When your agency or company decides to install a water tank, the decision is not as straightforward as you might think. You have a variety of choices and decisions. The size may dictate the type. Will it be fiberglass, prestressed concrete, welded precast steel, bolted steel, glass-fused steel, or stainless steel? Will it be underground or aboveground? What will you be storing–potable water, rainwater, stormwater, or wastewater? Have you analyzed life-cycle costs?
Analyzing life-cycle costs will require considering operations and maintenance costs over the lifetime of the tank. And several of the tank materials–fiberglass for example–require little or no maintenance, thereby lowering those life-cycle costs.
The City of Modesto, CA, provides a real-life example of the realities water agencies live through. The city began the design phase this spring for a new 250,000-gallon water storage tank, a booster pump, and 1,000-gallon-per-minute domestic water well. The storage tank, once built, will provide additional water storage for the Del Rio district two miles away. During a simulation, fire-flow water pressure in the area was found to be deficient, prompting the need for a new tank.
This 1-million gallon glass-fused steel tank was built by United Tank Systems, Stockbridge, WI, for Aqua Illinois, operator of the wastewater plant for University Park, IL. Glass-fused tanks are built from the top down with assembly at the ground level. Here, the final row is being installed.
The city owns and operates several water systems supplying well water to communities surrounding Modesto, but Modesto Irrigation District (MID) supplies water to the city itself, says Rob Christensen, a senior civil engineer with the city.
Modesto has built nine steel water storage tanks since 1990 and maintains them. It also has plans for two concrete storage tanks, with designs complete, on hold. Most of the tanks in operation are 1 or 2 million gallons, with the smallest at 220,000 gallons, and the largest at 4 million gallons. The concrete tanks will be 4 and 6 million gallons, respectively, and will be used to store surface water coming from MID to diversify water sources and save on ground water storage.
Christensen says they have not yet decided whether the new tank will be concrete or steel. A concrete tank can be built lower to cut down on visibility. A life-cycle cost analysis on the two types is to be completed before making the final decision.
Even though the new tank will be located in a rural setting, the few residents in the area have expressed concerns about its looks. Christensen says staff has been meeting with the residents about the visibility issue. Construction is expected to get underway in 2013 at a cost of $3.4 million.
An Urban Setting Challenge
Small water tanks–under 250,000 gallons–are built and owned by smaller utilities where budgets are challenging, says Christopher Cleveland, vice president at Carollo Engineers in Sacramento, CA. They are usually built above ground and are usually steel. Underground tanks have to be made of more expensive concrete, he says, because steel would corrode if buried. On a per-gallon of capacity basis, smaller tanks cost more, he says.
The City of Modesto, CA, installed the 220,000-gallon Grayson steel water tank in the early 1990s.
The future appears to be good for the industry. New water tanks will continue to be built and existing tanks will be refurbished, says Cleveland. He has seen two trends in the last five years nationwide. First, national regulations have increased the focus on water quality, leading to minimizing water age. Traditionally, utilities typically adopted filling and drawing water on a regular basis. In response to the new regulations, different operational methods for water mixing have been developed including the development of new technologies.
Carollo Engineers recently designed a new water tank for the City of West Sacramento in a challenging urban setting, and will, as part of a team, design the water tank for the City of Modesto, says Cleveland.
Cleveland says the challenges of building a water tank and pump station in an urban setting are similar for any size tank. The primary challenge is the impact a new water tank will have on neighbors, both during construction and for the long term. “Sometimes we can build partially or entirely underground, but limitations may lead us to build above ground,” he says.
The West Sacramento water tank, under construction as this story was being written, is 45 feet tall and 110 feet in diameter, and directly across the street from loft-style apartments. It is located on an irregularly shaped parcel shared with a park within one of the city’s largest redevelopment/revitalization areas. It will supply drinking water and fire protection to the newly developed district.
Public outreach and education is critical, says Cleveland. The city reached out to the community while the tank was being designed, and incorporated elements that were important to the community while acknowledging the need for the water tank.
