Most of the residents of Springfield, OH, are too young to remember Franklin Delano Roosevelt's Works Progress Administration (WPA) and the role it played in creating jobs while building the nation's infrastructure. As a result, they probably also don't know that the wastewater treatment plant they currently rely on was originally constructed as part of that program. Planning and timely renovation make it likely that the now modified historic structure can continue to efficiently serve needs of the community for another 25 years.
A three coat system encapsulated lead paint on Springfield's vintage WPA digesters.
Advances in paint chemistry contributed significantly to the recent renovation of the plant in several ways. The application of a state-of-the-art, three-coat paint system over an aging, lead-contaminated coating extended the service life of a digester dome, and eliminated the need for the city to invest in the expensive environmental controls required when lead-based paint is removed. The specification and application of a cold-weather coating system also permitted restoration work to proceed despite adverse weather conditions.
The assignment was completed through the teamwork of Dave Brown, Springfield's assistant superintendent for sewage works maintenance; the general contractor, Kokosing Construction of Fredericktown, OH, along with its applicator, Howard Painting of Defiance, OH; the engineer, Woolpert LLP of Dayton; and paint supplier, The Sherwin-Williams Company. Brown led the team by defining the performance and evaluation requirements for the paint systems.
Historic Beginnings
The Ohio Wastewater Treatment Plant's works and sludge stabilization facilities were put into service in 1935. In 1960, additional sludge facilities were constructed to manage the byproducts of biological treatment expansion. In the '70s and '80s, the city continued to improve the effectiveness of its treatment and of the quality of water discharged to the Mad River.
But by 1998, Springfield was faced with the problem of managing increasing volumes of solid byproducts and complying with more stringent regulations regarding their disposal. Looking ahead, the city selected a plan for renovating the digesters.
Weather had eroded the 1984 paint system, which showed rust blush.
The digester works consist of four tanks. Digesters 1 and 2, constructed in 1935, have a concrete-encased steel structural system supported from a steel center column and superstructure. The stabilization process is maintained by mechanical mixers that circulate the liquor through a steel draft tube. Digester 3, constructed in 1960, is a reinforced concrete structural system with mixing equipment similar to that in Digesters 1 and 2. Digester 4 is a secondary process tank where stabilized sludge is stored in a hopper silo and digester gas is stored in a steel dome on which the roof structure floats on digester gas.
Digester steel surfaces are exposed to severe service. The digester gas is a mixture of 40 percent methane and 60 percent carbon dioxide with trace concentrations of hydrogen sulfide. The mixed liquors are maintained at 95°F, resulting in a saturated gas phase above the liquor operating level, where the steel is continuously wetted with condensate saturated with carbon dioxide resulting in a low pH (acidic) solution. The rate of corrosion for the interior surfaces is controlled primarily by the lack of oxygen. The specification and application of a coating system to provide an effective barrier to corrosion is essential to extending the structures' service life.
Corrosion Assessment
The first step in plant renovation was a visual assessment, which consisted of a survey of interior surfaces. Digester 1 was emptied and the steel substrates inspected. The paint systems for the steel center column and draft tubes of Digesters 1 and 2 had failed due to chemical erosion and mechanical abrasion by the liquor.
The painting systems applied to Digester 3 had failed in a similar manner. The steel in Digester 4's interior dome surfaces and skirt areas were inspected by use of a closed circuit television camera. The pitting observed indicated that the coating systems were no longer effective. The exterior roof area of the Digester 4 dome had been refinished in 1984, and the lead-based primer system applied in 1960 had been removed. The 1984 paint system was being eroded by weather, and rust blush could be observed in the panel sections. Protecting all metal surfaces was essential to extend the equipment's service life.
The carbon steel interior digester surfaces were specified to receive a high performance coal tar epoxy recoating. The applicator, Howard Painting, selected Sherwin-Williams Hi Mil Sher Tar®. The coating was approved for application by the engineer based on its adhesion properties, its resistance to both organic and inorganic acids, and its resistance to abrasion by continuous exposure to grit at the mixer draft tubes. Two coats of the coating were specified to achieve a final thickness of 16 to 20 mils.
Digester 4 is located at the plant entrance. The digester gas storage dome exterior required specification of coatings for two environments:
- The dome roof exterior required a corrosion resistant coating system capable of exposure to sunlight and thermal stresses, as well as a good appearance due to the dome's high visibility.
- The digester skirt was alternately submerged in abrasive digested sludge or exposed to atmospheric UV deterioration. Long term protection, under such harsh conditions, is provided by the high performance coal-tar epoxy.
The specified dome roof exterior coating system consisted of an epoxy primer with a urethane finish coat. The applicator selected Sherwin-Williams Recoatable Epoxy Primer as the prime coat and the company's Corothane® II Satin as the finish coat. However, during mobilization for the dome roof refinishing, paint chips from the aging finish were analyzed and trace concentrations of lead were detected. While not environmentally hazardous, the aerosols generated by a conventional surface blast preparation could potentially exceed OSHA prescribed concentrations. As a result, alternate application techniques were proposed by the applicator and approved by the engineer.
A program of adhesion tests and paint thickness measurements was performed independently, both by the coating manufacturer and the engineer. The test results indicated good adhesion of the existing paint system to the dome steel, so it was decided to forego removal of the existing paint system. Instead, an overcoating system, also from Sherwin-Williams, that could be applied directly over the existing lead-based paint, was recommended.
As indicated by the paint manufacturer's specification, surface preparation consisted of mechanical scraping to remove loose scale, paint, and debris; solvent cleaning; and pressure washing, in accordance with SSPC-SP-1, SP-2, SP-3 and SP-12.
A three-coat paint system comprised of Sherwin-Williams Macropoxy 920 Pre-Prime, Macropoxy 646 fast cure epoxy, and a high solids polyurethane was then applied to the dome roof exterior. The key to the paint system's performance is the penetrating epoxy preprimer. The preprimer provides adhesion by penetrating through the rust into the steel substrate, so it provides the foundation for the application of subsequent coats.
After the application and curing of the preprimer, the intermediate coat was applied to achieve the required epoxy film thickness for weather, chemical, and abrasion resistance. The epoxy was then topcoated with a high-solids polyurethane. The polyurethane coat provides exterior gloss and toughness that withstands abrasion, weathering and UV light. Subsequent construction operations on the dome surface were completed without visible damage to the gloss.
Cold-Weather Painting
Due to the time needed to test and approve the overcoat system, favorable weather for applying coatings was lost. Therefore, the skirt coating system would have to be applied in November and December cold months in Ohio. The coldest early winter in 10 years made this project particularly challenging.
To accommodate the need for cold weather application, the specification for the skirt coating was changed to permit the use of a cold-weather curing epoxy designed for application at temperatures as low as 20°F. The applicator selected Corothane® I MCU Coal Tar Epoxy Urethane, manufactured by Sherwin-Williams. The paint system is a moisture-cure urethane coal tar with micaceous iron oxide. The paint system data sheet indicates cure time to provide suitability for quick immersion. This was of particular concern due to the rain, sleet, and snow squalls that intermittently buffeted the construction site with little warning.
The paint was applied in two coats with a specified dry film thickness of 10 to 14 mil. The actual paint thickness was measured as 14 to 16 mils. Solvent tests of the finish coat were conducted to monitor the proper cure of the film and resulted in the paint manufacturer's certification that the paint system was adequately cured to permit its immersion. The digester dome was then returned to service to provide the fuel gas reservoir required to maintain the sludge stabilization process through the winter months.
