LONG-TERM EVALUATION OF ALUMINUM HYDROXIDE-COATED SAND FOR REMOVAL OFBACTERIA FROM WASTE-WATER

The effectiveness of sand coated with aluminum hydroxide by in situ pr ecipitation was evaluated over a 4 month period during which the sand was exposed to wastewater, Biogrowth in one set of columns was prevent ed by chlorinating its wastewater influent, whereas a parallel set of columns received dechlorinated wastewater. Following an initial ''cond itioning effect'' (i.e. an increase in bacterial removal capacity), ba cterial removal capacity of coated sand gradually decreased to that of uncoated sand. Biogrowth on the sand accelerated this decline. The al uminum content of coated sands decreased by approximately 25% over the first two weeks: then remained relatively constant and well above tha t of uncoated sand. Similarly, zeta potential decreased over the first two weeks from above + 20 to below -70 mV, which was still significan tly more electropositive than that of uncoated sand. Zeta potential of coated sand without biogrowth subsequently remained approximately con stant, while that of coated sand with biogrowth increased gradually. N o apparent correlation was found between metal content or zeta potenti al and bacterial removal capacity. The results suggest that, absent of biogrowth, the effective lifetime :of the aluminum hydroxide coated s and is approximately 4 months, whereas with biogrowth, the effective l ifetime is reduced to approximately 3 months. This information is of i mportance for assessing the technological potential as well as economi cal implications of metallic hydroxide coating of filter media. (C) 19 98 Elsevier Science Ltd. All rights reserved.