Wastewater Treatment Plant (WWTP) is highly complex with the nonlinearity of control parameters and difficult to be controlled. The need for simple but effective control strategy to handle the nonlinearities of the wastewater plant is obviously demanded. The thesis emphasizes on multivariable model identification and nonlinear proportional integral (PI) controller to improve the operation of wastewater plant. Good models were resulted by subspace method based on N4SID algorithm with generated multi-level input signal. The nonlinear PI controller (Non- PI) with adaptive rate variation was developed to accommodate the nonlinearity of the WWTP, and hence, improving the adaptability and robustness of the classical linear PI controller. The Non-PI was designed by cascading a sector-bounded nonlinear gain to linear PI while the rate variation is adapted based on adaptive interaction algorithm. The effectiveness of the Non-PI has been proven by significant improvement under various dynamic influents. In the process of activated sludge, better average effluent qualities, less number and percentage of effluent violations were resulted. Besides, more than 30% of integral squared error and 14% of integral absolute error were reduced by the Non-PI controller compared to the benchmark PI for dissolved oxygen control and nitrate in nitrogen removal control, respectively
