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Reverse Osmosis vs Carbon Filter (2026): What Each Removes at the Kitchen Tap, Arsenic, Nitrate, Uranium, Sodium and Lead, Waste Water, Costs and Upkeep
Plumbing & Water
Reverse Osmosis or Carbon: Two Tools, and Only One Fixes the Scary Numbers
A carbon filter makes water taste better. A reverse osmosis unit takes things out of it that you cannot taste at all. Both fit under a kitchen sink and both get sold as “pure water”, which is how people end up with a carbon filter in front of an arsenic problem. This page covers what each one removes, what RO costs to run, why almost nobody puts RO on the whole house, and which lab results call for which.
Two Different Tools
Carbon is a sponge with an enormous surface. Taste, odour, chlorine from town water and a range of organic chemicals stick to it as the water goes through. It does nothing to anything dissolved like salt: nitrate, sodium and hardness walk straight past it.
Reverse osmosis pushes water through a membrane so tight that most dissolved things get left behind and flushed to the drain. RO units usually have carbon and sediment stages in front of the membrane to protect it, so an RO is really carbon plus a membrane.
| In your lab report | Health Canada guideline | Carbon | Reverse osmosis |
|---|---|---|---|
| Arsenic | 0.010 mg/L | no | yes |
| Nitrate | 10 mg/L as N | no | yes |
| Uranium | 0.02 mg/L | no | yes |
| Sodium | 200 mg/L aesthetic | no | yes |
| Lead | 0.005 mg/L | only filters rated for it | yes |
| Chlorine taste and odour | town water | yes | yes, in its carbon stage |
| Bacteria | E. coli 0 | no | not a substitute for UV or a fixed well |
Health Canada’s guideline summary table, December 2025. The removal columns are general; what a particular unit removes is on its own rating and spec sheet. Test the water after it goes in.
What It Costs
| Equipment | Installed | Upkeep |
|---|---|---|
| Under-sink reverse osmosis | $300 to $1,500 | filters $100 to $250 a year, membrane every 2 to 3 years |
| Arsenic filter, Simcoe-area dealer | $1,500 to $2,000 | ask the dealer |
| Whole-house carbon | $800 to $2,000 | part of $50 to $300 a year in filters |
| Whole-house reverse osmosis | $2,000 to $10,000+ | quoted with the system |
| Install labour, where quoted separately | $300 to $1,000 |
October 2026, before HST. Our default for an under-sink RO is $900. We found no published price for a replacement membrane, so we do not print one; ask for it before you buy, because you will buy three or four.
Why Ontario prices are all over the map: the dealers and price guides we checked for this page quoted very different prices for drinking water at one tap. A GTA dealer priced an under-sink RO at $300 to $1,500, and a Simcoe-area dealer $800 to $1,500 for a drinking water unit. For whole-house RO, the GTA dealer said $2,000 to $5,000 and up, and an Ottawa dealer $4,000 to $10,000 and up. The gap is the number of stages, a storage tank, a remineralizing cartridge, a pump for low pressure, the drain hookup, and whether a dealer is selling a unit or a service plan. Budget $900 for an under-sink RO, and get two or three quotes on the same written scope.
Ten years of an under-sink RO at our default, our estimate: $900 installed, plus filters at the middle of the range, 10 x $175 = $1,750. $900 + $1,750 = $2,650, plus three or four membranes at the dealer’s price.
Waste Water, the Tank and the Taste
RO does not catch what it removes; it flushes it. Some of every litre that goes into the unit goes down the drain with the rejected minerals. The ratio is on the spec sheet, and it varies a lot between units, so ask. On town water it is a line on the water bill. On a well with low yield, or a septic system, it is worth knowing before you buy.
The membrane is slow, so most units fill a small storage tank under the sink and a separate faucet on the sink draws from it. You need a hole in the sink or counter for that faucet and a drain connection under it. Plan it before the counter goes in, not after the granite is down; the kitchen renovation order of work shows where it fits.
RO water tastes flat to some people, because the minerals are gone. A remineralizing cartridge puts a little back for taste. It is a matter of preference, not health, and the dealer will be happy to sell you one either way.
Why Whole-House RO Is Rare
You drink and cook with a few litres a day. The other few hundred go to the toilet, the shower, the laundry and the garden, and none of them care about arsenic in parts per billion. Whole-house RO means treating all of it, paying $2,000 to $10,000 and up, and sending a share of every litre down the drain. Very pure water can also be hard on metal pipe and fittings, so the system usually needs more treatment after it. A good dealer will steer you to RO at the kitchen tap unless the water has a problem you cannot live with on the skin or in the air, and then ask for a second opinion.
Which One for Which Result
- Nitrate over 10 mg/L as N. Say the report says 14. Do not use the water for infant formula. RO at the kitchen tap, or a different source, and talk to the health unit. Find out why it is high, too: a septic bed or farm field nearby is a clue, not a coincidence.
- Arsenic over 0.010 mg/L. Say 0.025, two and a half times the guideline. RO at the tap or an arsenic filter, then test the treated water to prove it works. A softener does not touch arsenic.
- Uranium over 0.02 mg/L. RO at the tap, and the same retest.
- Sodium after a softener. If someone in the house is on a sodium-restricted diet, RO at the kitchen tap is the usual fix. The softener page covers the sodium question.
- Lead. In a house, lead usually comes from old plumbing or the service line, not the well. Fix the pipe if you can; the old pipe replacement page covers lead services. A filter rated for lead covers you in the meantime.
- Chlorine taste on town water. Carbon. That is what it is for, and it is the cheapest fix on this page.
None of these replace the bacteria test. Get the free test and a lab chemistry report first; the water testing and treatment guide covers the rest of the train.
Questions people ask
Does a carbon filter remove nitrate or arsenic?
No. Carbon handles taste, odour, chlorine and some organic chemicals. Nitrate, arsenic, uranium and sodium need reverse osmosis or, for arsenic, a dedicated arsenic filter.
How much does a reverse osmosis system cost in Ontario?
$300 to $1,500 installed under the sink, before HST, with $900 a fair budget, plus $100 to $250 a year in filters and a membrane every 2 to 3 years. Whole-house RO runs $2,000 to $10,000 and up.
Does reverse osmosis waste water?
Yes, a share of the water going in is flushed to the drain with what the membrane rejects. The ratio varies by unit; it is on the spec sheet.
Is RO water safe for baby formula if my well has high nitrate?
Do not use untreated water over 10 mg/L nitrate as N for infant formula. Talk to your health unit about RO at the tap or another source, and test the treated water.
Should I get whole-house RO?
Rarely. You only drink and cook with a few litres a day. RO at the kitchen tap does the job at a fraction of the cost and the waste water.
Find out which one your water needs
The water treatment selector reads arsenic, nitrate, uranium, sodium and the rest from your lab report, bacteria first, and tells you whether drinking water treatment belongs at your tap and which results mean call the health unit. If a dealer has already quoted, the $99 review checks one quote against your water test, or compares up to three.
Sources and further reading
- Guidelines for Canadian Drinking Water Quality: summary table: Health Canada limits for arsenic, lead, nitrate, uranium, manganese, iron, sodium and the rest, updated December 2025.
- Testing and treating private water wells: Ontario on free bacteria testing, private lab chemistry and disinfecting a well and the house plumbing.
