Reverse osmosis is the broadest treatment a household can fit at a tap, and the way it works explains the pattern of what it removes. The membrane rejects dissolved material rather than adsorbing selected compounds, so its coverage is defined by molecular size and charge rather than by chemistry, which produces a wide list with a few notable gaps.
Dissolved salts and metals
Sodium and chloride, which is why reverse osmosis is the technology used for brackish and saline supplies. Calcium and magnesium, so the treated water is soft. Dissolved metals including lead, copper, and arsenic in its charged form. Nitrate, which matters particularly for infants and which almost nothing else in a household lineup removes. Fluoride. And total dissolved solids generally, which is why a meter comparison across the membrane is the standard way to check that it is still working.
Particles and organisms
Anything larger than the membrane's effective pore is excluded physically, which covers suspended solids, bacteria and protozoan cysts comfortably. In practice most of that work is done by the sediment prefilter before the water reaches the membrane. It is worth noting that a system is not usually sold or certified as a microbiological treatment, because a compromised membrane or a fitting downstream could pass organisms without any visible sign, so it is not a substitute for disinfection on a contaminated supply.
What it does not remove
Chlorine, which is removed by the carbon prefilter instead, and which must be, because chlorine degrades the membrane permanently. Some small uncharged molecules and dissolved gases can pass, which is why systems include a polishing carbon filter after the tank for taste. And it removes nothing at all from the water in the rest of the house, since it treats one tap: showers, laundry and the water heater see the untreated supply and any hardness complaint there remains.
Proving it is still doing it
Performance declines gradually and silently as a membrane ages, so the useful check is a comparison rather than a single reading: take a dissolved solids measurement of the feed water and another of the treated water sampled before the storage tank. A wide gap means the membrane is rejecting properly, and converging readings mean it is spent. Change prefilters on schedule, because they are what keep it alive. Our reverse osmosis cost guide covers the systems and their upkeep.
Questions people ask about what does the reverse osmosis device remove
Does reverse osmosis remove nitrate?
Yes, which is notable because very little else in a household lineup does. It also removes fluoride, lead and dissolved salts.
Does it remove chlorine?
The carbon prefilter does, and it must, because chlorine degrades the membrane permanently.
Is it a treatment for bacteria?
It excludes them physically, but systems are not usually certified as microbiological treatment, so it is not a substitute for disinfection.
How do I know it is still removing anything?
Compare dissolved solids in the feed with the treated water sampled before the tank. Converging readings mean the membrane is spent.