How does a Reverse Osmosis Pressure Tank Work

Reverse osmosis (RO) is a process that purifies water and separates chemicals, pesticides, pharmaceuticals, and dissolved solids from the purified-water.

RO systems make use of tanks to use as storage for the water(purified water) that the RO membranes produce. These tanks are pressurized, so it could provide water immediately when there is a need for it. These tanks are also capable of turning off the RO system when it gets full because it has its shut-off valve.

All About Reverse Osmotic Tanks

Reverse osmotic tanks are hydropneumatic, meaning it is operating by both water and air or other gas(Merriam Webster). These tanks serve as the storage place, which collects the water that has been purified by the RO systems. The outer layer of these tanks are steel, but the inner layer or inside is composed of a substance called butyl. These prevent the water from having any contact with metal, which in terms also prevents contamination from metals.

The reverse osmosis system takes time before they could produce a decent amount of purified water. It is a water filtration method that uses membrane as the main base of its action. The water goes through a consecutive number of these membranes, which are the ones responsible for the filtration. The tanks provide a place to gather filtered water, which makes it easy to have immediate access to plenty of water.

Most RO tanks are small enough that they are possible to place in areas with limited spaces (examples are under kitchen sinks), but some are large enough that it could support a whole house or a building.

How do they Work?

These tanks use air pressure to transfer water instantly from the tank to a designated area where there is a need for water without requiring help from a booster pump. As stated in the introduction, these tanks are hydropneumatic because they not only keep water, but they also hold pressurized air. They contain a water chamber and an air chamber.

As the stored water increases, its weight increases, and then it starts to compress the air chamber, which would then increase the air pressure inside the tank. When you try to access water from your designated faucets, the air pressure transfers the water from the tank to those faucets. Without the air chamber, your tank will only be able to store water but not be able to send it anywhere you want, unless you have help from a delivery pump.

The tank size is the factor considered when you want to know where your water goes when it is inside the tank. The water goes on the top of the air chamber in smaller tanks. In larger ones(more than 10 gallons), the water goes under the air chamber or at the bottom part.

Importance of RO Tanks

RO tanks are vital because they supply two actions that are major in the whole process or RO systems. The first one is, it gives you purified water whenever you need it. The second one is, it activates the on and off of the whole RO system. These tanks sustain pressure within the entire RO system, they have a monitoring line pressure and sensory valve when it reaches ⅔ of the line, the RO System turns off automatically, and there is an immediate stop of water production. These actions help conserve a large amount of water. Without this feature, the RO system will continue to gather water and filter it through the membranes. Since there will be no available space for storage, the excess purified water will go to the drain directly. An additional benefit is it lessens the strain on your RO membrane.

Reverse Osmosis Capacity

The capacity of RO Tanks depends on their sizes. Keep in mind that gallon capacity stated by manufacturers includes not only the water capacity but also the space for the metal bladder and air. It is the total void volume of tanks internal spaces. These mean the definite amount of storage for water is affected by the feed pressure, air chamber, and valve on/offsetting.

How to Increase Tank Capacity

If your present storage tank fails to supply the amount of water you need and you want to increase its volume capacity, you can have an additional storage tank. These tanks will be connected and act as one. An increase in storage volume means an increase in water delivery capacity.

Booster Pump Kit

One more way to enhance tank storage capacity is by installing an RO booster pump. Usually, RO systems need feed pressure around 50 psi to perform efficiently. Lower psi results in slower water production, less efficient filtration, and an increase in waste. In addition to that, lower psi activates the on/off valve or shut-off valve faster. Improving the psi of your feed pressure will increase your tank storage capacity before it shuts off.

How to know when your tank is losing pressure

As time goes by, your tank will slowly lose some air from the air chamber, which will then decrease the pressure charge and water flow rate. These happen naturally to tanks over time. If you are having trouble acquiring sufficient water from your taps or faucets, You could do some tests to check if your RO tank is losing pressure.

-First, separate the tank and the RO system and check if your RO system can still produce filtered water.
– Then try to lift(for smaller ones) or move(for bigger tank), then check its weight.
-If you know your tank has water inside, and your RO can still produce filtered water, that usually means that it has a significant decrease in its air charge.

How to Re-pressurize Tank

The air valve(called Schroeder valve) of RO tanks is the same as the valve seen in bicycle tires, which means that you can also use a bicycle pump to bring back some pressure in your tank.

-First drain out the remaining water in your tank
-Then remove the cap that covers the valve
-Pump air into the tank valve(using a bicycle pump). These will drain out the remaining water
-Continue pumping until you reach 7 to 10 psi(standard recommended pre-charge), using a pressure gauge will help you with this.
-Make sure that you do not over pressurize

Adding more than the standard recommended pre-charge will not increase your water pressure. These will only lessen your tank water storage capacity.

When to Replace Your Tanks

If you are still unable to have water in your designated faucets even after re-pressurizing your tanks, then maybe it’s time to have your tank replaced. There are situations where-in tank problems are irreparable, and you would need to buy a new tank. However, you don’t need to worry much because most tanks have a 5years warranty and usually last another 5years before you need to have it replaced.


Your RO tanks work in tandem with the whole RO system. They work by making sure that you have a place to store your filtered water. They are also responsible for the water transfer from the RO system to every designated faucet in your house or building.
RO tanks check-ups are easy to do but do not hesitate to ask for professional help whenever you are unsure of what to do.

How does reverse osmosis work?