Long-term evaluation of aluminum hydroxide-coated sand for removal of bacteria from wastewater

The effectiveness of sand coated with aluminum hydroxide by in situ precipitation was evaluated over a 4 month period during which the sand was exposed to wastewater. Biogrowth in one set of columns was prevented by chlorinating its wastewater influent, whereas a parallel set of columns received dechlorinated wastewater. Following an initial “conditioning effect” (i.e. an increase in bacterial removal capacity), bacterial removal capacity of coated sand gradually decreased to that of uncoated sand. Biogrowth on the sand accelerated this decline. The aluminum content of coated sands decreased by approximately 25% over the first two weeks, then remained relatively constant and well above that of uncoated sand. Similarly, zeta potential decreased over the first two weeks from above +20 to below −70 mV, which was still significantly more electropositive than that of uncoated sand. Zeta potential of coated sand without biogrowth subsequently remained approximately constant, while that of coated sand with biogrowth increased gradually. No apparent correlation was found between metal content or zeta potential and bacterial removal capacity. The results suggest that, absent of biogrowth, the effective lifetime of the aluminum hydroxide coated sand is approximately 4 months, whereas with biogrowth, the effective lifetime is reduced to approximately 3 months. This information is of importance for assessing the technological potential as well as economical implications of metallic hydroxide coating of filter media.