BioChem Technology has developed an advanced aeration control system to effectively control the residual dissolved oxygen (DO) concentration in each aerobic zone of an activated sludge wastewater treatment process. Traditional aeration control systems use PID control loops to adjust the airflow and, hence, the DO level in the aerobic zones.
Experience has shown PID control to be a poor method of controlling residual DO concentrations. Normally, PID parameters are tuned to control average or typical influent conditions. Because the contaminant load to a plant is changing throughout the day, actual conditions are never “average” and one of two scenarios typically occurs depending on the current loading.
The first scenario occurs during low loading periods, resulting in PID control loop over-tuning and large oscillations in the DO concentration in the aerobic zones. The second scenario occurs during high loading periods when the PID control loop is under-tuned resulting in low DO concentrations as the control system tries to “catch-up” to the increase in oxygen demand.
The BACS does not use PID control loops, but instead utilizes a three pronged approach to maintaining a DO set-point.
- The first part of the BACS uses a direct measurement of the influent load in a feed-forward component to ensure that the aeration system never needs to “catch-up” to the current contaminant load.
- The second part of the BACS uses a feed-back component based on the DO concentration and a proprietary control law based on a model of the activated sludge process and not a PID control loop. These first two parts of the control system are combined to calculate an airflow rate for each aerobic zone.
- The third part of the BACS quickly and efficiently adjusts the control valves to each aerobic zone to provide the airflow calculated by the first two parts of the control system.
Using this approach, the BCT aeration control system is able to maintain the DO concentration very close to the DO set-point regardless of current loading conditions, a vast improvement over the performance of PID control loops.