Reverse Osmosis Water Filters: Pros and Cons

Short answer: A reverse osmosis (RO) water filter is one of the most thorough home treatment options. It removes germs and a long list of chemicals, including lead, and may reduce arsenic, fluoride and nitrate.^{Source [1]} The downsides are cost, upkeep and wasted water: a typical under-sink unit sends five gallons or more down the drain for every gallon of treated water it makes.^{Source [2]} The Environmental Protection Agency (EPA) notes that a simpler filter may be enough for many homes, so it pays to know what is in your water before you buy.^{Source [2]Source [3]}

This guide covers what RO removes, where it falls short, what it costs to own, and how to tell whether you need one.

How reverse osmosis works

Reverse osmosis uses pressure to push water through a semi-permeable membrane. The process creates two streams: treated water, called permeate, and reject water, called concentrate or brine, which carries the removed contaminants down the drain.^{Source [2]}

Most home units are point-of-use systems installed under the kitchen sink on a line used for drinking and cooking.^{Source [6]} They pair the membrane with other stages. The University of Missouri Extension describes a typical setup: a mechanical filter first removes dirt and sediment that would clog the membrane, then the RO unit, then an activated carbon filter that removes some organic compounds that pass through.^{Source [6]} Some systems store treated water in a tank under the sink, while tankless models make water on demand, usually with a booster pump.^{Source [3]}

Pro: it removes more than most filters

According to the Centers for Disease Control and Prevention (CDC), reverse osmosis filters remove parasites, bacteria and viruses.^{Source [1]} Most pitcher and fridge filters are not designed to remove germs at all.^{Source [1]}

For chemicals, CDC says RO removes lead, copper, chromium, chloride and sodium, and may also reduce arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium, nitrate and phosphorus.^{Source [1]} The University of Missouri Extension describes RO as removing substantial amounts of most inorganic chemicals, most microorganisms and many organic chemicals.^{Source [6]}

EPA lists lead, arsenic, PFAS and nitrate as common contaminants of concern for people shopping for an RO system.^{Source [3]}

Pro: a strong option for PFAS

In EPA research, reverse osmosis was one of three point-of-use technologies that greatly reduced PFAS, the "forever chemicals," along with granular activated carbon and ion exchange.^{Source [4]} To find one, look for certification to NSF/ANSI 53 or NSF/ANSI 58 for PFAS reduction.^{Source [4]}

One caveat: EPA says that as of April 2024, current certification standards did not yet show that a filter removes PFAS down to the levels in EPA's new drinking water standard.^{Source [4]}

Con: it wastes a lot of water

This is the biggest drawback. EPA's WaterSense program says a typical point-of-use RO system produces five gallons or more of reject water for every gallon of treated water, and some inefficient units produce up to 10.^{Source [2]} The Rhode Island Department of Health puts it at about 75% of the water that runs through the system, or about four gallons in for every gallon out.^{Source [5]}

Sources describe this differently. An EPA page on PFAS filters says RO wastes "about one gallon of water per every gallon treated,"^{Source [4]} while the WaterSense program describes five gallons or more for typical units and as little as 2.3 for labeled ones.^{Source [2]} Efficiency varies widely by model, which is why the ratings below matter.^{Source [2]}

How to cut the waste:

EPA estimates that a WaterSense-labeled system can save the average household nearly $50 a year in water and wastewater costs compared with a traditional model.^{Source [3]}

Because of the waste, WaterSense says it does not intend to promote RO for all uses, and that in some cases other methods such as filtration are enough.^{Source [2]}

Con: cost and maintenance

EPA lists RO systems at $150 to $1,000, not counting maintenance.^{Source [4]} Upkeep is ongoing:

• Prefilters: usually every six months.^{Source [3]}
• Membrane: every one to five years, depending on your water and how much you use.^{Source [3]}
• Storage tank: drain it about every two weeks to keep the water fresh and help the membrane hold pressure. Many manufacturers also recommend sanitizing the tank once a year.^{Source [3]}

