A reverse osmosis water tank is the small pressurized vessel under the sink that stores what the membrane has slowly produced, so the faucet delivers a usable stream instead of a trickle. Inside is a rubber bladder holding the water, with compressed air on the outside pushing it back out.
Why the tank exists
A household membrane produces water at a slow, steady rate measured in gallons per day, nowhere near the rate you want when filling a pot. The system therefore runs whenever the tank is not full, parking its output in the bladder against an air charge. When you open the faucet, that compressed air squeezes the bladder and pushes water out at a sensible flow. Storage is also why an under-sink unit can serve a refrigerator ice maker, which draws in bursts the membrane could never match live.
The air charge is a maintenance item
The air pre-charge is set with the tank completely empty of water, measured at the schrader valve on the tank shell, and it must sit below the system's operating pressure or the tank will refuse to fill. Air escapes slowly over years, and a tank that has lost its charge fills with water but delivers barely anything, because there is nothing pushing it out. Checking and topping the charge with a bicycle pump and a low-range gauge is a ten-minute job that solves a great many slow-faucet complaints.
Diagnosing a tank that will not deliver
Isolate the tank valve and open the faucet: if only a small amount comes out and then stops, the tank is the problem, not the membrane. Disconnect the tank, drain it fully and check the air pressure. If the charge is fine and the tank still will not deliver, the bladder has split, and water on the air side confirms it: air blows out wet at the valve. A ruptured bladder cannot be repaired. Also check the tank valve itself, which is often left half-closed after a cartridge change.
Capacity, weight and sanitation
Nominal tank capacity is not usable capacity, since the air charge means the tank never gives up everything it holds. A full tank is heavy, so it should stand upright on the cabinet floor or on a proper bracket, not hang from tubing. Water stored in a bladder picks up a flat taste, which is what the post-tank polishing carbon filter is for. Sanitize the tank when you change cartridges, following the maker's procedure, and replace the polishing filter afterwards rather than before.
Questions people ask about reverse osmosis water tank
Why is my reverse osmosis faucet flow so weak?
Most often a lost air charge in the storage tank. Drain the tank completely and check the pressure at its air valve before suspecting the membrane.
How do I know if the tank bladder has failed?
Water sprays or dribbles from the schrader air valve when you press it. A wet air side means the bladder has split and the tank needs replacing.
Can I run a reverse osmosis system without a tank?
Only if it is a tankless design built for it. A conventional membrane produces too slowly to serve a faucet directly, so removing the tank leaves you with a trickle.