Activated sludge, as the adsorbent of humid acid in solution, was considered as an environment-friendly material. The effects of three categories of metal ions on sorption of humic acid by activated sludge were studied. The results indicated that the salts (Na+), hardness (Ca2+ , Mg2+) and hydrolyzing metal (Al3+, Fe3+) ions were able to increase the efficiency of adsorption. In view of coagulant aids, the hardness metal ions would be a better choice at the concentration higher than 10(-2) mol/L. Addition of 10(-2) mol/L Ca2+ and Mg2+ to the solution resulted in the concentrations of humid acid of (0.64 +/- 0.17) mg/L and (2.32 +/- 0.40) mg/L, respectively, in 48 h with corresponding removal efficiencies of 98.4% and 94.2% . Upon increasing the concentrations of Ca2+ and Mg2+ in the solution to 10(-1) mol/L, the removal efficiencies were close to 100% after 48 h. Moreover, the effect of Ca2+ was superior to that of Mg2+. Decrease in thickness of the electric double layer due to repulsion forces, cationic bridge, destabilization and precipitation might be the major mechanisms to promote the removal of humic acid in solution. On the other hand, the addition of metal irons might have resulted in physical changes in sorption site on the activated sludge, even inactivation, and might have caused negative effects on humid acid adsorption.