Do home water filters work?
Yes, for specific things. The single most useful fact about any filter is what it is certified to remove — which is almost never everything.
Home filtration works. The problem is that filters are marketed as general-purpose purifiers and sold on the impression that they remove contaminants broadly, when each technology removes a specific and quite narrow set.
What each type actually removes
| Type | Removes | Does not remove | Certification |
|---|---|---|---|
| Jug filter (carbon + ion exchange) | Chlorine taste, some metals, some hardness | Nitrate, fluoride, bacteria, most PFAS, salts | NSF/ANSI 42; 53 only if specifically certified |
| Carbon block (plumbed-in) | Chlorine, pesticides, THMs, lead if certified, long-chain PFAS | Salts, nitrate, fluoride, viruses, short-chain PFAS | NSF/ANSI 42 and 53 |
| Reverse osmosis | Almost everything dissolved: salts, nitrate, fluoride, arsenic, lead, PFAS, pathogens | Some dissolved gases; nothing if the membrane is damaged | NSF/ANSI 58 |
| Water softener | Calcium and magnesium hardness | Everything else; adds sodium | NSF/ANSI 44 |
| UV steriliser | Bacteria, viruses, protozoa | Anything dissolved; provides no residual protection | NSF/ANSI 55 |
| Ceramic filter | Bacteria, protozoa, sediment | Viruses, dissolved substances | Varies |
| Distiller | Salts, metals, nitrate, fluoride, pathogens | Volatile organics, which distil over | Varies |
Choosing by problem, not by product
- Chlorine taste
- Any carbon filter, including a jug. The cheapest problem to solve.
- Lead
- A carbon block specifically certified for lead, or reverse osmosis. Best solved by replacing the lead pipework.
- Hardness and limescale
- Ion exchange softener for the whole house, or RO at the drinking tap. Note softeners add sodium.
- Nitrate
- Reverse osmosis or a nitrate-selective ion exchange unit. Carbon does nothing.
- PFAS
- Reverse osmosis is the most reliable. Carbon works for long-chain compounds and much less well for short-chain ones.
- Bacteria on a private supply
- UV, with pre-filtration to reduce turbidity — UV cannot work through cloudy water.
- Everything, on an unknown supply
- Test first. Buying treatment before knowing the problem is how people end up with the wrong technology.
Two things that go wrong
The first is cartridge replacement. Carbon has a finite adsorption capacity, and when it is exhausted it removes nothing. There is no visible change, no taste change and no warning. A long-overdue cartridge is a filter housing full of nothing.
The second is bacterial growth. A damp carbon bed full of adsorbed organic matter is a good environment for bacteria, and removing the chlorine residual removes the protection against them. Filtered water should be refrigerated and used within a day or two, and jug filters should not be left standing warm.
Sources
- NSF International / American National Standards Institute — NSF/ANSI drinking water treatment unit standards. Public but restricted · Publisher terms — public access, reuse not clearly granted
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO