This is called coolant spoilage. Coolant degradation results in a reduction of the ability of the coolant to lubricate and transfer heat away from the metal working process. Microbial Activity Bacteria and other microbial organisms thrive in the environment created by the impurities in the coolant. They feed upon mineral oils, fatty acids, emulsifiers, corrosion inhibitors, other additives and waxes in oil based and synthetic coolants. The corrosion inhibitors of synthetic coolants, for example, are consumed.
Aerobic bacteria require oxygen for metabolism and efficiently destroy the coolant.
Anaerobic bacteria grow in environments lacking oxygen. They feed upon the coolant and produce noxious byproducts such as hydrogen sulfide. This is commonly referred to as the Monday morning odor. Problems Associated With Microbial Spoilage Microbial action directly affects the coolant resulting in the splitting of emulsions, decreased pH, increased corrosion, degradation of the ingredients in the coolant and a loss of lubricating ability within the coolant itself. Odors may develop including hydrogen sulfide as a product of the bacteria's metabolism.