Research On Aeration Control Of Wastewater Treatment Based On The PWM Method

Now days, the technology of wastewater treatment is inferior and always spends greatelectrical energy in many wastewater treatment plants (WWTPs). In the21th century, energyconservation and sustainable development is becoming a focus, more and more newtechnologies are used in the industrial process control to reach the goal that is energy savingand carbon emission reduction. So, searching an aeration control method which is highefficiency and energy saving is becoming very important. In this thesis, a digital pulse widthmodulation (PWM) controller design method is proposed in order to reduce the energyconsumption. The main work of the thesis as follows:(1) The simplified static process model based on the activated sludge model No.1(ASM1) is developed, and an optimal linear model is constructed on the operating point usingthe optimal linearization technique. By linear quadratic regulator (LQR) method, an analogoptimal control law is designed. Since the speed of the control motor cannot change all thetime based on analog control law, so a PWM control law is designed.(2) The dynamic process model which contains inflow and outflow is analyzed, becauseof some disturbances, so the process of linearization has some difference with the static one,and the optimal linearization method should make some change, while the design of analogcontrol law and PWM control law is the same with the static process.(3) The model of nitrification is very sensitive to the dissolved oxygen (DO)concentration, and the DO fluctuates up and down caused by PWM controller, so much moretime is spent by the bioprocess and the wastewater treatment efficiency is low. In case of this,the modified PWM controller is designed and a changing-over control system is presented.(4) Because the input of actuator is limited, and the control signal designed in this thesismay beyond the range that the actuator could take in. If the control input is too powerful, thismay lead to cause some damages to the actuator, so the PWM controller with input constraintsis proposed.The PWM controller based on simplified ASM1model is proposed, and also iscompared to Multi-set point on/off controller and On/Off controller via some simulations, theresults show that PWM controller spends least energy and reaches the goal of energyconservation. Last, the modified PWM controller is designed, some simulations are made todemonstrate the high efficiency of it. From the result, modified PWM controller could solvethe problem caused by PWM controller well, and less energy and reaction time are spent.Also, the control input is limited by designing PWM controller with input constraints, andavoid damaging to actuator.