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Contaminants, i.e. minor impurities, are normally present in all fluids, natural or processed. Where the level of contamination in the fluid is significant, i.e. too great for that fluid to be consumed, or used for its proper purpose, or to be subsequently processed, then it becomes necessary (or in some cases legally obligatory) to remove these impurities from the carrying fluid, so as to reduce contamination to or below acceptable levels.

These contaminants may be rigid or deformable solids, when they can be separated by a filter, or perhaps by a sedimentation process. Distinct liquid droplet contaminants can also be removed in this way, using filter-like separators or sedimentation. As indicated in Section 1A, where these contaminants are completely mixed in the carrying fluid (gas dispersed in a gas, liquid dissolved in another liquid), then filtration or sedimentation are not suitable for their separation, and phase change processes such as adsorption (e.g. with activated carbon, or perhaps molecu lar sieves) must be used. The middle ground is occupied by some solid/liquid or liquid/liquid solutions whose value is such as to justify purifying by means
of a membrane separation process.

The kind of situation being discussed here involves the presence in a fluid of some material, solid or liquid, that would be harmful to the consumer or other user of that fluid if it remains in that contaminated state. Thus a homogeneous suspension may carry rogue particles of a size much larger than that in the emulsion – these can usually be removed quite easily by a scalping strainer. The coolant used to lubricate and cool the cutting zone of a machine tool is a quite complex mix of
liquids, which is too expensive to throw away. After use it will normally be contaminated with the metal particles (swarf) removed from the item being machined, so that before being recycled it must be freed of such contamination, in a filter, or settling tank or centrifuge. The air in the modern city street is quite highly polluted with diesel fumes among other things, and many people are now wearing personal respirator masks to protect themselves against these impurities in the air that they breathe. Surface water abstracted for drinking water production is frequently coloured to an unacceptable level by small quantities of colloidal solids, which may require ultrafiltration for their removal.

As can be seen from Figure 1.1, the kinds of impurity carried in the atmospheric air vary widely in size, with the greater proportion being invisible to the naked eye (the limit of human vision being about 20m). Such small particles, either in the
air or settled out into a liquid stream, are perfectly capable of sticking between adjacent machinery surfaces and causing wear, or of collecting together to block a liquid passage and so interfere with flow.

The separation of such contaminants from working or process fluids is very largely a task for filtration, in ways to be described later in this book. The only significant decontamination process not using filters is the large volume process of settlement of fresh or wastewater. Purifying processes on gas or liquid streams are often required to treat such large volumes of fluid. It obviously pays, therefore, to treat no more than is absolutely necessary – the personal gas mask, for example, rather than all of the ambient air. Another approach to the treatment of large flows for contaminant removal is by taking off a sidestream and cleaning this completely, before mixing it back into the main flow. With the correct ratio of sidestream to main flow, the final composition can be made acceptable.

It must be remembered that protection against contamination can often be a twoway process. Thus, a working space may need protection from the contaminants in ambient air by means of the provision of a controlled atmosphere. However, its
working materials may be sufficiently toxic that the external environment may need protection against unintentional emissions from inside the space, which means that all vents from the working space, of whatever size, will need to be provided with adequate filters or adsorbers.

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