4.2: Water Storage
Student Learning Outcomes
After reading this chapter, you should be able to:
- Explain the uses and requirements of water storage
- List the various types of water storage facilities
- List water storage facility equipment
- Describe the operations and maintenance of water storage facilities
Water storage is an important component of a distribution system. The main purpose is to provide sufficient amounts of water to average or equalize the daily demands on the system. Storage provides increased operating conveniences by providing sources of supply throughout the day and at night when utility employees are not typically at work. It levels out pumping requirements, can decrease power costs from pumping, provides a supply of water during power outages, fire storage, and other benefits. This chapter of the text will examine water storage systems, the various types of water storage, how water storage affects water quality, and a general overview of the operation and maintenance of water storage structures.
Why Water Storage Is Needed
Water demand is highest in the early more and evening hours of the day. In the morning, customers are showering, using toilets, and preparing meals, which require water. In addition, most irrigation systems are in use in the early morning hours before temperatures get too hot. In the evening hours, people are returning from work and school and water demand typically increases during these times. There is low use midday and during overnight hours. Without storage, the pumping capacity would be approximately two times the average requirement. Adequate storage allows for uniform pumping. Storage also allows minimal operator involvement during non-working hours. Without storage, workers would need to be available to operate the system if unexpected increases in demand occur. A utility would require various different size pumps to match changes in demand. Frequent on-off cycling of pumps causes increased wear on pumps and motors and higher costs for electricity.
Another benefit of water storage is the ability to store and supply water during emergencies and power outages. If a water utility solely relied on pumps to meet demands, anytime there is a power outage, water service would be interrupted. Having water stored in tanks allows for uninterrupted service during these times. Some distribution systems have long transmission water mains bringing the water supply into the community being served. Under these circumstances, any repairs made to these pipes requiring the water main to be isolated, would disrupt water supply to customers.
Fire fighting is another critical need of community water systems. Water utilities not only need to meet operational demands, they must also meet the demands to extinguish fires. Fire fighting may account for as much as fifty (50) percent of total storage. In addition, fire-fighting demands must be met during main line breaks, power outages, and maximum customer demands. Adequate pressure must also be maintained during fire flows.
Water Storage and Water Quality
Water storage can help with water quality and it can also contribute to water quality problems. As part of the Safe Drinking Water Act, a set of regulations called Surface Water Treatment Rule (SWTR), requires specific times that chlorine must be in contact with the water before the water reaches the first customer. Water storage tanks can provide detention time to allow the chlorine to remain in contact with the water long enough provide the required time. Water storage can also be an area where blending multiple sources of supply can take place. If an impaired source of supply is pumped into a storage tank, unimpaired sources can also be pumped into the tank providing adequate blending of the impaired source.
Water quality can also degrade in water storage tanks. While water storage tanks provide various benefits, storing too much water can lead to water quality degradation. Chlorine residuals can diminish and water can become stagnant if the water within storage tanks is not changed. Cycling tanks (allowing the level in a tank to rise and fall) can help avoid stagnation and water quality degradation.
What Type and Size of Storage Is Needed?
Water storage tanks come in various sizes and styles. Some of the factors to determine the type and capacity of storage in a distribution system depend on the size of the system, the topography of the distribution system, and how the distribution system is laid out (is the system spread out or concentrated in a small area). These and other criteria are used to design the water storage needs.
Several terms related to water storage should be understood.
- Average Day Demand (ADD)—This is the total demand for water during a period of time divided by the number of days in that time period. This is also called the average daily demand.
- Maximum Day Demand (MDD)—This is the highest total demand over a 24 hour period within a given year
- Peak Hour Demand (PHD)—This is the maximum demand over a one hour period within a given year
- Float on the System—This is a method of operating a storage facility. Daily flow into the system is approximately equal to the average day demand. When customer demands are low, the storage facility will be filling and when demands are high the storage facility will be emptying.
System hydraulics are directly related to the location of water storage facilities within a distribution system. If a water storage tank is located in close proximity to a pumping station, the head loss (pressure) to the farthest portion of the distribution system may be excessive through normal size piping. Additional transmission mains can help alleviate this type of pressure loss. If a storage tank is placed at the farthest end of a service area adequate pressure is typically received at the far ends and near the pumping stations. This type of set up avoids the need for increased main sizes. However, there must be enough capacity to the remote location to refill the tank during off-peak periods. In addition, if there is a great separation between the pumping facility and storage, lower pressures might occur in the middle of the distribution system. If possible, locating storage structures adjacent to the area with the lowest pressure is ideal. This typically provides enough available pressure to the entire service area and smaller diameter water mains can be used because flow from the tank is split into two directions.
