A reverse osmosis system in a St. Louis kitchen works in stages, and each stage wears out on its own schedule. Knowing which filter does what makes replacement easier to plan and keeps you from changing parts that still work while missing the ones that matter.
The parts that need changing
Most under-sink reverse osmosis units use several filters before and after the membrane.
- Sediment pre-filter: catches sand, rust flakes, and grit from the supply line. It is usually the first part to clog.
- Carbon pre-filter: reduces chlorine and some taste and odor compounds before water reaches the membrane, which helps protect the membrane.
- Membrane: the semipermeable part that separates dissolved minerals and many other substances from the water. Replace it when flow drops or a test shows it is no longer doing its job.
- Post-carbon polishing filter: a final pass that improves taste before the water reaches the faucet.
- Optional remineralization cartridge: adds some minerals back into the water and has its own schedule.
- Storage tank: not a filter, but a tank that loses air pressure or develops a leak changes both flow and taste, so check it along with the filters.
Start with the manufacturer schedule
Your unit's manual lists a replacement interval for each cartridge. Treat that interval as the baseline. Write the installation date on each filter with a marker so you are not guessing later. Schedules assume typical water, so a St. Louis home with hard water or recent work on nearby water lines may need earlier changes.
Signs it is time, even before the schedule
- Water flow at the faucet slows noticeably, which often points to a clogged sediment or carbon filter.
- Water tastes or smells different, especially if a chlorine odor returns.
- The sediment filter looks brown or gritty when you pull it out.
- The storage tank fills slowly, or the faucet sputters when you draw water.
- A handheld meter reads higher at the faucet than it did before, which can mean the membrane is letting more dissolved material through.
St. Louis conditions that shorten filter life
Many homes in the region draw water that carries minerals from limestone bedrock. Those minerals can scale pre-filters and the membrane surface over time. For most homes, though, the bigger issue is sediment after work on water mains or service lines. After a main break, a flushing program, or a boil notice in your area, change the sediment filter early, even if it is not yet due. If your home has a water softener upstream, the reverse osmosis pre-filters still see chlorine and grit, so the schedule does not get longer just because the softener is working.
Ask about the disinfectant type
Some St. Louis area supplies use chloramine rather than free chlorine. Carbon filters remove chloramine more slowly, so a carbon stage can wear out sooner than the label suggests. If you are unsure which disinfectant your supply uses, ask your water provider for its latest annual water quality report, which lists the treatment methods in use.
Use a simple test before and after
A handheld total dissolved solids meter lets you compare readings at the tap and at the reverse osmosis faucet. Take a reading from the cold tap, then from the reverse osmosis faucet, and keep a log. When the faucet reading climbs toward the tap reading, the membrane is likely near the end of its useful life. A meter does not show bacteria, lead, or other contaminants, so use a certified laboratory test for those. Chlorine test strips can confirm whether the carbon stage is still removing chlorine.
A simple replacement routine
- Shut off the feed water valve to the unit.
- Relieve pressure by opening the faucet until the flow stops.
- Replace the sediment filter first, then the carbon filter, then the membrane if it is due.
- Wipe the connections with a clean cloth and check the O-rings for cracks or flattening.
- Restore water, run the faucet to flush the new filters for the time the manual specifies, and follow its instructions for the first water drawn.
- Write the date on each new cartridge and update your log.
What to keep on hand
Store spare sediment and carbon filters in a dry spot in the kitchen or garage. Replacing a filter when you first notice a change, rather than waiting for flow to fail, keeps the water tasting right and helps the membrane last longer.
When to call for help
If the membrane needs replacing, the unit leaks at a fitting, or the tank repeatedly loses pressure, a local service technician who knows your model is the right call. Have the make and model number ready before the visit so the correct parts arrive the first time.