The contamination of water by arsenic (As) is a serious environmental problem, and many cost effective adsorbents have been developed to solve this problem. This study examines the effectiveness of reusing alum sludge from water purification facilities as adsorbents of As in aqueous systems. Adsorption kinetics and adsorption isotherm by temperature were the methods investigated. Kinetics revealed that the adsorption of As(III) and As(V) has rate-limiting steps such as surface diffusion and pore diffusion. In the equilibrium study, Langmuir isotherm was well fitted to experimental data and the adsorption of As was found to be dependent on the solution's pH, temperature, and competitive ions such as phosphates. Using the column experiment, the feasibility of alum sludge for As adsorption was also tested. The results showed that alum sludge can be used as an effective adsorbent of As in water treatment systems.