What is a Water Storage Tank and How Does It Work?

Posted by
John Woodard on April 01, 2024

A water storage tank holds clean water from your reverse osmosis system or other treatment systems. Pressurized storage tanks force water out on demand, while atmospheric tanks require a booster pump to supply pressure. Water storage tanks exist in a vast array of sizes, designs, and specifications, and can be used residentially, commercially, and for large-scale industrial or municipal enterprises. In this article, you can discover the answers to some common questions about water storage tanks.

What is a water storage tank? 

water storage tank is a collection container that receives water and stores it for later use and timely access. When you turn on your kitchen faucet, water is carried from the tank to your tap, providing you with fresh water on demand. Reverse osmosis systems work slowly, purifying water one drop at a time. A reverse osmosis storage tank ensures you can access this water whenever you need, without waiting for the system to painstakingly fill up your glass. Many wells are low-pressure and low-recovery and are tasked with providing pressurized water throughout a household or business. A well pressure tank ensures that when you turn your shower on or flush your toilet, you have immediate access to pressurized water. They also help extend the life of the well pump by protecting the pump from short-cycling.   

Water storage tanks of all shapes, sizes, and configurations are used for a variety of purposes across the water treatment industry. From a small 5-gallon reverse osmosis tank stored neatly under your sink to a towering 120-gallon well pressure tank, water storage tanks are an integral part of many household water systems. Large outdoor atmospheric tanks can store thousands of gallons of rainwater or well water that can be used for fire suppression or outdoor or indoor use. These tanks are usually equipped with UV inhibitors, preventing sunlight from facilitating algae and bacterial growth within the tank. Thermal expansion tanks prevent water heaters on closed water supplies from leaking and bursting. 

How does a water storage tank work? 

A water storage tank holds clean water from your reverse osmosis system until a demand for water is initiated in the house or business. Water is pumped into the tank from a water source, such as a well or a reverse osmosis system. The tank accumulates water until it reaches its maximum capacity. When you need access to water to fill a glass, run a bath, irrigate a field, or for any other use, the storage tank provides you with instantaneous access to water. 

Water storage tanks are generally either pressure tanks or atmospheric tanks. Point-of-use applications, like reverse osmosis storage tanks, usually implement pressure tanks. Outdoor applications or large-scale municipal water storage operations usually use atmospheric tanks.

Pressure tanks 

Pressure tanks use compressed air to create water pressure within the tank. A pressure tank is constructed with an air chamber or bladder and comes with a pressure pre-charge. As the tank fills up with water, the weight of the water will begin to compress the air. As the air continues to compress, the pressure within the tank builds. When the tank reaches a certain internal pressure, it signals the feed source to cease delivery. When you open a faucet in your kitchen, the air pressure bears down on the water as it is released from the tank, propelling pressurized water through your pipes and out of your kitchen sink. When water exits the tank, the air will start to expand again, reducing the tank pressure, and signaling to the water source to recommence delivery. If the pressure tank is being used to store well water, this means a pressure switch will signal the well pump to turn back on. If the storage tank is collecting RO water, lowering pressure will signal the reverse osmosis system to restart production. These tanks are known as hydropneumatic tanks because they use the combined power of water and air to generate pressure within a tank.  

Pressure tanks allow for pressurization without the aid of pumps. Since air will compress and water will not, the air within the tank will organically increase the pressure of the water as the tank fills up. There are also pressure tanks that do not rely on hydropneumatic pressure. These are known as constant pressure tanks. Constant pressure tanks provide city-like water pressure at all times, regardless of how many appliances are demanding water. A hydropneumatic tank can become overwhelmed if too much water is being drawn from it and sent to multiple sources concurrently. This results in a loss of pressure throughout the home. Constant pressure tanks use a sensor to monitor water pressure and keep the water in the tank at a stable, consistent pressure. 

Atmospheric tanks

Atmospheric tanks do not contain air bladders or diaphragms to pressurize the water within their reservoirs. Instead, atmospheric tanks hold water at ambient pressure. Ambient pressure is simply the pre-existing pressure corresponding with wherever the tank is located (usually about 0.5psi). There is no pressure exerted on the water within the tank. Atmospheric tanks can also be used to house liquids like crude oil, and some are capable of housing chemicals and acids. For example, the underground storage tanks at gas stations hold the gasoline at atmospheric pressure. 

To draw water out of an atmospheric storage tank, you need a water booster pump to move the water out of the tank and throughout the house. Since the water in these tanks does not have any pressure exerted upon it, you will need the assistance of a pump or gravity to move water out of the tank and through your pipes. UV-stabilized atmospheric tanks are commonly used in outdoor applications, such as rainwater collection or agricultural purposes like fertilizer storage. They can also be used for water distribution, transporting wastewater, or commercial chemical storage. Atmospheric tanks tend to be much larger than pressure tanks, holding thousands of gallons at a time. The ambient pressure and UV inhibitors of atmospheric tanks also lend them more weather resistance even under harsh conditions. 

