Municipal and corporate water tanks are typically sized according to a number of criteria, such as demand (both peak usage and total daily usage), fire protection needs, and local regulations. Increasingly, other metrics are being added, impacting the factors that determine sizing.
The biggest driver in sizing water tanks, says Kevin Gallagher, vice president of Caldwell Tanks—a manufacturer of field-erected, aboveground tanks for water, oil, and gas, and petrochemicals—is customer usage. “A tank has to provide sufficient pressure and capacity for the customer’s needs.”
Those needs include the customer’s purpose or usage—their daily demand—but are also augmented by “reserve for fire [protection], power outage, and a couple days’ worth of demand.” These days, that reserve is commonly increased to account for drought, population growth, and other concerns.
DEMANDING WORK
Peak demand is often the first and most important consideration when sizing a water tank. According to the American Water Works Association, most water supply sources operate on a 24-hour production basis, producing enough water in that period to equal demand—discounting clearwells because clearwell capacity is usually considered production reserve rather than distribution reserve.
Water in excess of the hourly demand must be stored in tanks, which are often filled during the early morning hours when demand is lowest. As demand increases throughout the day, the tanks feed water back into the system, helping to maintain a relatively constant pressure. Water tanks are filled and emptied daily, Gallagher elaborates, noting that replenishment of one-quarter to three-quarters capacity is normal.
Over the past 10 years, Bruce Coe, western regional water sales manager for ZCL | Xerxes, one of the largest manufacturers of watertight fiberglass underground tanks in North America, has noticed that standby water demand is increasing due to a shortage of pressure as well as a shortage of supply.
Supply shortages are directly related to increased demand—and the busy housing industry. Simply put, more homes are being built. “Twenty years ago, the growth rate after the recession put housing at maximum capacity,” continues Coe. Today, the trend of building more homes continues. He cites Portland, OR, as an example of the high demand for new urban housing because of new construction, including 25,000 apartments in the last five years. The influx of residents adds strain to the water system.
FIRE
Many times, fire protection is second only to demand in determining tank size. The AWWA notes that insurance underwriters have devised formulas to determine the necessary quantities, pressures, and flow duration for each application, taking into consideration the uses of all buildings in the area.
Storage requirements for fire protection are typically greater than those required for system regulation. Where fire flow is significant, additional pumping capacity may be required, sometimes in addition to the use of stored water.
Fire protection is just one of six markets in the water business listed by Peter Young, vice president of Flowtite Products for Containment Solutions Inc., manufacturer of double-wall underground tanks for gas and water. The other categories include rainwater harvesting, stormwater, septic, potable water, and grease interceptors.
BY THE BOOK: REGULATIONS
Fire protection requirements go hand-in-hand with national and local regulations. “Regulations drive the size of the tank,” states Young. With regard to fire protection, some subdivisions require onsite storage and sprinkler systems. “Los Angeles requires homeowners to detain all runoff into a tank, to be released at a certain rate.” Detainment is also common for parking lots.
Regulations also govern stormwater, although permanent stormwater retention is not common, Young explains. However, he acknowledges that tanks are increasingly being used for stormwater management. Most communities at least hold it until they can pump. Detention is a temporary solution during emergency weather events.
Most hospitals are required to have potable water and septic capacity for 72 hours for emergencies. Young mentions a “huge project in California” that Flowtite worked on to provide 72 hours’ worth of emergency storage of potable water and septic for hospitals to cover them in case of fire or other crisis situations.
Tank size is driven by local and national regulations, Coe emphasizes, and by owner and engineer requirements. Consultants study the systems, evaluate usage and demand, and make recommendations. “We don’t size tanks. The owner and engineer give us the requirements—how much they need to store for fire demand, drinking water, and stormwater. The engineer determines the storage requirements. We don’t dictate.”
The cost of real estate has a significant impact on tank size.
EXPANSION, GROWTH, AND THE FUTURE
Where practical, the AWWA advises, a tank should be appropriately sized to accommodate anticipated community growth that will eventually result in additional water demand. It’s particularly important in the design of water storage tanks because enhancing their storage capacity is not always possible, either physically or financially. “Proper sizing of a storage tank must also establish proper water turnover and circulation to ensure that water quality standards are met.”
Tanks may need to be bigger in developing neighborhoods in order to accommodate population growth and its attendant needs, such as the requirement for more reserve in case of drought and, simply, community water usage. But, points out Caldwell Tanks’ Gallagher, this denotes a change. “The number of new tanks is less, but the average size is growing significantly.”
The average water tower size has grown, he elaborates, because population distribution in the last 20 years has concentrated growth in suburban areas. “Population growth continues near large cities, which are already more densely populated [than rural areas],” says Gallagher, explaining that these communities don’t need as many tanks because the geographical areas are not as large, just more densely populated. He points to Washington DC, the Carolinas, Nashville, and Louisville as prime examples of rapidly expanding urban and suburban populations where water demand has increased substantially. These locations have opted for bigger water tanks rather than additional water tanks, due to the value of real estate. Therefore, the cost of real estate has a significant impact on the size of a tank.
