Quick answer
Reverse osmosis water filtration is a treatment method that uses water pressure to force water through a semi-permeable membrane, so treated water passes through and many dissolved contaminants are rejected and flushed down the drain.
Home systems are usually point-of-use units under the kitchen sink with a pre-filter, the membrane, a storage tank, a carbon post-filter and their own faucet. They make cleaner water slowly and send some water to the drain.
Quick facts
The key facts about Reverse Osmosis Water Filtration, with sources:
- Reject water (typical unit)
- 5 gallons or more for every gallon treated, per EPA WaterSense 1
- WaterSense label
- 2.3 gallons or less to the drain per gallon treated; membrane life at least 1 year 1
- Membrane life
- Typically 1–3 years, depending on conditions (MSU, 1990 archive) 3
- Not effective against
- Dissolved gases and most volatile organic chemicals, per Connecticut DPH 2
Key takeaways
- Reverse osmosis pushes water through a membrane that holds back many dissolved minerals and chemicals.
- Home units treat drinking and cooking water at one tap, not the whole house, because they work slowly and waste water.
- A typical unit sends 5 or more gallons to the drain for each gallon it makes; WaterSense models must send 2.3 or less.
- RO does not remove everything: dissolved gases and most volatile organic chemicals pass through, so a carbon stage is part of the system.
- Test your water first, buy a system certified for the contaminant you have, and test again after install.
Reverse osmosis meaning: osmosis run backward under pressure
Reverse osmosis uses pressure to push water through a membrane while most of what is dissolved in it stays behind. Plain osmosis runs the other way: water moves on its own from the less salty side of a membrane to the saltier side, and pressure on the dirty side reverses that flow 3.
The membrane is a thin plastic film. The EPA WaterSense program describes the two streams that come out: treated water, called permeate, and reject water, also called concentrate or brine, which carries the contaminants to the drain 1.
In a home, the word usually means a point-of-use (POU) system under the kitchen sink. Connecticut’s health department says it is not practical to treat all the water entering a home with RO because so much water is wasted, so treated water is used mainly for drinking and cooking 2.
Filtration vs reverse osmosis
A carbon or pitcher filter catches contaminants as water flows through and wastes almost no water. RO separates water from what is dissolved in it and always makes reject water. EPA WaterSense notes filtration is enough in some cases 1.
How a home RO system works, stage by stage
- Sediment pre-filter: Removes sand, silt and sediment so they do not foul the membrane 2.
- Carbon pre-filter: Often added to protect membranes that chlorine can damage 3.
- RO membrane: House water pressure pushes water through; contaminants are washed off into the reject stream 2.
- Storage tank: Treated water comes out much slower than tap water, so a small tank stores it 2.
- Carbon post-filter: Polishes taste and odor and takes out organic material and dissolved gases the membrane missed 23.
- Shut-off valve and faucet: A valve stops flow when the tank is full, and a dedicated faucet delivers the water 3.
- Sediment pre-filterSand, silt, rust
- Carbon pre-filterChlorine that can harm the membrane
- RO membraneDissolved minerals and many contaminants rejected
- Storage tankHolds slow-made treated water
- Carbon post-filterTaste, odor, leftover organics and gases
- Dedicated faucetDrinking and cooking water
Exact stages vary by model; read the performance data sheet for yours. Source: [2] [3]
Pressure drives everything. MSU notes higher pressure gives better rejection and more output, and that doubling the net pressure across a membrane more than doubles the flow of treated water 3. Cold water slows it down: a drop from 75°F to 45°F cut production nearly in half in MSU’s figures 3.
Types of reverse osmosis systems for homes
| Type | What it is | Note |
|---|---|---|
| Under-sink POU | Plumbed-in unit with tank and its own faucet | The common home setup 23 |
| Countertop | Sits on the counter and connects to the faucet | MSU lists countertop and under-sink models 3 |
| Tankless / high-flow POU | Makes water on demand without a storage tank | Covered by WaterSense if plumbed-in or faucet-mounted 1 |
| With booster (permeate) pump | Adds pressure to the membrane | Connecticut suggests it to cut reject water 2 |
| Point-of-entry (whole house) | Treats all water entering the home | Rare for homes; Connecticut calls whole-house RO impractical because of waste 2 |
Example: why a high removal rate can still fall short
This worked example comes from Michigan State University Extension, using a deliberately high nitrate level. It shows why you need both the removal rate and your own test result.
| Step | Figure | Source |
|---|---|---|
| Incoming nitrate (very high, for illustration) | 110 mg/L | MSU example 3 |
| Membrane rejection for nitrate | Up to 90% with some systems | MSU 3 |
| Nitrate left in treated water | 110 × 10% = 11 mg/L | MSU 3 |
| Federal limit for nitrate (as N) | 10 mg/L MCL | EPA 4 |
| Result | 11 mg/L is still above the 10 mg/L limit | MSU 3 |
A “90% removal” claim sounds safe, but it depends on what goes in. Test your water, then check the certified reduction for that contaminant on the system’s performance data sheet. Our total dissolved solids page explains the TDS number many RO units display.
