October 1, 2009

Coating Systems Help Protect Aging Wastewater Infrastructure

by Bob Murphy

Linings to Aid in Repair  of Aging Infrastructure *

Afflicted by pipe failures, sewer backups and sewage spills, America’s wastewater infrastructure is clearly showing its age. Most municipal sewer systems are 75 to 100 years old, and nearly all are compromised to some degree by corrosion, overburdening and structural fatigue. Even as these existing systems are deteriorating, population demands are pressing upon cities to expand their wastewater collection capabilities.

More and more municipalities are addressing this growing health and environmental challenge by paying increased attention to rehabilitating corroded manholes and infrastructure. By most industry estimates, leaking manholes account for an estimated 20% – 30% (or more) of the unnecessary groundwater infiltration and inflow that overtax and threaten the sanitary sewage system.

Once seen as secondary to mainline pipe repair, manhole rehabilitation is now recognized as essential to restoring a collection system’s overall integrity.

Advanced, high-performance polymer coatings technologies, including high solids epoxies, polyurethanes and polyurea, are increasingly finding a place in manhole rehabilitation due to their corrosion resistance and long-term effectiveness. Featuring higher film builds, faster dry times and other advantages, these new protective coatings formulations offer cost-effective solutions to municipalities seeking to reduce downtime, restore access and minimize the costs associated with sewer system repair.
Corrosion

A primary cause of corrosion in municipal sewer environments is MIC or Microbiologically Induced Corrosion. MIC occurs when bacteria of the genus Thiobacillus, “concrete-eating bacteria,” oxidize sulfur from hydrogen sulfide, “sewer gas,” and produce sulfuric acid as a by-product. Sulfuric acid is highly corrosive to concrete, steel and non-ferrous metals. When concentrated and heated by summer temperatures, it is extremely damaging to the concrete and non-ferrous metals used in most wastewater structures.

Manhole restoration can take several forms. For many years, manhole rehabilitation consisted mainly of spot repair or patching, typically offering only a temporary solution to the inevitable acid attack.

Today’s cementitious coatings are more sulfide-resistant, yet they still remain vulnerable to the corrosion that eroded the cement in the first place. Many municipalities have discovered that high-performance polymer coating systems offer the easiest and most effective way to rehabilitate manholes without having to excavate.

The coatings industry has developed corrosion-resistant systems designed specifically to withstand MIC because conventional epoxies and coal tar products are not as effective in highly corrosive environments. These hydrogen sulfide-resistant systems include tough, flexible aromatic 100% pure polyurea and polyurethane coatings and high solids epoxy coatings offering low or no VOCs and high film build. They can save certified applicators time and labor by reducing the number of coats needed to achieve the specified (or necessary) dry film thickness (DFT). These coatings are also suitable for other wastewater treatment facilities where hydrogen sulfide concentrations frequently can occur such as wet wells, lift stations, dewatering buildings and odor control buildings.

Polyurethane and polyurea coatings create highly elastic waterproof coating and lining systems that are resistant to chemicals and abrasion. These fast cure, no VOC, no odor coatings offer ultra-high film build and an extremely rapid dry time. When coated with polyurea or polyurethane systems, concrete and steel substrates can be submerged, buried or driven upon in just minutes. This fast reaction time is particularly appealing in municipal wastewater rehabilitation because it permits quick installation and a nearly immediate return to service.

Conclusion

According to various industry sources, there are anywhere from 18 million to 25 million manholes in the United States. It’s reasonably safe to say that a significant number of these are in some need of repair. Both high solids epoxies and advanced technology polyurea and polyurethanes offer cost-effective, nearly permanent solutions for manhole restoration to cities with limited funding. Factors including the structural condition of the manhole, application efficiencies, climatic conditions and economics may all determine which coatings system is ultimately the best for the job. Regardless, municipalities who elect to upgrade their sanitary sewer systems using these high performance coatings formulations can expect completed manholes that perform better than they did when they were new.

* Excerpted from New Coating Systems Help Protect Aging Wastewater Infrastructure, WaterWorld 3/2004

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