Reverse osmosis

Reverse osmosis (RO) removes the widest range of dissolved contaminants of any common home treatment, which is why it is the usual answer for PFAS, arsenic, nitrate and fluoride in drinking water. EPA says RO is “typically more than 90 percent effective at removing a wide range of PFAS, including shorter chain PFAS.”

It is slow and sends water down the drain, so it is almost always installed at one tap rather than for the whole house; the Minnesota Department of Health calls RO “more practical as a point-of-use treatment option.” For most homes, a certified under-sink or countertop unit is cheaper than any whole-house system that claims similar results.

How it works

Household water pressure pushes water through a semi-permeable membrane with pores the CDC puts at about 0.0001 micron. Dissolved salts, metals and many other contaminants stay on the feed side and are flushed to the drain as reject water.

A typical system has several stages:

1. Pre-filters (sediment and carbon) remove particles and chlorine, which Nebraska Extension says extends membrane life.
2. The RO membrane does the main work.
3. A carbon post-filter polishes taste and catches chemicals the membrane lets through.
4. A storage tank, or a pump in tankless designs, supplies a dedicated faucet.

Only part of the incoming water becomes drinking water. EPA says a typical point-of-use system sends five gallons or more to the drain per gallon treated.

What it removes and what it does not

The CDC says RO removes lead, copper, chromium and sodium, and may reduce arsenic, fluoride, radium, nitrate and sulfate. EPA lists RO as removing many inorganics, dissolved solids, radionuclides such as uranium and synthetic organic chemicals. For home buyers:

• PFAS: including shorter-chain PFAS that carbon handles poorly.
• Lead and copper.
• Arsenic: certified claims under NSF/ANSI 58 cover pentavalent arsenic (arsenic V) only. Wells with arsenic III may need an oxidation step first.
• Nitrate: Penn State puts rejection at 83 to 92 percent, depending on pressure, temperature and membrane.
• Fluoride.

What it does not handle well:

• Dissolved gases such as hydrogen sulfide, and some pesticides, solvents and VOCs, which Nebraska Extension says can pass the membrane. The carbon stages cover many VOCs.
• Bacteria. The membrane can hold them back, but Nebraska Extension says RO “is not recommended” for microorganisms because bacteria damage the membrane. Feed it only coliform-free water.

Certifications to look for

• NSF/ANSI 58: the RO standard. Every certified system must reduce total dissolved solids by at least 75 percent. Other claims, such as arsenic, nitrate/nitrite, lead, fluoride, cysts and PFOA/PFOS, are optional and must be named on the listing.
• NSF/ANSI 53 and 42: for the carbon stages, covering claims such as VOCs, PFOA/PFOS and chlorine.
• NSF/ANSI 401: emerging compounds such as some pharmaceuticals.
• EPA WaterSense: the efficiency label, introduced in November 2024. Labeled systems send 2.3 gallons or less to the drain per gallon treated and have a membrane rated to last at least a year.

Read the listing, not the box. IAPMO certifies the AquaTru Classic to NSF/ANSI 58 for a long list including arsenic V, nitrate/nitrite, fluoride, copper and lead, and to NSF/ANSI 53 for PFOA and PFOS. The Waterdrop G3P800’s PFAS certification covers PFOA and PFOS only, and the Waterdrop X16’s PFAS figures are lab test data, not a certification. EPA notes current PFAS certifications do not yet show removal to its 4 ppt limits.

Installation and upkeep

• Countertop units such as the AquaTru need no plumbing, only an outlet. The AquaTru lists a feed-water hardness limit of 10 grains per gallon.
• Under-sink units need a cold-water feed valve, a drain saddle and a faucet or countertop hole. Waterdrop describes the G3P800 as a DIY job of about 30 minutes.
• Whole-house RO needs a large storage tank, a repressurizing pump and professional installation.

HomeGuide puts prefilter changes at every 6 to 12 months and membranes at every 2 to 3 years. Maker schedules match: AquaTru’s pre-filter every 6 months, VOC filter every 12 and membrane every 24; Waterdrop’s membrane every 24 months.

Costs

HomeGuide puts under-sink RO at $300 to $950 installed, countertop units at $200 to $450, and whole-house RO at $4,800 to $8,000. Prices checked September 2026:

System	Type	Price	Filters
AquaTru Classic	Countertop	$475	About $130 a year
Waterdrop G3P800	Under-sink, tankless	$999 list	About $160 a year

Over 10 years the AquaTru comes to about $1,773 ($475 plus 10 years of filters). For a comparison with whole-house systems, see whole-house filter vs reverse osmosis, our AquaTru Classic review, how to remove PFAS from water and the product finder.

Pros and limits

Pros

• Broadest removal of any common home treatment, including short-chain PFAS, arsenic, nitrate and fluoride.
• Certified models are easy to find and verify.
• Cheap at one tap compared with whole-house treatment.

Limits

• Sends several gallons to the drain per gallon treated, unless WaterSense-labeled.
• Slow; sized for drinking and cooking, not the whole house.
• Not a disinfection method; needs coliform-free feed water.
• Hard water and chlorine shorten membrane life without pre-treatment.