This study was conducted to recover the aluminum from water treatment plant sludge containing alumina. The optimum reaction conditions about chlorination of sludge with $NH_4Cl$ are as follows: the weight ratio of sludge to $NH_4Cl$ is 4, the reaction time is 60min, and the temperature is $300^{circ}C$. And the result of leaching time test showed that the highest yield of alumina at $160^{circ}C$ was 96% but the result of leaching test at $160^{circ}C$ was little better than that of leaching test at $100^{circ}C$ while the leaching concentration of HCl was 4N. The optimum reaction conditions of chlorinated sludge with $NH_4Cl$, gave the highest yield of 95.41% based on aluminum.