What to check on an RO label, data sheet and quote
On the box or performance data sheet, look for the NSF/ANSI 58 mark and the list of certified reductions. Connecticut DPH advises checking that a system has been tested and certified by a third party, because RO membranes cannot be certified to remove 100% of anything 2.
Look for two water numbers: how much treated water it makes per day, and how much it rejects. Connecticut lists both among the questions to ask before you buy 2. A WaterSense label means the reject water is 2.3 gallons or less per gallon treated 1.
On a quote, check the stages, the tank size, a dedicated faucet, a drain connection with an air gap, and the price and schedule of replacement filters and membranes. Our water treatment service page covers what an install includes.
What reverse osmosis removes, and what it costs in water
Benefits
- Reduces a wide range of dissolved inorganics, such as sodium, nitrate, lead, fluoride and sulfate, per Connecticut DPH 2.
- Also effective on some detergents, uranium, some pesticides and some taste and color problems 2.
- EPA WaterSense lists lead, VOCs, PFAS, arsenic, bacteria and viruses among contaminants RO can remove 1.
- Works on house water pressure in many homes 3.
- Point-of-use design keeps filter and membrane costs lower 3.
Drawbacks
- Wastes water: 5 or more gallons per treated gallon in a typical unit 1.
- Not effective against dissolved gases and most volatile and semi-volatile organic chemicals 2.
- Not recommended alone for bacteria and other microbes, per Connecticut DPH 2.
- Slow: Connecticut found units need 2 to 78 hours to make a gallon, by model 2.
- Iron, manganese, hydrogen sulfide, chlorine or turbidity can foul the membrane 2.
What reverse osmosis does not fix: germs, gases and fouling
RO is not a germ-killing system on its own. Connecticut DPH says RO units alone are not recommended for bacteria and other microbes 2. EPA WaterSense lists bacteria and viruses among what RO can remove 1.
The safe reading is: if a well has a bacteria problem, treat it with a method designed for that, and use RO for what it is certified to reduce.
Gases and many volatile chemicals slip through the membrane, which is why the carbon post-filter matters 2. Our page on the activated carbon water filter covers what carbon does.
Hard water and iron shorten membrane life. Connecticut notes iron, manganese, hydrogen sulfide or turbidity may call for a point-of-entry treatment first 2. A water softener or iron filter upstream is a common pairing.
What drives the cost of reverse osmosis
Stages, certifications, pre-treatment and replacement parts set the cost of an RO system, and reject water adds to the water bill. We do not quote RO prices here, because fair numbers need dated local quotes. Connecticut DPH notes RO is often more costly than simpler methods, such as ion exchange for dissolved iron 2.
The drivers are the number of stages, tank or tankless design, certifications, a booster pump, any pre-treatment the water needs, the drain and faucet work, and how often filters and membranes must be replaced.
Reject water is a running cost on your water and sewer bill, which is where a WaterSense model helps; EPA says choosing one can save more than 3,100 gallons a year 1.
Our water usage calculator helps put the reject water in context, and the plumbing cost guide explains how quotes are built.
How to choose and install an RO system in five steps
- Test first: Connecticut DPH says to test at a state-certified lab before buying, to learn whether RO fits your problem 2.
- Match the certification: Pick a system certified for your contaminant, not just for TDS.
- Check the plumbing: The unit needs a cold supply tap, a drain connection and room for the tank. Pressure below what the maker lists may call for a booster pump 32.
- Install per the manual: Connecticut stresses installing and running the system per the maker’s instructions 2. Drain connections fall under your local plumbing code.
- Test again: After install, retest both raw and treated water at a certified lab, then at least yearly 2.
Maintenance: filters, membrane and sanitizing
- Replace pre- and post-filters on the maker’s schedule; MSU notes the interval depends on your water, especially solids 3.
