Filtration Through Size-Graded Media

Existing theory for filters containing media of one size has been extended to filtration through size-graded media. This theory demonstrates the advantages of filtering into the coarsest media first, then through subsequently finer media. This is the reverse of present (1965) practice in sand filters. Experiments compared down-flow (normal) with upflow (reverse) filtration, showing the operating advantages of the latter. Washing upflow filters gave practical difficulties. An experimental three-layer filter of coarse anthracite, on medium-size sand, on fine garnet sand, enabled filtration to proceed downwards but through the correct grading sequence suggested by the theory. This filter produced considerably lower head losses for comparable filtrate quality as compared with a normal sand filter with equivalent grain sizes. The three-layer format remained stable after several upflow backwashings that fluidized the media.