Carbon Filter vs Reverse Osmosis: Which Do You Actually Need?

Homeowners constantly ask whether they need a whole house carbon filter, an under-sink reverse osmosis system, or both. The two do different jobs in different places. A carbon tank on the main line takes chlorine, taste and smell out of the water at every tap in the house, and cuts down chemicals like herbicides and pesticides. A reverse osmosis unit (an RO) under the kitchen sink takes out what is dissolved in the water and gets past carbon, like salts, fluoride, nitrate and uranium, but only for the faucet you drink and cook from.

Caller "Do you have to go with the reverse osmosis if you just get a good, you know, pre-filter and a good activated carbon filter on it?"

Aidan "When I say carbon filter for the whole house, I mean a carbon tank. Like a 10 by 54 tank or a 13 by 54. You don't have to get the reverse osmosis. It's just a suggestion."

That caller is on a well. He put in a UV light last year, and this October he could smell and taste the lawn spray in his water after a rain. This page is the long version of my answer: what each one removes, where each one goes, and what each one costs to own.

Carbon filter vs RO side by side

A carbon tank treats every tap from the main water line, and an RO treats one faucet from under the kitchen sink. Here is how a whole house carbon tank and an under-sink RO compare on the things you live with every day.

The water-to-drain figure is the EPA's for a typical under-sink RO; units that earn the EPA's WaterSense label send 2.3 gallons or less. A carbon cartridge under the sink, on the wall or in a pitcher is carbon too, but it serves one faucet and fills up fast. In my carbon video I put a wall-mounted cartridge filter at maybe $80, and it's for a single fixture, not the house. When this page says carbon tank, it means a 10 by 54 or 13 by 54 tank on the main line.

Which one removes what

Both take out chlorine and bad taste. Only the RO takes out what is dissolved in the water, like salt, metals and minerals. This table is what each one does to the things people ask me about most.

The PFAS line comes from the EPA: carbon holds the longer-chain PFAS well and the shorter ones less well, and RO membranes are typically more than 90 percent effective on a wide range of them. Neither our carbon tanks nor our RO units are certified for PFAS, so test the treated water. If PFAS is why you're here, the PFAS guide covers testing and the systems made for it. For nitrate, the nitrate guide explains why the number after an RO depends on the number going in. For fluoride, see fluoride in drinking water.

How a carbon tank cleans water

A carbon tank holds a bed of carbon, and chemicals stick to the carbon's surface as the water passes through it on the way to every tap. Ours use Centaur, a catalytic carbon Calgon Carbon makes to break down chloramine faster than standard activated carbon. The Clack 2.5 holds 2.5 cubic feet of it in a 13 by 54 Vortech tank, with no electricity, no drain line and no backwash, and that one tank handles 2 to 10 people. For what carbon does and does not take out in more depth, see what a carbon filter removes.

Carbon does not last forever. When every spot on the carbon is full, contaminants start to pass through, and the University of Nebraska calls that breakthrough. In most homes our carbon lasts 4 to 5 years, and then the old carbon is dumped and the tank is refilled with 2.5 cubic feet of new carbon. There is no annual maintenance in between.

The tank ships empty: the 2.5 cubic feet of carbon ships separately and is poured in through the fill funnel at install. No drain line is included, and none is needed. I recommend having a plumber put it in, and I'm a phone call away for your plumber. There's no drain line and no electricity, just an inlet and an outlet, so it's about an hour or two of a plumber's time. If you've done some plumbing and would rather do it yourself, text me with any questions.

In the video below I show the Clack 2.5 and say a carbon tank will remove PFAS. Reduces is the right word: carbon lowers PFAS, and no carbon tank we sell is certified for it.

How reverse osmosis cleans water

An RO pushes water through a membrane that lets the water through and holds back what is dissolved in it, and what it holds back goes down the drain. It is not one filter but a row of them. The Pure-75 runs a 5 micron sediment filter, a carbon block, the membrane, and a second carbon block on the way to the faucet. The carbon in front matters: chlorine wears out the thin-film membranes in home RO units, so the carbon takes it out first.

An RO makes water slowly. The Pure-75's membrane is rated at 75 gallons a day, and it actually makes about 30 at standard test conditions. Cold water makes less: the University of Nebraska puts 45 degree well water at about half the output of 77 degree water. The 3.2 gallon storage tank keeps water ready at the faucet, and the whole system runs on water pressure with no electricity.

