Choosing the Right Water Storage for Your Community is an Important Decision
Nearly every community has one, emblazoned with the town’s name and towering above the community. A water tower often becomes a symbol of a municipality but for those who supervise and secure important infrastructure, the water tower is an important piece of equipment providing clean water to hundreds of thousands of residents.
The purpose of water storage tanks is usually to meet peak demands, such as fire flows and times of the day when water use is high. There are a few items to consider when selecting a new water storage tank for your community or industry.
Traditionally, it has been a common practice of many water utilities to keep storage tanks in a full or near full condition to be better prepared for peak water use periods and for emergencies such as fires. As a result, many storage facilities operate with larger water storage capacity than is needed for non-emergency usage. Also, some storage facilities – especially older ones – have an overflow elevation that is below the pressure of the water system. A valve prevents entry of water into the tank, and pumps are needed to move water from the tank into the distribution system. In these cases, water resides in the tank for a very long time, causing water quality problems.
Regardless of the type of tank facility, it is critical that the overflow elevation is properly matched to the pressure of the system, as this allows the water surface within the tank to “float” on the system. In other words, the water elevation in the tank itself creates the pressure in the distribution system. When tanks are being filled via pump stations, the pressure of the system is increased, and water flows into tank facilities. Normally, the water level in a storage tank routinely fluctuates on a daily basis through a fill and drain cycle. The difference in the fluctuation will vary depending upon the size (diameter and height) of the storage facility, system demand, and location in the distribution system.
Nevertheless, the water in the tank should fluctuate several feet prior to the start of the pump station or additional pumps coming online to ensure that proper circulation in the tank is occurring.
Ground Storage and Elevated Storage – Which is Right for you?
Required water distribution storage capacity for potable water systems is traditionally met by the use of ground, elevated or standpipe storage tanks or a combination of all three.
- Ground Storage Tanks – These are ground supported, flat bottom cylindrical tanks with a shell height less than or equal to its diameter and usually constructed of steel or concrete. A storage tank can be broken down into distinctive water storage components, each of which serves a particular purpose. Equalization storage is located at the top of the tank and is typically cycled on a daily basis. This meets the water system demands that are in excess of the pumping capacity of the system. Equalization storage can further be broken down into operational storage, low-end storage, and effective storage. Emergency storage is defined as the water level in the reservoir above which 20 psi can be maintained within the distribution system for fire flow and emergency service. The upper end of the emergency storage is typically defined as the bottom of the equalization storage component. The water below the emergency storage in the reservoir is considered to be “dead” storage volume and is not considered usable. This “dead” storage, especially if it is a relatively large volume and the tank is not turned over frequently, can lead to water quality problems in instances where internal tank mixing systems are not in place.
- Standpipe Storage Tanks – A standpipe storage tank is also a ground-level water storage tank that is available in a wide range of sizes. Standpipes are greater in height than diameter and can be designed with decorative elements to help blend the tank into the surrounding environment. Their taller design allows water above the operating range to typically provide gravity-fed pressure. The water below the operating range provides reserve storage.
- Elevated Storage Tanks – These consist of two primary components: the tank (or bowl which holds the water) and the supporting structure or tower which supports the bowl. These types of tanks are commonly constructed where the ground elevation is insufficient for the use of reservoir style tanks or where greater storage capacity than could normally be achieved with a standpipe is required. There are several common styles of elevated tanks including spheroid, fluted column and composite (steel bowl with concrete support structure). These tanks have commonly been constructed of welded steel; however, recently composite-style tanks are gaining prominence. Composite tanks utilize a steel bowl atop a concrete tower.
In order to pick the best solution for a community, there are many factors that need to be taken into consideration.
- Pumping Systems – The ground storage tanks may utilize either a direct or indirect pumping system. A direct pumping system pumps water from the reservoir to system pressures. In an indirect pumping system, water “floats” on the system and flows by gravity from the tank to system pressure. Newly designed elevated storage tanks are almost always indirect systems, and float at the connected system pressure. The indirect pumping system has several advantages over a direct pumping system in a water distribution system:
- Lowering peak pumping rates, maintain constant, reliable water supply and pressures.
- Stabilizing pressure variations as demands fluctuate.
- Balancing and leveling pump operations.
- Reducing need for wide range of pump sizes.
- Decreasing power costs – especially for time-of-day electricity pricing.
- Dampening extreme low and high low pressures (surge relief).
- Enhancing emergency response during water main breaks or power failure.
- Enhancing fire flow and pressure response.
- Size – Typically, a water tower is sized to hold approximately one day’s worth of water for the community served by the tower. If the pumps fail (for example, during a power failure), the water tower holds enough water to meet the community’s demand for about one day. In addition, the tower may play a major role during a fire and can affect the community’s insurance rates. During a fire, the water demand increases significantly and may greatly exceed the capacity of the pumps at the community’s water plant.
- Height – The elevation of the water tower determines the water pressure in the community. New systems are required to provide a minimum pressure of 20 PSI (pounds per square inch) at ground level at all points in the distribution system under all flow conditions. Typically, normal working pressure in a distribution system will be approximately 60 to 80 PSI and not less than 35 PSI. The water level in the tower must be high enough to supply that level of pressure to all customers in the zone served by the tower.
