4. Conclusions
This study has proved that the sludge of DWTP could be employed as a raw material of a low cost adsorbent to remove cadmium ion in the water. The presence of iron oxide and humic substances in the sludge were suggested as an important factor for the process of cadmium adsorption. The adsorbent material was dependent on the pH of solution, where pH 6–8 was favorable for adsorption of cadmium ion. The concentration of heavy metal ions in the sludge of DWTP is usually very low, therefore utilization of this material as a low cost adsorbent is safe and efficient.