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Biology of Cooling Waters

Home Domain Functional Sectors Cooling Water Biology of Cooling Waters

Water is widely used as a cooling media in the industrial heat transfer equipments. Cooling waters or Circulating waters, as it is generally called, contain all the naturally occurring living organisms and nutrients that are responsible for uncontrolled multiplication of the living organisms which is responsible for the formation of biofilm on the heat transfer surfaces. This biofilm acts as an excellent insulator so the water can't access the surface of the heat exchanger. The organic acids that are waste products of bacterial metabolism get between the biofilm and the heat exchanger surface and corrode the surface. This is Microbially Induced Corrosion (MIC).

The classes of microbiological organisms that proliferate in open cooling water systems are Algae, Fungi and Bacteria.

1) Algae

Algae are simple polyphyletic and paraphyletic group of organisms that range from very small, single-celled microalgae to macroalgae that group into very large organisms such as kelp. They are defined in differing ways, but are usually considered to be photosynthetic organisms excepting plants. There are about 30,000 species of algae.algaeThe vast majorities of algae are deriving energy from the sun and use chlorophyll to produce energy and biomass. Excessive growth of algae on the deck of cooling tower can choke the distributor nozzles and reduce the water flow through cooling tower. Algae, along with biofilm formation can be a problem inside the cooling tower basin and on nearby grids. These types of growth can have a negative influence on the efficiency of the cooling tower thereby resulting in direct / indirect commercial loss. In addition, algae can contribute to localized corrosion processes, especially microbiologically induced corrosion (MIC) resulting in corrosion of metal on water side of heat exchanger. Algae also influence the scale formation.

2) Fungi

Fungi are a group of simple plants that have no chlorophyll.

fungiTwo forms of fungi commonly encountered are molds (filamentous forms) and yeast (unicellular forms). Molds can be quite troublesome, causing white rot or brown rot of the cooling tower wood, The yeast do not cause wood rot, but instead can proliferate to high numbers and foul heat exchangers surfaces.

They can produce slime in abundant amounts and preferentially colonize wooden surfaces. Both yeast and fungi are commonly transported by air currents. They are relatively large and can be easily identified with microscopic analysis. There are more than 75,000 species of yeast and fungi, which include mold, smut, rust and mildew. They usually require less moisture and can survive at lower pH levels than algae or bacteria.

3) Bacteria

Bacteria are mostly unicellular organisms that lack chlorophyll (a green pigment found in plants that allows for the production of food) and are among the smallest living things on earth—only bigger than viruses. Multiplying rapidly under favorable conditions, bacteria can aggregate into colonies of millions. Many bacteria are so small that a million of them, laid end-to-end, would measure no more than about five centimeters (two inches).bacteria

The term ‘bacteria’ is the plural form of the word bacterium, which represents a single organism. Bacteria can often be found in cooling tower water. When cooling towers contain an open recirculation system, bacteria can spread from air to water. High levels of bacteria also leads to possible microbial influenced corrosion which reduces the efficiency of the cooling tower and thereby waste energy.

Microorganisms can rapidly multiply, when a substrate is present and conditions (pH, temperature, oxygen concentration and nutrients) are ideal for microbial growth. Both aerobic bacteria (which thrive in oxygenated waters) and anaerobic bacteria (which are inhibited or killed by oxygen) can be found in cooling systems. Sulfate reducing and iron metabolizing bacteria (Gallionella and Sphaerotilus) use soluble or ferrous, iron as an energy source, and convert it to an insoluble oxide or hydroxide form. These deposits create fouling and set up concentration corrosion cells and conditions under which anaerobic bacteria flourish. Biofilm coating on a heat exchanger will reduce the heat transfer efficiency of the exchanger.

The scale and biological deposits reduce the ability of refrigerant condensers and industrial process heat exchangers to transfer heat. Other bacteria that may be present in cooling water include Pseudomonas, Klebsiella, Eneterobacter, Acinetobacter, Bacillus, Aeromonas, and Legionella. Spore forming bacteria Denitrifying bacteria, or pseudomonas, can cause the loss of nitrite inhibitor in closed-water systems.

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