Reverse Osmosis

Reverse osmosis (RO) is a water purification process that forces pressurized tap water through a semi-permeable membrane with pores small enough to block dissolved salts, heavy metals, chlorine, fluoride, nitrates, and most other contaminants — producing water that is 95–99% free of dissolved solids. The filtered water goes to a storage tank; the concentrate (brine) with the rejected contaminants drains away.

How a residential RO system works

Most under-sink RO systems use a multi-stage process:

1. Sediment pre-filter: removes particles (rust, dirt, sand) that would clog or damage the membrane.
2. Carbon pre-filter: removes chlorine and chloramines that would degrade the RO membrane.
3. RO membrane: the primary filtration stage — blocks dissolved solids, heavy metals, bacteria, and most viruses.
4. Post-carbon filter: polishes taste and removes any remaining off-flavors from the storage tank.
5. Storage tank: holds 2–4 gallons of purified water ready for use.
6. Dedicated RO faucet: a small faucet at the sink dispenses purified water only.

What RO removes

RO membranes reject: lead (up to 98%), arsenic (up to 95%), fluoride (85–92%), nitrates (up to 94%), sodium (up to 95%), chlorine (when pre-filter is maintained), TDS (total dissolved solids) broadly. RO does NOT reliably remove volatile organic compounds (VOCs), some pesticides, or dissolved gases — a carbon filter stage handles those.

Maintenance

Pre- and post-filters need replacement every 6–12 months ($15–$40 each). The RO membrane lasts 2–5 years ($30–$100). Annual filter maintenance is the primary ongoing cost.

Cost

Under-sink RO systems cost $150–$600. Professional installation runs $200–$400. The system uses 3–4 gallons of water to produce 1 gallon of purified water (the rest drains) — a consideration in water-scarce areas.