The volume of water was fixed at 3.1 million gallons, and the tank would be tall–that couldn’t be avoided. However, the residents said, “We want you to make it as skinny as possible to give us more space in the park,” according to Cleveland. The developers were surprised to hear this, and subsequently increased the height from 35 feet to 41 feet, reducing the tank footprint.
According to Cleveland, this led to the philosophy “if it can’t be hidden, let’s celebrate it.” Water became the park theme and architectural elements accentuating that theme were incorporated into the park structure. The pipes connected to the steel tank were painted in different colors with LED lights attached and extended out into the park in creative lines. Park benches incorporated the water pipes into their design.
Design for new water tank in West Sacramento Park
The other challenge was the very poor soil on which the water tank was installed. Cleveland says construction involved driving concrete precast piles into the ground to support the tank and associated pump station. This impacted both the cost of the project and its schedule. “It’s also a very noisy operation,” he says, so communication with the neighbors and sensitivity to working hours were important considerations.
What Are Your Options?
Containment Solutions Inc. manufactures fiberglass tanks in sizes up to 60,000 gallons that can be tethered together for larger storage and buried underground. According to David Heiman, director of marketing, business for its tanks has been exploding in the last 10 years.
Heiman attributes this growth to the increased popularity of green projects, including water capture and water conservation, which are important for point collection with Leadership in Energy and Environmental Design (LEED) buildings applying for certification. Reducing the consumption of potable water for non-potable use by capturing rainwater and/or graywater for irrigation will earn points toward gold, silver, or platinum certification.
Historically, municipal water agencies were not interested in fiberglass water tanks, but Heiman says this is changing in a time with little or no funding. He says these agencies are looking for alternate water conservation methods and are encouraging customers to conserve water in tanks. New developments are installing fiberglass tanks in rural areas where there are no municipal services.
Customers prefer multiple large tanks, says Heiman. The ability to build a hybrid system for customers, such as schools and hospitals who are routinely buying fiberglass tanks creates opportunities for water conservation, particularly in summer peak demand periods. Rainwater can be captured in one tank, graywater in a second, and water condensate in a third. Heiman says the University of Binghamton in New York did just that. It recently bought four 50,000-gallon tanks with separate outlets to store rainwater in some and graywater in others.
American Structures designs, fabricates, installs, and services bolted and welded stainless steel storage tanks for the agricultural, industrial, municipal, and wastewater industries. Its water tanks are used to store potable, recycled, and stormwater, as well as water for fire protection. They are made to order, and come in all sizes.
Earl Wildenberg, president of American Structures, says, “People are getting really excited about stainless steel. It doesn’t have to be painted; it is resistant to corrosion, rust, and relatively maintenance-free; it is environmentally friendly; it blends into the landscape.”
Municipal water agencies don’t have to worry about refurbishing a stainless steel tank in five years, he adds. Water agencies are now looking at the cost of refurbishing existing tanks and comparing those numbers with the costs of a stainless steel tank.
Wildenberg says the company has installed a lot of smaller tanks, which can be designed for expansion later. The tank is unbolted at the bottom, the tank is jacked up and new panels are installed. They recently installed two 100,000-gallon tanks in North Dakota for a mining company. There are no differences in installing small or large tanks.
“It’s like building a garage,” he says. “Once you build an envelope, you can put anything in it.”
United Tank Systems is a distributor for Fusion Tanks, manufacturer of glass-fused steel tanks, headquartered in England. It also manufactures both epoxy-lined steel tanks and stainless steel tanks in all sizes. Joel Portmann, company president, says glass-fused bolted steel water tanks are like putting water in a glass.
Glass-fused steel tanks have advantages over other types of tanks. They can be built up to accommodate growth, they can be moved to another location for other uses, and they have near-zero life-cycle costs, compared to concrete and welded steel tanks, which must be recoated about every 10 years and be maintained to avoid biofilm growth, says Portmann.
What About the Price?
Glass-fused steel tanks are price competitive with concrete tanks, stainless steel and welded steel tanks as size increases beyond 200,000 gallons, but pricing is dependent on region and customer requirements, Portmann says. For example, in Wisconsin, a tank has to be built to handle snow on its roof, but in California, seismic regulations require a heavier tank–these variations all impact price. The cost of stainless steel tanks is tied to the price of the metal itself and becomes less competitive pricewise as the size increases.
