Application of automated online monitoring instruments in wastewater treatment

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Application of Automated Online Monitoring Instruments in Wastewater Treatment Authors: Qiu Mei, Huang Shiguang, Huang Huaxiang Abstract: Based on the operation conditions of Langdong Wastewater Treatment Plant in Nanning, this paper introduces the composition of automated online monitoring instruments and their application in wastewater treatment, and analyzes and summarizes the issues that should be noted as well as the operating procedures in the management of such instruments. Keywords: automated testing instruments, wastewater treatment plants, automated control systems, PLC controllers. Automated testing instruments are one of the key subsystems in automatic control systems. Typical automated testing instruments consist of three main parts: ① sensors, which use various signals to detect the analog quantity being measured ； ②The transmitter converts the analog signal measured by the sensor into a 4–20 mA current signal, which is then sent to the programmable logic controller (PLC) ； ③The display shows the measurement results visually, providing the outcomes. These three parts are integrated together organically; without any one of them, it cannot be considered a complete instrument. Automated testing instruments are widely used in industrial production due to their features such as accurate measurement, clear display, and simple operation. Moreover, since they have interfaces with microcomputers, they serve as an important part of automated control systems, being referred to as the “eyes of automated control systems”. With the advancement of science and technology, automated detection technologies have also seen significant development. Automated detection instruments are widely used in wastewater treatment, which not only helps wastewater treatment plants save a great deal of manpower and resources but, more importantly, enables timely adjustments to the treatment processes. This article will use the Langdong Wastewater Treatment Plant in Nanning as an example to introduce the application of automated monitoring instruments in wastewater treatment. 1. Project Overview: The Langdong Wastewater Treatment Plant project in Nanning was approved at the end of 1993, and construction officially began on November 27, 1997 ； It underwent a water flow test run on September 28, 1999, and began operating at full capacity normally in February 2000. The Langdong Wastewater Treatment Plant in Nanning has a design capacity for stage 1 wastewater treatment of 240,000 m3/day, and a capacity for stage 2 wastewater treatment of 100,000 m3/day. The design service area covers 30.5 km2, with a planned population of 343,000 people. The clean water purified by the Langdong Sewage Treatment Plant is partially discharged directly into the Zhupai Chong stream, and partially used for recharging South Lake in order to address the water pollution issues there. 2. Treatment Process: The Langdong Wastewater Treatment Plant in Nanning has adopted the most advanced wastewater treatment equipment from abroad. It uses the traditional activated sludge process with a two-stage biological treatment approach. Given the low concentration of pollutants in the wastewater from Nanning, the OOC process is employed in the core stage of treatment, namely the aeration process. This process has advantages such as low energy consumption, low operating costs, good water quality of the effluent, simple management, and stable operation. The wastewater collected from the external sewage main pipes and sent to the Langdong Wastewater Treatment Plant undergoes pre-treatment first. In the water intake pump room, a coarse screen is used to remove larger debris and floating objects from the wastewater ； The wastewater is pumped to the fine screen using 5 large sewage pumps to remove smaller floating particles ； Removing sand particles and oils from wastewater in an aeration grit chamber ； Then it enters the metering tank to measure the amount of wastewater treated. The pretreated wastewater undergoes primary treatment in a primary sedimentation tank, removing about 30% of the organic matter ； The effluent from the primary sedimentation tank enters the secondary treatment process, where organic matter is biodegraded first in the core part of the biological treatment process – the aeration tank ； The mixture from the aeration tank is transferred to the secondary sedimentation tank for sedimentation and separation of sludge from water. The upper clear liquid serves as the purified clean wastewater for discharge ； Part of the settled sludge is recycled by returning it to the aeration tank, while another part is returned to the primary sedimentation tank as excess sludge. The sludge from the primary sedimentation tank is pumped to the sludge thickening tank for further concentration. Through the sludge treatment system, the sludge in slurry form is dewatered and filtered under pressure to form dry sludge cakes. The process flow is shown in Figure 1. Figure 1: Process flow of