Deionized water does not contain minerals that in time may interfere with the coolant's efficiency. Tap water has minerals and ions in it as well as bacteria. Water based coolants will evaporate while sitting in the sump and vaporize while the coolant is being applied to the metal working process. The minerals present are not evaporated and accumulate in the sump increasing the concentration of minerals in the coolant. If tap water is continually used, the mineral content of the coolant continues to increase.
Concentrated minerals can form deposits and soaps, gumming up the metal working process.
During the metal working process, the heat of operation and the chemical reactivity of swarf fines initiates degradation of the metal working fluid. The particles chemically interact with the oils in the emulsions, weakening the emulsion and breaking the fluid down to its separate constituents. Swarf also facilitates the growth of the bacteria and fungi that feed on the coolant by providing a substrate for them to grow on. Bacteria use the emulsions, additives and oil for food and similarly contribute to coolant degradation. This is called coolant spoilage.