In a multi-bed filter why is the larger particle size bed placed over the smaller particle size bed?
Prakash Pimparkar, Environmental Consulting Services , email@example.com
These are depth filters and fed from the top. The top layer is made of carbon/hard coal and is course.The middle layer is made of filtration grade silica sand and the lower layer made of garnet sand.The relative densities are garnet> sand>carbon and the particle size range is carbon>sand>garnet. This traps the larger particles in the top layer medium in sand and very fine material in garnet. After some time, depending on the concentration of suspended solids ( SS ) in the feed, the bed starts to be filled with these trapped solids in the space between the media particles. As a result the bed becomes choked and exerts high back pressure. At such times the bed is washed ( back washing ) by reversing the flow of feed at almost 1.3 to 1.5 times the feed flow. Sometimes this is enhanced by using air to agitate and release the trapped particles for about 15 to 20 minutes. This dirty water is diverted for disposal. After this the bed is rinsed with clean water and the filter is ready to receive the feed .
The differential size of media thus helps in achieving better removal efficiency. To restore the bed, media layers of different densities are used to ensure that the bed retains the same configuration after back wash
The media resettles to coarse carbon on top, medium sand in the middle and fine garnet at the bottom, regaining the filtration efficiency. These are called multimedia filters to differentiate fron single media filters. Multimedia give better removal efficiencies and more consistent quality compared to single media filters.