Reverse osmosis, usually called RO, pushes household water through a very fine membrane that blocks a wide range of dissolved substances. Many St. Louis homeowners choose it for drinking and cooking water because the list of what it removes is long. The list of what it leaves behind is shorter, but it matters for your plumbing, your taste, and your next steps, so it is worth knowing before you commit.
What reverse osmosis removes well
RO does its strongest work on dissolved solids, the minerals that make water hard or give it a salty or bitter taste. Calcium and magnesium, the minerals behind most scale on faucets and shower heads across the St. Louis area, are largely blocked by the membrane. Sodium, chloride, sulfate, and nitrate are also reduced. Dissolved iron and manganese are reduced as well, though iron that has already oxidized into particles is better handled by a sediment filter placed ahead of the membrane. Dissolved metals such as lead are also reduced by a properly working membrane, which is why many homeowners pair RO with a lab test to confirm results.
What can still get through
No membrane is perfect, and RO is not a complete answer by itself. Small uncharged molecules, including dissolved gases such as carbon dioxide, pass through the membrane with the water. Volatile organic compounds, which evaporate easily and often come from fuels, solvents, or cleaning products, are only partly handled by the membrane, so many RO systems include a carbon stage for them. Chlorine and chloramine, which many water providers use to disinfect, can damage the membrane if they reach it, so a carbon pre-filter is standard in most designs. Bacteria are not reliably stopped by the membrane alone, which means the storage tank and faucet still need attention.
What it leaves behind in the water
- Very low mineral content, which some people notice as flat or sharp tasting water.
- Changes to water chemistry, since minerals that help buffer acidity are removed, which can affect how the water interacts with some pipes and fixtures.
- Dissolved gases, which can change taste and can make water fizz briefly when poured.
- Any substances the membrane does not block, which is why a test after installation is part of the plan.
What it leaves behind in the system
RO produces a waste stream, sometimes called reject or concentrate water, that carries the blocked substances down the drain. This is expected and is not a sign of failure, but the drain line must be routed correctly, and it is wise to check local plumbing requirements before connecting it. The storage tank is another place where things can build up. Sediment, scale, and bacteria can collect there over time, so the tank needs periodic sanitizing and the filters need replacing on the schedule the manufacturer lists. Missed filter changes can quickly change how RO water tastes.
Testing before and after
- Pull your water provider's annual water quality report for your address and note what it lists.
- Use a home test kit for hardness, iron, chlorine, and pH to get a quick picture.
- Send a sample to an accredited lab for metals, bacteria, nitrate, and organic compounds.
- Collect samples from a cold tap after running water, following the lab's bottle and handling instructions exactly.
- Test again after installing RO, sampling at the new faucet and at the tank.
Next steps for a St. Louis home
If scale is your main problem, a water softener usually handles hardness for the whole house, while RO handles only the drinking and cooking tap. If the test shows iron, sulfur, or bacteria, those need their own treatment, and RO alone will not fix them. Start with the test results rather than a guess, read the report with your household in mind, and decide the order of fixes from there. St. Louis water varies from one neighborhood to the next, so a result from one address does not describe another.