For hydraulic purposes, it is more ideal to have multiple smaller tanks instead of one larger tank. This allows for more stabilized and equal pressures throughout the distribution system. Head losses also increase whenever pumping is required over long distances and during peak demand conditions. Locating storage tanks near the center of a distribution system allows pumping stations to operate at or near average day demand conditions most of the time.
Types of Water Storage Structures
There are various types of water storage facilities. They are made of different types of materials and are designed in different shapes to serve various needs. While there are various storage structures storing raw water and within a treatment plant, this section only discusses storage structures found in distribution systems. The following are some of the more common water storage facilities within a distribution system:
- Elevated Storage Tanks—In regions with relatively flat topography, elevated storage tanks are commonly used. They are above ground tanks supported by a steel or concrete tower or pedestal. Most are made of steel and designed to float on the system. If they are not constructed tall enough, they can overflow and provide inadequate pressures.
- Hydroneumatic Storage Tanks—These tanks are used in very small systems with in adequate pressure. They are kept partially full with compressed air to provide water in excess of the pump capacity when required. These types of systems will also provide water for a limited time if a pump fails.
- Standpipes—These tanks are constructed directly on the ground and have a height greater than the diameter. They are commonly used to equalize storage near a source of supply like a well field. They can also be used to provide additional fire protection.
- Above Ground Storage Tanks—These are the most commonly used forms of storage tanks along the west coast of the United States. They can store large quantities of water and are located where the topography is such that they can be constructed on hillsides. The main downside is that they require a fairly large area of land.
Components of Elevated and Above Ground Storage Tanks
These tanks have very similar components. This section will review the major components and address any differences between the two styles of storage structures.
Inlet and Outlet Pipes
Elevated storage tanks generally have a common inlet and outlet pipe, while above ground storage tanks can have either common or separate inlet and outlet pipes. The purpose of these pipes is to bring water in and allow water to exit the tank. The purpose of having a separate inlet and outlet-piping configuration is to help water circulate inside the tank. The common pipe (called a riser) for an elevated storage tank typically runs up the middle of the support structure holding the tank. The inlet and outlet piping of an above-ground storage tank typically enters the tank along the bottom portion of the tank. In the separate inlet/outlet configuration, the location of the inlet and outlet connection is typically at opposite ends of the tank.
Overflow Pipe
Each type of tank is equipped with an overflow pipe. It is designed to allow water to exit the tank to the atmosphere in the event the water-level controls fail. They are commonly constructed to discharge into a catch basin and should never be directly connected to a sewer or storm drain. They should have a proper air gap separation from the area they are discharging into and should be screened or have a weighted flap to prevent animals from entering the pipe.
Drain Connection
All tanks need to be inspected periodically. While some inspections can occur with water in the tank, it is common to drain a tank for inspection, cleaning, and repairs. Water in a tank can be lowered by preventing pumps from turning on to fill the tank. However, they can only be lowered to a level the height of the outlet pipe and at no time should a tank be drained completely while in service. Once the water is brought down as far as in can while in service, a separate drain pipe can be opened to drain the rest of the water.
Monitoring Devices
Water storage tanks, just like other water distribution facilities are commonly equipped with monitoring devices. Details about these monitoring devices are covered in another chapter. However, the will also be discussed here briefly. One of the most important things to monitor on storage tanks is the water level. Therefore, most are equipped with either a physical site gauge mounted on the outside of the tank and/or level sensors which can transmit tank levels to remote locations. These devices are commonly furnished with high and low water level alarms.
Valves
In order to isolate a tank from the distribution system, a valve must be furnished along the inlet/outlet piping coming in to the tank. This valve can then be closed to take the tank out of service for maintenance and repairs. Sometimes a tank will be furnished with a valve referred to as an altitude valve. This valve is designed to close preventing the tank from overflowing.
