RO faucet (dispensing faucet)A separate small faucet that delivers treated water only.
Storage tank (RO tank)Stores treated water under pressure so the faucet flows at once.
Tank valve (tank ball valve)A quarter turn of its lever shuts the tank off.
Sediment pre-filter (stage 1)First stage: catches silt and other particles.
RO membrane (membrane element)Sheets wound in a spiral; water passes, dissolved solids mostly do not.
Post filter (polishing filter)Carbon that removes leftover taste and odor after the tank.
Feed water adapter (feed valve)Tees off the cold water supply and has its own small valve.
Drain saddle (drain clamp)Clamps round the sink drain pipe above the trap.
System under the sink 8
Filter housingUnscrews from the head so the cartridge inside can be changed.
TubingFlexible plastic line; makers color-code each run.
Membrane housingOne inlet at the cap end, two outlets at the other.
Drain line (reject line)Carries reject water, also called concentrate or brine, to the drain.
Carbon block pre-filterFine carbon molded into a solid block; it takes out chlorine.
GAC pre-filter (granular carbon)Loose granules of activated carbon; chlorine would harm the membrane.
BladderHolds the stored water; the air around it pushes the water out.
Air valve (tank air stem)Sits under a cap; the tank's air pre-charge is set here.
Membrane and valves close-up 7
Automatic shut-off valve (ASO valve)Closes the feed to the membrane once the tank is full.
Check valveStops tank water from pushing back into the membrane.
Flow restrictor (drain flow restrictor)Slows the reject flow, which sets how much water is recovered.
Quick-connect fittingTubing pushes in to lock; pressing the collet ring releases it.
Tee fittingJoins the tank line to the inlet of the post filter.
Membrane housing capUnscrews so the membrane can be pulled out and replaced.
O-ringA rubber ring in a groove that seals a screwed joint.
How this list was compiled: Part names and every detail line were checked, for facts only and with nothing copied, against pages read on 2026-09-20. A state university extension guide: https://extensionpubs.unl.edu/publication/g1490/na/html/view (typical systems have a pretreatment filter, the RO membrane, a flow regulator, a post-treatment filter, a storage tank and a dispensing faucet; sediment pre-filters remove silt and other large particles; activated carbon pre-filters remove chlorine that may harm the membrane; the membrane can be spiral wound; the storage tank is pressurized to give adequate flow with the tap open; post-filters remove taste and odor; shut-off valves stop the flow when the storage tank is full; the flow regulator sits on the waste stream side and a slower waste flow gives a higher recovery rate; a separate faucet is installed at the sink). The federal water-efficiency program: https://www.epa.gov/watersense/point-use-reverse-osmosis-systems , https://www.epa.gov/system/files/documents/2024-11/ws-products-watersense-ro-systems-specification.pdf and https://www.epa.gov/system/files/documents/2024-11/ws-products-ro-systems-mini-report.pdf (permeate; reject water, also called concentrate or brine; RO membrane, pre-filters, post-filters and storage tank; the automatic shut-off device stops reject water being discharged when the system is not treating water, and shuts off the incoming water when the storage tank fills). A water treatment trade association's glossary: https://wqa.org/education/glossary-of-terms/ (air gap, check valve, flow control and flow controller, permeate, reject water also called brine, concentrate or retentate, storage tank, polishing filter, feedwater, drain line, activated carbon in block and granulated form, activated carbon block filter, spiral-wound, module as the membrane element and its housing). Maker A's installation manual: https://cdn.shopify.com/s/files/1/0674/1304/9597/files/ROES-50-V7.2.pdf?v=1749750244 (sediment pre-filter and housing, carbon block pre-filters, membrane and housing, in-line carbon filter as the last stage, storage tank, tank ball valve, automatic shut off valve, check valve that blocks tank water from flowing back into the membrane, T-fitting on the last stage filter with one end to the tank valve, feed water adapter with shut off valve, drain saddle above the trap, drain tubing with the flow restrictor, faucet and the optional air-gap faucet with its three tubes, quick connect fittings and the collet ring, housing O-rings, membrane housing cap, the tank bladder, the air valve under a cap and the pre-charge; stage 1 is the right-hand housing). Maker B's instruction manual: https://cdn.shopify.com/s/files/1/0025/0963/5637/files/RO_System_Instruction_Manual_2022.pdf?v=1651620803 (feed water adapter valve, filter housing, automatic shut-off valve that reads the back pressure of a full tank and shuts off the incoming water to the membrane, RO membrane housing and its cap, check valve, drain flow restrictor, drain saddle, stem run tee on the inline post carbon filter, tank valve, water storage tank with a bladder and an air valve on its lower side, RO faucet, sediment, GAC and carbon block filters, quick connect collet; the sediment housing is the rightmost). Wikipedia: https://en.wikipedia.org/wiki/Reverse_osmosis (household use, thin-film composite membranes, spiral-wound modules, feedwater, permeate and concentrate; the article no longer lists the stages of a household unit, so no part name rests on it alone). Every part asked has two of these sources or more. Names with one source are in leftOut. Licence: own work
The drawing: Drawn for this site on 2026-09-28: a composite teaching system, not any product; nothing traced or copied. Licence: own work
How it works
The parts of a reverse osmosis system sit in flow order: a feed water adapter, three pre-filters, the RO membrane, a storage tank, a post filter and the faucet. This drawing opens the sink cabinet and enlarges the membrane housing with its small valves.
Read the full guide
Reverse osmosis parts in the order water meets them
Cold water leaves the feed water adapter and crosses the sediment pre-filter, then two carbon pre-filters that take out chlorine. At the RO membrane it splits. Permeate, the treated part, passes the check valve to the storage tank, and crosses the post filter when the faucet opens. Reject water passes the flow restrictor and runs down the drain line to the drain saddle.
What shuts a reverse osmosis system off?
The automatic shut-off valve. It reads the back pressure of a full storage tank and closes the feed to the membrane, so no more reject water is made until the faucet draws the tank down.
Pick the group Membrane and valves close-up for its 8 small parts, from the automatic shut-off valve to the flow restrictor. The drawing zooms in.
Start here: 8 main parts covers what shows with the cabinet open, from the RO faucet to the drain saddle.
Questions and answers
What are the stages of a reverse osmosis system?
This drawing has five: a sediment pre-filter, a granular carbon pre-filter, a carbon block pre-filter, the RO membrane and a post filter. Some systems add stages, such as an ultraviolet lamp, and this drawing leaves those out.
What is the tank for on a reverse osmosis system?
The storage tank keeps treated water ready, because the membrane fills it slowly. Inside, a bladder holds the water, and the pre-charged air around the bladder pushes water up to the faucet as soon as it opens.
What is an air gap faucet on a reverse osmosis system?
A faucet with a third tube. Reject water is routed up through the faucet, across an open gap, and back down a wider line to the drain saddle, so drain water cannot flow back into the system. This drawing shows a standard faucet.
Where does the waste water from a reverse osmosis system go?
Reject water, also called concentrate or brine, leaves the membrane housing, passes the flow restrictor, runs along the drain line and enters the sink's drain pipe at the drain saddle, above the trap.