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One of the more predominant types of bacteria occurring in recirculating cooling water systems are slime-forming bacteria. These bacteria are aerobes and are naturally present in the soil. Consequently, they continuously infect cooling systems. 

These bacteria may flourish throughout the cooling system, including the heat exchanger surfaces and the cooling tower. These are typically encapsulated bacteria which produce slime layers outside the cell walls. These slime-encased cells attach to the available surfaces. 

Then they reproduce, form additional slime and develop into boideposits or "biofilms". The biofilm layer is composed mostly of water, hence it presents a significant barrier to heat exchange. The biofilm mass, which also frequently contains filamentous bacteria, can serve as the nuclear for agglomeration and deposition of water borne suspended solid. These combined biological-mineral deposits dramatically reduce heat exchanger efficiency and also create differential aeration cells which can result in high localized corrosion rates.

Corrosion of metal surface is also caused by several types of anaerobic bacteria. These bacteria flourish beneath biofilm and other deposit layers where oxygen is not present or readily replenished. The most notable of the anaerobic corrosive bacteria are the sulfate reducing bacteria (SRB). 

These bacteria reduce sulfate to corrosive hydrogen sulfide. They also are thought to cause cathodic depolarization by removal of hydrogen from the cathodic portion of corrosion cells. Acid producing bacteria (APB) produce organic acids. These metabolic processes cause localized corrosion of deposit laden distribution piping and also provide the potential for severe pitting corrosion of heat exchanger surfaces. This entire process is called microbiologically induced corrosion, or MIC. 

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