What is the storage tank in reverse osmosis water filters and how does it work?

What is a RO storage tank and how does it work?

Nowadays, most of us resort to water purifiers to get pure water for ourselves. And, the most popular of those is the reverse osmosis or RO water filters.

All of the reverse osmosis water filters come with various other instruments packaged inside. They will have a variety of pre and post-filters, a reverse osmosis membrane, and so on.

Another notable element that comes with all the RO water filters is a storage tank. In truth, this storage tank plays a significant role in water purification.

That’s why you need to know how this storage tank works. In turn, this can help you when your system is not working as per your expectation.

While troubleshooting, you need to make sure the storage tank is working well. That’s why you need to get some basics about it. 

In this article, we’ll be discussing what a RO storage tank is and how it works. We’ll also cover some other issues regarding it. So, let’s go.

What is the storage tank in a RO water filter?

Now, let’s start by discussing what a storage tank is. In general, a tank works to store things for us; the thing may be water, air, or something else.

For example, an oxygen tank contains oxygen for us. On this note, we’re not talking about the tanks used in the war.

So, a storage tank is a cylindrical device that holds the purified water from a RO water filter for later usage.

Every reverse osmosis water filter comes with a water storage tank of its own. This one stays in between the reverse osmosis membrane and the post-filter.

That means purified water from the reverse osmosis membrane will continue to gather at the tank and as soon as you open the faucet, water will travel to the faucet through the post-filter.

So, now that you know what the storage tank is, let us have a closer look into what is inside a storage tank.

What are the components of a storage tank?

Now, let’s take a look inside a RO water storage tank. All conventional under sink RO water filters will have two compartments inside.

The first one holds the purified water coming from the reverse osmosis membrane. The second one contains air.

Therefore, you can see, there are a water chamber and an air chamber inside all the RO storage tanks. A horizontal elastic diaphragm separates the two chambers.

In most cases, the air chamber remains in the lower portion of the tank. In that case, the other one will be above the diaphragm.

On the other hand, some reverse osmosis water filters will have the air chamber at the upper portion of the tank while the water chamber will remain underneath.

Moreover, there is a valve in the air chamber. With this valve, you can regulate the air pressure inside the storage tank.

In general, pure water from the reverse osmosis membrane will travel through a tube and enter the water chamber. Furthermore, there is another valve at the top of the tank.

This valve will regulate the water flow coming out of the tank. A tube from the water chamber will take the stored water, pass through the valve, and enter into the post filter.

So, you get the picture. In short, there is a water chamber, an air chamber, one incoming tube, one outgoing tube, and two valves in a water tank. Now, let’s see how the tank works.

How does the RO storage tank work?

Now, let’s look into how the storage tank works. As you’ve already seen in the previous section, there are two compartments separated by a bladder.

The purified water from the reverse osmosis membrane will enter into the water chamber. On the other hand, the air chamber already has a definite amount of air inside.

The air chamber is already enclosed. As the air valve is tightly enclosed, there is no way for the air to leak in any way. 

Now, as more and more water seeps into the water chamber, this will continue to put pressure onto the air chamber. This pressure will translate into energy.

The air chamber will store this energy. This is almost similar to the mechanism of how spring works. As you squeeze a spring, it will accumulate mechanical energy.

Now, as soon as you release the pressure, the spring will bounce back. The storage tank follows the same principle. As more water enters, more pressure develops in the air chamber.

When you open the faucet, the air chamber will propel water through the tube with this stored up energy. In most cases, the storage tank stays on the ground.

The energy from the air chamber helps water fight against gravity to reach the post-filter or the faucet.

So, you see, the RO storage tank saves up energy in the form of pneumatic pressure and then, throws the stored water to the post-filter using this energy. This is how the storage tank works.

Why is there a storage tank in RO water filters?

Now, the natural question that can come into your mind is whether this storage tank is necessary or not.

To understand that, we have to take a look at how reverse osmosis works. Reverse osmosis is a process in which water molecules are forced to pass through a membrane.

The RO membrane has a selective property. Only water molecules can seep through. Therefore, other dissolved molecules like ions or others get left out.

If you would like to know more about reverse osmosis, you should visit our post here.

