The potential of using two different kinds of air drying of activated sludge (aerobic activated sludge and anaerobic activated sludge) for the removal of Cu2+, Mn2+, Zn2+, and Fe3+ from aqueous solutions was assessed. Results indicated that the maximum biosorption occurred at pH = 5.0 for Cu2+, Zn2+, and Mn2+ and pH = 3.0 for Fe3+. The kinetic parameters of biosorption data were found to be best fitted to the second-order equation. Also, it was found that the best dosage for biosorption was 0.2 g for both aerobic activated sludge and anaerobic activated sludge. The experimental results were fitted well to the Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherms. The maximum biosorption capacities of Cu2+, Mn2+, Zn2+, and Fe3+ for aerobic activated sludge were 65.789, 44.843, 64.935, and 75.756 mg/g, respectively, while they were 59.880, 49.020, 62.500, and 69.444 mg/g for anaerobic activated sludge, respectively. The mean free energy values evaluated from the D-R model indicated that the biosorptions of studied heavy metal ions onto activated sludge were taken place by chemical interaction. The results of this study provided valuable information on the biosorption of heavy metals by activated sludge that may contribute in wastewater treatment.