Under-sink water filter vs. reverse osmosis: which system do you have?

Look for a storage tank

A rounded pressurized storage tank is the clearest sign of a traditional reverse-osmosis system. RO filtration is slower, so treated water is commonly stored in a tank until the dedicated faucet is opened.

A direct under-sink filter usually has one to three cartridges and no storage tank. It may feed a separate drinking-water faucet or connect to the main cold-water faucet.

Identify the RO membrane stage

Traditional RO systems often have several vertical prefilter housings plus a long horizontal membrane housing. The membrane is a separate stage with its own part number and capacity rating. Tankless RO systems may enclose these parts in one cabinet, so the model label becomes especially important.

• Storage tank: strong clue for traditional reverse osmosis
• Horizontal membrane housing: common on open-frame RO systems
• One to three vertical cartridges without a tank: commonly direct filtration
• Enclosed cabinet with drain connection: may be tankless RO

Confirm before choosing replacements

Appearance helps identify the system type, but it does not prove cartridge compatibility. Photograph the system model and all installed filter numbers, then compare them with the manufacturer's replacement schedule.

Reverse-osmosis systems also connect to a drain. If you are unsure which tubing is which, do not disconnect it simply to identify the system.