- Plan on a new membrane every 1–3 years in older guidance 3; WaterSense-labeled membranes must last at least one year 1.
- Sanitize the system when you change filters, as the maker directs; MSU notes some units include a sanitizing procedure for bacteria that build up inside 3.
- Keep a logbook of filter changes and repairs, as Connecticut DPH advises 2.
- Use a unit with an automatic shut-off so it stops making reject water when the tank is full 2.
Use the water daily
MSU notes output and rejection drop as the tank fills, so a tank sized to your use and emptied daily works best 3.
Signs an RO system needs filters, a membrane or a repair
- Treated water flow slows to a trickle or the tank never fills.
- The drain line runs all the time, which can mean a failed shut-off valve 2.
- Taste or odor returns, often a sign the carbon stage is spent.
- A built-in TDS monitor shows treated water rising toward tap water levels 2.
- Leaks under the sink at fittings, the tank or the drain saddle.
A slow leak under a sink can rot a cabinet floor before anyone sees it. Our leak detection and repair service covers what a visit includes.
NSF/ANSI 58, WaterSense and EPA limits for RO
Product standard: NSF/ANSI 58 covers point-of-use RO systems. Connecticut DPH points buyers to NSF certification as proof that claims were tested 2. For lead, EPA’s consumer tool lists filters certified to NSF/ANSI 53 and 42 for reducing lead to 5 ppb or less 6.
WaterSense (EPA, Version 1.0, November 2024): Labeled POU RO systems must send 2.3 gallons or less to the drain per gallon treated, have a membrane that lasts at least a year, meet a baseline TDS reduction, and be certified for any extra contaminant claims.
EPA issued technical clarifications in August 2026 covering packaging and documentation 1.
Drinking water limits: EPA’s National Primary Drinking Water Regulations apply to public water systems. Nitrate (as N) has an MCL of 10 mg/L, arsenic 0.010 mg/L, fluoride 4.0 mg/L, and lead an action level of 0.015 mg/L, falling to 0.010 mg/L from November 1, 2027 4.
Private wells: The EPA says private wells are not regulated under the Safe Drinking Water Act, and owners are responsible for their water’s safety 5.
Plumbing: The drain and supply connections fall under the plumbing code your jurisdiction adopted. Ask your installer how the drain is protected against backflow.
Reverse osmosis vs other home water treatment
| Method | Main job | Wastes water? | Limit |
|---|---|---|---|
| Reverse osmosis | Dissolved inorganics and many other contaminants 21 | Yes, reject water 1 | Not for gases or most VOCs alone 2 |
| Activated carbon filter | Taste, odor, chlorine, some organics; lead if certified to NSF/ANSI 53 6 | Little or none 1 | Does not remove dissolved salts |
| Whole-house filter | Sediment or chlorine at the point of entry | Little or none | Usually not a drinking-water health treatment |
| Ion exchange softener | Hardness minerals and some dissolved iron 2 | Uses water to regenerate | Does not remove most health contaminants |
Reverse osmosis myths
- Myth RO removes 100% of contaminants.
- Reality Connecticut DPH says RO membranes cannot be certified to remove 100 percent of anything 2.
- Myth RO is the best choice for every water problem.
- Reality WaterSense says filtration is enough in some cases and uses little or no water 1.
- Myth An RO unit alone makes well water safe from germs.
- Reality Connecticut DPH does not recommend RO alone for bacteria and other microbes 2.
- Myth A high removal percentage means safe water.
- Reality MSU’s nitrate example shows 90% removal can still leave water above the limit 3.
- Myth Whole-house RO is the upgrade to buy.
- Reality Connecticut says treating all household water with RO is not practical because of the waste 2.
Where wells and septic systems change the picture
The biggest regional variable is a private well. The EPA says about 15% of the U.S. population relies on private wells, which are not regulated under the Safe Drinking Water Act, so testing and treatment fall to the owner 5.
Homes on septic face a second limit: Connecticut DPH says up to 30 gallons a day of RO reject water to a septic system is considered incidental, and larger flows may cause problems 2.
Hard-water regions also wear membranes faster and often need a softener first.
When reverse osmosis makes sense, and where to start
RO fits when a lab test shows a dissolved contaminant it is certified to reduce, such as nitrate, arsenic or high TDS, and you want treated water at one tap. It is less useful if your only complaint is chlorine taste, where a carbon filter may do the job with no reject water.
