A reverse osmosis system looks like a row of cartridges and a tank, and inside that arrangement are eight distinct components. Knowing which is which is worth the ten minutes, because the faults owners find hardest to diagnose almost always come from three small parts rather than from the membrane everybody suspects first.
The filtration path
A sediment cartridge comes first, catching grit so nothing abrasive reaches what follows. One or two carbon stages come next, and their job is removing chlorine before the membrane, since chlorine destroys thin film composite material quickly. The membrane itself is a spiral wound element in a horizontal housing, and it is where the separation happens. A polishing carbon cartridge sits at the very end, between the storage tank and the faucet, correcting the faint taste stored water picks up from the tank bladder.
The tank and the faucet
The storage tank is a pressure vessel with a butyl bladder and a cushion of compressed air behind it, because the membrane makes water far too slowly to serve a tap directly. It fills against that cushion and the cushion pushes the water back out. The tank connects through a single tee, so the same line fills and empties it. The dedicated faucet delivers the treated water, and on air gap installations it also routes the drain line up through its body and back down.
The three small parts that cause most faults
The flow restrictor on the drain line maintains pressure across the membrane by limiting the reject stream, and a missing or wrong one either wastes water or fouls the membrane. The automatic shutoff valve senses tank pressure and stops production when the tank is full, so a failed one leaves the system running to drain permanently. The check valve on the permeate line stops tank water flowing backwards through the membrane, and a failed one produces a system that never fills. All three are inexpensive.
Diagnosing by part
A weak trickle from the faucet with a heavy tank is almost always a lost air charge, checked with the tank completely empty. A system running to drain constantly is the shutoff valve or the check valve, separated by isolating the tank and seeing whether the flow stops. Stale tasting water is usually the polishing cartridge rather than the membrane. Falling water quality is the membrane, confirmed by comparing feed and permeate readings. Our whole house reverse osmosis guide covers commissioning.
Questions people ask about reverse osmosis system parts
Why does an RO system have carbon before the membrane?
Chlorine destroys thin film composite membrane material quickly, so it must be removed before the water reaches the membrane.
What does the flow restrictor do?
It limits the reject stream to maintain pressure across the membrane. Without the right one the system wastes water or fouls.
My tap gives only a trickle, which part is it?
Almost always the storage tank's air charge. Empty the tank completely and check it with a low pressure gauge.
The water tastes stale, is the membrane finished?
Check the polishing carbon cartridge between tank and faucet first. A spent one is a far more common cause than a failed membrane.