Abstract: Based on the characteristics of a wastewater treatment production process and the requirements of production process control, and combined with the actual situation of wastewater treatment plants in China, a distributed computer control system for the entire wastewater treatment production process is designed using the "EIC three-electric integration" computer distributed control technology. On this basis, by adopting advanced technologies such as optimized setting control technology and adaptive self-tuning PI control technology, the key technical problems such as large lag, nonlinearity, and coupling in the control of urban wastewater production processes are solved, and automatic control of wastewater lifting, flotation dosing, sludge dewatering, and other wastewater treatment production processes is realized. Keywords: wastewater treatment; automatic control; programmable logic controller; optimized setting control 1. Overview Water pollution is one of the most urgent problems that mankind needs to solve. Compared with other industrial processes, wastewater treatment control methods and means are relatively backward, mostly concentrated on simple on/off control and PID control, or even manual operation. The operation and running stability are poor, the effluent indicators fluctuate greatly, the treatment efficiency is low, the energy consumption is large, and it is difficult to achieve optimization and energy saving. The overall level of wastewater treatment is closely related to the degree of application of ICA (instrumentation, control, automation) in wastewater treatment plants[[i]]. Based on the characteristics of the wastewater treatment process and the requirements of process control in Shenyang, a PC+PLC hierarchical distributed automatic control system was designed according to the actual situation of the plant. This paper describes the overall system architecture and functional characteristics, and provides reasonable automation solutions for the difficult problems in the control process. 1.1 Wastewater Process Control Requirements The primary task of a wastewater treatment plant is to ensure that the effluent quality is stable and qualified, meeting the national discharge standards, while minimizing the cost per ton of water treated. The following points need to be considered in the design and operation of a wastewater treatment plant: (1) Productivity: Improve the operation mode and increase the wastewater treatment capacity of the unit process; (2) Reliability: Reduce the failure rate of the process, especially to ensure the normal operation of the wastewater treatment process under interference events (drought, rainstorm); (3) Stability: When the process is highly stable, occasional interference may have a serious impact on the effluent quality, but the impact of events such as drought and rainstorm can be avoided through control technology; (4) Operating Costs: Reduce chemical reagents and energy consumption to reduce production costs. Consider the dynamic characteristics of the receiving water body, apply control, and match the assimilation capacity of the receiving water body. 1.3 Requirements of Wastewater Treatment Plants for Automatic Control Systems The process and management methods of urban wastewater treatment plants differ significantly from other industrial processes, and the control and operation are far from standardized and optimized. Therefore, when designing a computer automatic control system, it is necessary to consider the dynamic characteristics of the process and the control requirements in order to achieve the desired effect of the control system. The main requirements of wastewater treatment plants for automatic control systems are as follows: (1) The computer control system should have good adaptability. (2) The computer control system should adopt a distributed structure and have good communication capabilities. (3) The computer control system should have a good human-machine interface and good backup capabilities. (4) The computer control system should have good openness and statistical and computational capabilities. (5) Automatic detection equipment should be used to measure parameters such as water quality online. (6) The control system should be able to take into account both process and unit control. (7) The computer control system should have comprehensive fault diagnosis and alarm recording capabilities . 2. Computer Control System Architecture and Functions 2.1 Overall Structure of Computer Control System Based on the process characteristics and control requirements of the wastewater treatment process, and considering the specific characteristics of the plant such as low investment and small footprint, the computer control system adopts a PC+PLC hierarchical distributed control form[[ii]]. The system adopts a control structure combining local operating stations and main PLCs, and uses fieldbus technology to achieve communication processing, thereby saving investment, improving system security, and realizing the concept of distributed control. The overall system structure is divided into three levels: (1) Upper level: central control room computer monitoring system; (2) Middle level: main PLC controller as central control unit; (3) Lower level: seven local control stations distributed in two sub-control stations, flotation and sludge treatment. The middle level main PLC controller is connected to the upper level monitoring system via ETHERNET; the lower level field control unit PLC collects the working status of each device and sends it to the main PLC via Genius BUS. The main PLC completes the processing, calculation, interlocking logic, PID control and other functions of the field data and sends control commands to each sub-control station; at the same time, it exchanges process and equipment data and control commands with the central control room computer control system. After the control commands are sent to each sub-control station, they are output to the field control equipment through the output module to complete the control process. 2.2 Automation System Functions Under computer control, the system functions mainly include the following aspects: (1) Data acquisition and equipment control; (2) Process control; (3) Process monitoring. As shown in Figure 2. Equipment and loop control are integrated together for control, which constitutes the process control level in the integrated automation system. The start and stop of electrical equipment can be realized through soft keys on the computer screen, and loop control can realize automatic closed-loop regulation and soft manual regulation. The optimization setting of control parameters aims to realize the optimization setting of the process control level and the operation guidance of the process monitoring level based on the optimized control model of the sewage treatment production target and the advanced control ideas and control technologies. [align=center] Figure 2 Functional structure diagram of monitoring system[/align] (1) Data acquisition and equipment control level Data acquisition mainly realizes the real-time detection of process parameters, such as: sludge concentration, liquid level, flotation dosage, sludge dewatering dosage, temperature, pH value and other process parameters acquisition, alarm and other functions. Equipment