What reverse osmosis actually removes
Updated August 2026 · Certification claims referenced against NSF/ANSI 58
When somebody says they have filtered water, they usually mean a small unit under the kitchen sink with its own little faucet beside the main one. That is reverse osmosis, and it is the strongest drinking water treatment you can put in a house. It is also the piece people are most likely to buy for the wrong reason. Here is what it does in plain terms.
How it works
Your house pressure pushes water against a thin sheet of plastic film rolled up inside a cartridge. The openings in that film are smaller than the dissolved minerals, metals, and salts the water is carrying. Picture a screen so fine that a single water molecule slips through and almost nothing else can follow it. Clean water collects on the far side and fills a tank or a line to the faucet. Whatever gets turned away rinses off the film and leaves through a small drain line.
Carbon does the other half of the work. Every real RO unit includes carbon stages ahead of the membrane and usually one after it. The carbon protects the film from chlorine and chloramine, and it handles taste on the way out.
What it takes out
The certification is where the real answers live. NSF/ANSI 58 is the standard written specifically for reverse osmosis, and a certified system carries claims only for the contaminants it was actually tested against. NSF/ANSI 53 covers the health claims a carbon block can earn on its own.
Metals matter most in older houses. Lead reduction is a standard NSF/ANSI 58 claim, and in most homes the lead enters the water from the pipes and fixtures inside the house. Certified RO systems also carry claims for arsenic, chromium, and copper. Several small St. Tammany systems have published lead results above the federal action level of 15 parts per billion in recent monitoring, including one Covington-area system with a 90th percentile result of 39.2 ppb. Certified filtration at the drinking tap is the piece built for that exact problem.
Dissolved solids are the second thing the membrane handles, and that covers the whole mineral load including sodium, fluoride, and nitrate. This is the story on a lot of Northshore groundwater. The River Park system in Covington publishes sodium at 202 milligrams per liter with a pH of 9.16. Rigolets Estates in Slidell publishes 160. Dissolved mineral at those levels is what leaves the white film on a glass after it dries, and anybody watching sodium at the dinner table should be watching the water too. On Mississippi River water in Metairie and Kenner, hardness runs about 78 milligrams per liter.
Taste is what most people notice first. The carbon stages strip the disinfectant, and around here that is usually chloramine. New Orleans runs a chloramine residual around 3.3 parts per million, while a swimming pool is kept between 1 and 3. We wrote about what's actually in New Orleans tap water and why your tap water tastes like a pool if you want the longer version.
PFAS is worth asking about directly. Systems certified for PFAS reduction exist, and the way to know whether a specific unit qualifies is to ask for the certification on that exact model.
What it leaves alone
An RO system covers a narrow slice of the house.
- It serves the single tap it is plumbed to, so every shower, every load of laundry, and every other faucet in the house gets untreated water. Covering the whole house is the job of a whole-home carbon system, and both pieces are on what we install.
- Bacteria on a private well call for ultraviolet treatment, because an RO membrane is not the primary answer for that problem.
- Some water goes down the drain. A typical home unit sends a few gallons to drain for every gallon it makes, and efficient modern units do considerably better than the old ones.
Tank or tankless
A tank system stores its finished water in a small pressurized tank in the cabinet, usually holding two or three gallons. You get water the instant you open the faucet because it is already made and waiting. The tradeoff is cabinet space, and the flow slows down as the tank empties during a big pot fill.
A tankless system skips the tank and uses a booster pump with a higher-flow membrane to make water on demand. It frees up the cabinet and it gives you fresher water at a steadier rate. It needs an electrical outlet under the sink, and it costs more up front.
What maintenance looks like
Carbon cartridges get changed once a year on most residential systems. The membrane itself lasts several years, typically three to five, and how long depends on how hard your feed water works it. Well water with sediment or iron needs pretreatment ahead of the unit and more frequent prefilter changes. A cheap handheld meter tells you when the membrane is fading, because the dissolved solids reading starts climbing back toward the raw water number. Our ownership page lays out the full schedule.
Questions people ask
Does RO remove healthy minerals? Yes, it takes out most of the mineral content along with everything else. People get the overwhelming majority of their dietary calcium and magnesium from food. If you miss the mineral taste, a remineralization cartridge adds a measured amount back before the water reaches the faucet.
Why does RO water taste different? Minerals carry flavor, and taking them out leaves the water tasting cleaner and lighter than what you are used to. Most people adjust within a week. Coffee and tea are usually where the change is most obvious.
Does it work on well water? It works well on dissolved contaminants in well water. Wells need the supporting pieces to go with it, meaning sediment and iron filtration ahead of the membrane and UV if bacteria is a concern.
Do I still need a whole-home filter? That depends on what your utility reports. Whole-home carbon does more for you when the issue is disinfectant taste at every faucet in the house. RO is the piece that matters when the issue is lead or dissolved solids in the water you drink and cook with.
Find out whether RO is the right piece for your house
Every system on the Northshore and around New Orleans publishes its own numbers, and those numbers decide whether reverse osmosis belongs in your plan or whether your money is better spent elsewhere. Get the report for your address and see what your water is actually carrying.