The Activated Sludge Process (ASP) exhibits highly nonlinear properties. The design of an automatic control system that is robust against disturbance of inlet wastewater flow rate and has short process settling times is a challenging matter. The proposed control method is an I-P modified controller automatic control system with state variable feedback and control canonical form simulation diagram for the process. A more stable response is achieved with this type of modern control. Settling times of 0.48 days are achieved for the concentration of microorganisms

(reference value step increase of 50 mg·L

-1

) and 0.01 days for the concentration of oxygen (reference value step increase of 0.1mg·L

-1

). Fluctuations of concentrations of oxygen and microorganisms after an inlet disturbance of 5×10

3

m

3

·d

-1

are small. Changes in the reference values of oxygen and microorganisms (increases by 10%

20% and 30%) show satisfactory response of the system in all cases. Changes in the value of inlet wastewater flow rate disturbance (increases by 10%

25%

50% and 100%) are stabilized by the control system in short time. Maximum percent overshoot is also taken in consideration in all cases and the

largest value is 25% which is acceptable. The proposed method with I-P controller is better for disturbance rejection and process settling times compared to the same method using PI controller. This method can substitute optimal control systems in ASP.