Why are water storage tanks elevated? 

Atmospheric water storage tanks are elevated to assist them in generating water pressure. If a water storage tank is elevated high enough, gravity alone can trigger water pressure identical to that of large water booster pumps. For example, community water towers act on this principle. Water is pumped up into the water tower and then held until demand is initiated. When you turn on your faucet, the water tower can deliver your home high-pressure water because of its elevation. Water towers are usually around 165 feet tall. For each foot the water travels downward, it will take on about 0.43 psi of pressure, meaning water reaches your showerhead and kitchen tap between 50 and 100 psi. 

This is all achieved by gravity alone, meaning the city doesn’t have to pay for immense and expensive water pumps or the fuel to keep them running. This can also be mimicked residentially. Keeping an atmospheric tank away from your home and at an elevation can generate an increase in water pressure as the water is forced out of the storage tank.  

elevated water storage tank

What is an underground water storage tank?

An underground water storage tank, typically called a water cistern, is used to safely store potable drinking water underground. Underground water storage tanks can hold thousands of gallons of water for agriculture or a few hundred gallons to support the water needs of a small home. Most commonly seen outside of the United States, cisterns are popular water storage tanks around the globe. Cisterns are usually made from ribbed plastic resin. The ribbing allows the storage tank to be buried underground without disrupting the soil. It’s important for water cisterns to never be completely drained of water. The pressure from the earth could warp the plastic resin and distort the tank. The water helps equalize the pressure between the tank and the surrounding soil. 

In addition to drinking water, cisterns can also be used to store wastewater and rainwater. Rooftops can be equipped with rainwater collection troughs that harvest rainwater and channel it underground into a cistern. For people living in areas where groundwater is unsuitable for drinking and municipal water is unavailable, this is a very viable water source if rainfall is plentiful. For example, mining towns along the Eastern seaboard of the United States have adopted cistern rainwater collection because of the damage done to local groundwater by mining. The rainwater can then be pumped into the home, like from a well, and used for bathing, cooking, and drinking. In this case, the water must first be passed through a water filtration system.

Learn more: Above ground vs underground water storage tanks

What are water storage tanks used for? 

Water storage tanks are used across a variety of applications where water needs to be stored for ready access. Household water storage, commercial food and beverage preparation, agriculture and irrigation, fire suppression, and industrial manufacturing all utilize water storage tanks to streamline water delivery. The size and specifications of the tank will vary based on the application the tank is being used for and the daily water demand of the home or business.   

Well water systems

Well pressure tanks are used to maintain water pressure throughout households that rely on well water. Well pressure tanks also preserve the longevity of well pumps by protecting them from rapidly cycling on and off. When a pump is forced to turn on and off repeatedly, undue duress is placed on the pump, causing it to prematurely fail. Well pressure tanks insulate the pump by providing a quantity of water for appliances to draw upon before asking the well pump to kick into action. This extends the time between pump cycles. When you turn on a faucet to quickly rinse off your hands, water from the pressure tank flows to the sink. The well pump isn’t forced to turn on and then shut off to complete such a brief task. When a pump is forced to turn on and off every time your home demands water, it is known as short cycling. Short cycling puts tremendous strain on your well pump, and with a well pressure tank, you will find yourself replacing your pump far too often. 

Well pressure tanks also help maintain consistent water pressure throughout your house. These are hydropneumatic tanks, meaning they provide sufficient water pressure. The tanks air chamber compresses as water fills the storage tank and release pressurized water into your home when demanded.

Learn more: 4 well pressure tank problems and how to fix them

Reverse osmosis systems

Reverse osmosis storage tanks work on the same principle as well pressure tanks on a smaller scale. They are also hydropneumatic water storage tanks designed to provide pressurized water without the aid of a booster pump. They protect the reverse osmosis unit overall by ceasing water production when the tank capacity is full. Once the pressure tank’s psi reaches 2/3 of the water line pressure, the ASO valve will shut the system. This protects the RO membrane from being in constant use and prevents the system from perpetually sending wastewater to the drain. Reverse osmosis tanks are made of rolled steel, but internally lined with an inert material called butyl. This protects the purified water from contacting any metallic surface. 