As we’ve already mentioned, you need to apply an external force to purify the water through the reverse osmosis chamber. This comes in the form of water pressure.

Most of us get our water through municipal water supplies at a moderate flow rate. As it arrives through the distribution system, the flow rate translates into water pressure.

This water pressure is enough to purify water molecules through the reverse osmosis. So, you can realize filtration through the RO membrane is a slow process.

As a result, if you are suddenly in need of water and the faucet is directly linked with the output of the membrane, it will take a lot of time to fill your glass.

That’s why you need to preserve the water beforehand. And, that’s where the storage tank comes in. 

The purified water will keep on gathering inside the tank. Now, as soon as you open the faucet, the air pressure will send water into your glass at a moderate rate.

That’s why all the RO water filters come with a storage tank of their own to accommodate the latency in the RO chamber.

How much water does a storage tank hold?

Now that we have covered all the trivial questions about the storage tank in RO water filters, it is time to dig into more about some analytics.

So, let’s first take a look at the standard size of these RO filter tanks. To be completely honest with you, there is no definite answer as to the size of the water tank. 

The capacity of the storage tank varies from one brand to the other. But, in general, most of the under-sink RO water filters out there come with a tank capacity of 3-5 gallons.

Now, one thing you have to remember is that here the tank capacity stands for the total space inside the tank. You already know that the tank also has an air chamber.

As a result, the water chamber will occupy a space smaller than the total capacity of the tank. 

Another factor that can influence the tank’s capacity to hold water is the input water pressure at the tank.

If the incoming water pressure at the tank is high, then the tank will contain more water. Let’s take a look at the following example of the APEC water filter.

At water pressure higher than 70 PSI, a 4-gallon tank will contain water of 3.1 gallons. If the pressure is at 60 PSI, it will fill up to 2.8 gallons.

If the input water pressure falls to 50 PSI, the amount of water will go down to 2.5 gallons, while if the pressure is at 40 PSI, only 1.9 gallons of water will get into the tank.

We’ve compiled a list showing you some of the best under sink water filters and how much water they can hold in their tanks respectively. Take a look.

Name of the water filterMaximum Water holding capacity of the storage tank
APEC RO Water filter3.2 gallons
Express water systems RO water filter2-2.25 gallons
iSpring RO water filter2.5 gallons

How much time does it take to fill up the tank?

Now, let’s move onto another important aspect of these RO storage tanks. Yes, you guessed it. Let’s take a look at how fast these tanks fill up with purified water.

In general, most of the reserve tanks for reverse osmosis water filters would fill up within 1.5 to 3 hours. 

The estimated timeline to fill up the tank depends on so many different factors like the brand, size, or input water pressure of the water filter.

For example, it would take about 2-3 hours to fill up the tank of an Express Water Systems RO water filter. On the other hand, the same thing would happen at around 2.5 hours in a Home Master RO water filter.

So, you see, the duration will vary from one brand of filter to the other. We have compiled a list showing you the duration to fill the tank vs. brand of the water filter. Take a look.

Name of the water filterApproximate duration to fill up the tank
Express Water system RO water filter2-3 hours
APEC RO water filter1.5-2 hours
iSpring RO water filter1-3 hours
Home Master RO water filter2.5 hours

The duration will also depend on the size of the reserve tank. The higher the volume of the storage tank, the slower it will fill up.

For example, a 4-gallon water tank will take approximately 2.5-3 hours to fill up. On the other hand, a 3-gallon reserve tank will fill up within 1.5-2 hours.

There is another factor at play here- the input water pressure. If the input water pressure is too low, it would take too much time to attain the complete volume of the water tank.

Therefore, you see, the brand, size, or the water pressure can significantly influence the duration to fill up. But, on average, it takes about 1.5-3 hours when everything is working fine.


And, with this, we’re ending our discussion here on the reserve water tank that comes with all the RO water filters.

In this article, we explained the basics about the RO storage tank- how it works, how much water it can hold at a time, or how much time it takes to fill up.

On this note, we also showed you some valid data about various brands of RO water filters. We hope that our article has answered all your queries about RO storage tanks.

If you still have a question or two, don’t forget to leave those in our comment section. Also if you would like to read more informative posts like this, check out our blog page here. Cheers!!!

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