Microbes need nutrients for growth and to obtain energy for growth. Therefore they cannot grow in pure water. However most cooling water contains a few nutrients and these become more concentrated if water is lost by evaporation and this water loss is made up by the addition of more water. If the cooling water contains additives for scale and corrosion control these chemicals provide more nutrients; the effectiveness of these additives will be lost if they are attacked and degraded by the growing microbes.
Microbial influenced corrosion, fouling and additive depletion are common in many types of cooling water system and are a particular problem in the cooling water of large diesel engines and closed water building systems.