Water filter type
Reverse osmosis, tank
A membrane system under the sink that produces treated water to a dedicated tap and stores it in a small pressure tank. The widest contaminant coverage of any residential option, at one tap.
Why reverse osmosis covers more than anything else
Reverse osmosis pushes water through a semi-permeable membrane with pores small enough to reject dissolved ions. Where carbon works by adsorption, catching molecules on a vast internal surface area, a membrane works by exclusion. That difference is why the contaminant list is so much longer.
Nitrate, arsenic, fluoride, dissolved solids, and PFAS all pass through carbon but are rejected by a membrane. For drinking water, this is the broadest residential option available, and it is why we recommend it when a test has found something specific rather than a general taste complaint.
A tank system stores treated water in a small pressure vessel under the sink so you get a normal flow rate at the dedicated tap. Production itself is slow, a few gallons an hour, which is why the tank exists.
The wastewater question, answered honestly
Reverse osmosis sends water to drain. Typical residential systems send somewhere between two and four gallons to drain for each gallon they produce, and older or poorly maintained units can be worse.
People are often surprised by this, so we raise it before the quote rather than after the install. In practice the volume is small next to a shower or a washing machine cycle, but it is real and it appears on a metered bill.
Permeate pump models reduce the ratio meaningfully and cost more up front. Tankless models with booster pumps are also generally more efficient. If the waste ratio matters to you, say so and we will specify accordingly.
What it does not do, and the safety limit
It treats one tap. Your shower, bath, and washing machine are unaffected, so if the goal is scale control or chlorine removal throughout the house, this is the wrong tool on its own.
It does not soften the rest of the building, so a water heater on hard water keeps scaling.
Most importantly, a standard reverse osmosis system is not a microbiological barrier and must not be used to make unsafe water safe. A membrane can develop a flaw that gives no visible sign. If the supply is not microbiologically safe, that must be addressed separately, usually with ultraviolet disinfection, before any of this matters.
What it reduces, and under which standard
Each row names the NSF/ANSI standard the category is tested under. A standard describes what the test covers. It does not mean every unit on the market carries that certification, so check the specific model against its listing before you buy.
| Contaminant | Standard | Note |
|---|---|---|
| Total dissolved solids | NSF/ANSI 58 | |
| Lead | NSF/ANSI 58 | |
| Nitrate and nitrite | NSF/ANSI 58 | |
| Arsenic V | NSF/ANSI 58 | Arsenic III must be oxidised first or it passes the membrane. |
| PFOA and PFOS | NSF/ANSI 58 | |
| Fluoride | NSF/ANSI 58 | |
| Cysts including cryptosporidium and giardia | NSF/ANSI 58 |
What it does not do
Every filter has a boundary. These are the things this one will not fix, listed so you can rule it out quickly if your problem is on the list.
- Water at any tap other than the one it feeds. Showers and laundry are untouched.
- Hardness in the rest of the house, so scale continues elsewhere.
- Bacteria, unless the unit is specifically certified for it. Do not treat unsafe water with RO alone.
- Dissolved gases such as hydrogen sulfide, which pass through the membrane.
When it fits
- You want the broadest drinking water coverage at the kitchen tap
- A test has found nitrates, arsenic, or PFAS in the supply
- Total dissolved solids are high enough to affect taste
When it does not fit
- Your only issue is chlorine taste, where a carbon filter costs far less
- There is no cabinet space or no drain connection
- You object to the wastewater ratio, which is typically 2 to 4 gallons sent to drain per gallon produced
Space, services, and upkeep
Most of an under-sink cabinet. The storage tank is roughly 15 inches tall.
A drain connection is required. It needs no power. This is not a rental option, because it fixes to the building and stays there.
Replacement interval. Pre-filters and post-filter every 6 to 12 months. Membrane every 2 to 5 years depending on feed water.
Problems this is used for
We recommend this equipment against these confirmed problems. Each page explains how to confirm the problem before spending anything, and when treatment is not warranted.
- Chlorine and chloramine taste. Utilities add a disinfectant residual and are required to maintain it through the distribution network.
- Lead in drinking water. Lead does not normally come from the source water.
- PFAS. A large family of manufactured fluorinated compounds used in firefighting foam, non-stick coatings, and industrial processes.
- Nitrates. Nitrate enters groundwater from fertiliser, manure, and septic systems.
- Arsenic. A naturally occurring element in bedrock across parts of the Northeast.
Property types where this comes up
- Brownstones and pre-war buildings
- Single-family homes on city water
- New construction
- Offices
- Medical and dental practices
Common questions
Does reverse osmosis remove beneficial minerals?
Yes, it removes most dissolved minerals along with the contaminants, because the membrane rejects by size rather than by whether something is desirable. In dietary terms the contribution of drinking water to mineral intake is small next to food, so for most people this is not a practical concern. If it bothers you, remineralising cartridges are available as a final stage and some systems include one as standard.
How much water does it waste?
A typical residential system sends two to four gallons to drain for each gallon it produces, and older or poorly maintained units can be worse. A permeate pump or a pumped tankless model improves that ratio significantly. We raise this before quoting rather than after installing, because people are reasonably surprised by it and it does show up on a metered bill.
Can it make unsafe water safe to drink?
No, and it should never be relied on for that. A standard reverse osmosis system is not certified as a microbiological barrier, and a membrane can develop a flaw with no visible symptom. If the supply is not microbiologically safe, that has to be addressed separately, normally with ultraviolet disinfection certified to NSF/ANSI 55 Class A, before anything else is considered.
Other filter types to compare
If the notes above rule this out, these are the neighbouring options worth reading before you decide.
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