LIGHTWEIGHT FOR FLEXIBILITY
Because fiberglass tanks have more flexibility in diameter size, they can get into tighter spaces with a smaller footprint than other types of tanks, says Jim Merchlewitz, business development manager for water and wastewater, ZCL | Xerxes. Flexible design options allow for horizontal expansion. Available in standard sizes of 6, 8, 10, and 12 feet in diameter, fiberglass tanks are also customizable up to 60,000 gallons per tank, based on an engineer’s decision about volume and site constraints.
In addition, fiberglass is lightweight, making the material ideal for building larger tanks. Fiberglass tanks are easier to move, maneuver, and install than tanks made of other materials. They require smaller equipment, thanks to their lighter weight. But their weight is not indicative of their strength. Fiberglass tanks are strong, Merchlewitz says.
They can also be buried deeper than other alternatives. “The integral ribs and convex domes reinforce the strength of the tanks to allow for deep burial.”
Centennial College’s new Centre for Aerospace and Aviation in Toronto, Ontario (ON), Canada, didn’t meet code for fire protection, so they turned to ZCL | Xerxes for lightweight, underground fiberglass water storage tanks. The fiberglass tanks cost less to ship and install than concrete tanks and arrived fully assembled, unlike concrete tanks that require partial assembly during installation. In addition, because fiberglass tanks have greater capacity than concrete tanks, fewer were needed to supply the water volume required: three, as opposed to six to eight concrete tanks.
One option that can accommodate future growth or complicated sites is a multi-tank installation, such as tanks in parallel or tanks in a series and then connected hydraulically. “In small communities with tight budgets, this provides an option to add capacity later for future growth,” says Merchlewitz. The tanks—typically 10,000 to 50,000 gallons—are plumbed together to fill and drain at the same rate in unison.
Alternatively, multi-compartment tanks let customers purchase one tank instead of many, and adjust the size of the tank in compliance with the site’s footprint as needed.
BIGGER IS BETTER
The standard formula to determine the size of a tank has been to combine the 10-year rainfall average and the size of the roof, Young says. But formulas based on the expected amount of rain have changed.
Why? Because weather patterns have changed. “We now experience fewer but larger storms,” contends Young. Thus, historical rainfall charts are no longer accurate. Because of the change, new regulations require bigger tanks. “Tanks continue to get larger. Fifteen years ago, 90% of the tanks were eight-foot diameter tanks. Then, the same percentage were 10 feet in diameter. In the last few years, we’re seeing requests for 12-foot diameter—our largest product—and capacity has increased from 15,000 to 20,000 gallons to 30,000 to 50,000 gallons.”
One reason for the uptick in size and capacity is economics. The cost of tank installation is calculated per gallon; therefore, Young says, the larger tank you get (made of fiberglass), the cost per gallon drops. “As tanks get larger, fiberglass gets more competitive cost-wise with concrete and HDPE.”
Another factor contributing to the increased size of tanks is the decentralization of water treatment. “Less water is sent into treatment,” states Young. More septic is being treated onsite. More graywater is being treated onsite and reused. More stormwater is being treated onsite. “There is more decentralized septic treatment today because of the lack of capacity at treatment plants, so some [facilities] treat stormwater onsite.”
Rainwater harvesting has not developed because the payback is not there, Young believes, despite a slow move toward the use of rainwater treatment systems for irrigation, which can reduce the amount of treatment needed for potable water usage. In the mid-Atlantic, he says some corporations are using rainwater. “They will post-treat it and turn it into potable water.” That requires two tanks: one for rainwater, one for potable water.
But there’s been an even bigger shift to graywater systems because graywater is better at post-treatment, Young says. “Specific needs drive the size of the tank, but some locations now use two systems: potable [water] and graywater.” The separation of supplies for different purposes can determine size.
BACKUP
Proper sizing of tanks usually relies on establishing daily usage numbers for typical or average usage as well as emergency needs. However, some situations complicate that metric, such as seasonal usage, rainfall reuse and recycling, and conservation.
Some facilities divert wastewater for irrigation in order to reduce the need for potable—treated—water. Some industry estimates range from 60% to 80% of municipal supply that is used for non-potable purposes…but water utilities treat 100% for potable use. Conservation and reuse are cost-effective and noble options, but require more onsite water storage.
Many car washes reuse water. “They filter it in a rudimentary way and mix it with clean water,” explains Young. Better technology to treat water decreases the amount of potable water required from 75% to 25%, but separate tanks are needed for potable and non-potable water.
More tanks are going in at hospitals, car washes, hotels, and commercial buildings, Young lists, for reasons as diverse as fire protection, regulations, expansion, and backup. Mission-critical facilities often store “backup” water in extra tanks if their regular supply isn’t sufficient for peak demand, emergencies, or seasonal usage, he reveals. Some companies store water as part of conservation efforts.
The National Parks Service stores water for seasonal use. “They have two needs: potable for visitors during the season if their well system doesn’t produce enough gallons per minute to meet demand, and a septic holding tank,” explains Young. They can pump out of the well overnight to fill a storage tank in order to have sufficient capacity to meet peak demand during the tourist season.
