Reverse Osmosis vs Distilled Water: What Each One Removes

Both processes get quoted in the same breath as "the purest water you can make at home," and both can, in a well-maintained unit, get total dissolved solids down to a handful of parts per million. That's where the similarity ends. Reverse osmosis pushes water through a membrane under pressure; distillation boils water away from everything it's carrying and condenses the vapor elsewhere. Different mechanisms leave different things behind — and let different things through — which is the actual question worth answering before you buy either one, rather than treating "purified" as a single grade.

This guide covers how each process physically works, the specific contaminant category each is weakest against, what a gallon actually costs once electricity and wasted tap water are counted, and which one makes more sense for drinking, cooking, an aquarium, or the specific appliance in your house asking for one by name.

Despite landing on a similarly low mineral count, reverse osmosis and distillation don't share a single step. One is a filtration process happening at room temperature under pressure; the other is a phase-change process happening at a rolling boil. That mechanical difference is exactly what explains the different gaps each one leaves.

Reverse osmosis: pressure against a membrane

Tap water — pressurized by your home's plumbing, or boosted by a small internal pump on many countertop units — is forced against a thin-film composite membrane whose effective pore size is small enough to reject the large majority of dissolved ions, minerals, heavy metals and larger organic molecules while still letting water molecules through. The water that makes it across is the purified stream, called permeate. The water that carries the rejected material away is the reject or concentrate stream, and it goes down the drain rather than into your glass — which is why under-sink RO systems publish a drain ratio, and why running one always uses more tap water than it delivers as finished drinking water.

A membrane on its own is a poor match for dissolved gases and for chlorine specifically, which degrades the thin-film material over years of exposure. That's why practical RO systems build carbon stages around the membrane instead of relying on it alone: carbon ahead of the membrane strips chlorine before it can cause damage, and carbon after it polishes taste. Both AquaTru units in the picks below describe themselves as 4-stage systems for this reason — the membrane is one stage among several, not the whole story.

Distillation: boiling water away from everything else

Distillation is mechanically older and simpler: water is heated to its boiling point in a chamber, the steam is routed through a condensing coil, and the condensed water is collected as distillate. Water boils at 212°F (100°C) at sea level, and the great majority of what's dissolved in tap water — calcium, magnesium, sodium, chloride, silica, and heavy metals such as lead and arsenic — does not vaporize anywhere near that temperature, so those substances stay behind while the water itself leaves as vapor. That's why a countertop distiller develops a visible mineral scale ring over weeks of use, and why the process also leaves behind essentially all bacteria and protozoa, which don't evaporate either. There's no membrane, so there's no reject stream and no drain ratio to publish.

"Purified" is a spectrum, not a threshold, and both technologies have specific, well-documented weak points rather than being flawless. Knowing where each one is weakest matters more than the headline percentage on the box.

The gaps in reverse osmosis

Membrane rejection isn't uniform across every dissolved substance — it depends heavily on molecular size and, more importantly, on ionic charge. Multivalent ions such as calcium, magnesium and sulfate reject very well, often above 98%. Monovalent ions such as sodium and chloride reject well too, just a notch lower. The two substances water-treatment literature consistently flags as harder for a standard RO membrane to reject are boron and nitrate. Boron exists mostly as uncharged boric acid at ordinary tap-water pH, and uncharged species pass a membrane far more readily than charged ones, so a standard single-pass membrane commonly rejects boron only in a roughly 50–70% range. Nitrate is also rejected less completely than divalent ions, which is why well owners dealing with a nitrate problem are usually told to test the RO product water specifically. And because a membrane does very little to a dissolved gas, chlorine and chloramine pass through relatively freely unless a carbon stage intercepts them first — exactly why the systems in the picks section below build carbon around the membrane instead of leaning on it alone. Our explainer on what RO certifications actually mean is worth reading before you weigh a contaminant-removal claim on a listing against a third-party test.

The gaps in distillation

Distillation's blind spot is close to the mirror image of RO's. Anything with a boiling point at or below water's, or that vaporizes readily well before water reaches a full boil — many volatile organic compounds, some pesticides, chlorine and several of its disinfection byproducts — can travel with the steam and recondense right back into the distillate on the other end. That's the textbook weak point of pure distillation, and it's the reason most distillers built specifically for drinking water add a small vent or an activated-carbon post-filter to intercept those compounds, rather than leaving boiling point alone to do the whole job.

This is worth its own section because it's the single most misunderstood exception to "distillation removes everything." Distillation separates by boiling point, full stop — it doesn't distinguish "good" from "bad" among substances that happen to vaporize in the same range as water. Minerals and heavy metals have boiling points hundreds of degrees above water's, so the separation is clean and dramatic. Some volatile organic compounds have boiling points below or close to water's 212°F, so the separation is the opposite of clean: they leave the boiling chamber alongside the steam and re-form as liquid the moment the vapor cools, ending up in the same collection vessel as the water you're trying to purify.