“There have been times when stainless steel is a lot cheaper and times when it is more expensive,” says Portmann. Operations and maintenance is similar to that recommended for glass-fused steel tanks.
Wildenberg agrees that stainless steel is cost-competitive initially and has low operations and maintenance costs since it is resistant to corrosion and rust, making it cheaper to maintain.
Another element affecting price is the ability to assemble the tank at the factory if it is small enough and can be trucked to the site, thereby decreasing costs. The cutoff is 20,000 gallons, Portmann says. However, transportation of the tank materials outside of the region where they are manufactured also increases the total price.
Portmann says epoxy-lined steel tanks are the least expensive type, and are usually purchased by private companies for chemical and other liquid storage uses. New improved epoxies are now coming on the market. The original epoxies are holding up well on tanks, he says, although some require special coatings. It is still too soon to say how well the new epoxies will endure, he says.
Pricing can be a challenge, says Heiman, and can vary depending on the fittings and accessories customers request for the tanks. Fiberglass is easy to customize, but still economies of scale rule, he says. If changes are called for in the field during installation, they can be made there, in contrast to concrete or steel tanks.
Prices for fiberglass tanks are comparable to those for concrete tanks, and with water rates rising, paybacks are improving, Heiman says. Life-cycle costs for fiberglass tanks are much lower, since they don’t rust, because biofilm such as bacteria and algae can find no home in the underground environment due to their smooth inner surfaces. Heiman says biofilm needs a textured wall to grow on, as well as sunlight. Once installed correctly, fiberglass tanks will operate for decades without maintenance, he says.
Heiman contractors like fiberglass tanks because of the ease with which they are installed. Anchors can be transported on the same truck as the tanks, and smaller cranes are needed.
Good O&M Will Protect Assets
Operations and maintenance (O&M) standards for water tanks developed by the American Water Works Association (AWWA) have been established long enough that they have become the industry standard and adopted by virtually all the states and some countries abroad.
AWWA M42, Manual for Steel Water Storage Tanks is referenced several times in this story. Standards are found in D100 for welded steel, D102 for coating steel water storage tanks, C652 for disinfection of water storage facilities, and G200 for distribution systems operation and management. Standards for concrete tanks can be found in D110 and D115, and standards for fiberglass tanks are found in D120-09.
The following discussion was drawn from a white paper written by Randall L. Moore, vice president of special products and technical services at Utility Service Company, and an April 2012 Web seminar organized by AWWA featuring Moore as moderator with speakers Kirt Ervin, vice president for Water Quality at Utility Service Company, and Mark Johnson, director of metropolitan water operations at the Massachusetts Water Resources Authority.
The white paper, “A Sustainable Approach to Potable Water Quality Management,” can be found on the PAX Water Technologies website, under resources/white papers. The Web seminar, “Tank Inspections and Sustainable O&M Best Practices to Manage Water Quality” is available through AWWA’s education services. Additional comments were provided by Moore and Johnson in interviews.
Moore cautions that it is best to confirm you are viewing the most recent updates when you read the standards. The most recent update of AWWA’s reference book, Steel Water Storage Tanks: Design, Construction, Maintenance, and Repair, for example, was released in 2010, he says.
Moore, Ervin, and Johnson stressed chemical cleaning, active mixing, and tank asset management as tools that will provide plant managers and operators the ability to comply with the myriad new regulations governing contaminants and disinfection by-products.
Inspections Are Important
In the Web seminar, Ervin recommends that all water agencies have a written inspection plan outlining frequency, procedures, and maintenance of records. A good inspection program can extend the life of an existing tank indefinitely, he says.
Types of inspections include weekly visual, plus once-per-year inspections via remotely operated vehicles if sedimentation is a problem, in which tanks can remain full and sediments can be removed. Dive inspections can be done every three years or longer. When tanks are drained for chemical washouts, every aspect of the tank, and distribution system can be inspected.
