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Municipal sewer systems can be hotbeds for corrosion. One type in particular, where hydrogen sulfide gas (H2S) generated from the anaerobic decomposition of sewage is converted by Thiobacillus bacteria into sulfuric acid (H2SO4), presents a unique challenge. Sulfuric acid is extremely corrosive to the concrete used in most sewer systems. The process, known as microbiologically induced corrosion (MIC), causes a gradual loss of concrete and reduces the service life of the affected structures in sewer systems. It is also referred to as biogenic corrosion, bacterial corrosion, bio-corrosion, microbial induced corrosion, and other similar names; and if left unchecked, the corrosion can lead to structural failures such as collapse.
In this article, we examine MIC in detail, discuss the shortcomings of spray-on liners, and explore the benefits of AGRU Concrete Protective Liners.
Microbiologically induced corrosion: The bane of sewer systems
MIC is a significant problem. The designed service life of most concrete structures in a city’s sewer system is between 75 and 100 years. However, that estimate can be cut to less than 10 years if the structure is exposed to corrosion. The need to rehabilitate these sewer systems ahead of their designed service life has led to a significant capital drain. In the United States, for instance, sewer system rehabilitation costs municipalities nearly $14 billion every year. In some cases, sewer system service life is cut even more drastically, because the MIC rate—the rate at which the concrete is corroded—is affected by nearly a dozen factors.
The MIC rate is affected by wastewater pH, high sulfates, retention time, turbulence, flow velocity, wastewater and ambient temperature, and more. Because there are so many pieces to the MIC equation, engineers have not found a practical way to alter the sewer environment to prevent the MIC reactions from taking place. Instead, sewer system designers have sought to treat the surface of the concrete to be less susceptible to corrosion.
Spray-on liners: A costly temporary solution
Beyond surface washes, sewer design engineers have the option of using spray-on coatings. While these spray-on coatings do help prevent concrete corrosion, they each present a number of disadvantages. For instance, coatings with limited life expectancies require regular reapplication including treatment and preparation of the concrete surface. After several treatments, the cost of replacing the sacrificial material can be significant.
Coating concrete with a spray-on liner that is resistant to corrosion uses chemicals that can be harmful to the environment. Furthermore, these coatings only offer limited resistance to groundwater backpressure and, therefore, do not adequately protect against infiltration and inflow which is a significant problem with many wastewater systems. Finally, perhaps the biggest pitfall of spray-applied coatings is the frequent need to place portions of the sewer system into maintenance. Each time the spray-on coating must be reapplied on the concrete, the pipe must be emptied, cleaned and prepared; and the sewage flow must be bi-passed.
Overall, spray-on coatings are a fine short-term solution, but for those seeking long-term protection against MIC consider the strengths of a polyethylene (PE) liner.
- Wu L., Hu C., Liu WV, “The Sustainability of Concrete in Sewer Tunnel—A Narrative Review of Acid Corrosion in the City of Edmonton, Canada.” 10:517 (2018). doi:10.3390/su10020517.