Reverse Osmosis (RO)
A water treatment process that forces water through a semipermeable membrane to remove up to 99% of contaminants — including lead, PFAS, nitrate, arsenic, and most other dissolved substances.
How It Works
Reverse osmosis is one of the most effective water treatment technologies available. It works by applying pressure to push water through a membrane with pores so small that most dissolved contaminants cannot pass through. RO removes: PFAS (>95%), lead (>95%), nitrate (>90%), arsenic, fluoride, bacteria, viruses, and most dissolved solids. The drawback is waste water — RO systems typically produce 3-4 gallons of waste for every gallon of treated water. Point-of-use RO systems (under-sink units costing $200-500) provide treated water at a single tap. Whole-house RO systems are available but more expensive. Many water systems use RO at the treatment plant level, particularly in areas with high dissolved solids or PFAS contamination.
Related Terms
- Activated Carbon Filtration — A water treatment method using porous carbon material to adsorb contaminants — effective for removing chlorine, many organic chemicals, some PFAS, and improving taste and odor.
- PFAS (Forever Chemicals) — Per- and polyfluoroalkyl substances — a class of thousands of synthetic chemicals that don't break down in the environment, accumulate in the human body, and are linked to cancer, immune disorders, and developmental problems.
- Lead in Drinking Water — Lead contamination from aging pipes, solder, and fixtures — there is no safe level of lead exposure, and even low levels can cause irreversible developmental damage in children.
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About This Definition
This definition is part of the IsWaterSafe Drinking Water Safety Glossary — 22 terms explaining water contaminants, treatment methods, and safety standards. Written for homeowners, renters, journalists, and public health professionals.