Similarly, a splash pad playground in Utah has a 130,000- to 140,000-gallon capacity to accommodate regular business. “It’s a lot,” acknowledges Young, “but to prevent closure if there’s a drought, they added capacity.”
For utilities that have rates based on the amount of electricity used to pump water, controlling pumping can help reduce the power demand. Part-time use of more or larger pumps can be cost-effective, states the AWWA, if pumping is done during off-peak hours. Filling storage tanks when rates are low saves money…but can only be accomplished if there is sufficient storage capacity.
An installation at a VA hospital in Aurora, CO
GOING DEEP
Recently, a slight shift has occurred from aboveground to underground water tanks, Young observes. He lists causes as the cost of real estate—especially on the coasts—and maintenance, as well as temperate controls, particularly in northern areas.
For some, it’s the “environmental impact” of the tank’s appearance. While the AWWA indicates that this impact can be somewhat mitigated by design and exterior coating systems created to help a tank blend in with its surroundings, burying a tank hides all ugliness. (Site location and site development are also important factors to consider in reducing any adverse environmental impact.)
“In municipal applications, our tanks can be stored underground, unlike the large, unsightly concrete tanks you see aboveground,” according to ZCL | Xerxes.
That said, the Water Storage Tank Market Report: Trends, Forecast and Competitive Analysis report (www.researchandmarkets.com/research/gww8z3/global_water?w=5) predicts that concrete water storage tanks will remain the largest segment over the forecast period due to low cost, durability, and minimal maintenance requirements. Conversely, the research predicts that fiberglass water tanks will experience the highest growth because of their light weight, high degree of chemical resistance, service life, and low maintenance cost.
While most underground tanks are concrete, Young indicates that fiberglass tanks don’t leak or crack and have better long-term value. In addition, he says, underground tanks provide usable space. “You can drive over them, so you can put a parking garage on top or put a tank next to the foundation, creating a grass area for recreation.” It might also be important to know that underground fiberglass water storage tanks can contribute to satisfying multiple credits of the LEED green building rating system.
As important as LEED credits are on some projects, cost is critical on all projects. The cost of a tank varies with material, capacity, site, and position above or below ground. Aboveground reservoir-type tanks are the most economical, according to the AWWA. However, “the cost of a standpipe depends on its ratio of height to diameter. A tall, small-diameter standpipe will cost more than one of the same capacity [with] a diameter only slightly greater than its height. Two elements influence this cost differential. First, the minimum weight of steel to contain a given capacity is usually found in tanks that have a diameter equal to their height. Second, taller tanks cost more per unit weight of steel to erect because of the difficulties in lifting the steel and conducting assembly operations at greater heights.”
For elevated tanks, the cost per unit volume decreases significantly as the tank capacity is increased. For example, a 100,000-gallon elevated tank costs approximately twice per unit volume of a 500,000-gallon elevated tank. For reservoirs and standpipes, an increase in capacity also lowers the cost per unit volume, but the unit cost levels out at a capacity of approximately 5 million gallons.
ADDITIONAL DRIVERS
Water quality is a consideration in water storage capacity because it’s important to move water to keep it fresh while still maintaining fire flow and adequate pressure.
Thus, the age of water has become an issue, leaving water utilities reconsidering their distribution systems and tank designs to ensure that they’re moving water around efficiently in order to maintain high quality and sufficient pressure, while eliminating nitrification issues.
It can be even more of a concern when facilities conserve or recycle water.
Because tank design and pipe configuration can impact water quality, it’s important to consider water circulation and water flow during the design phase. The AWWA says, “Water age can be managed through a well-designed system that ‘exercises’ the tank.” Considerations include water turnover, altitude valves, pumping management, and other components for maintaining fresh water in the tank and mitigating water quality issues.
Highland Park Rain Water Tank
COATINGS, LININGS, AND MATERIALS
Most tanks store potable water, although graywater is held for specific purposes, such as irrigating golf courses, Gallagher points out. The difference is important not only for how water is used, but how it’s stored.
Many water tanks are made of welded carbon steel or stainless steel, but in some circumstances, pre-stressed concrete may make more sense, Gallagher indicates. Factors for material choices include capacity, service life, and environmental conditions.
If the tank is made of steel, it is often coated or painted. Coatings and paint are used instead of linings, other than bladders sometimes used in some tanks. The purpose of the coatings and paint is to protect the long-term integrity of the steel, Gallagher explains. Environmental conditions, including coastal weather, cold weather, and warm weather, threaten unprotected steel. But, he hastens to add, a different coating is used on tanks that store graywater than is used on potable water tanks.
Regardless of the kind of water stored inside, most tanks have multiple coats. “Three is most common,” reveals Gallagher. The coatings last for decades and undergo required inspections in most states.
Most communities have more than one tank, so when a tank needs recoating or repainting or other maintenance, there are redundant storage tanks to meet demand. Gallagher says that ground level, temporary tanks can be brought in for emergencies or maintenance situations when necessary.
TRENDS
Water tank sizing generally follows demographic patterns. With that in mind, Gallagher sees nothing that would reverse the trend of increasing tank sizes.
