A novel method is utilized to maintain the desired MLSS concentration in each stage of the activated sludge system by automated sludge wasting. First, sludge is wasted every 8 hours on a timer. This is important to steady the operation of the biological system and reduce the potential for upset, especially over weekends and holidays when the plant is not fully staffed. Excess sludge from the first stage is wasted to the second stage; excess sludge from the second stage is co-settled in the primary settling tanks. Second, the amount of sludge wasted is based on the difference between the MLSS setpoint and the current reading from a ViSolid sensor located at the tail end of each stage. Third, the amount of sludge wasted is adjusted based on the most recent half hour settling test result entered by the operator to prevent too much or too little wasting if sludge settleablity changes or the sensor calibration drifts unexpectedly. The UltraClean ultrasonic cleaning system prevents fouling of the sensor. In total, maintenance requires not more than 15 minutes of operator attention per week. Dissolved oxygen setpoints are also maintained automatically. Modulating valves on the air supply piping to each of the first and second stage aeration basins are adjusted directly in response to readings from TriOxmatic dissolved oxygen sensors installed at the tail end of each stage. Implementation of DO control in the activated sludge system, in addition to other improvements including installation of a smaller blower, has allowed realization of substantial energy saving. In addition to verifying compliance with the discharge permit, measurements from the online DO sensor in the effluent are used to directly maintain compliance. An automated valve opens to increase air flow to the post aeration tank if effluent DO falls below the set point. The plant’s existing IQSN system was expanded after the original project to provide online monitoring of the treatment process. It was a simple matter of purchasing additional sensors, mounting them into the desired locations, and connecting them into the existing IQSN network. Two VisoTurb turbidity sensors were installed, one in the influent to the tertiary filters and the other in the final effluent channel to monitor the performance of the second stage activated sludge system and the tertiary filters. The turbidity sensors detect small changes in the quality of wastewater before it becomes visually apparent. Operators trend the measurements from the online turbidity sensors to stay ahead of the process and enable proactive adjustments before bigger problems could occur.
An IQ SensorNet module mounted tank side along with sensors for continuous process monitoring and control.
Results The utility has not raised user fees in 15 years. A big reason for that is the commitment of the wastewater department team. It was their progressive attitude and desire to “stay ahead of the curve” that gave rise to the project to install the SCADA system and online sensors. The investment has paid off. At the time of project completion, user fees were above the median for the state. Today, user fees are 12 percent below the median. A high level of performance is also achieved despite substantial industrial wastewater loadings. Over 98% of TSS and CBOD5 are removed on average and monthly average effluent ammonia-nitrogen did not exceed 0.3 mg/L during the most recent three-year period.
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IQ SensorNet
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