Prefiltration can be described simply as any filtration step incorporated into a manufacturing process prior to the final filtration.
In practically, a final filter is a microporous membrane, which is manufactured from high-tech polymers. It is normally in pore size designations of 0.04 – 8 micron. These filters retain particles of sizes larger than their pore size ratings with great reliability. The final filters will not only remove submicron particle that is designated to, it will also capture loads of large particles if the feed fluid is highly contaminated.
We can prolong the filtration life time of final filters by adding larger filtration area (more filters used). However, this will increase filtration cost. Hence, in order to reduce overall filtration cost, pre-filters is used prior to final filter to remove portion of larger impurities. Generally it is more cost effective to remove the majority of the contaminants from the fluid stream by using pre-filters prior to the final filter removing the reminder.
Pre-filters are not intended to be completely retentive. It is designed to accommodate only a portion of the particulate load, permitting the remainder to impinge upon the final filter. In the process. the life of the final filter is prolonged by the use of pre-filters.
Depth-type filters are usually used for pre-filtration. However, microporous membranes of higher pore size ratings may serve as pre-filters for finer porosities.