Reverse osmosis produces the cleanest drinking water most households can make at home, but the word pure does a lot of unearned work in the marketing. A membrane rejects the large majority of dissolved contaminants and passes water molecules, and how close the result gets to pure depends entirely on what was in the feed water and what treatment stood in front of the membrane.
What the membrane actually rejects
A thin film composite membrane rejects most dissolved ions and larger molecules: lead, nitrate, arsenic in its common forms, fluoride, sulfate, sodium, dissolved salts and the mineral load that makes water taste heavy. It is also an effective physical barrier to bacteria and cysts, though it is not sold as a disinfection method because any breach or downstream contamination undoes that. What it does not reject well are some small uncharged molecules, which is why a carbon stage is always paired with it.
The stages around the membrane are not optional
A sediment prefilter protects the membrane surface from blinding by particles. A carbon prefilter removes chlorine, which attacks thin film membranes directly and will destroy one within months on chlorinated supply. A carbon post filter polishes taste after the storage tank. Skipping or neglecting any of these turns the most expensive element in the system into a consumable. Manufacturers publish their systems as staged configurations with rated production, and that staging exists for exactly this reason.
On a well, pretreatment decides everything
Well water arrives untreated, so the membrane meets whatever the aquifer contains. Iron and manganese foul membranes and need oxidizing treatment upstream. Hardness scales the membrane surface and shortens its life, which is why a softener ahead of the drinking water system is common on hard wells. Sediment loads need a proper whole house stage rather than a small cartridge. And bacteria are a whole house question, since private wells are not federally regulated under the Safe Drinking Water Act and nobody is testing them but you.
Remineralization and the taste question
Water with almost nothing dissolved in it tastes flat to many people, and low mineral water is slightly aggressive toward metal fittings and can sit at a lower pH. A remineralization cartridge after the membrane adds back a small amount of calcium and magnesium, restoring body to the taste and raising pH a little. It is a preference rather than a necessity, and it should sit before any branch feeding a refrigerator so both the faucet and the ice benefit from it.
Judging whether it is working
Take a dissolved solids reading of the feed water and of the treated water on commissioning day and write both down. A healthy membrane drops the reading substantially, and a treated reading that climbs toward the feed reading over time means a failing membrane, a bypassing seal or a leaking shutoff valve. Change prefilters on schedule so the membrane is never doing their job. Our best RO system for well water comparison covers matching a system to a pretreated well supply.
Questions people ask about pure water ro system
Does RO water count as pure water?
It is very low in dissolved contaminants but not laboratory pure. Rejection depends on the feed chemistry, pressure and the condition of the membrane.
Is RO water bad for you because minerals are removed?
Diet supplies far more calcium and magnesium than water does. Many households still add a remineralization cartridge because they prefer the taste.
Can I run an RO system straight off a well?
Only after pretreatment. Iron, manganese, hardness and sediment foul membranes quickly, and bacteria need addressing at the whole house level first.
How do I know the membrane is still good?
Compare feed and treated dissolved solids readings against the figures recorded at commissioning. A treated reading creeping upward signals a failing membrane.