The sludge extracted from chemical sedimentation basins shows a high concentration of aluminium which was added in the rapid mix tank. Therefore, the sludge volume after activation treatment with hydrochloric acid can be reused in order to coagulate turbid matter and humic substances in influent water. This study has focussed on examining the effectiveness of this sludge recycling process. HPLC with a RI detector was used to determine the molecular weight (M. W.) of PAC in the pH range of 3.0∼5.0. Results indicated that PAC was composed of two kinds of aluminium complexes at pH 3, having a M. W. of about 11,200 and 6,400. The M. W. of PAC in the pH variation and after the repetition of the activation treatment was also determined. Moreover, the abilities of PAC activated under the variable pH values to coagulate simulated natural surface water containing kaolin or humic acid were evaluated though jar-tests. When the pH in activated PAC was set at under 4, the coagulability of PAC did not deteriorate even if the activation treatment was repeated three times. In conclusion, the effectiveness of this process in practical use has been made clear from three points of view: the ph, the M. W., and the coagulability of PAC.

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