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.