Reverse osmosis pushes water under pressure through a membrane with openings so small that water molecules pass and almost everything dissolved in the water does not. It's the only common residential technology that removes things carbon can't touch: nitrates, sodium, arsenic, fluoride, PFAS, lead and total dissolved solids.
It goes under the kitchen sink, not on the whole house. Two reasons — you don't need drinking-water purity to flush a toilet, and RO water is slightly aggressive toward copper pipe and appliances, so you don't want it in the whole distribution system.
The Delta Clarifi system I install most is tankless, which is the meaningful change in this category. Older tanked systems waste roughly four gallons for every gallon produced. The Clarifi runs about 2:1 and fits in about a quarter of the space, because there's no storage tank under the cabinet.
What the stages actually do
| Stage | Handles |
|---|---|
| Sediment prefilter | grit, rust |
| Carbon block | chlorine, VOCs |
| RO membrane | TDS, metals, PFAS |
| Carbon polish | taste, odor |
| Remineralizer | Ca, Mg, pH |
Chlorine destroys an RO membrane, so the carbon stage ahead of it isn't optional — it's what protects your most expensive cartridge. The Clarifi is certified to five NSF standards (58, 42, 53, 401 and 372) and tested against 90-plus contaminants.
"Doesn't RO strip the good minerals out?"
It does remove calcium and magnesium along with everything else — that's the mechanism, not a defect. But the amount of dietary mineral you get from tap water is small compared to food, so for most people it isn't a health issue.
It is a taste issue. Fully demineralized water tastes flat. That's what the remineralizer cartridge is for, and I'd generally recommend it — it puts calcium and magnesium back and brings the pH up. It's an optional cartridge on the Clarifi, and worth the money.
Maintenance and where it can run
- Carbon and RO cartridges on the Clarifi run about two years, and change in under a minute each.
- Indicator lights and an audible alert tell you when a cartridge is due.
- It can be plumbed to your refrigerator's water and ice dispenser, not just the sink faucet.
- The dedicated faucet comes in finishes that match Delta fixtures, so it doesn't look like an add-on.
- On well water, the RO goes at the end of the chain — after iron, hardness and sediment are already handled, or the membrane fouls fast.