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Reverse osmosis systems utilize pressurized tanks to store purified water until the demand for the water is initiated. Reverse osmosis storage tanks also keep the RO system efficient by turning the system on and off as the tank fills with water and pressure increases. Join our Master Water Specialist, as we explore how reverse osmosis tanks work, how to repressurize your tank, and how to achieve your ideal tank capacity. 

What is a reverse osmosis tank? 

A reverse osmosis tank is a hydropneumatic pressure tank designed to fit beneath the sink and inline with your reverse osmosis system. The reverse osmosis tank is a storage tank, collecting the water being gradually purified by the reverse osmosis membrane. The reverse osmosis filtration process is a slow one. Water is pushed through the semipermeable membrane one drop at a time. The tank allows water to accumulate, so, when you go to pour yourself a glass of water you can have immediate access to plenty of water. 

Reverse osmosis tanks are made out of rolled steel and internally lined with an inert material called butyl. The butyl lining ensures that the purified RO water never comes into contact with the steel. 

How does a reverse osmosis tank work? 

Reverse osmosis tanks use air pressure to deliver water from the tank to the dedicated faucet above your sink. Reverse osmosis tanks are hydropneumatic, meaning the tank is holding not only water but also pressurized air. Hydropneumatic storage tanks are capable of delivering pressurized water swiftly and on-demand, without necessitating the assistance of a booster pump

Inside the reverse osmosis tank is both an air chamber and a water chamber, divided in the center by a bladder. Though water does not compress, air will. As the reverse osmosis system feeds water into the storage tank, the weight of the water begins to compress the air chamber. As air compresses, it will continue to increase in pressure. When you open your faucet, this air pressure propels the water out of the tank and up through your faucet. If the tank did not have an air chamber, the tank would fill with water but unable to transport it anywhere without the assistance of something like a delivery pump. Hydropneumatic storage tanks allow you to have a delivery system without an electric pump. 

The size of the tank predetermines where the water will exist within the tank. In smaller tanks, between one and ten gallons, the water sits on top of the pressurized air chamber. As tanks get larger and larger, the water sits on the bottom of the tank. Water weighs about nine pounds a gallon, so as the volume of stored water increases, the tank needs gravity to assist with the air compression and propulsion of the water out of the tank. On smaller tanks, where the air chamber rests at the bottom, the diaphragm is able to push down into the air chamber and compress the air. 

Why are reverse osmosis tanks important?

The reverse osmosis tank provides two very important functions in the context of the RO system as a whole. Primarily, the tank provides you with pressurized water whenever you require it. But it’s secondary function is no less important to the system’s operation. The reverse osmosis tank maintains pressure throughout the reverse osmosis system and actuates the on/off cycling of the system by monitoring line pressure. The reverse osmosis system is equipped with a sensory valve that halts the production of water when the pressure in the tank reaches 2/3 of the line pressure. If your feed pressure is 60 psi, the membrane will continue to filter water and fill the storage tank until the compressing air within that tank reaches 40 psi. When the tank senses that 2/3 of line pressure has been reached, it will move the valve to the closed position, and water production will cease. The automatic shutoff valves, (stylized as ASO valves) work in tandem with the storage tank to conserve thousands of gallons of water per year. If the system did not automatically shut off when the tank filled, water would continue to be sent through the membrane and down the drain. 

Explore more about how storage tanks work.

What is the tank capacity of a reverse osmosis tank?

The actual tank capacity of an RO tank will differ from the gallon capacity the manufacturer indicates the tank is capable of holding. When the label says a storage tank is 4.5 gallons, it is referring to the total void volume of the tank. But, the internal space of these tanks is occupied by a metal bladder and air, not only water. So, this 4.5 gallon tank will actually hold around 2.5-3 gallons of water. Furthermore, the exact amount of water the tank is capable of holding will differ based on the feed pressure and the settings on the shutoff valve. 

How do I increase my tank capacity? 

If you are unsatisfied with your current holding capacity, you can increase the volume of stored water by adding another storage tank. These tanks, when teed together, will work in tandem as one tank. By increasing the volume of stored water, you will also enhance the delivery capabilities of your reverse osmosis system. If you are running water from your RO system to the ice machine in your refrigerator, it is likely that the ice is producing slowly because of the distance the water has to travel. Increasing your tank capacity with a second tank will increase the overall pressure. This is because as the tank empties, the air pressure within that tank will also start to drop down to its standard pre-charge pressure as water exits and the air expands. When you add a second tank, the line running to your ice maker will pull from both tanks simultaneously. As water is drawn from both tanks, the amount of water leaving each tank is reduced, enabling higher pressure for a longer period of time. 

Another way to increase your tank capacity is to add an RO booster pump. Reverse osmosis systems require at least 50 psi to perform optimally. If your feed pressure is below that, your system will produce water much slower, your waste will increase, and the filtration will not be as thorough. Furthermore, the storage tank will both accumulate water slower and shut off faster. If your feed pressure is only 45 psi, it will only take 30 psi in the tank to actuate the shutoff valve. By increasing your feed pressure, you will increase how much water your tank can hold before shutting off. 

Learn more about how water booster pumps work. | Learn more about how demand and delivery pumps work.     

How do I repressurize my reverse osmosis tank? 

