Potable water priorities

CE May 2010 » Products » ENGINEERED SOLUTIONS
Raw water condition and storage tank integrity require well-engineered systems.
Bob Drake

Growing demand for potable water, greater scrutiny of water quality, depleting drinking water sources in some areas, and an emphasis on water recycling are all impacting the design of and technologies used for water treatment and storage systems.

The starting point for design of a potable water treatment system is the condition and contents of the raw water, according to Daryl Gisch, Ph.D., applications development leader for Dow Water & Process Solutions (DW&PS). “Two universal needs are required for potable water: The treated water should not make you sick, and the water should be palatable,” he said. “On the health and wellness side, keeping pathogens, turbidity, and chemicals out are essential for the safe sourcing of drinking water, while at the same time providing water that meets consumer’s needs for acceptable taste, odor, and color.”

But, as water demand grows, falling water tables and greater intrusion from surface water into groundwater wells can require use of dirtier raw water that requires more, and often specialized, treatment. “Design flexibility will be a key issue that civil engineers face as they look to supply potable water from a wider range of raw water sources,” Gisch said. “As raw water varies, so do the treatment needs. These variances may be seasonal shifts due to spring runoff or late season drought.”

According to Gisch, source aquifer sustainability and replenishment are key to keeping the raw water source healthy for the long term. “This means more management of the watershed and, in many cases, limiting the water draws that can be taken from a given location,” he said. “Flexibility for treating a wider range of raw water types will be essential for meeting future water needs.”

To provide that flexibility to potable water system design engineers, DW&PS offers a wide range of water treatment tools, including ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) membranes; ion exchange resins; and specialty media designed for salt and selective contaminant removal. System components can be tailored to meet the specific needs of a given source water for cleaner treated water and better pathogen removal. The company is also seeking new and more cost-effective ways to remove metals and chemicals. Gisch said that DW&PS can help reduce and eliminate threats found in untapped raw water sources, such as pathogens, by filtering with UF membranes or by removing trace contaminants such as perchlorate, nitrate, or arsenic by using selective media.

Raw water storage can also affect its quality, according to SolarBee, Inc., a division of Medora Environmental, Inc., that develops and installs solar-powered circulation (SPC) equipment in raw water reservoirs, lakes, and storage tanks. A report recently published in Harmful Algae, a peer-reviewed journal, noted the efficacy of SPC in suppressing freshwater harmful algal blooms (FHABs). The incidence of FHABs is increasing worldwide because of excessive nutrient input and declining flow rates, Solar Bee said, and efforts to control algal blooms have included watershed management to limit nutrient input, chemical algaecide treatments to terminate blooms, and SPC technology to prevent blooms.

Burial of this 1.1 million-gallon, DYK prestressed concrete recycled water reservoir in Antioch, Calif., maximizes tight site space and minimizes visual impact.
Aerialsondemand.com

Research was based on data collected at three water utilities where high nutrient levels and periodic seeding with cyanobacteria caused annual blooms. The studies demonstrated that SPC of the upper portion of the water column where blooms occur strongly suppressed FHABs, even in nutrient-rich waters.

Storage systems
Once treated, storing drinking water for distribution presents its own range of challenges. Design engineers must address aesthetic, structural, and maintenance issues for both concrete and steel water tanks.

Gregory Tseng, business development, DYK Incorporated, noted that as communities grow, sites for water storage tanks are becoming more visible to residents. “To accommodate the aesthetic need of the communities while providing proper storage and distribution, DYK prestressed [concrete] tanks can either be buried or architecturally treated,” he said. Prestressed tanks don’t require interior or exterior coatings, so they can take advantage of such applications, according to Tseng.

DYK cites buried tanks, such as a 1.1 million-gallon recycled water reservoir in Antioch, Calif., as examples of owners maximizing tight site space while minimizing visual impact. In addition, burying water storage tanks eliminates the need for a retaining wall. If the reservoir is to be built on a hillside, the contouring slope can be backfilled directly against the reservoir. “If a tank cannot be buried because of hydraulic reasons, architectural features can be implemented,” Tseng said.

Tseng emphasized that it is important during design to keep the need of the end user in mind. “For most owners, having a low-maintenance tank is expected when they come to DYK,” he said. “DYK uses galvanized strand pulled to 14,950 pounds of force to keep the high-strength concrete core wall in compression at all times. It is our goal to also make routine inspections just as effortless. DYK designs a sloped floor for proper drainage and a flat slab roof for safer access. Typical floor slopes are 1.5 percent, with the high point at the center, reducing the amount of piping by having the low point near the wall. Flat slab roofs, in addition to their aesthetic and durability advantages compared to a dome, provide a much safer platform for access to the owner.”

Corrosion protection is particularly important for steel water tanks, but some concrete tanks may require protective coatings during their lifetimes. Tony Ippoliti, business development manager — water storage, Sherwin-Williams, said that engineers should be familiar with the following American Water Works Association (AWWA) standards used for tank construction and painting:

  • D100: Standard for Welded Carbon Steel Tanks for Water Storage;
  • D102-06: Standard for Coating Steel Water-Storage Tanks;
  • D103: Standard for Factory-Coated Bolted Steel Tanks for Water Storage;
  • D110: Standard for Wire- and Strand-Wound, Circular, Prestressed Concrete Water Tanks; and
  • D115: Standard for Tendon Prestressed-Concrete Water Tanks.

“The most complete standard, covering the greatest number of individual systems and the most innovative coating technologies,” Ippoliti said, “is the D102 standard for welded steel tanks. It currently includes surface preparation, six generic classifications of Outside Coating Systems (OCS), and five generic classifications of Inside Coating Systems (ICS).”

According to studies conducted at three water utilities, solar-powered circulation technology from SolarBee suppresses freshwater harmful algal blooms, helping to reduce costs of algaecides and chemicals used in producing drinking water.

The D102 standard also alerts design professionals to the importance of ventilation, tank disinfection, and the first-year anniversary inspection, he said. Improper ventilation of a welded steel tank while the coatings dry and cure, for example, can lead to undesirable taste and odor problems once the tank goes into service (see AWWA D102 A.7.5).

“Two of the most innovative systems are also two of the most recently recognized by the D102 standard,” Ippoliti said, “ICS No. 4 and OCS No. 4. ICS No. 4 refers to a polyurea or polyurethane NSF 61-acceptable elastomer with a minimum dry film thickness of 25 mils. When properly applied, the adhesion, elasticity, and water resistance offered by this coating type can provide 30 years of service life. OCS No. 4 incorporates a fluorourethane topcoat that delivers the longest-lasting color and gloss retention, resisting UV light deterioration for up to 30 years.”

AWWA D102 committee members evaluate the most promising coating technologies that comply with environmental regulations of varying stringency. Ippoliti, an AWWA D102 committee member, said that the move toward solvent reduction has resulted in improved resins for coatings. “These improvements allow coatings manufacturers to offer the specification community coatings that are easy to apply and have performance characteristics often superior to the higher-VOC coatings being replaced,” he said. “And in the next revision of the D102 standard — expected out sometime this year — two coating systems will be included that confirm the ability of the coatings industry to offer high performance with low environmental impact.”

The new coating systems are a waterborne epoxy/urethane exterior system that uses harmless water as its main “solvent,” and an interior epoxy system that is 100-percent solvent free, replacing the current ICS No. 3.


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