Reverse osmosis is the only domestic treatment that removes dissolved inorganic contaminants at scale. If your water test shows nitrate, arsenic, fluoride or high total dissolved solids, RO is the answer and carbon filtration is not. If it shows none of those, RO is usually more system than your water requires.
That is the whole decision, and it should be made from a test result rather than from a sense that more filtration is better.
How it works
Mains pressure forces water through a semi-permeable membrane with pores small enough to reject dissolved ions. Water molecules pass; most dissolved salts, metals and larger molecules do not. NSF/ANSI 58 is the standard covering reverse osmosis systems, which reduce contaminants regulated by the EPA and Health Canada.
A domestic system is a train of stages rather than a single filter:
- Sediment pre-filter — protects everything downstream from particles.
- Carbon pre-filter — removes chlorine, which damages common membrane materials. Skipping or neglecting this stage destroys the membrane.
- RO membrane — the stage doing the dissolved-solids work.
- Storage tank — because production is slow, treated water is accumulated under pressure.
- Carbon post-filter — polishes taste after storage and catches anything that passed the membrane.
Understanding stage two matters practically: a neglected carbon pre-filter is the most common cause of premature membrane failure, and the membrane is the expensive part.
What it removes, and what it does not
RO is strong on dissolved inorganics — nitrate, arsenic, fluoride, many heavy metals, and total dissolved solids generally. It also carries PFOA, PFOS and Total PFAS reduction claims under NSF/ANSI 58, which absorbed those claims from the earlier NSF Protocol P473.
It is weaker than people assume on some organics. Small, volatile organic molecules can pass a membrane more readily than large dissolved ions, which is why a carbon post-filter is part of the design rather than an upsell. In a well-designed system the carbon stages handle the organics and the membrane handles the inorganics — they are complementary, not redundant.
It does not disinfect. RO membranes reject organisms physically, but an RO system is not a substitute for NSF/ANSI 55 Class A ultraviolet treatment on a microbiologically unsafe source. On a well with bacterial contamination, UV is the stage that addresses it.
The costs, stated plainly
Wastewater. Rejecting dissolved solids means concentrating them into a reject stream that goes down the drain. Domestic systems produce several gallons of reject for each gallon of treated water, varying by design and by incoming pressure. In a drought-prone area this is a genuine consideration, and it is the specification manufacturers describe most vaguely. Look for a stated recovery ratio.
Production rate. An RO membrane produces slowly — a matter of gallons per day, not per minute. That is why the tank exists. Draw a large volume quickly and you empty the tank, after which flow at the tap drops to the membrane’s production rate. Filling a stockpot is a noticeably slow business.
Space. A sediment filter, a carbon filter, a membrane housing, a post-filter and a pressure tank. Under a typical sink, that is most of the cabinet.
Plumbing. A cold feed connection, a drain saddle for the reject line, and usually a separate dedicated tap through the worktop or sink. Also a drain air gap in many jurisdictions — check local plumbing code, or have it fitted by a plumber.
Multiple replacement schedules. Pre-filters typically every six to twelve months, post-filter annually, membrane every two to five years. Miss the pre-filters and you shorten the membrane’s life, so the cheap consumables protect the expensive one.
Pressure dependence. Membrane performance falls with incoming pressure. Low household pressure means slower production and poorer rejection. A permeate pump or booster pump addresses it, reduces wastewater, and adds cost and complexity.
Two things frequently claimed, and what is actually true
“RO removes beneficial minerals.” It does — calcium and magnesium are dissolved solids and the membrane rejects them along with everything else. Whether that matters nutritionally depends on your overall diet rather than your water, and it is a question for a dietitian rather than for us. What is not in doubt is the taste: water with almost no dissolved solids tastes flat to most people, and that is why remineralization cartridges exist. They add minerals back for taste, and are a preference rather than a health measure.
“A TDS meter tells you if the system is working.” Partly. A TDS meter measures total dissolved solids, so it will show you the membrane is rejecting dissolved solids, and a rising reading over time is a genuine warning that the membrane is failing. What it cannot do is tell you whether a specific contaminant is being reduced — arsenic at a few parts per billion is invisible to a TDS reading. Use it as a membrane health indicator, not as a safety test.
When RO is the right choice
| Your test shows | RO? |
|---|---|
| Nitrate above the limit | Yes — carbon cannot address it |
| Arsenic | Yes, with attention to which arsenic species and the system’s specific claim |
| Fluoride you wish to reduce | Yes — one of the few effective domestic options |
| High TDS, brackish or salty taste | Yes |
| PFAS | Either RO under NSF/ANSI 58, or carbon or anion exchange under NSF/ANSI 53 — choose on the named claim |
| Lead only | No — a certified carbon block is simpler, faster and cheaper to run |
| Chlorine taste only | No — carbon, at a fraction of the cost and none of the wastewater |
| Cysts only | No — a carbon block with a cyst claim |
| Bacteria on a well | Not alone — UV Class A is the stage that handles this |
| Nothing of concern | No |
If you buy one
- Check it is certified to NSF/ANSI 58 and read the performance data sheet for the contaminants you care about, not the standard number alone.
- Find the recovery ratio — treated water against reject — before buying, not after.
- Check your incoming pressure, and budget for a permeate pump if it is low.
- Measure the cabinet. Including the tank, upright.
- Confirm the drain connection and whether your local code requires an air gap.
- Price all the cartridges plus a membrane over five years.
- Set reminders for the pre-filters. They are what protect the membrane.
- Take a TDS baseline when the system is new, so a later rise means something.
Related guides
- How to choose a water filter, starting from your own water
- NSF/ANSI 42 vs 53 vs 401
- Pitcher, faucet, under-sink or RO: choosing by contaminant
Sources: NSF, Standards for Water Treatment Systems, for the scope of NSF/ANSI 58 and of NSF/ANSI 55 Class A; NSF, Forever Chemicals and the Advancement of Filtration Standards, for the PFAS claims in Standards 53 and 58 and their origin in Protocol P473. Wastewater ratios, production rates and replacement intervals are typical ranges that vary by design — take the figures for any system you are considering from its own documentation. Nutritional questions about mineral content are outside our scope and belong with a qualified professional.