminant from feed water.

On top of that, RO systems do not rely solely on the RO membrane but rather have pre and post-filter stages to remove additional water contaminants not suited to membrane filtration – first and foremost sediment and chlorine/chloramine.

blue reverse osmosis membrane

As shown above, reverse osmosis systems are capable of removing a wide range of contaminant particles including:

  • Heavy metals
  • Chlorine and chloramine
  • Pesticides and herbicides
  • Sediment, silt, and rust
  • VOCs
  • Minerals and salts
  • Bacteria, viruses, and protozoa
  • Radioactive elements

Reverse Osmosis Removes Healthy Minerals

Due to the ultra-fine reverse osmosis membrane, RO systems will remove nearly all particles over 0.0001 microns. Unfortunately, this means they’ll remove some beneficial minerals like magnesium, potassium, and calcium along with harmful contaminants.

There is a simple solution to this issue, however. An optional remineralization stage can be added to your system as one of the final stages – which will add back these beneficial minerals to your water.

Contaminants Not Removed By Reverse Osmosis Alone

While reverse osmosis is incredibly effective, like all water purification methods it isn’t 100% effective against all types of contaminants. So in certain situations, you may need to use an additional filter type.

Contaminants not removed by RO alone:

  • Dissolved gasses such as radon, chlorine, and carbon dioxide won’t be removed by reverse osmosis alone. This includes hydrogen sulfide, a bothersome contaminant that gives off a rotten egg odor. Keep in mind some of the above contaminants like radon and chlorine will be removed through activated carbon filtration – which is why these are common pre-filters for RO systems.
  • Certain pesticides, namely atrazine and trichlorobenzene.
  • Organic compounds such as benzene and total trihalomethanes (THMs)

Pre and Post-Filtration in RO Systems

The need for pre and post-filtration in an RO system is twofold. Firstly, certain contaminants like chlorine and sediment can clog or damage the RO membrane and render it ineffective. Secondly, as RO membranes aren’t perfectly effective against all water contaminants, the pre and post-filters fill in the gaps and remove pollutants that the membrane doesn’t target.

The two primary pre-filter types are sediment filters and activated carbon filters.

Sediment filters – as you might expect – remove large particles like sand, silt, and rust chips from your feed water. Carbon filters remove a wide range of contaminants, but the most important one they remove is chlorine. Chlorine is commonly found in municipally supplied water as the chemical is used as part of the disinfection process.

There are a variety of post-filters that can be used with RO systems, too, but the most common one is a carbon block filter. This filter is used to “polish” the water’s flavor as well as remove any contaminants remaining after the previous filter stages.

Water Testing

Before you purchase any water filtration system, it’s recommended to get your water professionally tested to see exactly what you’re dealing with in terms of contaminants. This will enable you to tailor your filter system to remove the actual contaminants in your feed water rather than simply guessing.

Water testing typically costs between $50 and several hundred dollars depending on the number and type of tests you request.

Water Testing Report

What Is a Reverse Osmosis Filter System and How Does It Work?

Reverse osmosis systems for home use are typically installed under your kitchen sink and supply filtered water through a separate faucet on the kitchen countertop. There are also whole house systems available, which filter the entire water supply to your home, but these are less widespread due to their high cost and maintenance requirements.

Reverse osmosis systems combine the primary filter membrane stage with pre and post-filter stages. Typical under sink systems will have between 3 and 5 stages in total, in addition to a storage tank to keep several gallons of filtered water ready for use at all times.

Reverse osmosis filtration is a slow process, which is why a storage tank is required to keep enough water on hand for use as needed.

Pros and Cons of RO

Let’s take a look at the benefits and drawbacks of using an RO water filter.


Advanced Filtration

First up, reverse osmosis systems are unparalleled at broad-based contaminant removal. They will remove up to 99% of non-organic particles such as lead, arsenic, fluoride, chlorine/chloramine, and nitrates/nitrates, which are the most widespread contaminant type in municipally supplied water.

They also remove a wide spectrum of organic contaminants.

Water Aesthetics

RO filtration removes nearly everything from water leaving you with incredibly pure, clean-tasting water. This includes hardness minerals like calcium and sodium, which can give water a bit of an off taste to some people.

Healthy Drinking Water

In addition to its aesthetic appeal, RO water is exceptionally clean and healthy drinking water. As nearly all contaminant particles are removed through the filtration process, you’re left with as close to pure H₂O as you can get.

Easy to Install and Maintain

RO systems are relatively easy to maintain, only requiring occasional filter changes and period cleaning/sanitizing. Under sink units are also fairly simple to install yourself, provided you have basic DIY skills and can follow directions.

Sediment and carbon filters generally need to be changed every 6 months or so, while an RO membrane only needs replacement every 2 to 5 years.


Many RO systems feature modular designs – which means they can utilize optional filters depending on your needs and preferences.

If you’re somewhat of a tinkerer, then you can build an RO system from scratch using parts commonly available from water filtration suppliers. This won’t save you a ton of money over purchasing a pre-assembled system but will allow you to customize the setup to your desired specifications.



One of the unavoidable downsides of RO water filtration is that it generates a sizeable portion of wastewater. Standard RO systems generate wastewater at a ratio of 4 parts waste to 1 part filtered water.

This can be mitigated to a large degree by installing a permeate pump. This will reduce the amount of water sent to the drain by about 90%.

Mineral Loss

As mentioned previously, RO systems will remove a large proportion of essential minerals in addition to removing harmful contaminants. While this may be a concern, it is less of an issue than it first seems – optional remineralization stages will add back all of the healthy minerals removed by the RO membrane.

Filtration Speed

RO systems are relatively slow at filtering water, which is why a storage tank is used to store filtered water on hand for immediate use. If your system is operating very slowly, it’s generally an indication that your filters or membrane are clogged or there is an issue with the storage tank pressure.

Space Needs

An average under sink RO system requires 2-foot x 2-foot of space underneath your sink. This includes the system and the storage tank. For most typical kitchen sinks this is a non-issue, but if yours doesn’t have the necessary space, you may need to install the system in an adjacent cabinet.

plumber installing reverse osmosis system under sink

Extra Faucet

RO systems require a separate faucet from your main sink faucet. This will require installation in your sink or countertop, and unless you have a spare hole for a soap dispenser, this means drilling a new hole through the sink or countertop.

Drain Connection Required

As a consequence of the filtration process, wastewater flows from the RO membrane outlet and must be directed to a drain pipe. That means installing a drain saddle on your sink’s drain pipe and connecting the wastewater tubing to it.

If you have any thoughts about the question, what contaminants does reverse osmosis take out of water, please don’t hesitate to leave a comment below!

About the Author Alexandra Uta

Alex is a content writer with an affinity for research and a methodical attention to detail. Since 2020, she has fully immersed herself into the home water treatment industry only to become an expert herself. Alex has been using water filters and similar products for years which has gained her lots of hands-on experience.
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