Transcript
Multimedia Filtration 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 t he backwash flow. fter the backwash ba ckwash cycle cycle,, the bed is allowed to settle se ttle before the filter f ilter is ret urned to to service (i.e., normal flow). "filter!to!waste" cycle is used following the settling to assure the filtration 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 f ilter" that typically uses one grade of sand alone as the filtration media. #n 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 and heavier particles, stratify proportional to their mass lower in the filter. $his 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 %!& inches of the top where the filter media particles have the least space between them. $he 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 water filters typically
utilize three layers of media for multimedia filtration' anthracite, sand and garnet. $hese media are often chosen for use in multi media filters due to the distinct differences in their densities. nthracite is the lightest filtration media per unit volume, followed by sand, and then garnet. $he 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. $his layering of the f iltration 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. $rapping contaminants in this manner allows for more efficient turbidity removal and for longer run times between backwash cycles . simple sand filter can be epected to eliminate particles down to &!* microns in size, as compared to a multi media filter that can remove particles down to %*!& microns. +perating at higher pressure differential is liable to drive particles so deeply into t he media bed that backwash is not able to remove them all. +ver time the build!up of dirt deep in the filter will cause shortened filter runs and high differential pressures. ilter backwash may include air scour to help loosen packed dirt in the media bed. -hen 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. locculants coagulants may be used upstream of the filter to induce the tiny dirt particles to oin together to f orm particles large enough to be removed by the f ilter. $his process is called "agglomeration" and, with proper chemical dosage, ade/uate miing and ade/uate
contact time, it will enable the filter to remove particles below %* microns in average diameter. THE BENEFITS OF MULTIMEDIA FILTRATION OVER CONVENTIONAL SAND FILTERS
0nlike traditional sand filters, multi-media water filters are composed of three filtration media, ordered in decreasing porosity. 1ecause of their multi!layer design, multi!media water filters are able t o trap and retain a far larger number of particles than traditional sand filters before backwashing becomes necessary. $rapping sediment and particulates throughout the entire depth of the filter bed, allows multi!media water filters to operate for much longer periods of time than conventional sand filters. $he process of m ultimedia filtration produces high /uality, filtered water at much faster flow rates than traditional sand filtration.
