Reverse Osmosis
A pressure-driven filtration process that pushes water through a semipermeable membrane, leaving dissolved minerals, salts, and most contaminants behind in a concentrate stream.
Reverse osmosis is the polishing stage that separates serious water-purification systems from carbon-only filtration. The membrane has pores small enough (effectively ~0.0001 µm) to reject monovalent ions like sodium and chloride — well past what any depth filter or carbon block can do.
How it works
In ordinary osmosis, water moves from a low-solute side of a membrane to a high-solute side. Reverse osmosis applies pressure on the high-solute (incoming tap-water) side, forcing pure water through the membrane in the opposite direction. The dissolved material is left behind in a small "concentrate" or "reject" stream, which goes down the drain.
What it removes
A healthy RO membrane reduces:
- Total Dissolved Solids by 90–99%
- Heavy metals (lead, arsenic, chromium) by 95–99%
- PFAS compounds by 90–99% — RO is the only widely-available residential method that meaningfully removes them
- Fluoride, nitrate by 85–95%
It does not sterilize water — viruses can occasionally pass undamaged membranes, so RO is rarely the right answer for microbially compromised supplies.
What it doesn't do
RO removes minerals indiscriminately, which is why some systems re-mineralize the output for taste. The membrane is also slow — typical residential systems produce 50 gallons of permeate per day, which is why every household RO has a small storage tank.