Start with a lab test, then compare certified systems on the reduction you need, daily output and reject water. The guide below covers four common starting points.
- If Well with nitrate or arsenic
- Then Test, then fit a certified RO unit at the kitchen tap
- If Only a chlorine taste
- Then Try carbon filtration first
- If Bacteria in a well
- Then Use a disinfection method; RO alone is not the fix
- If Home on a septic system
- Then Keep reject water under 30 gallons a day
Questions about Reverse Osmosis Water Filtration
What does a reverse osmosis system remove?
Reverse osmosis reduces many dissolved inorganic contaminants. Connecticut DPH lists sodium, nitrate, lead, fluoride, sulfate, manganese, chloride and total dissolved solids, plus some pesticides and uranium. EPA WaterSense also lists VOCs, PFAS and arsenic. What a specific unit removes depends on its certification, so check its performance data sheet for your contaminant.
What does reverse osmosis not remove?
Connecticut DPH says RO is not effective against dissolved gases and most volatile and semi-volatile organic chemicals, including some pesticides and solvents. That is why most systems include a carbon post-filter. It also says RO alone is not recommended for bacteria and other microbes, so wells with germs need a disinfection method.
How much water does reverse osmosis waste?
EPA WaterSense says a typical point-of-use RO system sends five gallons or more to the drain for each gallon of treated water, and inefficient models up to 10. WaterSense-labeled systems must send 2.3 gallons or less. EPA estimates a labeled model saves more than 3,100 gallons a year.
Is reverse osmosis water safe to drink?
Yes, RO water is commonly used for drinking and cooking. The questions are whether the system is certified for the contaminants in your water and whether it is maintained. Connecticut DPH advises testing both raw and treated water at a certified lab after installation and at least yearly after that.
How often should an RO membrane be replaced?
Older MSU Extension guidance says RO membranes typically last one to three years, depending on operating conditions, membrane type and how well the pre-filters protect them. WaterSense-labeled membranes must last at least one year. A built-in TDS monitor or a lab test is the surest way to know when it is time.
Do I need a whole-house reverse osmosis system?
Rarely. Connecticut DPH says it is not practical to treat all water entering a home with RO because about 75 percent of the water can be wasted. Most homes use a point-of-use unit at the kitchen tap for drinking and cooking, and treat other problems, such as hardness, at the point of entry.
Does reverse osmosis remove lead?
RO can reduce lead, and EPA WaterSense lists lead among the contaminants RO can remove. Check that the specific system is certified for lead reduction. EPA’s consumer tool also lists filters certified to NSF/ANSI 53 and 42 that reduce lead to 5 ppb or less, which can be a lower-waste option.
Can I connect RO reject water to a septic system?
Usually, in small amounts. Connecticut DPH considers 30 gallons a day or less of reject water to a septic system incidental and unlikely to cause harm, but larger flows may cause problems. A WaterSense-labeled unit, a booster pump and an automatic shut-off all cut reject water.
Why is my RO system so slow?
RO makes water slowly by design, which is why it has a storage tank. Connecticut DPH found units take 2 to 78 hours to make a gallon, by model.
Low house water pressure, cold water, a clogged pre-filter or an old membrane all slow it further. MSU notes cold water from 75°F to 45°F nearly halves output.
Sources
- U.S. EPA WaterSense, Point-of-Use Reverse Osmosis Systems, retrieved .
- Connecticut Department of Public Health, Publication 21: Reverse Osmosis Treatment of Private Drinking Water Systems (2009), retrieved .
- Michigan State University Extension, Reverse Osmosis for Home Treatment of Drinking Water (WQ24, 1990, archive), retrieved .
- U.S. EPA, National Primary Drinking Water Regulations, retrieved .
- U.S. EPA, Private Drinking Water Wells, retrieved .
- U.S. EPA, Consumer Tool for Identifying Point-of-Use and Pitcher Filters Certified to Reduce Lead, retrieved .
Expert review
Written by the HyrePlumber Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: System stages, removal scope, reject water, septic limit and buying questions come from Connecticut DPH Publication 21. Reject-water benchmarks and WaterSense criteria are from EPA WaterSense.
Pressure, temperature, membrane life and the nitrate worked example are from an archived 1990 MSU Extension bulletin, labeled as such. Limits are from EPA’s NPDWR table; well facts from EPA. Where Connecticut DPH and EPA differ on microbes, both are shown. Old dollar figures in the MSU bulletin are not used.
Suggest a correction. We fix errors and say what changed.