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What is a Service Reservoir?

By C. Martin
Updated May 17, 2024
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A service reservoir is a water storage container that holds clean water after it has been treated in a water plant, and before it is piped to the end users. These containers are covered, and are designed to keep the water safe from contamination. Their main purpose is to provide a buffer within the water supply system so that water supplies can be maintained across periods of varying demand.

Service reservoirs typically need to be situated at a sufficient height to maintain enough pressure in the downstream pipe network to provide a good flow to the area being supplied, and to enable the water to be raised up to the top of buildings. For this reason, they are sometimes built in the form of water towers. Where water service reservoirs can be built on elevated ground, however, they are often situated underground. A water service reservoir structure is usually based on concrete or steel. There is often an inner lining to protect the water from contamination from the structure itself.

Service reservoir storage requirements that affect the volume necessary for a service reservoir water supply include operating requirements, equalizing requirements, and fire or emergency requirements. Operating volume requirements are dictated by the amount of water that is needed to maintain flow and water pressure for normal domestic and industrial use in the area being supplied. Equalizing requirements are worked out using the volumes of water needed to refill the reservoir after daily periods of peak demand. Fire or emergency storage volume requirements are based on estimates of the amount of water that may be needed in the area in exceptional circumstances. In many areas, fire authorities and water companies work together to ensure that the service reservoirs have sufficient emergency storage for the area being supplied.

Most reservoirs also have a certain amount of water volume, called “dead” volume, which is the volume of water that lies at the bottom of the container and that cannot be practically used due to low pressure, or due to dead flow zones where the water has become stagnant and of insufficient quality. Dead storage volumes are usually largest in tall service reservoirs. Flow modeling can be used to study the water flow that occurs in various sizes and shapes of water reservoirs, and to help design structures that can minimize the occurrence of dead flow zones, and improve water service reservoir storage efficiency.

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Discussion Comments

By anon264739 — On Apr 29, 2012

For a 100,000-gallon capacity water tank, normally 60' of height is required to supply water through the gravity system. The size should be 48'x 32'x10'. --sajjusweet

By matthewc23 — On Sep 12, 2011

I see water towers all the time in smaller cities but never (or very rarely) in big cities. Do bigger cities usually use underground service reservoir systems, or is there some other method?

I have seen in pictures where large skyscrapers and apartment buildings will have their own towers. The same with big factories.

My guess is that normal water towers would take up a lot of valuable space, not to mention they would have to be huge. It still begs the question of how the water is supplied to normal homes, though.

By Izzy78 — On Sep 11, 2011

@ - Great point. My town just got a new water town about ten years ago, and there were huge debates about not only where it should be located but also what color it should be painted and what should be put on it.

In the end, they decided on paining it white and putting the name of the town with a picture of the American flag. The flag itself drew a lot of criticism, surprisingly.

Unfortunately, one of the things they didn't consider was that the water tower was located next to several fields and an interstate, which means that it gets extremely dirty in a hurry, and they have to spend more money cleaning and painting it every year.

By stl156 — On Sep 11, 2011

@Emilski - I guess it does seem kind of counter-intuitive to pump water to the top of the service reservoir just to let it go back down, but what you have to consider is the huge amount of water pressure formed by the tower being as tall as it is.

The water in the tower is constantly causing downward pressure, so whenever you turn on a faucet, it pushes water through. If you had the pumps at ground level, there would have to be a pump constantly running to create the right water pressure. With the water tower system, the pump can just run when the tower needs filled. I hope that makes sense.

Of course it isn't the main purpose of service reservoirs, but they give towns the opportunity to display their name.

By Emilski — On Sep 10, 2011

I understand the principle of how this system is supposed to work, but how does the water get to the top of the service reservoir to start with, assuming we're talking about a water tower?

Water has some ability to push itself upward through small diameter pipes, but there is no way a water tower that is 100 feet tall could pull up enough water to fill itself without a pump. If you have to expend all the energy from the pump to get the water to the top of the tower, why not just use a surface level reservoir and save some of the energy that goes into the pump? I'm sure I am missing something in this process, but it just doesn't make sense to me.

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