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Multi-Media Filtration (MMF) is a water treatment process that employs a pressure filter vessel utilizing three (3) or more different media. It is commonly used in industrial filtration applications and water treatment plants.
The terms “multi-media”, “multilayer,” “in-depth,” and “mixed media” apply to a type of filter which is graded by size and density. Coarse, less dense particles are at the top of the filter bed, and fine, more dense particles at the bottom. Downflow filtration allows deep, uniform penetration by particulate matter, permits high filtration rates and long service runs. Because small particles at the bottom are also more dense (less space between particles), they remain at the bottom. Even after high-rate backwashing, the layers remain in their proper location in the MMF bed.
In MMF, pressure vessels with sand and other loose media are used. 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).
Typically, MMF use three (3) layers of media: anthracite, sand, and garnet. These media are chosen due to their distinct differences in densities. The layers encourage larger contaminants to become trapped in the first layer of the filter, with smaller particulates sifting farther down into lower layers. This allows for more efficient turbidity removal and longer run times before backwash cycles. Anthracite is the lightest media per unit volume, followed by sand and then garnet.
A MMF differs from a “sand filter” that typically uses one grade of sand alone as the filtration media. In a sand 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 lower in the filter. This results in very limited use of the media 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.
Note that MMF can remove particles down to 10-15 microns (µm) in size compared to a simple sand filter that can eliminate particles down to 25-50 microns (µm) in size.

Types of Media

Quartz sand, silica sand, anthracite coal, garnet, magnetite, and other materials may be used as filtration media. Silica sand and anthracite are the most commonly used types. When silica is not suitable (e.g., in filters following a hot process softener where the treated water is intended for boiler feed), anthracite is usually us.
The size and shape of the filter media affect the efficiency of the solids removal. Sharp, angular media form large voids and remove less fine material than rounded media of equivalent size. The media must be coarse enough to allow solids to penetrate the bed for 2-4 inches. Although most suspended solids are trapped at the surface or in the first 1-2 inches of bed depth, some penetration is essential to prevent a rapid increase in pressure drop.
Sand and anthracite for filters are rated by effective particle size and uniformity. The effective size is such that approximately 10% of the total grains by weight are smaller and 90% are larger. Therefore, the effective size is the minimum size of most of the particles. Uniformity is measured by comparison of effective size to the size at which 60% of the grains by weight are smaller and 40% are larger. This latter size, divided by the effective size, is called the uniformity coefficient; the smaller the uniformity coefficient, the more uniform the media particle sizes.
Finer sands result in shallower zones for the retention of suspended matter. The most desirable media size depends on the suspended solids characteristics, effluent quality requirements and specific filter design. In general, sand filters use sand with an effective size of 0.35-0.60 mm (0.014-0.024 in.) and a maximum uniformity coefficient of 1.7. Coarse media, often 0.6-1.0 mm (0.024-0.04 in.), are used for closely controlled coagulation and sedimentation.
A large particle bed supports the filter media to prevent fine sand or anthracite from escaping into the underdrain system. The support bed also serves to distribute backwash water. Typical support beds consist of gravel or anthracite in graded layers to a depth of 12-16 inches.

Typical Construction

Conventional gravity and pressurized MMF operate in the downflow direction. The filter media is usually a 30-36 inches deep bed of sand or anthracite or sufficient media to allow for 50% expansion during backwash. Single or multiple grades of sand or anthracite may be used.

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