What the drain line is doing
If you have a reverse osmosis unit under a St. Louis kitchen sink, you have probably noticed a thin stream of water going down a separate drain line while the faucet is off. For most systems, that is normal. It is part of how the unit keeps working, and it is worth understanding before you decide something is wrong.
Reverse osmosis pushes household water through a semipermeable membrane. Clean water passes through. Most dissolved material, including the minerals that make water hard, does not pass through easily, so it stays on the feed side. A membrane that only collected rejected material would clog quickly, so the system keeps a steady flow of water moving across the membrane surface. That flow carries the rejected material out to the drain. The water going to the drain is called concentrate or reject water.
Why St. Louis tap water matters here
St. Louis area water is commonly described as hard. The minerals behind that hardness come from the limestone and related rock under the region, dissolved as water moves through it. Hard water gives a reverse osmosis membrane more dissolved material to reject. To keep that material from building up on the membrane, the unit has to send more concentrate to the drain. Homeowners in the city, St. Louis County, and St. Charles County may notice this more than people whose supply is softer.
Private well water is a different case. If your home in the wider St. Louis area runs on a well, the feed water can carry iron, sediment, or other material that the pre-filters and membrane have to handle. A fouled pre-filter or a clogged membrane can change how the unit behaves, so drain flow alone does not tell the whole story.
What the flow restrictor and storage tank do
Most home units include a flow restrictor, a small fitting in the drain line that sets how fast concentrate leaves the system. It keeps enough pressure on the membrane for good output and keeps the drain flow from running so fast that it wastes pressure that could go toward producing clean water. Changing the restrictor shifts the balance between clean water output and drain flow, so it should not be swapped casually.
The storage tank fills with treated water while the unit runs. When the tank reaches its set level, a shutoff valve closes and the drain flow should stop. If the drain keeps running when no water is being drawn and the tank is full, something needs attention.
Normal drain flow versus a problem
Here is a practical way to sort out what you see:
- A steady trickle to the drain while the tank is filling is normal.
- Drain flow that stops once the tank is full is normal.
- Drain flow that never stops, even when the tank is full and the faucet is closed, may point to a faulty shutoff valve or a stuck check valve.
- A sudden drop in clean water output, or a sharp rise in drain flow, may mean the membrane or pre-filters need attention.
- Water pooling at the drain fitting or dripping from the drain line needs prompt attention.
Checks a homeowner can make first
- Read the owner's manual for the expected drain behavior and the location of the flow restrictor.
- Watch the faucet and tank for a few minutes with everything closed to see whether drain flow stops.
- Look at the drain fitting and tubing for leaks, kinks, or a loose connection under the sink.
- Check the pre-filter housings for sediment and note the date of the last change.
- If you have a home test kit for hardness, run it on your tap water so you know what the membrane is working against.
When to call a qualified person
Call a qualified service person if the drain never stops, if clean water output drops sharply, if you see leaks at the fittings, or if you are unsure which part is failing. Ask the person to explain the cause in plain terms and to show you the part being replaced. Keep notes on what you observed and when, since that history helps anyone diagnosing the unit.
Keeping the drain balance healthy in St. Louis
Routine care keeps the system predictable. Change pre-filters on the schedule in the manual, or sooner if your water carries visible sediment. Watch the tank for changes in how quickly it refills. Keep the drain line clear and well supported, since a sagging or kinked line can back up. If your home has a water softener, talk with whoever services it about how it affects the water reaching the membrane, because softening changes what the membrane has to reject.
The drain line is not a problem by default. It is part of how the unit protects its membrane in a region where hard water is common. Knowing what normal looks like makes it much easier to spot the drain behavior that actually needs attention.