Waterborne Coatings for Water and Wastewater Treatment Plants: Part 1
Most Americans view clean water as an investment in public health and the economic well being of our nation. As a result of this concern for water quality and the passage of the Clean Water Act, America’s water and wastewater treatment plants are among the best in the world. Still, deteriorating infrastructure is a major national concern, and with the continued growth of suburban areas, plant expansion is continuing and new plants are needed.
Much of the funding to update these facilities must come from local governments that are finding themselves in the difficult position of having to do more with less. As a result, many water and wastewater treatment facilities are focusing on operation efficiency. The title of a recent WEF conference was "Plant Operations: Maximizing the Performance of Small and Medium-Sized Wastewater Treatment Plants," and the keynote address focused on the importance of plant operations and maintenance. The service conditions in water and wastewater treatment plants cause facilities and equipment to be susceptible to degradation. The role of paints and coatings is to protect metal substrates from corrosion and concrete from chemical attack and deterioration. This is especially important given the current focus on operation and maintenance efficiency.
During the past 20 years, the processes performed in water and wastewater facilities have remained basically the same. They include collection/grit removal, flocculation/coagulation, clarification, purification, transmission and distribution. Today, plants are using these processes even more efficiently, meaning that far more gallons per day can be treated using these same processes.
Similarly, the protective coatings used in water and wastewater treatment facilities during the past 20 years remain basically the same. Epoxies and urethanes remain the workhorses and most common types used. However, VOC regulations have been the catalyst for changing the formulation of these coatings to reduce VOC emissions by moving from low volume solids to high volume solids. These regulations also have spurred advances in the formulation and usage of waterborne coatings. In recent years, there has been a tendency to over-specify solvent based epoxies and urethanes where other generic coatings will provide acceptable performance at reduced cost.
Coatings are being improved in ways that allow them to better accommodate construction schedules and to better meet maintenance painting needs. During the past 20 years, materials of construction also have changed in water and wastewater treatment facilities. There is greater use of concrete, non-ferrous metals and PVC/FRP. These materials have their own surface preparation needs and require coatings that will adhere to and protect these surfaces.
Benefits of Waterborne Coatings
New polymers allow today’s waterborne epoxy and urethane coatings to be abrasion, chemical and moisture resistant at prices competitive to conventional coatings. Complementing these resins are additives that help the coatings overcome flash rusting, so they can be applied directly to bare steel. In addition, most waterborne coatings are tolerant of damp concrete surfaces. Due to their good adhesion, some waterborne coatings can be applied directly to hard-to-coat surfaces that previously required special primers. Two examples are acrylic coatings applied directly to galvanized surfaces and acrylics or waterborne epoxy coatings applied directly to PVC/FRP surfaces.
During the last several years, high performance cementitious waterborne block fillers have been developed. These block fillers provide a one-step application that will smooth the surface of concrete that has voids and "bugholes" and provide better sealing and impermeability to water while providing a suitable surface for subsequent coatings.
Much of the appeal of waterborne coatings is related to the application benefits they offer. These include low odor, enabling application without affecting other trades and allowing accelerated construction schedules. Waterborne coatings also have high flash points that can reduce fire hazards and can be cleaned up with soap and water, eliminating the high cost of disposal of organic solvents that are considered hazardous wastes. Unlike some difficult-to-apply solvent-borne coatings, waterborne industrial maintenance coatings can be applied easily by brush, roller or spray. Since they do not contain quantities of organic solvents that may lift, wrinkle or soften existing coatings, waterborne coatings may be applied directly to existing coatings without the fear of lifting or wrinkling the existing system.
Waterborne resins based on new technologies continue to be introduced into the marketplace, permitting new coating choices with improved performance. One type allows cross-linking to take place within a single-component coating as it cures. Therefore, film integrity is improved, leading to increased chemical and corrosion resistance. Another type introduces an innovative hardener for a waterborne epoxy primer, allowing for colder temperature application, faster dry times for shop application, faster cure and increased chemical and corrosion resistance.
These advantages can result in significant benefits for the owners and operators of water and wastewater treatment facilities. With the exception of immersion (where waterborne coatings usually are not recommended), these coatings, once considered less effective than solvent-borne systems, are becoming desirable alternatives.
However, there are a few limitations to the usage of waterborne coatings. For example, waterborne coatings should not be applied to wet or sweating surfaces. In addition, temperature and relative humidity can affect waterborne coatings more than their solvent-borne equivalents. Unless specially formulated, they generally should not be applied below 50° F, above 90 percent relative humidity and not less than 5° above the dewpoint.
Dry times for waterborne coatings are more dependent on relative humidity than dry times for solvent-borne coating systems. In times of high humidity, curing and film formation of these coatings may be slowed because of the nature of water. There is no such thing as faster evaporating water or slower evaporating water. In moisture saturated atmospheric service (wherever surfaces are exposed to continuous high humidity and condensation during application), waterborne coatings are generally not recommended, even if the condensed moisture is removed from the surface prior to painting.
Very low temperature application of waterborne coatings also is not possible because of the freezing point of water. Ice or frost may form on surfaces to be coated and this may prevent film formation and adhesion. While some waterborne products may be applied at temperatures as low as 35° F, at this time waterborne coatings cannot be formulated for application at temperatures below 32° F.
Considerations in Specifying Coatings for Water and Wastewater Treatment Plants
While The Clean Water Act spawned a surge in the building and updating of water treatment plants, The Clean Air Act of 1970 was the impetus for enormous change in the paint and coatings industry. Due to amendments passed in 1990, a new proposed maximum VOC content level for architectural and industrial maintenance paints and coatings took effect in 1999. Waterborne coatings for industrial maintenance use comply with these fee types of industrial maintenance coatings are available with water as their solvent. As stated earlier, these coatings can be applied easily, and pose no solvent hazard. Waterborne primers are available for these topcoats, and they offer the durability and service life expected of these coating types. Digesters may have concrete bases and steel tops. A waterborne coating system may be used for both of these substrates.
Digester Coating System Example:
• Exterior Steel Cover–WB Rust Inhibitive or WB Zinc Rich Primer / WB Acrylic or WB Urethane Topcoat
• Exterior Concrete Walls–WB Cementitious Filler/WB Acrylic Topcoat
In concrete secondary containment or chemical storage areas, cementitious waterborne coatings can be used as block fillers or surfacers. Topcoats for these structures include waterborne epoxies or waterborne urethanes. Be sure that the system is resistant to the specific chemicals and the concentrations at which they are stored.
Waterborne coatings also provide the necessary protection against attack from chlorine gas and hydrogen sulfide (Table 1).
For water storage tank exteriors, one coating system offers the highest level of corrosion protection and durability available. It consists of a zinc-rich primer, an epoxy intermediate coat and a urethane topcoat. It is estimated that this protective coating system can offer more than 20 years of durability. (See NACE International Corrosion 98 Paper No. 509.) This same system is available using waterborne protective coatings and may be considered for use on these structures (Table 2).
Part 2 of this article will appear in the February issue and will focus on new types of coatings as well as other advances in coating technology.