Pressure vessels with sand or other loose media are widely used in industrial filtration applications. During the cleaning cycle, called "backwash", the bed is lifted (or "fluidized") to loosen the filter media and release trapped dirt which is removed in the backwash flow. After the backwash cycle, the bed is allowed to settle before the filter is returned to service (i.e., normal flow). A "filter-to-waste" cycle is used following the settling to assure the media has sufficiently re-stratified and that any loose dirt is removed from the underdrain / collectors.
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 have three layers, consisting of anthracite, sand and garnet. These are often the media of choice because of the differences in mass between the materials. Garnet is by far the heaviest per unit volume, sand is intermediate while anthracite is the lightest. The idea behind using these three media of differing densities is that anthracite media, with the largest particle size, will stratify on top following backwash while the intermediate size media (sand) will settle in the middle and garnet, the heaviest but having the smallest particle diameter, will settle to the bottom.
This filter media arrangement allows the largest dirt particles to be removed near the top of the media bed with the smaller and smaller dirt particles being retained deeper and deeper in the media. This allows much longer filter run times between backwash and much more efficient dirt or turbidity removal. Sand filters typically remove particles down to 25-50 microns while a well-operated multi-media filter may remove particles from 10-25 microns.
Multi-media filters are frequently operated at loading rates of 3-8 gpm per square foot of surface area and must be backwashed at 12-15 gpm per square foot to adequately lift and loosen the bed. Due to the variation of water density with temperature, higher flows are required with warmer water temperatures so manufacturer’s specifications should be consulted during design. Filters should be backwashed at differential pressure less than or equal to 12 psi (that is, if the pressure differential at design flow across a clean filter bed is 4 psi, the filter should be backwashed at a delta p less than or equal to 16 psi). Operating at higher pressure differential is liable to drive particles so deeply into the media bed that backwash is not able to remove them all. Over time the build-up of dirt deep in the filter will cause shortened filter runs and high differential pressures.
Filter backwash may include air scour to help loosen packed dirt in the media bed. When this step is included, it is preceded in the backwash cycle by a "drain down" period for water to be bled out of the filter vessel.
Flocculants / coagulants may be used upstream of the filter to induce the tiny dirt particles to join together to form particles large enough to be removed by the filter. This process is called "agglomeration" and, with proper chemical dosage, adequate mixing and adequate contact time, it will enable the filter to remove particles below 10 microns in average diameter.
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