Enhanced Particle Capture through Aluminum Hydroxide Addition to Pores in Sand Media

Abstract

In this study, precipitation of Al(OH)3(am) was used to modify a sand filter medium by fluidized-bed pretreatment. A mixture of alum, sodium hydroxide, and tap water was applied to the filter bed in the last stage of the backwash cycle. The placement of Al(OH)3(am) in the filter pores was evaluated for both alum-treated raw water (contact filtration) and untreated raw water. The filter pretreated with Al(OH)3(am) achieved better than 99.98% removal of an untreated clay suspension, with a filter effluent turbidity below the detection limit of 0.01 NTU. Al(OH)3(am)-pretreated filters that were challenged with clay and humic acid achieved ≥99.8% turbidity removal efficiency for 14 h of operation in the contact filtration mode. Pretreatment with Al(OH)3(am) also enhanced turbidity removal efficiency (up to 99.8%) when the filter was challenged with clay and humic acid, even when the raw water was not coagulated. The aluminum concentration in the filter effluent of an Al(OH)3(am)-pretreated filter was below the EPA secondary drinking water maximum contaminant level (200 μg/L for aluminum) when the raw water pH was between 6 and 7; the pretreated filter had the best performance at pH 6.