The demonstration
Most of it is calcium and magnesium. It is harmless, and nobody selling you a filter wants to say so out loud.
Here is the arithmetic, then the part that actually matters.
What you are actually looking at
Total dissolved solids is the sum of everything dissolved in your water: calcium, magnesium, sodium, bicarbonate, silica. Boiling drives off the water and concentrates all of it until it crystallises out as scale.
A high TDS reading does not mean your water is dirty, and a low one does not mean it is safe. TDS does not detect PFAS, lead, bacteria, or industrial chemicals at the concentrations that matter. Those are measured in parts per trillion. Your meter reads parts per million.
This matters because a whole category of water marketing depends on you not knowing it. The classic version is the electrolysis demo, where two electrodes are dropped in a glass and the water turns brown. That colour is the iron electrode corroding. It happens in clean water too.
We are telling you this because a demonstration you can debunk is worthless to everyone, including us.
So what removes it
Leaves TDS essentially unchanged
Carbon adsorbs chlorine, many volatile organics, and (when certified for it) lead and PFAS. Dissolved mineral ions pass straight through. Test before and after a carbon pitcher and the meter barely moves.
Boil the filtered water and you get the same pot.
Removes 90 to 99% of TDS
RO pushes water through a semi-permeable membrane that rejects dissolved ions along with the contaminants you actually care about. It is the only common household method, other than distillation, that changes the boil test.
This is the only category we cover, for that reason.
Certification is what separates a real contaminant claim from a story. Look for NSF/ANSI 58 for reverse osmosis, and 53 for specific contaminant reduction including total PFAS. Note that NSF P473 no longer exists as a standalone protocol · it was folded into 53 and 58, so a product still advertising "P473 certified" is quoting a retired standard.
Countertop reverse osmosis
Prices captured from listings on 29 July 2026. We have not yet tested these units ourselves. When we have, this table will say what we measured and how.
| System | Price | Vessel | Certification as listed |
|---|---|---|---|
| Waterdrop C2SL | $159.99 | Glass pitcher | NSF/ANSI 372 |
| DREO countertop RO | $179.99 | Plastic | None listed |
| SimPure Y7T-A | $229.49 | Plastic | NSF/ANSI 58 |
| Bluevua RO100ROPOT-Lite | $299.00 | Borosilicate glass | WQA and NSF |
| AquaTru Carafe | $375.00 | Borosilicate glass | IAPMO to NSF 42, 53, 58, 401 |
| Bluevua RO100ROPOT | $389.00 | Borosilicate glass | WQA and NSF |
On money, before you click anything
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Check our work
It costs nothing and it takes one pot. If our diagram is wrong, this is how you catch us.
Two litres of cold tap water into a clean, dry, heavy-bottomed pan. Note the volume · the arithmetic depends on it.
Simmer uncovered until roughly a centimetre of liquid remains. Drop the heat near the end. Scorching adds carbon and ruins the read.
Lowest heat until the last of the water goes. What remains is the dissolved solids from two litres. In average US tap water that is about half a gram.
Same pan, same volume, same method. A carbon pitcher should look about the same. Reverse osmosis should look dramatically emptier. That difference is the entire claim.
Do not read this as a safety test. It measures dissolved minerals, not contamination. It is a demonstration of what a filter does and does not remove.
Same pan, same volume, tap against reverse osmosis, timelapsed, with meter readings on camera. Real footage · we do not simulate demonstrations. Leave an email and we will send it once, when it is done.