PLC was developed based on the simulation of the original relay control principle. In the 1970s, PLC only had switch logic control and was first applied in the automotive manufacturing industry. It stores instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations; And control various types of machinery or production processes through digital input and output operations. The control program developed by the user expresses the process requirements of the production process and is pre stored in the user program memory of the PLC. Execute the stored program content one by one during runtime to complete the operations required by the process flow.

At present, PLC control cabinets are widely used in sewage treatment plants in major cities and rural areas.

The PLC based sewage treatment automation system has three control modes: remote "manual" control, remote automatic control mode, and local control mode. Local control mode: When the "local/remote" switch on the on-site control box or MCC control cabinet of the equipment is selected as the "local" mode, the equipment can be started and stopped through the buttons on the on-site control box or MCC control cabinet.

Remote control is also known as' centralized control '. This operation is divided into automatic control and manual control. "Automatic control" mode: on-site control box or MCC control cabinet When the local/remote switch is selected for remote mode and the automatic/remote control of the on-site control station is set to automatic mode, the PLC automatically completes various controls including continuous loop control based on the actual situation on site, without the need for manual intervention.

Remote manual control mode, on-site control box or MCC control cabinet Select remote mode for the local/remote switch on the device. At this time, the operator monitors the process data and operates it in real-time to ensure safe operation of the equipment.

1. Ultrasonic level difference gauges are installed before and after the coarse and fine grids, and the real-time liquid level difference between the coarse and fine grids is displayed on the computer monitor in the field and central control room. According to the treatment water quality and quantity of our factory, process values have been set. When the current and subsequent liquid level differences are greater than or equal to this process value, automatic interlocking start stop of coarse and fine grids can be achieved.

2. An electromagnetic flowmeter is installed in the inlet pipe to measure the inlet flow rate in real time and display the liquid level value of the pre-treatment inlet pump station on the on-site and central control room computer monitor. The three inlet lift pumps are automatically controlled to start and stop according to the high or low liquid level value, so that the operation time of the three lift pumps is basically balanced, and the start and stop status of each lift pump is displayed on the computer. A pH meter is installed in the regulating tank to measure the instantaneous pH value of the incoming water, in order to reflect whether the incoming water quality meets the treatment requirements. The required range for setting the pH value of the inlet water is 4.0-9.0. When the pH value of the inlet water is not within this range, the central control room computer will issue an audible and visual alarm signal and automatically close the inlet gate to ensure the quality of the outlet water.

The DO in the anoxic tank is generally around 1.0mg/L; The DO of the aerobic tank is generally around 2.0-2.5mg/L. When the monitored real-time value is not within the set value range, the central control room computer will emit an audible and visual alarm signal, and the staff can decide the number of blowers to turn on and the amount of aeration based on this, saving energy consumption for users while ensuring that dissolved oxygen is within the normal range. And by switching the operating status of the main and auxiliary machines, the cumulative working time of the three blowers is basically equal.

When the concentration is low, the sludge reflux rate will be appropriately increased. The above control processes can be remotely controlled from the central control room based on monitoring data.

5. Install a mud level gauge in the secondary sedimentation tank. When the online monitoring mud level value is high, it can automatically adjust the scraper suction machine and sludge discharge equipment to discharge the remaining sludge and prevent sedimentation sludge from spoiling.

6. Install one pH meter, one COD online measuring instrument, and one sludge concentration meter in the effluent tank, and monitor and display the measured values in real-time on site and in the central control room. The staff can keep track of the effluent quality at any time