Abstract: Based on the operation of the Langdong Wastewater Treatment Plant in Nanning City, this paper introduces the composition of automated online monitoring instruments and their application in wastewater treatment. It also analyzes and summarizes the issues and operating procedures that should be noted in the management of these instruments. Keywords: Automated online monitoring instruments. Automated monitoring instruments are one of the key subsystems in an automated control system. A typical automated monitoring instrument consists of three main parts: ① a sensor, which uses various signals to detect the analog quantity being measured; ② a transmitter, which converts the analog signal measured by the sensor into a 4-20 mA current signal and sends it to a programmable logic controller (PLC); ③ a display, which visually displays the measurement results. These three parts are organically combined; the absence of any one part renders the instrument incomplete. Automated monitoring instruments, with their precise measurement, clear display, and simple operation, are widely used in industrial production. Furthermore, their internal interface with a microcomputer makes them an important part of the automated control system, often referred to as the "eyes" of the automated control system. With the development of science and technology, automated detection technology has also made great strides. Automated detection instruments are widely used in wastewater treatment, saving wastewater treatment plants not only a significant amount of manpower and resources, but more importantly, allowing for timely adjustments to the process. This article will take the Langdong Wastewater Treatment Plant in Nanning City as an example to introduce the application of automated detection instruments in wastewater treatment. 1 Project Overview The Langdong Wastewater Treatment Plant project in Nanning City was approved at the end of 1993 and officially started construction on November 27, 1997; it began trial operation on September 28, 1999, and reached full-load normal operation in February 2000. The first phase of the Langdong Wastewater Treatment Plant in Nanning City is designed with a primary wastewater treatment capacity of 240,000 m3/d and a secondary wastewater treatment capacity of 100,000 m3/d. The designed service area is 30.5 km2, with a planned service population of 343,000. The clean water purified by the Langdong Wastewater Treatment Plant is partly discharged directly into Zhupaichong and partly used for the reinjection of Nanhu Lake to improve the water pollution problem of Nanhu Lake. 2. Treatment Process The Nanning Langdong Wastewater Treatment Plant has imported the most advanced water treatment equipment from abroad, employing the traditional activated sludge process of a two-stage biological treatment process. However, considering the low pollutant concentration in Nanning's wastewater, an OOC (Out-of-Coke) process is used in its core component—the aeration process. This process boasts advantages such as low energy consumption, low operating costs, good effluent quality, simple management, and stable operation. Wastewater collected from the external sewage trunk line to the Langdong Wastewater Treatment Plant undergoes pretreatment. At the influent pumping station, it passes through a coarse screen to remove larger debris and floating matter. Five large sewage pumps then lift the wastewater to a fine screen to remove smaller floating matter. Sand and oil are removed in the aerated grit chamber. The wastewater then enters a metering tank to measure the wastewater volume. The pretreated wastewater undergoes primary treatment in a primary sedimentation tank, removing approximately 30% of the organic matter. The effluent from the primary sedimentation tank enters the secondary treatment process, first undergoing biodegradation of organic matter in the aeration tank—the core component of the biological treatment process. The mixed liquor from the aeration tank is then transported to the secondary sedimentation tank for sedimentation and sludge-water separation. The upper clarified liquid is used as clean discharge water after purification; part of the settled sludge is returned to the aeration tank for reuse, and part is returned to the primary sedimentation tank as surplus sludge. The sludge from the primary sedimentation tank is pumped to the sludge thickening tank, and further thickened. Through the sludge treatment system, the sludge in slurry state is dewatered and filtered to form dry sludge cake. 3 Introduction to main applied instruments 3.1 Ultrasonic level gauge, differential level gauge, flow meter (1) Bar screen operation control. One ultrasonic differential level gauge is installed in each of the coarse and fine bar screens. The degree of bar screen blockage is reflected by the difference in liquid level before and after the bar screen and transmitted to the PLC controller for analysis and calculation. When the liquid level difference exceeds the preset value, the bar screen is controlled to operate, remove garbage, ensure normal water flow, and reasonably reduce equipment wear. (2) Booster pump operation control. In order to realize the automatic control of the inlet booster pump, two ultrasonic level gauges are installed at the inlet pump well to measure the water level of the pump well and transmit it to the PLC controller and host computer in real time for system analysis. According to the control program corresponding to the measured value, the operation combination of the booster pump is automatically controlled. In this way, the pump operation status can be adjusted accurately and timely according to the external water volume, reducing equipment fatigue; at the same time, the manpower consumption of the traditional three-shift pumping station can be eliminated. (3) Real-time monitoring of flow rate and treatment volume. For the operation and management of sewage treatment plants, water volume is an important control parameter. Accurate and timely grasp of the influent volume is of great importance for process control and improving the sewage treatment plant's ability to resist hydraulic load impact. Traditional water volume measurement uses weir plates or Venturi flow channels, which have the disadvantage of not being able to monitor and display in real time. The metering channel of Langdong Sewage Treatment Plant uses an ultrasonic flow meter combined with a Venturi flow channel, which can display the flow rate and cumulative treatment volume in real time on site and on the host computer, achieving the purpose of accurately measuring the treated water volume and providing real-time flow for operation and management. 