A reverse osmosis system is one of the more common under-sink water treatment setups in St. Louis homes, and it can look like a mystery when the cabinet door is open. Behind the faucet is a small group of parts that work together. This guide explains what each part does, what happens to the water on the way through, and what a St. Louis homeowner can check before deciding whether the system fits their kitchen.
What the system is
Reverse osmosis, often shortened to RO, separates water from dissolved material. Water is pushed under pressure against a thin semipermeable membrane. The membrane lets water molecules pass while holding back many dissolved substances. Those substances leave with a smaller stream of water that goes down the drain. Under a kitchen sink, the system is usually a stack of filter canisters and a storage tank, connected to the cold water line and to a separate faucet mounted on the sink or countertop.
How water moves through the stages
Most under-sink units follow the same general order. Water enters from the cold line and passes through a sediment filter first. This catches particles such as sand, rust flakes, and small debris that could clog the membrane. Next, a carbon filter reduces chlorine taste and odor and removes some organic compounds. The membrane comes after these pre-treatment stages because it performs best when the water reaching it has already been cleaned of material that would foul it.
Once water passes the membrane, the purified stream fills a storage tank. The concentrated stream carries the rejected material to the drain line through a small restrictor, which controls how much water leaves that way. When you open the RO faucet, water flows from the tank. The tank then refills slowly over time, so a large draw of water at one time will empty it faster than it can recover.
The storage tank
The tank holds a small reserve of treated water under modest pressure. Its size and the air charge inside it affect how steady the faucet flow feels. A tank that has lost its air charge can make the faucet sputter or run weak, and that is one of the first things to check if the flow changes over time.
The faucet and the drain line
The RO faucet is a separate tap, so it needs its own hole in the sink or countertop. The drain line carries the concentrated stream away, usually into the household drain pipe through a small fitting. Because the drain line carries rejected water, it should be routed and secured so it does not kink or back up.
What St. Louis homeowners should check first
St. Louis homes draw from a regional supply that is treated before it reaches the tap, and the water can differ by neighborhood, by age of the building, and by the condition of the service line. Before choosing any treatment system, it helps to gather a few facts:
- Read the annual water quality report that your water supplier publishes. It lists what was measured in the treated water and how it compares to standards.
- Ask whether your home has lead service lines or older plumbing, since that affects what you may need to test for at the tap.
- Order a lab test of your own tap water if you want results for a specific substance. A lab test shows what is present in your water, which is more useful than guessing from taste or appearance.
- Measure the space under the sink, including the depth of the cabinet, the position of the shutoff valve, and the clearance around the drain pipe.
- Check whether the cabinet sits against an exterior wall, since cold spots can affect plumbing in winter.
A reverse osmosis system removes many substances, but it is not a substitute for knowing what is in your water. Testing gives you the information needed to decide whether a system addresses a real concern in your home.
Maintenance and what to expect
An under-sink RO system needs regular filter changes. Sediment and carbon filters wear out first because they capture material from the incoming water. The membrane lasts longer but still needs replacing on a schedule set by the manufacturer of your unit. Keep a simple log with the date of each filter change so you can plan ahead and avoid running a system with spent filters.
Expect some water to go to the drain as part of normal operation. Expect the storage tank to refill over time after heavy use. If the faucet slows noticeably, check the tank air charge, the filters, and the feed valve before assuming the membrane has failed.
Next steps
Start with the water quality report and a basic lab test if you have a specific concern. Measure the space under your sink and note where the water line and drain line would connect. With those facts in hand, you can compare systems on the details that matter for your St. Louis home, including filter schedules, space needs, and the testing results for your own tap.