Reverse osmosis (RO) is one of the most effective technologies available for reducing a wide range of dissolved contaminants in drinking water. Understanding how it works—and its tradeoffs—helps homeowners decide if an RO system is the right fit for their kitchen or point-of-use needs.
The Basics: What Is Reverse Osmosis?
Osmosis is a natural process in which water moves through a semipermeable membrane from a less concentrated solution to a more concentrated one until balance is reached. In reverse osmosis, pressure is applied to the more concentrated side (your tap water) so that water is forced through the membrane in the opposite direction. The membrane has extremely tiny pores—small enough to block many dissolved salts, minerals, metals, and other contaminants while allowing water molecules to pass through. The result is two streams: treated water (permeate) for drinking and a concentrate stream (reject) that carries the removed substances away, usually to the drain.
RO is used at scale in desalination and industrial water treatment, and in smaller systems for residential drinking water. Home systems typically sit under the kitchen sink and deliver filtered water through a dedicated faucet.
How a Typical Home RO System Is Built
A standard under-sink RO system usually includes:
- Prefilters: Water first passes through one or more sediment and carbon filters. These protect the membrane from particles and chlorine (which can damage the membrane) and improve taste.
- RO membrane: The core stage. Pressurized water is pushed through the membrane; many dissolved contaminants are rejected and sent to the drain with a portion of the water.
- Storage tank: Many systems use a small pressurized tank so you get water on demand without waiting for the membrane to produce it in real time.
- Postfilter: A final carbon or polishing filter can improve taste of stored water before it reaches the faucet.
Flow and pressure in your home affect production rate; that’s why a tank is common for residential use.
What Reverse Osmosis Removes (And Doesn’t)
RO is very effective at reducing many dissolved inorganic and organic contaminants, including:
- Dissolved salts and minerals (e.g., sodium, calcium, magnesium)
- Heavy metals such as lead, copper, arsenic, chromium
- Nitrates and nitrites
- Many bacteria and viruses (by size exclusion; RO is not a disinfection device, so pairing with a disinfecting step may be recommended for non-municipal water)
- When certified for it, PFAS such as PFOA and PFOS (check NSF/ANSI 58 and product claims)
It does not remove all contaminants. Volatile chemicals that readily turn into gas (e.g., some VOCs) may pass through; some systems include additional carbon or venting. RO also does not disinfect; for well water or other non-disinfected sources, additional treatment may be required. Always match the system to your water quality and look for certifications that apply to the contaminants you care about (e.g., NSF 58 for lead, PFOA/PFOS).
Waste Water: The Reject Stream
RO produces a concentrate stream that must go somewhere—usually the drain. The ratio of treated water to waste water depends on water pressure, temperature, and system design; 1:1 to 3:1 (three gallons treated per gallon of reject) is a common range. So “waste” is a real consideration: more efficient systems and proper maintenance can improve the ratio. Some homeowners reuse reject water for irrigation or flushing if local codes and practicality allow, but it should not be used for drinking. When evaluating an RO system, ask about the efficiency ratio and how it may change with your water conditions.
Maintenance: Filters and Membrane
For RO to perform as designed:
- Prefilters and postfilter need to be replaced on the manufacturer’s schedule (often every 6–12 months). Skipping changes can reduce flow, worsen taste, and shorten membrane life.
- The RO membrane typically lasts longer—often 2–3 years or more—but should be replaced when flow drops or quality degrades. Some systems have indicators or reminders.
Using certified replacement cartridges and membranes keeps the system within its certified performance claims.
When Reverse Osmosis Is a Good Fit
RO is a strong choice when:
- You want the highest level of reduction of dissolved contaminants at a single faucet (drinking and cooking).
- Your water has elevated lead, arsenic, nitrates, or PFAS and you want a certified at-home solution.
- You’re okay with under-sink installation, a dedicated faucet, and periodic maintenance.
- You accept the reject stream and are fine with the water use (or have a more efficient unit).
It may be less ideal when:
- Your main concern is taste/odor/chlorine only; a carbon filter might be enough.
- You need filtered water at many taps; RO is usually point-of-use, though whole-house RO exists at higher cost and complexity.
- Water pressure is very low; some RO systems need a minimum pressure or a booster pump.
Summary
Reverse osmosis uses a semipermeable membrane and pressure to separate water from many dissolved contaminants, producing a treated stream for drinking and a concentrate stream to the drain. Home systems typically include prefilters, membrane, storage tank, and postfilter. RO is highly effective for many dissolved contaminants when properly maintained and certified; the tradeoffs are under-sink space, waste water, and maintenance. Matching the system to your water quality and goals—and keeping up with filter and membrane changes—ensures you get the benefits of RO where they matter most: at the tap.