As part of efforts to upgrade automation throughout water treatment and reclamation facilities in Kent, Ohio, Thoma Technologies implemented a three-phase modification project for aeration controls (to see the full story on Kent’s upgrades, see “Kent’s Water Utility Keeps Automation Fluid”) at the city’s Water Reclamation Facility (WRF).
For the first phase, the integrator implemented an automation system for control of the two centrifugal blowers using AutomationDirect D0-06DR controllers. Six dissolved oxygen sensors (with provisions) determine the airflow needed for the optimal amount of dissolved oxygen in the mixed-liquor suspended-solids tanks. The oxygen is critical for supporting the proper levels of bacteria required to perform the activated sludge processes.
“When flows are low, especially at night, the oxygen demand goes down. The automation system reduces the blower speeds at this time,” explains Robert Thoma, president of Thoma Technologies. “The blowers are conversely sped up when the sludge presses are working and about to load more oxygen demand to the system.” This increases efficiency of the process and equates to less power demand overall and power cost savings.
Thoma Technologies created a control scheme that controls both the centrifugal blowers as well as the turbo blower using the same algorithm. The centrifugal blowers have their inlet valves ratcheted opened or closed a selected amount at a determined time interval to achieve a direct readable dissolved oxygen level. The reduction of airflow into the blower reduces the work the impeller must do, lowering the power demand of the 350 hp motor on each blower.
The second phase of aeration automation consisted of a power savings project in which a small turbo blower was installed that uses the same controls as the centrifugal blowers. Instead of valve controls, though, the turbo blower uses an airflow set point in minimum cooling airflow that allows the blower to ratchet up and down by a determined amount at time intervals set by the plant staff.
The third phase of aeration automation is comprised of automated valves that balance airflow in the mixed-liquor suspended-solids tanks to achieve maximum efficiency of produced airflow. “There are multiple motor-operated valves that use discrete control with 100 percent duty cycle motors to provide the required level of control—a much less expensive option than modulating valves,” Thoma says.