Water Science · Buying Guide
They look alike in the showroom. They cost dramatically different amounts. And they remove dramatically different things. Here's which one actually solves your water.
The short version
Carbon removes chlorine, taste, odor, some lead, and some PFAS. Reverse osmosis removes all of that plus dissolved solids, arsenic, nitrates, fluoride, and essentially all PFAS — at the cost of wastewater, counter space, and a higher price tag. Test your water, then decide.
Carbon filters work by adsorption — water flows over a very large surface area of activated carbon, and certain molecules stick to it. Good for organic compounds (chlorine byproducts, pesticide residues, PFOA, PFOS) and for taste and smell. Not good for dissolved inorganic compounds (nitrate, arsenic III, fluoride, TDS).
Carbon block filters are the denser, more tightly-packed version. They're what you want for lead reduction — loose granular carbon doesn't have enough dwell time to do the ion exchange reliably.
RO pushes water through a semipermeable membrane under pressure. The membrane pores are small enough (≈0.0001 μm) to exclude essentially everything that isn't a water molecule. Salts, metals, PFAS, microplastics, pharmaceuticals — all stay on the wrong side of the membrane and get flushed to drain.
RO systems always include carbon stages too, because the membrane needs protection from chlorine. So an RO system is really a carbon system plus a membrane.
| Contaminant | Carbon Block | Reverse Osmosis | Notes |
|---|---|---|---|
| Chlorine / chloramine | ● Full | ● Full | Either works. Carbon is cheaper. |
| Lead | ● Full (certified block) | ● Full | Carbon needs to be certified to NSF 53 specifically for lead. |
| PFOA / PFOS | ● Full | ● Full | Carbon works for the classic long-chain PFAS. |
| Short-chain PFAS (GenX, PFBS) | Partial | ● Full | The real gap between the two technologies. |
| Arsenic (III / V) | ○ None | ● Full | If you're on a well, you need RO. |
| Nitrates | ○ None | ● Full | Ag runoff. RO only. |
| Fluoride | ○ None | ● Full | Depends what you believe about fluoride. |
| Microplastics ≥1 μm | ● Full | ● Full | Both fine pore structures catch these. |
| Pharmaceuticals, PPCPs | Partial | ● Full | RO wins. Carbon catches some compounds. |
| TDS (total dissolved solids) | ○ None | ● Full | Mostly cosmetic — TDS isn't a direct health metric. |
Start by pulling your utility's most recent Consumer Confidence Report. Look at three lines: detected PFAS (and which ones), arsenic, and nitrate. If any of the three are flagged, or if PFAS includes anything other than PFOA/PFOS, go RO.
If your CCR is clean and your concerns are chlorine, taste, and lead (from old plumbing), a certified carbon block is simpler, cheaper, and equally effective. Don't buy filtration you don't need.
Our top carbon pick for these situations is the Aquasana Claryum 3-Stage, which we cover in full in our under-sink water filter guide.
For RO, the Waterdrop G3P800 is our editor's pick — tankless, fast, and non-detect on every PFAS compound we tested. Renters should look at the AquaTru Countertop instead.
Technically true — RO removes calcium and magnesium along with everything else. Nutritionally irrelevant for anyone eating a normal diet; you get dramatically more of these minerals from food than from tap water. If it bothers you, buy an RO system with a remineralization post-filter.
Carbon filters can harbor bacteria if you leave them wet and unused for weeks. In everyday use, water flow and residual chlorine keep them from becoming a problem. Follow the manufacturer's filter-change schedule and you're fine.
Older RO systems ran 3:1 or worse — three gallons down the drain for every gallon at the tap. Modern systems are 1:1 or better; the tankless systems we tested averaged 1.3:1. Still not zero, but small next to outdoor irrigation.
The short version
If your utility's CCR shows detected PFAS — go RO, and the Waterdrop G3P800 is our pick. If it's clean and you mostly want to cut chlorine and lead, the Aquasana Claryum is simpler and cheaper. Both are third-party certified and install without a plumber.
Full write-ups
See our best-of guide
Head-to-head lab data, pros and cons, and install notes for five under-sink PFAS systems.
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