Water
Reverse Osmosis
RO
A treatment process that forces water through a semi-permeable membrane under pressure, leaving most dissolved substances behind. It produces a reject stream, so some water is discharged for every volume treated.
In more detail
Reverse osmosis reverses the direction water would naturally move across a semi-permeable membrane. Pressure on the feed side pushes water molecules through while most dissolved ions, larger organic molecules and many contaminants are held back and carried away in a reject stream.
The consequences of that mechanism define the technology. Rejection is broad rather than selective: the membrane does not distinguish a contaminant from a dissolved mineral, so it removes both. It needs pressure, which is why undersink systems are slow and usually paired with a storage tank. And it discharges water — the recovery ratio, the share of feed water that leaves as product, is a real operating cost and a real number to ask about.
A residential unit is a train rather than a single element. Sediment and carbon prefilters protect the membrane, the membrane does the separation, and a post-filter polishes the stored water. Rejection is stated per contaminant on a new membrane under test conditions, and it degrades as the membrane fouls, which makes service intervals part of the specification rather than an afterthought.
Why it matters
RO is the most capable point-of-use treatment available to a house and the one most often installed without deciding what it is for. It is the right answer to specific dissolved contaminants and an expensive answer to water that was already fine. It also changes the water: low-mineral water tastes flat to some people and is more aggressive toward some plumbing materials, which is why remineralization stages exist. Deciding after a water test costs far less than deciding after installation.
The common misunderstanding
That RO removes everything. Rejection is high for most dissolved salts and many organic compounds and lower for some small, uncharged molecules — which is why residential systems pair the membrane with carbon stages rather than relying on it alone. Published rejection figures also describe a new membrane at test pressure, not a permanent property of the installed system.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.