Prefiltration involves the use of a filter designed to remove particulate matter (dirt, sediment, etc.) from water prior to further treatment. Prefiltration is important for two reasons. First, larger particulate matter has the potential to clog or prematurely exhaust the filter(s) which follow it  in the system. Second, the effectiveness of further treatments (e.g. chemical or mechanical) can be significantly reduced in the presence of particulate matter. 

A variety of prefilter types are available. The most important way in which these filter types differ is in their “fineness”, or the size of the smallest particle they are able to filter out. Water filter fineness is typically expressed in micron size. “Micron” refers to the filter pore diameter in micrometres (µm). 

Ultimately, the fineness of the prefilter required for a particular application will vary, and will depend upon the quality of the source water involved (which will help identify the size of particles present in the water that need to be removed). Multiple levels of filtration are often required – prefiltration before fine filtration, for example – as much larger particles must be filtered out before water reaches the filter intended for finer particles. Without prefiltration, these large particles may damage the finer filter(s). 

It is also important to choose the correct size of filter. The “size” of the filter refers to the maximum flow rate that it is able to accommodate. There are often multiple filter sizes available for each level of fineness. The water pressure of the system must also be taken into consideration. Each filter model will have a limit to the maximum pressure at which it can operate. Also, the filter’s minimum operating pressure should be noted to ensure this pressure will be maintained to optimize system performance.

Deciding whether to add another level of filtration is a cost/benefit decision. While any form of prefiltration requires extra cost, the operator must take into consideration the costs or losses that would be incurred without prefiltration. Prolonging the life of more expensive downstream fine filters, for example, may be well worth the investment.

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