Iron bacteria usually enter a cooling tower or a closed-loop system through the makeup water supply or through airborne contaminants. Iron bacteria are also known as iron-reducing or iron-related bacteria. Once iron bacteria have attached themselves to a metal surface, they can be resilient and difficult to remove.
“Iron Bacteria” are capable of withdrawing iron from its aquatic environment and depositing it in the form of Fe(OH)3 (hydrated ferric hydroxide) or in its extracellular secretions. Iron-reducing bacteria can even be more aggressive than sulfate-reducing bacteria in various environments. The brownish-red slime byproduct of iron bacteria is almost always found on the distribution pans, in the sump, and at the ends of the cooling tower. If you were to draw your name in the slime layer and come back a few days later, it would be completely filled in by extracellular secretion.
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Iron bacteria cause microbiologically influenced corrosion (MIC). Iron bacteria are tenacious and are extremely hard to get rid of. They are sessile, meaning they easily adhere to surfaces. The slime layer of iron bacteria often sloughs off, forming deposits that affect the industrial process.
Also, the conditions they promote provide the perfect environment for sulfur-reducing bacteria to thrive. As the sulfur-reducing bacteria multiply in the anaerobic conditions underneath the iron-related bacteria, they begin to form corrosion nodules in the piping. This presents a new problem for the system. To measure the damage caused by iron bacteria, it is recommended to use corrosion coupons to track corrosion over time. Corrosion coupons track this based on the amount of metal missing on the coupons inserted.
Field or lab testing should be utilized to test for iron bacteria, as there are many bacteria that cause microbiologically influenced corrosion, and specific treatment will differ.
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IRB slime layer peeling away during chemical cleaning process. |
Cooling Tower distribution pans following Chardon’s IRB eradication process. |
The chemicals that should be used to fight iron bacteria are chlorine and a combination of two biocides. The biocides utilized should include one oxidizing and one non-oxidizing to fight bacterial strains that are resistant to one or the other. There should be multiple doses used, and the chemicals should be alternated to effectively kill the iron bacteria.
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