the wastewater treatment plant 3. Introduction to main measuring instruments 3.1 Ultrasonic level gauges, level difference meters, flow meters (1) Grid operation control. One ultrasonic level difference gauge is installed on each of the coarse and fine grids; the degree of grid blockage is determined by the level difference before and after the grids, and this data is sent to the PLC controller for analysis and calculation. When the level difference exceeds a preset value, the control grid is activated to remove debris, ensuring smooth water flow while also reducing equipment wear in an appropriate manner. (2) Improve pump operation control. To achieve automatic control of the feedwater lift pump, 2 ultrasonic level gauges were installed in the pump well to measure the water level there; the data is transmitted in real time to the PLC controller and the upper-level computer for system analysis. Based on the measured values and the corresponding control program, the operating combination of the lift pump is automatically controlled. This allows for the accurate and timely adjustment of the pump’s operating conditions based on the water volume coming from outside the plant, thereby reducing equipment fatigue ； It can also eliminate the labor costs associated with the three-shift system in traditional pumping stations. (3) Real-time monitoring of traffic and processing volume. For the operation and management of wastewater treatment plants, water volume is an important control parameter. Accurately and timely monitoring of the water inflow plays an important role in process control and enhancing the capacity of wastewater treatment plants to withstand hydraulic load shocks. Traditional water volume measurement methods, such as weirs or Venturi flow channels, all have the drawback of being unable to provide real-time monitoring and display. The metering tank at the Langdong wastewater treatment plant uses an ultrasonic flow meter in combination with a Venturi tube, which enables real-time display of flow rates and total amount of water treated both on-site and on the supervisory computer. This approach ensures accurate measurement of the water volume treated and provides real-time flow data for operational management purposes. 3.2 Dissolved oxygen meter, redox potential meter, sludge concentration meter (1) Dissolved oxygen control in the aeration tank. The Langdong Wastewater Treatment Plant in Nanning uses an OOC-enhanced version of the traditional activated sludge process; dissolved oxygen sensors with a measurement range of 0.05–10 mg/L are installed in the aerobic zones of the four circular aeration tanks to monitor the dissolved oxygen concentration in real time, with the data being transmitted to the PLC and the upper-level computer system. When the measured concentration is lower than the set value, the automatic control system activates the blower to supply oxygen to the aeration tank ； Conversely, when there is an adequate supply of oxygen, the blower stops operating. By using a dissolved oxygen meter to control the blower, it is possible to precisely regulate the start and stop of the blower based on the dissolved oxygen requirements of the aerobic microbial community. This ensures optimal biochemical performance of the microbes, reduces energy consumption, protects the equipment, and enhances the decomposition capacity of the aerobic microbial community. (2) Control of the aerobic and anoxic sections in the aeration tank. Redox potentiometers with a measurement range of -500 to 500 mV are installed at the interface between the aerobic zone and the anoxic zone on the outer perimeter of each aeration tank. By measuring the redox potential, it is possible to control the high-speed operation of the blowers in order to supply oxygen to the outer perimeter, thereby creating an alternation between periods of intense aerobic aeration and anoxic conditions, and thus enhancing the efficiency of phosphorus and nitrogen removal in the treatment process. If a redox potentiometer is not installed. Therefore, the operation of the blower can only be controlled by time, which significantly reduces the effectiveness of phosphorus and nitrogen removal. (3) Control of sludge concentration in the aeration tank. The sludge concentration in the aeration tank is an important process parameter. In traditional wastewater treatment plants, sludge concentration is monitored using old laboratory testing methods, which present significant shortcomings in terms of the timeliness and accuracy of the data provided. It is difficult to make timely adjustments to the amounts of return sludge and excess sludge, which results in errors in terms of timing and accuracy. The Langdong Wastewater Treatment Plant in Nanning installed an online sludge concentration meter with a measurement range of 0.5–10 g/L in each aeration tank, which effectively solved this problem. Installing a sludge concentration meter allows for the timely adjustment of the aeration tank’s operations based on accurately measured sludge concentrations, while also reducing the workload for laboratory staff. 