3.2 Dissolved oxygen meter, oxidation-reduction potentiometer, sludge concentration meter (1) Dissolved oxygen control in aeration tank. The Langdong Wastewater Treatment Plant in Nanning City adopts the OOC modified process of the traditional activated sludge method. Dissolved oxygen meters with a measurement range of 0.05 to 10 mg/L are installed in the aerobic zone of the inner circle of the four circular aeration tanks to monitor the dissolved oxygen concentration in real time and transmit the data to the PLC and host computer. When the measured concentration is less than the set concentration, the automatic control system starts the blower to oxygenate the aeration tank; conversely, when the oxygen is sufficient, the blower stops running. By controlling the blower through the dissolved oxygen meter, the start and stop of the blower can be accurately controlled according to the dissolved oxygen demand of the aerobic bacteria. This ensures the good biochemical ability of the bacteria while saving energy, protecting the equipment, and enhancing the decomposition ability of the aerobic bacteria. (2) Control of the aerobic and anoxic sections of the aeration tank. Oxidation-reduction potentiometers with a measurement range of -500 to 500 mV are installed at the boundary between the aerobic and anoxic zones on the outer ring of each aeration tank. The measured oxidation-reduction potential can be used to control the high-speed operation of the blower, supply oxygen to the outer ring, and form an alternation between the strong aerobic aeration stage and the anoxic stage, thereby improving the phosphorus and nitrogen removal capacity in the treatment process. If oxidation-reduction potentiometers are not installed, the operation of the blower can only be controlled by time, which will significantly reduce the phosphorus and nitrogen removal effect. (3) Sludge concentration control in aeration tanks. Sludge concentration in aeration tanks is an important process parameter. In traditional sewage treatment plants, sludge concentration is monitored by laboratory testing using old experimental methods, which has significant deficiencies in the timeliness and accuracy of data. It is difficult to make timely process adjustments for the amount of returned sludge and excess sludge, resulting in errors in time and accuracy. The Langdong Sewage Treatment Plant in Nanning City has installed an online sludge concentration meter with a measurement range of 0.5 to 10 g/L on each aeration tank, which has solved this problem well. Installing sludge concentration meters allows for timely adjustments to the aeration tank process based on accurately measured sludge concentrations, while also reducing the workload of laboratory staff. 3.3 Electromagnetic Flowmeters and Gas Flowmeters: Five electromagnetic flowmeters with a measurement range of 0–1200 m³/h were installed in the return sludge and waste sludge pipelines at the Nanning Langdong Wastewater Treatment Plant to measure the flow rate of return sludge and waste sludge. After installing the flowmeters, operators can determine the proper functioning of the return sludge pump and waste sludge pump based on the displayed flow rate, solving the problem of not being able to easily determine the proper functioning of submersible pumps. Furthermore, electromagnetic flowmeters are easy to install and maintain. Four gas flowmeters with a measurement range of 0–4000 m³/h (standard conditions) were directly installed on the air pipeline between the blower and the aeration tank. The installation of the gas flowmeters allows operators to monitor the amount of gas supplied to the aeration tank by the blower at any time. 4 Operation and Management Experience 4.1 Operation and Management Experience The Langdong Wastewater Treatment Plant in Nanning City officially started operation in February 2000. In more than two years of operation and management, we have summarized the following experiences in ensuring the normal operation of automated detection instruments: (1) Keep the sensors of automated detection instruments clean. Regularly clean the probes by designated personnel to ensure the accuracy of data acquisition. Because the instruments work in the wastewater environment, the cleaning of the instruments is particularly important, especially the analytical instruments such as dissolved oxygen meters, oxidation-reduction potentiometers and sludge concentration meters that are in direct contact with wastewater. In order to ensure the normal operation of the instruments, we regularly have designated personnel clean them. The instruments are thoroughly cleaned once every 7 days. Soft materials are required for cleaning to avoid damaging the instruments. (2) Regularly calibrate various instruments. Instruments will inevitably produce measurement errors during long-term operation. This requires regular calibration to ensure the accuracy of instrument measurements. We have formulated a system of regular calibration once every two months for analytical instruments. Moreover, laboratory staff are required to use analytical methods to analyze the corresponding detection items and compare them with the monitoring results of the instruments on site. If the deviation is too large, the instruments should be calibrated in time to ensure accuracy. (3) Ensure the stability of the instrument power supply voltage and extend the service life of the instrument. Instantaneous high voltage surges often cause the instrument to burn out easily. During the operation of the Langdong Sewage Treatment Plant in Nanning City, there have been many instances where the ultrasonic level differential and ultrasonic level transmitters were damaged due to unstable power supply voltage, which affected the normal operation of the automatic control system. The Langdong Sewage Treatment Plant in Nanning City is carrying out technical transformation to avoid damage to the instrument caused by unstable power supply voltage, reduce operating costs, and improve economic efficiency. 4.2 Some Experiences During the operation, we also have the following experiences: (1) To improve the economic efficiency and management level of the sewage treatment plant, it is not enough to only have the above instruments. We should also add monitoring instruments such as blower outlet pressure gauges and primary sedimentation tank sludge pump outlet flow meters to monitor and adjust parameters such as the outlet pressure of the large blower and the inlet flow of the thickening tank. (2) The spare parts and components of imported instruments are expensive and difficult to purchase, which affects the use and maintenance of the instruments. For example, repairing an ultrasonic level transmitter damaged due to unstable power supply voltage costs over 8,000 yuan, while replacing it with a new one costs over 15,000 yuan; replacing an oxidation-reduction potentiometer electrode costs over 2,000 yuan, and the electrode's lifespan is generally only two years; most companies find it difficult to afford such high costs long-term. Standardization and calibration of imported instruments are difficult, and most quality inspection departments do not accept testing of wastewater treatment instruments. The widespread adoption of domestically produced instruments is still lagging behind, and their price advantage cannot be fully realized, which to some extent affects the application of automated monitoring instruments in wastewater treatment plants. Overall, automated monitoring instruments play a significant role in wastewater treatment plants, but some problems still exist in practical applications. We believe that automated monitoring instruments will play an even greater role in China's environmental protection efforts in the future.