The cleaning process is really just an exercise of removing contamination from the surface being cleaned and moving it to a segregated location for collection and disposal. This is accomplished in a number of ways depending on the volume and nature of the contamination. Solid contamination that is more dense than the cleaning media (usually water) is commonly segregated using gravity separation or filtration. Larger chunks of stuff like metal chips, large pieces of scale and sand, for example, collect in sumps and at the bottom of cleaning tanks and are collected and removed by periodic “dredging” of the sump or tank bottom. Solid contamination that is less dense than the cleaning media floats and can be collected and removed by periodic manual or automated skimming of the surface of a cleaning tank. Smaller particles of solid contamination often remain in suspension in constantly moving cleaning liquids and are removed by filtration. A pump is used to route contaminated liquid through a filter media which is designed to allow passage of liquid but retain solid particles. The filter media is chosen depending on the size and volume of particles present, the degree of cleanliness needed and the nature of the cleaning solution. There are a number of filtration configurations available. Liquid contaminants may include those that are soluble in the liquid cleaning media and others that are not soluble in the cleaning media. Contaminants that are soluble in the cleaning media simply accumulate over time and are finally removed only when the cleaning solution is dumped and replaced. Insoluble contaminants (like most oils) are handled in one of two ways depending on the chemistry of the cleaning media. Chemistries known as “splitters” preferentially wet the surface being cleaned thereby displacing the insoluble liquid contaminant. The displaced contaminant then either floats to the surface of the cleaning liquid or sinks to the bottom where various schemes are used to remove it. Other chemistries known as “emulsifiers” or “saponifiers” divide, surround and isolate small globules of otherwise insoluble contamination and effectively prevent them from being re-deposited on the cleaned surface. These emulsified contaminants build up just like soluble contaminants and are removed by draining and replacing the cleaning liquid. There are, of course, other means of separating and collecting other “special case” contaminants (magnetic particles for example) but the above pretty much covers the everyday cases of contamination collection and disposal. Future blogs will cover the details of filtration and chemistry selection in more depth.