If your reverse osmosis tank has lost its pressure, you can easily add pressure back to the tank with a bicycle pump. The air valve on an RO tank is called a Schroeder valve, and its the same valve you’ll find on your bicycle tire. Before repressurizing your RO tank, open the faucet and drain all of the water out of the tank. If your tank has lost pressure entirely, unscrew the cap covering the valve and start pumping air into the tank using the bicycle pump until water starts coming out. Once the tank is drained, continue adding pressure until you reach around 7 psi. This is the standard recommended pre-charge for most reverse osmosis tanks. You can monitor this by using a pressure gauge. Be mindful not to overpressurize your tank. 

Why is my reverse osmosis tank losing pressure? 

Over time, air will seep out of the tank, causing the storage tank to lose its pressure charge and your water’s flow rate to decrease significantly. This is a normal occurrence and will happen to any tank over time. If you are no longer receiving water at the tap and you believe your tank has lost pressure, it’s always wise to do a quick troubleshooting test. Isolate the storage tank and run the reverse osmosis system to make sure it is still capable of producing water. Then, lift the tank up and test its weight. If your tank is heavy and filled with water, and your RO system is still capable of filtering water in isolation from the tank, that typically means the tank has lost its air charge. 

Will adding more pressure to my reverse osmosis tank increase my water pressure? 

No, adding more pressure to the tank will merely reduce your tank capacity. The recommended pre-charge setting for a reverse osmosis storage tank is 7-10 psi. If you add more pressure than that to the tank, you will start to significantly limit how much water that tank can hold. The reverse osmosis system will stop producing water when the tank pressure reaches 2/3 of the line pressure. If 2/3 of your line pressure is 40 psi, and you pump your tank up to 20 psi, that shutoff will occur much faster and you will have less water at your disposal.

Learn more reverse osmosis troubleshooting tips

When do I need to replace my reverse osmosis tank? 

If you repressurize your reverse osmosis storage tank and water still fails to come out of your faucet, it’s a good indication it’s time to replace the tank. If water is coming out of the Schroeder valve when you test the air pressure, this means that the bladder within the tank has failed. This is not a repairable problem, and you will have to purchase a new tank. 

However, generally speaking, reverse osmosis tanks tend to last quite a long time. They are usually warrantied for 5 years, but most of them will continue to operate well beyond that. Usually a tank will not need to replaced sooner than ten years. 

How often should I sanitize my reverse osmosis tank? 

You should sanitize your storage tank once a year. Since you have to change out the reverse osmosis filters every 12 months, it’s a good idea to pair your annual tank cleaning with the filter change. Sanitizing the tank purges it of any kind of build-up that may have accumulated within the tank over the past year, and ensures a sterile environment for your purified RO water.

How to sanitize your reverse osmosis storage tank: 

    1. Wash your hands. Since reverse osmosis is producing water of such high purity, you want to make sure you don’t contaminate any part of your system. Ensure the area is clear of any dirt or dust, and consider wearing sanitary gloves.
    2. Turn off the water supply line to your RO unit. You don’t want water spilling out all over your floor, so be sure there’s no water running to the reverse osmosis system. 
    3. Disconnect any lines running to the refrigerator or icemaker. If you use RO water in your fridge to prepare crystal clear ice, make sure this is disconnected before proceeding. 
    4. Open the system’s faucet and drain the tank. You want to make sure you drain the system entirely of water. Running the faucet will also depressurize your storage tank. Leave the faucet open until all water drains, then shut it off. 
    5. Remove all prefilters and the membrane. Open up all the filter housing and remove the filters. There should be no filters or membranes inline your system during the sanitization process except for the postfilter. If you are exchanging them for new filters, discard the old filters at this point. 
    6. Reconnect the filter housings to the system. Replace all housing back on the system, except for the pre-filter housing. 
    7. Pour the sanitizing solution into the pre-filter. We recommend using the NSF-certified sanitization solution called Sani-System. It is the only EPA approved RO sanitization solution, and it requires no measuring or mixing. It doesn’t contain any chlorine and will neutralize any bacteria growing within your tank. Attach the pre-filter housing back to the system. 
    8. Turn the water supply back on. Allow the storage tank to fill up with water. Turn on the RO faucet until water begins to flow out, alerting you the tank has filled. Turn off the faucet. 
    9. Allow the solution to sit. This will vary based on the sanitization solution you are using. If you’re using Sani-System, you need to wait for at least one minute. 
    10. Flush the entire system. Open the faucet and allow the reverse osmosis system to flush itself. Wait around five minutes. Allow the tank to again fill with water before flushing it a second time to ensure all traces of sanitization solution exits the tank. 
    11. Disconnect the water supply and drain the system. Disconnect the water supply and open the faucet. Wait until all the water drains out of the system and the tank completely depressurizes. 
    12. Replace all the filters and membranes. This is a great time to replace your filters, as pairing the maintenance with filter replacements ensures your system is regularly cleaned and your filters are replaced on time. 
    13. Reconnect the water supply. If you disconnected any icemaker or refrigerator connections, reattach them at this time. Allow the storage tank to refill with water, and you’re ready to use your reverse osmosis system again.

Learn more about how reverse osmosis works. | Explore 5 of our favorite reverse osmosis systems


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