Vents
Air ventilation is usually provided at the tops of tanks to allow air to escape as the tank is filling and air to enter and the level in the tank drops. These air vents must be large enough to prevent the tank from collapsing and they must be properly screened with a minimum mesh size of ¼”.
Access Hatches
Access inside a tank also needs to be provided. There are at least and sometimes multiple access hatches on the top of storage tanks. These allow works to enter the tank for inspection and maintenance. These hatches must be properly constructed with rims under the cover to prevent surface water runoff from getting into the tank. There are also manways at the bottom of the tank for access when a tank has been drained and taken out of service for maintenance and inspection.
Ladders
Access needs to be provided to the tops of tanks and inside tanks. Ladders usually provide this access. Some above ground water storage tanks use spiral staircases as opposed to ladders. Elevated storage tanks are usually equipped with three (3) different ladders. The first one runs up the leg of the tower from the ground to the balcony around the tank. The second ladder runs from the balcony to the top of the tank roof. The third ladder runs along the inside of the tank for inside access. Outside ladders should be installed six (6) to eight (8) feet off the ground or have a locked metal shield around the bottom to prevent unauthorized access.
Cathodic Protection
Interior tank walls are subject to corrosion, especially in the upper portions, which are not constantly submerged in water. Cathodic protection can reduce this interior corrosion in coated steel tanks. Cathodic protection reverse the flow of current that tends to dissolve iron from the tank surface causing rust and corrosion. Electrodes with a direct current (DC) are used and will corrode instead of the tank walls. In warm climates, the electrodes can be suspended from the tank roof. In cold climates, the electrodes must be submerged. The anodes can last up to ten (10) years, but should also be inspected annually.
Tank Coatings
Since steel can oxidize and deteriorate and since water is considered the “universal solvent”, it is important to properly coat the interior and exterior of tanks. Interior coatings must be able to withstand constant emersion in water, varying water temperatures, alternate wetting and drying periods, ice abrasion, high humidity, heat, chlorine, and mineral content in the water. Exterior coatings must endure similar conditions and maintain a good appearance. All interior coatings must be NSF approved.
Operation and Maintenance
The American Water Works Association recommends that all water storage structures be completely inspected every three (3) to five (5) years. Elevated and above ground storage tanks should be periodically drained, cleaned, inspected, repaired, and painted. The interior surfaces should be cleaned thoroughly with a high-pressure water jet or by sweeping and scrubbing. All dirt and debris should be removed from the tank and a complete or spot re-coating should also occur.
All tanks need to be disinfected before being placed in service. This includes new construction and tanks taken out of service for maintenance. There are three (3) basic methods for disinfecting storage tanks:
- The first method involves filling the entire tank with water and held at a disinfectant residual level of 10 mg/L. If the water is disinfected before entering the tank the detention time is six (6) hours and is the tank is filled and then disinfected, the detention time is twenty-four (24) hours.
- The second method involves spraying the interior walls with a disinfectant solution concentration of 200 mg/L
- The third method requires six (6) percent of the tank to be filled and disinfected to a residual of 50 mg/L. The tank is then completely filled and held for twenty-four (24) hours.
The tank must then be sampled and analyzed for total coliform bacteria. If the results come back positive, additional disinfection is required until two (2) consecutive samples are negative. New and recoated tanks must also be sampled and analyzed for volatile organic compounds.
Routine inspections should also be conducted at water storage tanks. The overflow piping, vents, hatches, ladders, and locks should be monitored frequently for damage and vandalism. Ladders should be in good condition and replaced if deemed unsafe. The roof and access points should also be checked for cracks and holes to prevent surface water leaking into the tank.
Sample Questions
1. An altitude valve is used to ___________.
- Prevent storage tanks from filling too fast
- Prevent storage tanks from overflowing
- Separate the inlet and out let flows
- None of the above
2. Water storage reservoirs should be completely inspected?
- Every year
- Every other year
- Every 3 to 5 years
- Every 5 to 10 years
3. Without storage, pumping capacity would be approximately ___________.
- Twice the average requirement
- Three times the average requirement
- Less than the average requirement
- None of the above
4. Fire demand may account for as much as ___________.
- 10% of storage
- 25% of storage
- 50% of storage
- 100% of storage
5. It is recommended that storage tanks have ___________.
- Separate inlet and outlet piping
- Common inlet and outlet piping
- Outlets twice as large as inlet piping
- There are no recommendations