On a well, an RO can't go in by itself. Iron, high hardness and heavy sediment have to come out first or they foul the membrane, and if there are bacteria in the water, a UV light goes in first. Reverse osmosis on well water covers that order in full.

Could you run the whole house on RO? You'd need a commercial-size unit in the basement, several storage tanks, likely a pump to push the water back out, and often more treatment after it because RO lowers the pH. That's $10,000 to $15,000, which is why the RO stays at the kitchen sink. For the full list of what a membrane does and does not take out, see what reverse osmosis removes.

Which one to buy for your water

Buy a carbon tank for what you taste and smell at every tap, and an RO for what is dissolved in the water you drink. You don't need both to get carbon at the sink: the Pure-75 already runs a carbon block before and after its membrane. So the real question is whether the rest of the house needs a $1,995 carbon tank too.

When a carbon tank is enough

City water that tastes or smells of chlorine is the most common case. Chlorine and chloramine are what a carbon tank does best, at every faucet and shower. On a well, a carbon tank takes out a taste or a light rotten egg smell, and herbicides and pesticides, like the lawn spray that caller tasted. In my own house I don't have any under-sink filter at all. I prefer to do the whole house: I have a big carbon tank and a water softener, because the water where I live tastes bad without them. When a caller on a well asked me about adding carbon this spring, I told him, "It makes a big difference in the way the water tastes, so you can cut back on buying bottled water if you're doing that now." If you are on a well and not sure a carbon tank is worth it, whether a well needs a carbon tank goes through the four cases where it does.

When an under-sink RO is enough

When the problem is something dissolved in the drinking water, the RO is the answer and a carbon tank won't touch it. If your TDS is 500 or higher, the only way to bring it down for drinking water is an RO; that is one of the main things an RO removes that a regular carbon filter doesn't. The same goes for salt, fluoride and nitrate. A caller this summer on a well had a trace of uranium on his lab report, and I told him, "the best thing to do as far as drinking water goes is just put a reverse osmosis in for any water you're gonna drink or make coffee with." A renter in an apartment who called in March asked me for a countertop PFAS filter. My answer: "The best thing to get is a reverse osmosis. The most of the countertops units are just carbon units, and they don't have much capacity."

When to put in both

Both makes sense when you want every tap drinkable and something dissolved in your drinking water too. A homeowner in Virginia on a well with PFAS already had an RO in the kitchen. He asked about adding a carbon tank, and I told him it "improves the quality of the water throughout the entire house, so you can ... take a drink at any faucet." On a well with iron and sulfur, the order I give people is an iron filter, a softener, then a carbon tank, then a UV light, so you have safe water to drink anywhere in the house; the RO goes under the sink only if the test shows something dissolved that those don't take out.

If you don't know what is in your water yet, your city's yearly water quality report lists TDS, nitrate, fluoride and the rest. On a well, a certified lab test ($199) reports them all from one sample, and there is a city water version for $179.

Have a water report or a taste problem?

Text Aidan your water report

Send a photo of your report, or tell him what you taste and smell. He will tell you whether you need a carbon tank, an RO, both, or neither.

Text Aidan443-277-2204 Call800-460-5810Aidan

8 AM to 5 PM Eastern, seven days a week. A real person, not a call center.

Where each one goes in your plumbing

On city water the carbon tank goes first, before a softener, because chlorine shortens the life of softener resin. On a well it goes after the iron filter and the softener, and the UV light comes after the carbon. The RO goes under the kitchen sink in both cases, on the cold line, after everything else.

Most wells need only part of that row; your test decides which pieces go in. On city water it is usually just the carbon tank on the main line, a softener after it if the water is hard, and the RO under the sink. The order for well water treatment explains each position.

What each one costs to own

A carbon tank costs more up front and almost nothing after that, and an RO costs less up front but needs new cartridges twice a year. These are today's prices, with shipping included to the lower 48 states.

Buying both, the Clack 2.5 and the Pure-75, is $2,590. Neither one uses electricity. With either one I recommend a plumber, and if you'd rather do it yourself, text me with any questions. For every carbon tank we sell and the house size each one fits, see which carbon tank fits your water; every RO is in our reverse osmosis systems.

What buyers wrote

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