EPA says a well-maintained RO system can last 15 years.^{Source [3]} Skipping maintenance matters: CDC says any filter needs regular upkeep to keep working and to stop germs from growing in it.^{Source [1]}

Con: it also removes things you may want

Filters don't distinguish between good and bad substances. CDC notes that some filters remove beneficial chemicals such as fluoride, which prevents cavities.^{Source [1]} RO may also reduce minerals such as calcium, magnesium and potassium.^{Source [1]}

Con: slow output and one tap only

An RO unit makes a limited amount of water each day. EPA suggests checking the daily production rate to make sure it covers your household's drinking and cooking needs.^{Source [3]} Tankless models can take a little longer to fill a glass.^{Source [3]}

RO is designed for drinking and cooking water only.^{Source [3]} The Rhode Island Department of Health says whole-house RO is not practical because of how much water it wastes.^{Source [5]}

Do you need reverse osmosis?

Start with what is in your water. CDC says that if your tap water doesn't contain harmful germs or chemicals, you may not need a filter at all.^{Source [1]}

• On public water: read your utility's annual water quality report.^{Source [1]} Our guide on how to read a water quality report walks through it.
• On a private well: test at least once a year, and use a state-certified lab.^{Source [1]Source [5]} See how to choose a water test.
• Worried about lead: test first. Our lead in water testing guide explains how.

EPA's consumer guide says a simple tap filter or pitcher may be all you need.^{Source [3]} If your main concern is taste or a single contaminant that a cheaper filter is certified to reduce, compare water filter pitchers first.

RO makes more sense when testing shows contaminants that simpler filters don't address, such as nitrate, arsenic or PFAS, and the system you choose is certified for them.^{Source [1]Source [3]Source [4]}

For people with weakened immune systems: CDC says to talk with your healthcare provider and a water disinfection specialist, and if you get a system, choose one that removes Cryptosporidium. CDC also advises not changing filters yourself.^{Source [1]}

How to choose an RO system

1. Check certification. NSF/ANSI 58 is the standard for reverse osmosis systems.^{Source [1]} Review the product's performance data sheet or packaging table to see which contaminants it is certified to remove.^{Source [3]}
2. Match it to your water. Choose a system certified for the specific contaminants in your test results.^{Source [1]Source [3]}
3. Look for the WaterSense label to limit reject water.^{Source [2]}
4. Check the daily production rate against your household's needs.^{Source [3]}
5. Ask before you buy. The Rhode Island Department of Health suggests asking about total costs, gallons produced per day, how much water is rejected, and maintenance requirements.^{Source [5]}
6. Test after installing. The same tip sheet recommends testing treated water at a state-certified lab to make sure the system works.^{Source [5]}

Frequently asked questions

Is reverse osmosis water good for you?

Reverse osmosis water is drinking water with many germs and chemicals removed. CDC notes that RO may also reduce minerals such as calcium, magnesium and potassium, and filters can remove fluoride. Whether you need RO depends on what is in your water, so test or read your water report first.

Does reverse osmosis remove PFAS?

EPA found that the reverse osmosis point-of-use systems it studied can greatly reduce PFAS. Look for certification to NSF/ANSI 53 or 58 for PFAS reduction. EPA notes that, as of April 2024, certifications did not yet show removal down to its new drinking water standard levels.

How much water does a reverse osmosis system waste?

EPA's WaterSense program says a typical under-sink RO system sends five gallons or more down the drain for every gallon of treated water, and inefficient units up to 10. Systems with the WaterSense label send 2.3 gallons or less.

How often do you change reverse osmosis filters?

EPA says most RO systems need a prefilter change about every six months and a membrane change every one to five years, depending on water quality and use. Follow the manufacturer's schedule for your model.

Can you put reverse osmosis on the whole house?

Generally no. The Rhode Island Department of Health says whole-house RO is not practical because the systems waste about 75% of the water that runs through them. Point-of-use units are designed for drinking and cooking water.