RO tanks come in a variety of sizes, but in general, they are designed to fit snugly beneath your kitchen counter alongside your reverse osmosis system. Reverse osmosis takes a long time to purify water, so RO storage tanks also act as simple collection container for treated water so you can access it at your convenience. If you want to increase your reverse osmosis system’s tank capacity, the addition of a second tank will expand your water storage. You can also add an RO booster pump to the water’s feed pressure. The reverse osmosis system will stop producing water when the tank’s internal pressure reaches 2/3 of the line pressure. If you have low water pressure running through your reverse osmosis system to begin with, your tank will shut off much sooner. Increasing the feed pressure will raise the maximum capacity of your storage tank. Use a pressure gauge to monitor the air pressure within your RO tank. 

Learn more: How reverse osmosis storage tanks work | How reverse osmosis works

Water heater thermal expansion

Thermal expansion tanks protect your plumbing system by increasing the capacity of your traditional tank-style water heater. Through a process called thermal expansion, your water heater will expand as it heats water up. For example, if your 40-gallon water heater is filled with water, once that water increases in temperature, it will expand the sides of the tank. This means your 40-gallon tank’s volume will increase to around 42 gallons when heating the water. If these extra two gallons have nowhere to go, they will push against the walls of the water heater. This expansion and contraction strains the walls of the tank. Given enough exposure to these increased pressures, the tank can even burst. Replacing a water heater is incredibly expensive, not to mention repairing the damage of a basement flooded with hot water. 

Thermal expansion tanks attach directly to the water heater, providing relief for those extra 2 gallons of water. Thermal expansion tanks control pressure build-ups in closed water systems. To ensure the pressure is stabilized, make sure that the pressure charge in the thermal expansion tank is the same as the pressure in your water heating system. 

Learn more: What is a water heater thermal expansion tank?

How to size a water storage tank

The size water storage appropriate for your home will depend on your water usage and the application. For example, if you are using a reverse osmosis storage tank to provide water throughout your home, estimate how much demand will be on that system and size accordingly based on RO output and storage. If your RO system is serving an office, consider what times of day the water demands are the highest. Fill the tank first thing in the morning so people will have fresh water to brew coffee or espresso, and make sure the tank is large enough to refill by lunch so people can fill up their glasses and water bottles. When sizing an RO tank, you want to make sure you have enough tank capacity to support the habits of everyone in the building. You can always increase the storage capacity and water pressure of the RO tank by connecting to a second tank. 

Sizing a well pressure tank is a more complex task because you need to account for more than just drinking water habits. You must ensure there is enough water to support bathing, flushing toilets, and running appliances. Some tank manufacturers recommend estimating your tank size by counting the number of water-using fixtures the tank will be servicing and multiplying that number by three. For example, if you have two showers, three toilets, five faucets, one dishwasher, one laundry machine, and a refrigerator, you’d have a total of thirteen appliances demanding water. Multiplied by 3, this means you would want at least a 39-gallon tank to support the home. When in doubt, it is best to purchase a larger tank thank you think you need. Properly sized well tanks extend the life of the well pump by protecting the pump from cycling on and off too rapidly. 

Sizing a tank properly requires specific information about your home and water, including flow rates, tank drawdown capacity, pressure switch setting, and the minimum runtime of your pump.

Learn more: How to size a well pressure tank

How to calculate water storage tank capacity

While many water storage tanks come with a stated gallon capacity on the label, this is not always the tank capacity of the water storage tank. If this is a pressure tank or a reverse osmosis tank, the tank comes equipped with a metal diaphragm and an air bladder. If the tank is a 32-gallon tank, that number is referring to the total void volume of the tank. Tank capacity is the actual amount of water the tank can hold. 

The tank capacity that is best suited for your needs depends upon your individual needs. Water storage tanks are always sized based on demand. If you are a cafe supplying water from a reverse osmosis tank to several coffee makers, espresso machines, and ice makers, your tank capacity is going to differ dramatically from a family of two using an RO tank in their home. Finding a water storage tank that can supply you with water throughout the day without interruptions or reduced pressure indicates you have found the proper tank capacity for your needs. 

well pressure tank

How to install a water storage tank

The storage tank installation process differs depending on the application of the tank. Reverse osmosis tanks are very simple to install, requiring only a piece of plastic tubing and a couple of plastic quick-connect fittings to connect your tank’s control valve to the air gap faucet.

Though installing a well storage tank system is more involved than an RO storage tank, it is not an overly difficult or time-consuming task. A well tank installation requires several parts to ensure the system runs smoothly. 

