Read a reverse osmosis diagram as one path with a branch. Water enters from the cold supply, passes through prefilters into the membrane housing, and there it splits: treated water goes on to a storage tank and the faucet, while the concentrated stream leaves through the drain line. Everything else on the drawing controls that split.
The feed side: valve, prefilters and their order
The diagram starts at the cold water angle stop under the sink, where a feed adapter tees off the supply. The line runs first to a sediment prefilter, which catches grit and rust so the finer stages are not blinded, then to one or more carbon prefilters. Those carbon stages are not optional: chlorine damages common membrane materials, so the carbon is protecting the most expensive component in the system. Any diagram showing water reaching the membrane before the carbon has the order wrong, and any system plumbed that way will lose its membrane early.
The membrane housing and the split
Inside the housing, a semipermeable membrane is wrapped around a central tube. Line pressure pushes water against it, and the membrane passes water molecules while rejecting the majority of dissolved solids. This is where the single line on the diagram becomes two. The permeate, the treated water, exits through the center tube toward the tank. The concentrate, carrying the rejected material, exits separately and goes to drain through a flow restrictor, which is the small component that maintains pressure across the membrane. Remove or lose that restrictor and the system produces almost nothing while running constantly to drain.
Drain line, check valve and the automatic shutoff
The concentrate line joins the sink waste at a drain saddle or a fitting on the waste tee, always above the trap so nothing can siphon back. Two control components sit near the membrane on most diagrams. A check valve on the permeate outlet stops stored tank pressure pushing water back through the membrane. An automatic shutoff valve senses tank pressure and closes the feed when the tank is full, which is what stops the system running to drain around the clock. A system that never stops draining almost always has one of those two at fault.
Storage tank, postfilter and faucet
Because a membrane produces slowly, treated water is stored in a small pressurized tank containing an air bladder. Air pressure on one side of the bladder is what pushes the water back out when you open the tap, so a tank that delivers only a trickle usually needs its air charge checked with the tank empty. From the tank the line runs through a final polishing carbon postfilter, which takes out any flat or stale taste picked up in storage, and then to a dedicated faucet on the sink deck. Many diagrams also show a tee to the refrigerator or ice maker.
Whole house layouts read differently
A whole house reverse osmosis diagram adds an atmospheric storage tank, a repressurization pump and often a post treatment stage, because treated water has to be pushed back up to household pressure and reverse osmosis water is aggressive toward some plumbing materials. The membrane array is larger and pretreatment is heavier, typically including softening to stop hardness scaling the membranes. That is a very different drawing from an under sink unit, and our whole house reverse osmosis guide covers when a household actually needs one rather than a point of use system.
Questions people ask about reverse osmosis diagram
Why does a reverse osmosis system have a drain line?
The membrane rejects dissolved solids into a concentrated stream, and that stream has to leave somewhere. Without the drain line and its flow restrictor, the membrane would foul quickly.
What order do the stages go in?
Sediment prefilter, then carbon prefilter, then the membrane, then the storage tank, then the carbon postfilter, then the faucet. Carbon before the membrane protects it from chlorine.
What does the automatic shutoff valve do?
It senses pressure in the storage tank and closes the feed when the tank is full, stopping the system from producing and draining continuously.
Why is my reverse osmosis faucet flow so weak?
Usually low air pressure in the storage tank bladder, a clogged postfilter, or a tired membrane. Check the tank air charge with the tank drained before replacing anything expensive.