What is a Reverse Osmosis Tank and How Does it Work?

Posted by
John Woodard on November 06, 2023

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, how to achieve your ideal tank capacity, and some of the best reverse osmosis tanks. 

What is a reverse osmosis tank? 

reverse osmosis tank is a hydropneumatic pressure tank that stores treated water from a reverse osmosis system. Reverse osmosis is a slow process. Water is pushed through the semipermeable membrane one drop at a time. The tank allows water to accumulate, supplying immediate access to plenty of water. RO tanks are designed to fit underneath the sink and inline with your reverse osmosis system.

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

The inside of a reverse osmosis tank contains 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. Without an air chamber, the tank would fill with water but not be able 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 can 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. The primary purpose of an RO tank is to provide water whenever it is needed. Its 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 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.

Learn more: 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 can hold. 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 RO tank 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 must 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. 

Does an RO booster pump affect tank capacity?

An RO booster pump can increase your tank’s capacity if your feed pressure is low. Reverse osmosis systems require at least 50 psi to perform optimally. If your feed pressure is below 50 psi, 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: How water booster pumps work | How demand and delivery pumps work    

How to choose the right reverse osmosis tank

When looking for a reverse osmosis tank, look for two criteria: capacity and material.

The capacity of an RO tank should provide enough water for your kitchen sink on even the busiest day. The stated capacities of RO tanks are the total volume of the inside of the tank. Because air is needed inside the tank to maintain pressure, these tanks cannot work while completely full of water. As a general rule, assume an RO storage tank can hold about half of its stated capacity. While RO tanks can usually hold more than half of their total capacity, you can guarantee you are purchasing a large enough tank if you assume their real capacity is about 50%.

The material used for an RO storage tank determines its longevity. The most common materials used for RO tanks are stainless steel and polypropylene. Polypropylene is a thermoplastic polymer that is used for a variety of purposes, such as plastic packaging, machinery, and textiles. Polypropylene RO tanks are typically less expensive than stainless steel tanks, but they are also less durable. In contrast, you can expect a stainless steel RO tank to last you a decade or longer.

How to repressurize a 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, the same valve you’ll find on your bicycle tire. Before repressurizing your RO tank, open the faucet and drain all 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. Next, 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 higher pressures to the tank, you will 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, leaving you with less water.

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 is time to replace the tank. If water exits 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 need to purchase a new tank. 

Reverse osmosis tanks typically last a long time. They are usually warrantied for 5 years, but most of them will continue to operate well beyond that. In most cases, 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 must change out the reverse osmosis filters every 12 months, you can 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 a reverse osmosis storage tank

    1. Wash your hands. Since reverse osmosis produces 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 ice, make sure this is disconnected before proceeding. 
    4. Open the system’s faucet and drain the tank. 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 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, signaling 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 exit 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 your RO system is ready to use again.

Best reverse osmosis storage tanks

Here are some of the best reverse osmosis tanks for your reverse osmosis system.

PAE RO-132 Powder Coated Steel Reverse Osmosis Tank

The PAE RO-132 is a 4.5-gallon tank features a polypropylene liner, high-grade chlorobutyl diaphragm, and a stainless steel 1/4" MPT water connection. Its diaphragm is post cured to ensure the quality of your water is not compromised during storage. The stainless steel exterior of the tank is coated with a two-part epoxy primer and polyurethane paint finish that protect the tank from damage and extend its life. For home reverse osmosis systems with low demand for high volumes of water, the PAE RO-132 is a perfect fit.

Amtrol RO-10 Stainless Steel Reverse Osmosis Storage Tank

The Amtrol RO-10 is a 10-gallon reverse osmosis tank manufactured to last for many years with your system. This tank comprises of stainless steel water connections, a high-grade chlorobutyl diaphragm, and a polypropylene liner. Its butyl water reservoir guarantees water has minimal contact with rubber, preserving the water's fresh taste and odor. Each Amtrol RO-10 is individually tested and comes with a 5-year warranty, making it a safe and reliable storage tank for your reverse osmosis system.

ROMATE RO-80 80-gallon Reverse Osmosis Storage Tank

The ROMATE RO-80 is a reverse osmosis storage tank made of heavy-duty butyl that resists chlorine and is lead-free, preventing chemicals and other unwanted elements from being introduced into your water. It comes with a 5-year warranty for the tank body and a 1-year warranty for the bladder. This tank will suit your needs as a reliable, well-constructed reverse osmosis tank.

ROMATE RO-120 120-gallon Composite Reverse Osmosis Storage Tank

The ROMATE RO-120 is an excellent option for those with high-volume reverse osmosis needs. It is WQA certified for lead free compliance, and its heavy-duty butyl aircell construction resists chlorine and other unwanted chemicals. Like the RO-80, this tank comes with a 5-year warranty on the tank body and a 1-year warranty on the bladder. Because of its higher capacity, this tank is excellent for household or small-scale commercial reverse osmosis applications.

Learn more: How reverse osmosis works | 3 of our favorite reverse osmosis systems 

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