Wastewater treatment plants are inherently dynamic because of variations in influent wastewater flow rate, concentration, and composition. A better understanding of the dynamic behavior of these plants and the use of control systems to convert unsatisfactory to satisfactory dynamic behavior has significant potential for solving operational problems as well as reducing operational costs. The increasing use of instrumentation, control and automation (ICA) accompanied with the rapid development of computer technology triggered the application of automatic control technology to wastewater treatment plant operation.
The objective of this study is to develop an automatic control technology of activated sludge process for coke wastewater. A discrete type autotuned proportional-integral(PI) controller using an auto-regressive exogeneous (ARX)model as a process model was developed to maintain the DO concentration in aerators by controlling the speed of surface aerator motors. Also a nonlinear pH controller using the titration curve was used to control the pH of influent wastewater.
This control system was tested in a pilot-scale plant equipped with variable speed surface aerators, speed controllable pumps, and various sensors such as pH, DO, MLSS, and ORP meters. All of the analog and digital signal data were collected and pH, DO were controlled with an IBM personal computer. During this pilot plant experiment, there was small deviation of pH and the electric power consumption of surface aerators could be reduced up to 70% with respect to the full operation when the DO set point was 2.0 mg/l.
This automatic control system was applied to an existing activated sludge process plant treating 9,000㎥/day coke wastewater. In this plant, DO is supplied by mechanical aeration with 37 kWh AC motor. From the real-time record of DO, it was known that there was large fluctuation with manual on-off control.
Inverters and DO sensors were installed to control DO concentrations in aeration tank automatically. The automatic control of DO was done with the automatic control program tested in simulation plant by modifying only several input/output points. The monitoring of more than 100 input/output signals and the control of DO were accomplished with an IBM PC.
During the installation of automatic control instruments, there was an accident in the treatment plant. Rapid decline of pH in the aeration tank and the rise of microorganisms in the biological settling tank caused by unintended nitrification after altering the upstream process resulted in the upset of activated sludge process. The function of activated sludge process was recovered after maintaining the DO concentration in the aeration tank lower than 1.0 mg/l by controlling the speed of the surface aerator motor via inverter.
In case of real plant operation with this system, the discrete PI controller showed good tracking for set point change. The electricity saving was more than 40% of the electricity consumption when considering surface aerators. As a result of maintaining the DO constantly at the set point by the automatic control system, the fluctuation of effluent quality was decreased and overall improvement of the effluent water quality was achieved.