Johnson says that remotely operated vehicles (ROVs) are doing a great job now. A crawler tank can soak up sediment to produce a clean tank as a result. Once materials dredged from the tank are allowed to settle and dry out the remaining liquid can be discharged, he says.
Areas that should be inspected on a regular basis, says Ervin, include the roof and its coverings, low spots on roof plates, access hatches that may get unlatched and vents, which should be kept in good condition. Overflow pipes should have a good catch basin.
Openings through roofs or access hatches have allowed birds, squirrels, beer cans, and, yes, even eels, to get into the water. Roofs with low spots are prone to rust where water and snow can gather. Vents should be inspected at least once a year, and kept in good condition. A good vent has a fine mesh screen preventing wind-borne particles from entering the tank.
Johnson recommends taking outbound water samples from tanks weekly, and having them laboratory tested for bacteria. The monitored data and chlorine residuals, for example, can be plotted each week. He recommends using 1.0 milligram per liter (mg/l) chlorine residual as a measurement for an action threshold.
Furthermore, this schedule is a weekly opportunity to confirm access is intact, gates are locked, the fences are secure, there are no obvious tank problems, screens, vents, and hatches all secure, and there is no leakage due to rust or penetrations. Finally, Johnson cautions that tank ladders should not become cable trays for cell phone vendors. Renting space to cell phone vendors is a great source of needed income, he says, but don’t forget that these are water tanks, not antenna bases.
Johnson recommends AWWA’s G200-09, “Distribution systems Operations and Maintenance,” section 4.3.1, which provides a great inspection checklist.
Maintain Water Quality
When potable water sits in a storage tank, problems occur and the water stagnates. Johnson says to try and keep the age as low as possible. He quoted a white paper published by EPA and a Water Research Foundation study, which reviewed turnover rates and found they varied from one to three days and as much as five days. The Water Research Foundation study found that a turnover rate of 2.5 days of 40% of water in the tank became the optimum level.
Tanks that are well mixed are sustainable, because they consume less disinfectant, minimize the formation of disinfectant by-products, and avoid the need to empty the tank should nitrification occur, which depletes disinfectant residual. With active mixing, the common operational practices of “deep cycling” storage tanks, onsite chlorine boosting, and tank/main flushing used to remedy problems due to poor mixing can be reduced or eliminated within the storage tank.
Also, an active mixing system can minimize icing in tanks located in northern climates and reduce ice damage to the tank interior coatings, thereby helping to minimize biofilm growth and corrosion by-products, ultimately improving water quality.
Three water quality management tools can be used to maintain water quality. Washouts at least every three years are recommended in AWWA’s M42 Manual for Steel Water Storage Tanks to remove sediments and clean the tank. But neither pressure washing nor the disinfection process will kill and remove biofilm on interior surfaces.
As biofilm builds up, an increasing amount of disinfectant is required. However, a low-pressure NSF standard 60-certified chemical washout can remove all biological growth from the empty tank, as well as the mineral staining typically associated with the biofilm.
Removing biofilm and stain, while the water tank is empty makes inspection and failure repairs in the coating much easier and more thorough. Maintaining the integrity of the coating will minimize biological regrowth by eliminating anchor sites where colonization occurs. Eliminating surface roughness will reduce biofilm growth.
An active mixing system certified by NSF Standard 61 which operates 24 hours a day seven days a week is highly recommended to eliminate thermal stratification and achieve homogenous water chemistry within the storage tank. Moore sites a study, which shows that passive mixing of the water when the tank fills will restratify both thermally and chemically within four hours after pumping water into the tank.
A 15,000-gallon fiberglass tank, manufactured by Containment Solutions Inc., being lowered into place
The upper stratified layer has the oldest, warmest water with the lowest (often zero) disinfectant residual. This upper layer is the most likely to have a bacteriological problem and high concentration of disinfectant by-products.
The Utility Service Company publication recommends a highly energy efficient six-inch, “Lily” impeller that can mix up to 7 million gallons and uses the same amount of power as three 100-W light bulbs, or roughly at a cost of one dollar per day. It can be connected directly to the power grid or solar panels can be utilized where grid power is not available, or by choice.
