Enhanced filter performance by fluidized-bed pretreatment with Al(OH)3(am): Observations and model simulation

Fluidized-bed pretreatment of a rapid sand filter with alum [Al ₂(SO₄)₃14H₂O] has been shown to significantly improve performance during the initial portion of filter operation. Two alternative mechanisms are hypothesized by which alum pretreatment acts to improve particle removal: (1)Al(OH)3(am) coats the sand filter medium, and alters its porosity. (2)Precipitated Al(OH)3(am) embedded within the media pores acts as an additional filter medium that increases the particle capture efficiency. Conceptual models for these two mechanisms are developed and compared with experimental data for particle removal and head loss. Model predictions indicate that particle removal by filtration through Al(OH)3(am) flocs is the mechanism that significantly enhances the filter performance.