BLOG: Steel Coating Technology for Wastewater Tankage
Posted on by Hugh B. Mickel, P.E.
Onsite wastewater treatment systems like package treatment plants are typically manufactured within a steel tank. How the steel is prepared and coated contribute to the performance and durability of the system.
History of Steel Coating Technology:
The U.S. Bureau of Reclamation and other government agencies have studied the performance of steel coatings for over 100 years, contributing significantly to the evolution of usage guidelines, usage limitations, new coating product development, and application standards. As the knowledge base grew, so did the evidence that coating type, surface preparation, and application conditions were all highly important for the long term performance of wastewater treatment systems.
Early tank systems using mop-applied coatings were replaced with coal tar enamels and more effective surface cleaning. Coal tar epoxies entered the picture, as did mechanical surface preparations, which became the norm. By the late 1980’s sand blasting had become widely available to improve coating adhesion, and that has given way to the use of manufactured abrasives. Meanwhile, refinements in epoxy coatings and other technologies have resulted in a plethora of options for decentralized wastewater systems.
Still, surface preparation may be the single most important factor in the long-term service life of coated steel wastewater tanks. Other key factors during manufacturing include dust removal techniques, timing, atmospheric conditions in the blasting and painting bays, and the blasting material selected. Steel grit media has been found to be the most effective for removing oxidation, scale and other surface density occlusions that could prevent a strong adherence of initial coating layer.
Our Approach to Steel Coating:
Although sandblasting is still a common and generic term for the acceleration of abrasive media using compressed air and a nozzle, sand is not the best media. In fact, the term used in wastewater tank specifications should, more accurately, be ‘grit blasting.’ Aside from the steel grit used by Delta, common abrasives include rounded steel shot (which peens the surface), aluminum oxide (which is more brittle than steel grit), and a host of other less aggressive media (glass bead, crushed glass, melamine plastic, ceramic grit, and copper slag). Steel grit works the best on A36 mild steel because it offers the right amount of hardness, allowing for a high degree of angularity in the particle shapes to achieve high surface sheering/cutting and low peening or denting. Recyclability, along with minimal dust creation from the grit, is also an advantageous feature. The surface profile is targeted to be 3 mils and the surface texture is etched allowing for optimum adhesion of high-performance coatings, such as those used on Delta’s wastewater tankage.
How long will they last?
We believe wastewater tankage used in modern decentralized wastewater treatment plants will last more than 40 years and here’s why. Coated steel wastewater systems built using less sophisticated coating application techniques have demonstrated service lives beyond 30 years, and the coating application process didn’t generate near the adherence levels achieved today.
Withstanding the test of time. The steel package plant to the right was manufactured using A36 steel and was coated with coal tar epoxy. The surface preparation involved sandblasting and air cleaning. It was placed in service in Louisiana in 1986. The photos to the right were taken 32 years later in 2018.