All three authors recommend a full service asset management program that complies with Governmental Accounting Standards Board Rule 34. Filter media cleaning for consistent efficient performance, biofilm removal in storage tanks and active mixing systems to eliminate thermal stratification can each be implemented separately. However, a full service asset management program will have the greatest impact on water quality since each of the management tools will work synergistically to minimize disinfectant demand and produce improved stability and consistency of water quality throughout the system.
Moore says water agency managers need to take a holistic approach, not only with the water tank, but to the whole distribution system, including wells and mains to avoid reseeding the tank with bacteria. Commenting in an interview, Moore says water agencies can create their own asset management program or companies, such as Utility Service Company, can be hired to design and deliver this service.
“The real beauty is they are sustainable over the long term,” he says. “Proactive maintenance is always cheaper than reactive maintenance.”
There are economic, environmental, and social benefits to a sustainable asset management program. It assures that ongoing maintenance needs are funded providing long-term security and protection of water supplies. By maintaining the exterior coating system on water tanks the time between sand blastings is maximized and the environmental impacts to air, water and the ground are minimized. And water quality and public health are assured by maintaining sanitary water tank conditions.
Linings and covers that come into contact with potable water have to comply with NSF International and ANSI Standard 61 for drinking water system components and EPA’s Safe Drinking Water Act. Also, the American Water Works Association has developed Standard D130 for geomembrane materials for potable water applications, which the following manufacturers adhere to.
Environetics Inc. manufactures covers and baffles for both concrete and steel water tanks. Tank and floating covers control algae growth, odor containment, and thermal retention. Richard Winters, vice president of marketing, explains that baffles are used to create mazes to better mix water and reduce need for chemicals. Water travels through the mazes to prevent stagnant water from remaining in corners.Environetics’ geomembrane material for its tank covers has a 25-year lifetime and can be reskinned, says Winters, extending the lifetime up to 35 years. Covers are custom manufactured to fit the profile of new or existing tanks. Frames are made of clear span structural aluminum.Liners have two purposes: they are designed to provide new life for old tanks, and they will extend the life of a new tank. For the most part, Flexi-liner Corp. lines old tanks. These include redwood, galvanized steel tanks and bolted steel tanks. It also lines plastic or polyurethane tanks typically used for chemical storage. A liner lasts 15 to 20 years and are 10% to 15% of the cost of replacing the tank, according to Kyle Phelps, a salesman with the company. Liners designed for potable water are made of modified vinyl based terpolymer.
Burke Environmental Products, a division of Burke Industries headquartered in San Jose, CA, manufactures and fabricates liners and covers for multiple applications. Its flooring products division manufactures rubber flooring and stair treads and carpet base.
Burke’s floating covers for potable water reservoirs, are manufactured of 5-ply Hypalon, a polyethylene material. According to company literature, it is weather resistant, especially to oxidation and immune to ozone and ultraviolet light. It also reduces required levels of treatment chemicals in addition to retarding evaporation. Hypalon pond liners are formulated to a grade that meets EPA requirements for potable water storage. Burke Roofing Systems also manufactures single ply Hypalon roofs for new or existing water tanks. Furthermore, it manufactures Ecoseal reservoir liners and covers based on a thermoplastic polymer alloy suitable for most secondary containment applications.
PAX Water Technologies manufactures active submersibles for potable water storage tanks. The tank mixers and aeration systems stir stagnant warm water sitting in the top of water tanks with cooler water standing below, and they remove disinfectant by-products, such as trihalomethanes, from potable water stored in water tanks.
PAX commercialized its tank mixing technology six years ago when PAX Water Technologies was founded by PAX Scientific. Aeration hardware, sitting on a tripod in the bottom of the tank, combined with laboratory and computer software and modeling creates a pumped circulation loop that shoots a jet of cool water upwards from the bottom of the tank. This jet of cool water forms the center of a toroidal flow structure that moves the entire tank volume stabilizing water temperatures. It is effective in any size tank, according to PAX website documentation.
In the case of volatile compound removal, sprayers and other air handling equipment are used to infuse water with air to remove volatile compounds like trihalomethanes, keeping water fresh.