A plain tabletop distiller with no vent stage and no carbon polish will pass a meaningful fraction of these compounds into the distillate rather than trapping them the way it traps calcium or lead. Reverse osmosis doesn't share this failure mode, because carbon adsorption — not boiling point — is what catches volatile organics, and a carbon stage ahead of an RO membrane is standard rather than optional in systems built for drinking water. It's not that distillation is worse across the board; RO has its own gaps, covered above. It's that this one category is where the two technologies' mechanisms diverge the most, and it's why a distiller marketed for drinking water almost always advertises a vent or carbon stage rather than relying on the boil alone.

The sticker price on either a countertop RO unit or a countertop distiller is one number. What it costs to run, gallon after gallon, is the number that actually compounds over a few years of ownership — and it's the one most buying guides skip.

Reverse osmosis's running costs

Two line items make up an RO system's marginal cost per gallon: wasted reject water going down the drain, and electricity for a booster pump on units that use one. Neither is large. Municipal tap water in the US typically runs somewhere under a cent per gallon depending on your utility, so even a reject stream several times the size of the purified stream adds only a couple of cents to one finished gallon. A small booster pump isn't heating anything — it's only pressurizing water for a minute or two — so its electricity draw is a fraction of what it takes to boil water to 212°F and hold it there. The cost that actually matters for RO ownership is filter replacement on a schedule, and it varies by system and by how hard your water is; we don't have a sourced replacement-cartridge price for the three countertop units in this guide, so check the specific listing before you buy. Readers weighing a plumbed alternative should also see our guide to tankless under-sink systems, which run on a different cost structure again.

Distillation's running costs

Distillation's marginal cost is dominated by electricity, because bringing a batch of water to a rolling boil and holding it there while a gallon or more evaporates takes real, sustained energy — nothing about the process is passive the way pushing water through a membrane is. A widely cited rule of thumb in the water-treatment world puts a home countertop distiller at roughly 3 kWh of electricity to produce one finished gallon. At a commonly cited US national average residential rate around $0.16 per kWh, that works out to roughly $0.48 in electricity alone for a single gallon, before the unit's own price or any cleaning supplies are counted. Your own utility rate and your unit's efficiency will move that number, sometimes by a lot, so treat it as an order-of-magnitude figure rather than a quote.

Put the two side by side and the gap is stark: something on the order of a couple of cents per gallon for a maintained RO system against roughly half a dollar per gallon for a boiling distiller, with electricity as the single biggest line item separating the two. The trade RO makes for that running-cost advantage is the membrane itself, the plumbing or booster pump, and a filter-replacement schedule; the trade distillation makes for its mechanical simplicity is the electric bill. If the purchase price is what's holding you back rather than the running cost, our roundup of reverse osmosis systems under $250 is a reasonable place to start, and a cheap handheld TDS meter — covered in our guide to what a TDS reading actually means — tells you whether either system is still doing its job months after installation.

Drinking and cooking

For everyday drinking, most households land on reverse osmosis, remineralized or not, mainly because it's continuous rather than batch — you draw water when you want it instead of waiting on a cycle. Distilled water is perfectly fine too; the near-total absence of dissolved minerals in unremineralized water is why some people describe it as tasting flat, a subjective complaint, not a safety one. For cooking, especially boiling and steaming where mineral-laden tap water leaves scale on pots and kettles over time, both are equally effective, and the choice comes down to whichever is already sitting in a pitcher in the kitchen.

Aquariums

Reef and planted-tank keepers reach for RO or RO/DI water far more often than distilled, mainly for throughput: a membrane producing a slow but continuous stream is easier to build a water-change routine around than refilling a distiller's boil chamber batch by batch. Either source starts near zero TDS, and that's the point for hobbyists but also the trap for beginners — near-zero-mineral water is not safe to add directly to a stocked tank without reconstituting the calcium, magnesium, alkalinity and trace elements a given species actually needs, using purpose-made aquarium remineralizing salts rather than a drinking-water mineral cartridge like the ones on the M6SL or the alkaline AquaTru. That reconstitution step is identical regardless of whether the water started out as RO or distilled.

Appliances: irons, humidifiers, CPAP machines

Steam irons and humidifiers both benefit from either source for the same reason: minerals in ordinary tap water cause the white scale and mineral dust these appliances are known for, and both RO and distilled water are low enough in dissolved solids to prevent it. CPAP machine manufacturers are the one category that consistently names distilled water specifically in their manuals rather than "purified" or "RO" water generically, largely because distillation gives them a single, standardized process to point to rather than a moving target that depends on a membrane's age and condition. If your CPAP humidifier's manual calls for distilled water, it's worth following that instruction rather than substituting a well-maintained RO system, even though the two are close in practice on total dissolved solids alone.