- Location – Available land for new water storage tanks and the location go hand in hand to also determine the required height of the tank. Water towers are typically located on high ground and tall enough to provide the necessary pressure. By choosing a high site, the height of the tower can be reduced, thus reducing the cost of construction. However, many factors can complicate location selection. For instance, overhead or underground utility cables may need to be relocated. Utility relocations are time-consuming and need advance planning and coordination with the utilities and regulatory agencies. In locating an elevated tank, the following criteria are recommended for consideration:
- To maintain a pressure of 60-80 PSI in the distribution system, the tank height is limited to 200 feet (ground to overflow). So higher ground elevations are preferred.
- Location within a pressure zone: To maximize the benefits from hydraulic water pressure to a pressure zone, the tank must be located in the central portion of the zone.
- Distance from existing water transmission main: Tank sites further from the water transmission main may require a connecting main at additional cost
- Site area, availability and accessibility: Adequate site area will be required to provide clearance for construction and future maintenance of the tank. The site needs to be available to purchase, preferably undeveloped, to overcome resistance from other property owners. The site shall provide adequate access for construction and maintenance equipment. The costs associated with acquiring the parcel must be considered.
- Foundation Design – The requirements for Foundation Design make choosing the location of a new water tower important, and sometimes difficult. Engineers must complete detailed geotechnical investigations to understand the underlying soil properties at potential sites. This is an important step to prepare the foundation design for the water tank.
- Water Age – Water quality is impacted by keeping freshwater in the system. This means turning over the water in the tower. Increased water age can lead to deteriorating water quality such as the increased formation of disinfection-by-products (DBPs), nitrification, and loss of disinfection residual. Taste and odor problems can also result from excessive water age. Therefore, the location for the water tower must be selected with an understanding of how water moves through a water distribution system. Poor site location can have a seriously detrimental effect on distribution system water quality.
- Maintenance – Once a location is found, proper maintenance is crucial. Water tanks need to be periodically inspected and cleaned to maintain good water quality in the community’s water distribution system and to help extend the life of the storage tank. Regular tank inspections can also help to identify small problems and prevent major problems from developing that can create health-related issues, leading to expensive repairs or premature structural failure of the tanks. Tanks that are not periodically cleaned can cause contamination events that can harm human health or generate aesthetic complaints. Typically a community should plan on performing an interior and exterior inspection of their water tanks once every three to five years. Some states require that the interior and exterior of all water storage facilities of 10,000 gallons or larger shall be inspected and maintained a minimum of every five years by a professional tank inspection firm or by a registered professional engineer. In addition, the interior and exterior coatings on steel elevated water storage tanks should be inspected by a person trained to evaluate the integrity of the paint system. Although rare today, tanks painted before the late 1970s could still have a lead paint coating if the original paint had not been removed by sandblasting to bare metal. Making sure this has been done should be the focus of any inspection of older tanks.
The timing of inspections can be tricky, because a tank needs to be taken out of service for interior inspections. The first question to be answered is whether the tower can be taken offline and, if so, for how long. A test shutdown can be performed to reveal any potential challenges prior to conducting the work. Numerous factors will dictate when you can most easily take your tank out of service. Location of the tank, required flow rates, peak demand, the ability to isolate the water tower from the system, and the possibility of ice in the water tower (in winter and spring months) are all considerations which affect the final decision. Elevated storage tanks are best inspected during the warmer weather between late spring and early fall. If the water tower is susceptible to the formation of ice, inspection is not an option until the ice is completely out of the tower. There are methods of inspection that can occur without taking the tower out of service. One method is to use a remote-operated vehicle (ROV). Alternatively, inspection divers can be hired to visually inspect the interior surfaces. A permit is generally not required from the regulatory agencies to inspect, clean, repair or paint a tank. However, a permit will be required if any modifications are made to the tank such as installing vents, hatchways, overflows or any type of mixing devices. It is recommended that the local regulatory agencies be informed before tank inspection or cleaning. After underwater inspection and/or cleaning, and prior to placing the tank in service, water samples from the tank must be analyzed for chlorine residual and or coliform bacteria. If coliform bacteria are absent and chlorine residual is acceptable, the tank can be placed back into service. If not, continue water sampling until two consecutive samples for coliform are negative. If necessary, the tank should be disinfected.
The most common work performed on water towers is repainting. Given the height and shape of water towers, and choices in coating systems with different life expectancies, this can be a complicated process. Many tanks will require the exterior of a tank to be prepared with a commercial sandblast to remove all existing coating. If the tank is located near parks, residences or businesses, sandblasting operations would require a protective containment system, with the canopy over the tank during the work. Prior to sandblasting to remove the existing coatings, environmental sampling is recommended to test for the presence of lead in existing coatings. If lead is present in the existing coatings, appropriate containment systems need to be designed and provided.
HR Green has extensive experience in designing, constructing, maintaining and refurbishing water storage tanks for both public and private uses. To learn more contact Ravi Jayaraman, PE (in Illinois), Heath Picken, PE (in Iowa), Mark Hardie, PE (in South Dakota), Adam Salo, PE (in Minnesota), and Michael Halde, PE (in Texas).