food grade product line by Water Services Ltd

Bacteria

Bacteria are single cell, microscopic organisms that usually reproduce by binary fission. Bacteria can be described as aerobic, anaerobic, or facultative, depending on whether they flourish in an oxygenated environment, and environment void of oxygen, or in both types of environments, respectively.

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

Legionella bacteria, which can pose potential health hazards, can also proliferate in cooling towers and condensers. Legionella bacteria are normally found in low numbers in water containing systems and can be isolated from most natural aquatic and soil environments. Cooling towers are therefore easily contaminated with these bacteria. When Legionella are expelled from towers as an aerosol in the drift, there is the potential that people may contact a certain type of pneumonia called Legionnaires' Disease or Legionellosis. Although people inhaling these bacteria-containing aerosols may get severe pneumonia and require antibiotic therapy in a hospital, the conditions necessary to produce the disease from cooling tower aerosols are certainly not well established. A less serious form of the disease is called Pontiac Fever. Legionella bacteria are not slime-forming and do not appear to cause fouling or corrosion in the cooling towers.