Reverse Osmosis

The most comprehensive residential water filtration technology, removing 95-99% of dissolved contaminants by forcing water through a semipermeable membrane.

Updated: 2026-05-16Written by: TapWaterGuide Editorial Team

What does reverse osmosis remove from water? Reverse Osmosis is effective at removing Lead, PFAS (forever chemicals), Arsenic, Chromium-6. It does NOT remove Dissolved gases (radon, hydrogen sulfide) without additional treatment, Does not soften whole-house water (point-of-use only for most systems), Volatile organic compounds require carbon pre-filter (which most systems include). Residential systems typically cost $150–$5000 (Under-sink: $150-800. Tankless: $500-900. Whole-house: $2,000-5,000+) and are available as under-sink, countertop, whole-house configurations.

Last updated: 2026-05-16 · Source: NSF International, manufacturer testing data

How Reverse Osmosis Works

Reverse osmosis works by applying pressure to push water through a semipermeable membrane with pores approximately 0.0001 microns in size — small enough to block virtually all dissolved contaminants while allowing water molecules to pass through. A typical residential RO system has 3-5 stages: a sediment pre-filter to protect the membrane, an activated carbon pre-filter to remove chlorine (which damages RO membranes), the RO membrane itself, and one or two post-filters for polishing taste. Some systems add a remineralization stage to add back beneficial minerals for improved taste. The rejected contaminants are flushed away in a waste stream.

What It Removes — and What It Doesn't

Effectively removes

✓ Lead
✓ PFAS (forever chemicals)
✓ Arsenic
✓ Chromium-6
✓ Fluoride
✓ Nitrate
✓ Uranium
✓ Radium
✓ Total Dissolved Solids (TDS)
✓ Sodium
✓ Pharmaceuticals
✓ Microplastics
✓ Bacteria and viruses

Does NOT remove

✗ Dissolved gases (radon, hydrogen sulfide) without additional treatment
✗ Does not soften whole-house water (point-of-use only for most systems)
✗ Volatile organic compounds require carbon pre-filter (which most systems include)

Pros and Cons

+ Most comprehensive contaminant removal of any residential technology
+ Removes dissolved contaminants that carbon filters cannot (nitrate, fluoride, arsenic, TDS)
+ NSF 58 certification standard provides strong third-party verification
+ Modern tankless systems offer high flow rates and minimal space requirements
+ Relatively low ongoing maintenance costs for the breadth of protection
− Wastes 2-4 gallons of water per gallon produced (improving with newer models)
− Removes beneficial minerals along with contaminants (can be addressed with remineralization)
− Slower flow rate than carbon filters, especially tank-based models
− Higher upfront cost than carbon-only systems
− Requires adequate water pressure (40+ PSI) to function efficiently
− Under-sink only in most residential installations — does not treat shower or laundry water

Cost

System cost
$150–$5000

Under-sink: $150-800. Tankless: $500-900. Whole-house: $2,000-5,000+

Ongoing maintenance
$50-100/year for filter replacements. Membrane replacement every 2-3 years ($30-80).

NSF Certifications to Look For

NSF 58 (Reverse Osmosis)NSF 42 (Aesthetic — taste/odor)NSF 53 (Health Effects)NSF 401 (Emerging Contaminants)

Always verify NSF certification on NSF's official database. Manufacturer claims without certification should be treated with skepticism.

Best Reverse Osmosis Systems

We've compared the top reverse osmosis systems on removal performance, NSF certifications, and total cost of ownership.

View our top picks →

Frequently asked questions