Multimedia filtration refers to a pressure filter vessel which utilizes three or more different media as opposed to a “sand filter” that typically uses one grade of sand alone as the filtration media. In a single media filter, during the “settling” cycle, the finest or smallest media particles remain on top of the media bed while the larger, and heavier particles, stratify proportional to their mass lower in the filter. This results in very limited use of the media depth since virtually all filterable particles are trapped at the very top of the filter bed or within 1-2 inches of the top where the filter media particles have the least space between them. The filter run times are thus very short before the filter”blinds” or develops so much head pressure that it must be backwashed to avoid seriously impeding or stopping the flow.
Multi media filters typically utilize three layers of media for multimedia filtration: anthracite, sand and garnet. These media are often chosen for use in multi media filters due to the distinct differences in their densities. Anthracite is the lightest filtration media per unit volume, followed by sand, and then garnet.
The idea behind using media with differing masses is that during backwashing the lightest media with the largest particles (anthracite) will naturally stratify at the top of the filter, while the intermediate sized media (sand) will settle in the middle, and the heaviest media with the smallest particles (garnet) will settle to the bottom.
This layering of the filtration bed encourages the very largest contaminants to become trapped in the first layer of the filter, with smaller particulates sifting farther down into the lower layers. Trapping contaminants in this manner allows for more efficient turbidity removal and for longer run times between backwash cycles. A simple sand filter can be expected to eliminate particles down to 25-50 microns in size, as compared to a multi media filter that can remove particles down to 10-25 microns.