    • Connect the tank to a water inlet from the well. You’ll need to make sure the pressure tank is connected to the water line running to the well. The well pump should be pumping water directly into the tank. Install fittings to route the plumbing from the well directly to the storage tank. 
    • Make sure a check valve is in place. Check valves allow water to flow in only one direction. A check valve will let the well pump push water into the tank, and prevent it from flowing back out of the tank and down into the well once the pump has turned off. 
    • Install a pressure relief valve. Pressure relief valves are designed to open when a specified pressure is reached and allow water to flow through until the pressure drops to the desired level. These prevent excessive pressure build-up that could cause the storage tanks to exceed their design limits
    • Ensure the tank is connected to a boiler drain. In the event that you need to drain the entire tank, a boiler drain allows you to quickly and efficiently do so. Rather than turning off the well pump and running your faucet until the tank empties out, boiler drains release all the water with the turn of a knob. 
    • Attach a pressure switch. Pressure switches tell the well pump when to turn on and off based on the pressure building within the tank. Without this, the pump will not know when to stop delivering water to the tank. For example, a 40/60 pressure switch allows water to drain out of the tank until the internal pressure reaches 40psi. This then tells the well pump to activate and start pumping water. The well pump will continue to fill the tank until the tank reaches 60psi, at which point the pump will turn off until the next pressure drop.

All of these components can be easily purchased and installed together as one unit called a tank fittings package. You can select your desired pressure switch settings and get all the required fittings in one convenient package. 

Learn more: Plastic tubing glossary | How quick-connect fittings work 

How to clean a water storage tank

All water storage tanks require periodic cleaning and maintenance to keep your water and storage tank clean. Routine cleaning flushes debris from the storage tank and eliminates any bacteria or algae growth blooming within the reservoir. Reverse osmosis tanks require annual cleaning cycles to rid them of any slime that has accumulated within the tank. Since the RO’s pre-filters and post-filters must be replaced every 12 months, many RO owners choose to pair the cleaning cycle with their filter replacements. For reverse osmosis storage tanks, a sanitization kit like Sani-System can be run through the system. This kit is entirely chlorine-free and NSF-certified to eliminate e.coli and other harmful bacteria. This will keep your RO running smoothly and alleviate any fear of bacteria spreading within the tank. If you have an atmospheric storage tank, periodically add drops of chlorine to disinfect the water and eliminate any bacterial growth. 

Sediment from your well water will accumulate in your well tank over time. In general, placing a sediment filter between the pressure switch and the well pump is discouraged, as it can interrupt the on/off cycling of the well pump. However, if enough sediment collects within your well pressure tank, you may notice drops in household pressure. To rid your tank of any build-up, you will need to periodically flush your system. 

How to flush a well pressure tank 

    • Isolate the tank. Make sure the tank is no longer running water into your home before you begin flushing it. Locate the ball valve on the tank package or on the pipe running from your home to the tank. Turn the valve 90 degrees so that the water supply is shut off. 
    • Turn off the well pump. Find the switch or breaker controlling the well pump and cut all power to it. The storage tank should now be totally isolated. 
    • Connect a hose to the boiler drain. Grab a plastic bucket so you can observe the water being discharged by your well tank and position the hose over it. 
    • Drain the well tank. Watch as the water exits the tank. If the water is significantly cloudy, you know you’ll need to repeat the process several times. If there’s a significant build-up of sediment, it's also a good indication you should flush your storage tank more regularly. 
    • Restore power to the well pump. Once you’ve drained the tank, allow the well pump to fill your pressure tank back up with several more gallons of water. 
    • Repeat the flushing process. Continue doing this until the water running out of the boiler drain is clear and there’s no visible sediment exiting the tank. 

To disinfect your well pressure tank, use unscented, NSF-certified bleach to rinse out the tank of bacteria growth. The industry standard is to use one gallon of bleach for every thousand gallons of water (which would break down to one quart for every 250 gallons, or 2 cups for every 125 gallons.) This solution creates a chlorine concentration of 50ppm. Letting this solution sit in your tank for 24 hours will effectively neutralize any extant bacterial presence. 

Learn more: How sediment filters work | How sump pumps work

How to prevent a water storage tank from freezing

To keep your water storage tank from freezing, make sure it is properly insulated in the winter, placed in a location like a barn, shed, or garage. Consider installing an aeration pump to keep the tank’s water circulating. If you have an outdoor water storage tank in a part of the country prone to frigid temperatures, you will need to take precautions to keep your water from freezing over. Moving the tank to an indoor location will protect it from direct exposure to extreme temperatures. Placing an insulated cover over the tank will trap heat within the tank and prevent the water from chilling too quickly. Perhaps the best way to ensure that your water stays in liquid form is to keep the water in perpetual circulation. Standing water freezes much quicker than moving water, so the addition of an aeration pump to the tank will keep the water in motion. 

You can also always upgrade to a larger tank. The larger your atmospheric tank’s reservoir of water is, the longer it will be able to withstand freezing temperatures. Round tanks are also less likely to freeze compared to rectangular tanks, as they have less surface area and therefore emit less heat. Barrel or drum heaters can be wrapped around tanks and distribute uniform heat throughout the water storage tank. 


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