Filtration Through a Trimedia Filter

An historical review of existing filter theory indicates the advantages of filtering from coarse to fine media. A study was made of filtration through trimedia beds and the filtration characteristics exhibited by the mineral garnet. Two three-layer filters using different size gradations of anthracite, sand, and garnet were compared with a conventional sand filter. The first series of tests was run on filters comprised of 8-in. layers of 1.19-mm anthracite, 0.590-mm sand, and 0.420-mm garnet. The second series of tests was run on filters of 8-in. layers of 1.00-mm anthracite, 0.707-mm sand, and 0.590-mm garnet. The conventional filter contained sand with an effective size of 0.460 mm and a uniformity coefficient of 1.44. The filter tube had an ED of 3-1/2 in., provided for the monitoring of the pressures within the bed and for obtaining samples of fourteen points. The trimedia filters permitted a head loss reduction of nearly 50%, with no reduction in filtrate quality, as compared with the conventional sand filter under the same operating conditions. The trimedia filter remained intact after repeated backwashing, but required somewhat more wash water and higher wash rates to clean the bed effectively.