Reverse osmosis, explained without the sales pitch

Reverse osmosis is the most oversold and under-explained product in residential water treatment. It's also genuinely excellent at what it does. Both of those things are true, and the gap between them is where people get taken advantage of.

How it actually works

Osmosis is the natural tendency of water to move across a semi-permeable membrane from a less concentrated solution to a more concentrated one. Reverse osmosis does what the name says: it applies pressure to push water the other way, through the membrane, leaving dissolved contaminants behind.

The membrane's pores are extraordinarily small — small enough to pass water molecules while rejecting the large majority of dissolved salts, metals and many organic compounds. What's rejected goes down the drain. What passes through is your drinking water.

A typical four-stage residential system runs like this:

1. Sediment prefilter — catches particulates that would otherwise foul the membrane.
2. Carbon prefilter — removes chlorine, which is important because chlorine degrades most RO membranes. This stage protects the expensive part.
3. RO membrane — the actual separation.
4. Carbon postfilter — final polish on taste before the water reaches the faucet.

Treated water collects in a small pressurized storage tank, because production is slow — a 50 GPD system makes roughly 50 gallons across a full day, not on demand. The tank is what lets you fill a glass instantly.

The certification question, which is where the industry gets slippery

This is the part worth reading twice.

NSF/ANSI 58 is the standard covering point-of-use reverse osmosis systems. When someone says a system is "NSF certified," the useful follow-up questions are: certified to which standard, and certified as what?

Four different things get described as "certified," and they are not equivalent:

And baseline certification isn't a blanket

Here's the second trap. NSF/ANSI 58 certification at baseline covers total dissolved solids reduction. Additional contaminant claims — lead, arsenic, nitrate, PFOA and PFOS — are separate claims that must appear on that specific product's listing.

So "NSF 58 certified" followed by a list of contaminants the salesperson recites is not, by itself, evidence of anything. The EPA says this directly in its guidance on identifying filters certified to reduce PFAS: check the certifier's listing to see whether the product treats the specific contaminant you care about.

Ask for the certifier's listing number. Any legitimate supplier can produce it in seconds, and you can look it up yourself in NSF, WQA, IAPMO, UL or CSA's public database. If nobody can point you at a listing, treat the claim as marketing.

What RO does and doesn't do

The remineralization question

RO produces very low-mineral water. Some people find it tastes flat — there's genuinely less in it.

A remineralization stage adds a small controlled amount of mineral content back, which most people find improves the taste. It's worth knowing this is available, and worth knowing it's a taste preference rather than a health necessity. Anyone presenting remineralization as medically important is overselling.

It matters slightly more in Birmingham than in some markets, because the water is already moderately low in dissolved solids before treatment.

What to ask before you buy one

• Is the complete system certified, or just components?
• What's the certifier's listing number? Then look it up.
• Does that listing include the specific contaminant you care about?
• Who makes it? If the answer is a brand name that exists only on that installer's truck, your filter supply and service options are whatever they decide.
• Where do replacement filters come from, and what do they cost? A cheap system with proprietary annual cartridges is not cheap.
• Is there leak protection? A mechanical leak detector is inexpensive and sits under a cabinet you rarely open.

That last one gets skipped constantly. Under-sink equipment fails quietly, and the first sign is usually a warped cabinet floor.

Where we land

Our Pure 50 is $995 installed. It's a four-stage system, nominal 50 GPD, with an approximately 4-gallon metal storage tank and a standard chrome air-gap faucet. IAPMO R&T certifies this configuration to NSF/ANSI 58 and CSA B483.1, including PFOA and PFOS reduction. That comes from the manufacturer's performance data sheet, which names the model directly. We publish no microplastics claim for it - no listing we hold names them.

We also offer the Cahaba Performance 75 — nominal 75 GPD, five stages, with an integrated booster pump for homes where incoming feed pressure needs support. The manufacturer identifies that platform as certified to NSF/ANSI 58. That is a different certifier and a different listing from the Pure 50's, which is exactly why we print them separately and never move one onto the other. More production is not the same as more reduction.

If your problem is drinking water, this is the product. If your problem is chlorine in the shower, it isn't — and we'd rather tell you that before you buy.