Predicting Fluidization and Expansion of Filter Media

Granular media filtration is of growing importance in both water and wastewater treatment. A variety of filter media are being adopted for some filters. The common filter materials (sand, anthracite, garnet, illmenite) are all angular in shape. Some have low sphericity (very angular) while others have high sphericity. The sphericity of the material greatly influences the fluidization behavior of the media. It influences the fixed bed head loss, the minimum fluidization velocity, and the expansion of the media as a function of upflow rate. All of these are important design aspects in backwashing of filters. The physical properties of the several common filter materials are presented, including the sphericity, density and porosity. The Wen and Yu equation is adequate for predicting minimum fluidization velocity for all of the filter media studied. A new model for predicting expansion of filter media is presented which includes the sphericity or dynamic shape factor of the filter media as explicit variables.