3.3 Electromagnetic flowmeters and gas flowmeters: Five electromagnetic flowmeters with a measurement range of 0–1,200 m3/h were installed at the Langdong Wastewater Treatment Plant in Nanning to measure the flow rate of returned sludge and excess sludge in the respective pipelines. After installing the flow meter, the duty personnel can determine whether the flow rate displayed is correct, thereby assessing whether the return sludge pump and the excess sludge pump are operating properly. This solves the problem of it being difficult to determine whether submersible pumps are functioning correctly. Additionally, electromagnetic flow meters are easy to install and simple to maintain. 4 gas flow meters with a measurement range of 0–4000 m3/h (under standard conditions), installed directly on the air pipeline between the blower and the aeration tank. The installation of a gas flow meter allows the operators to know at any time the amount of gas that the blower supplies to the aeration tank. 4. Insights into Operation Management 4.1 Experience in Operation Management Since its official commissioning in February 2000, the Langdong Wastewater Treatment Plant in Nanning has, over more than two years of operation and management, accumulated several pieces of experience for ensuring the proper functioning of automated monitoring instruments, as follows: (1) Keep the sensors of automated monitoring instruments clean. The probe should be cleaned regularly by a dedicated person to ensure the accuracy of data collection. Since the instruments operate in sewage environments, it is particularly important to keep them clean. This is especially true for analytical instruments that come into direct contact with sewage, such as dissolved oxygen meters, redox potential meters, and sludge concentration meters. To ensure the proper functioning of these instruments, they are cleaned regularly by specialized personnel; a thorough cleaning is carried out every 7 days. Soft materials should be used during cleaning to avoid damaging the instruments. (2) Regularly calibrate various instruments. Inevitably, measurement errors occur in instruments over time as a result of long-term operation; therefore, regular calibration is necessary to ensure the accuracy of their measurements. For analytical instruments, we have established a schedule for calibration every two months ； Furthermore, laboratory staff are required to use analytical methods to analyze the corresponding test items and compare them with the results obtained from on-site instruments; if the discrepancy is significant, the instruments should be calibrated in a timely manner to ensure accuracy. (3) Ensure the stability of the power supply voltage for the instrument, thereby extending its service life. Sudden high-voltage surges can easily damage instruments. During the operation of the Langdong Wastewater Treatment Plant in Nanning, there were multiple instances where unstable power supply voltage caused damage to the ultrasonic level gauges and their transmitters, thereby affecting the proper functioning of the automatic control system. The Langdong Wastewater Treatment Plant in Nanning is undergoing technical upgrades to prevent damage to instruments caused by unstable power supply voltage, thereby reducing operating costs and improving economic efficiency. 4.2 Some Insights During operation, we have also gained the following insights: (1) To improve the economic efficiency and management level of wastewater treatment plants, these instruments alone are not sufficient; it is also necessary to add monitoring instruments such as pressure gauges at the outlet of blowers and flow meters at the outlet of sludge pumps in the primary sedimentation tank, in order to monitor and adjust parameters such as the outlet pressure of large blowers and the inlet flow rate of thickening tanks. (2) Spare parts and components for imported instruments are expensive and difficult to obtain, which affects the use and maintenance of these instruments. For example, the repair cost of an ultrasonic level difference transmitter that has been damaged due to unstable power supply voltage is over 8,000 yuan, while replacing it with a new transmitter costs over 15,000 yuan ； Replacing a redox potentiometer electrode costs over 2,000 yuan, and the typical service life of such electrodes is two years ； It is difficult for ordinary companies to cover this high cost over the long term. Standardized calibration of imported instruments is difficult, and ordinary quality inspection departments do not accept the testing of wastewater treatment instruments. The widespread adoption of domestic instruments still lags behind, and their price advantages cannot be fully realized, which to some extent affects the use of automated testing instruments in sewage treatment plants. Overall, automated monitoring instruments play a significant role in the operation of sewage treatment plants; however, there are still some issues in their practical use. We believe that in the future, such instruments will contribute even more to environmental protection efforts in China.