For example, swarf is particulate metal created during the grinding or cutting of metals. The presence of swarf creates friction, defeating both the lubricating ability and cooling capabilities of the coolant. The increased concentration of tramp oil similarly interferes. Some coolant manufacturers state that their coolant can use any type of water while others specify deionized water or mineral free water. 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.