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 0.1 mg·L−1
). Fluctuations of concentrations of oxygen and microorganisms after an inlet disturbance of
5 × 103
m3
·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.
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