Water Conditioning & Purification Magazine

Media

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Materials that form a barrier to the passage of certain suspended solids or dissolved liquids in filters. Media stones, pebbles or plastic that provides a substrate for the growth of a film of microorganisms in wastewater treatment; plastic, ceramic or metal shapes or surfaces that improve gas-liquid contact in gas scrubbing. Also called depth filters. A variety of media types may be employed in this type of filtration device, depending on the objective of this part of the water treatment system. Sand has been employed in this type of filter for many decades. Media type filters are recommended for applications where large quantities of suspended particles need to be removed economically. Usually the media is permanent, so maintenance costs are very low. The device is typically designed with graded media beds, arranged according to size. The spaces between the particles of sand or other media determine the size of particles that will be removed. The depth of the filter base is also a critical design criteria. Water flows downward through a distributor, through the graded media beds, and is collected by another set of distributors installed in the bottom of the tank. The particles are left behind in the media beds. Cleaning is accomplished by backwashing, or reversing the flow. This flow lifts the media beds and dislodges the particles trapped there. These are then washed away with the backwash fluid and discharged. Media filters are often employed as one of the first parts of the pretreatment system in membrane type water treatment systems. Usually, additional filtration afterwards is also required, as the particle size removed by media filters is not small enough to provide good membrane protection.

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