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Microorganisms are found everywhere in nature; they are distributed in air, water and soil and play a number of vital roles in the health of man and animal alike. Some microorganisms are beneficial, while others, called pathogens, cause diseases such as typhoid fever, amoebic dysentery, cholera or hepatitis. 

Microbial life affects many industrial processes. The nature of the particular microorganism and industrial process will determine whether the relationship is beneficial or destructive. 

For example, the zoogloeal bacteria found in activated sludge wastewater treatment plants are beneficial; they produce a polysaccharide slime which aids other bacteria to digest organic matter that would otherwise be discharged into a receiving stream and cause pollution. 

Conversely, the microorganisms which inhabit commercial or industrial cooling water systems can adversely affect the efficiency of the operation either by their sheer numbers, metabolic waste products generated or deposits created. This discussion will focus on problems created by microbial infestations in cooling water systems and the control procedures used to prevent them.

As an isolated single cell, a microorganism presents no problem to a water supply to be used for cooling purposes. However, bacteria reproduce by an asexual process called binary fission, where one cell divides to become two. Thus, when we speak of bacterial "growth," we mean an increase in the number of cells. An entire population will double in size during one division. As these divisions continue

until controlled by physical or chemical conditions, growth is exponential. Systems which store water may provide a potential environment for their uninhibited growth. The cooling tower is an excellent example of a contained water system that provides optimum conditions for microbial growth. Temperature and pH are usually within the ideal ranges, and there is generally an abundance of nutrients in the form of organic matter, inorganic salts and sunlight required for their growth. 

Significance is often attached to the fact that microorganisms, like mineral salts, will concentrate in open evaporative systems. This arithmetic increase is inconsequential, however, when compared to the natural bacterial logarithmic reproduction cycle. Although mineral salts may concentrate six times in a tower, it is conceivable that bacterial concentrations may increase six million times during an equal time span.