control level mainly realizes logic control and sequential control, such as: the start, stop, interlock of main electrical equipment, equipment status alarm and other functions. (2) The main control loops at the process control level include wastewater lifting pump station control technology, pump station load distribution control, flotation, dewatering dosing self-tuning PID control, and flotation, dewatering dosing optimization setting control. (3) The process monitoring level This monitoring system is a secondary development on the industrial control software platform Citect. The system has rigorous engineering and good versatility. It has the following characteristics: ① The control screen is vivid and intuitive, accurately reflecting the wastewater treatment process flow. The process parameters are simple and clear, and the system is highly practical; ② The control mode is flexible, convenient, safe and reliable. Operators can realize process interlock control, unit interlock control and manual switch control through the monitoring screen, which greatly facilitates the optimized operation of the system and equipment maintenance. ③ The alarm function is complete. The monitoring system can provide nearly 300 digital alarms and more than 30 analog alarms, covering almost all production equipment in the plant area. The alarm display is equipped with sound and light display, automatic screen switching and printing and storage functions, which greatly facilitates alarm processing and fault diagnosis and analysis. ④ The data processing capability is strong and the system is easy to optimize. The system can provide trend storage and calculation functions for all process parameters, and provide start-up count accumulation and power accumulation functions for important equipment. Production reports and statistical reports are indispensable important production adjustment bases for the plant. 3. Key automation control technology The process characteristics of sewage treatment plants require the control system to be able to take into account both process and unit control. Unit control is the foundation of computer control system and a condition for realizing system optimization control. Therefore, it is necessary to study and develop unit control methods for important links in sewage treatment plants. According to the process control requirements, the control system of sewage lifting pump station and flotation dosing link is studied in detail. (1) Sewage lifting pump station control technology based on fuzzy control (liquid level control) Sewage lifting pump is a key piece of equipment in sewage treatment plant. Its condition, maintenance and operation effect directly affect the sewage treatment effect. Pump station process requirements: (1) The sewage lift pump station has a fast response capability to ensure that all sewage enters the plant; (2) The water level of the pump station collection well changes within a certain range; (3) The load of the five lift pumps is evenly distributed; (4) The pump start interval is greater than 10 minutes. The traditional level change graded control method for pump stations only considers the change of liquid level and does not consider the rate of liquid level change, resulting in frequent pump starts and severe wear. Moreover, it cannot meet the requirements of rapid response when the sewage flow rate changes greatly. In recent years, pump station control methods based on PID or continuous control algorithms have received widespread attention. These methods have high control accuracy, but the five submersible pumps on site cannot be controlled continuously due to the lack of frequency converters. Although traditional control theory cannot play an effective role, fuzzy control based on operator or expert experience can achieve satisfactory control results that other control methods cannot achieve. Fuzzy control adopts a graded control method for control variables and output quantities. Under the premise that the control progress requirements are not high, it is very suitable for pump station control. The sewage lift pumps are controlled by introducing fuzzy control methods and adopting a PD control structure to control the number of pumps in operation based on the water level difference and the rate of change of water level in the collection well, thereby improving the pumping station's response capability to changes in influent flow. The fuzzy controller uses three fuzzy variables: water level difference LE; water level change rate CE; and number of pumps to be started MC. Among them, LE and CE are input fuzzy variables with different domain ranges, respectively; and MC is the output fuzzy variable. The structure diagram of the fuzzy controller is shown in Figure 3. [align=center] Figure 3 Fuzzy controller structure diagram[/align] The above fuzzy control idea is implemented by PLC programming. The PLC detects the water level in the collection well at 10-second intervals, calculates the quantization level of water level LE and water level difference change CE according to the quantization formula, converts the level into the control table position, and queries to obtain the actual number of pumps to be started. The control table is stored in a series of consecutive units starting from the main PLC memory address AR, and the quantization levels of LE and CE are placed in the PLC memory addresses BR and CR. The control table unit position is calculated according to formula (1), and the number of pumps to be started is sent to the PLC memory address DR for control use. The control table unit position = Equation (1) (2) Pump station load distribution control method When controlling the pump start and stop, the system no longer operates on a specific pump, but continuously detects the number of pumps started and compares it with the output of the fuzzy controller to determine whether the system should perform pump start or stop operation. To facilitate program control, the 5 booster pumps are distinguished as 1#-5# respectively. The start and stop sequence number of each pump is set in the PLC and stored in the corresponding memory unit. When the system determines that the pump needs to be started or stopped according to the output of the fuzzy controller, the program detects the corresponding start and stop sequence number, starts or stops the corresponding pump according to the start and stop sequence number and modifies the relevant sequence number of the pump. The control rule is that the five pumps start in sequence, the pump that starts first stops first, and the pump that stops first starts first; thus ensuring the 5 pumps are used in a cyclical manner and avoiding the situation where one or several pumps are in operation for a long time while other pumps are idle. The initial setting and modification method of the pump start and stop sequence number is as follows: First, when the pump needs to be started, the start and stop flag of pump #1 is detected as 1, and pump #1 is started; at the same time, the start flags of each pump are modified, the start sequence number of pump #1 is modified to 5, and the others are reduced by 1. The next start process is the same, starting pump #2, #3, #4, and #5 in sequence. The stop process is similar. In pump station control, the problem of a certain pump working for a long time or pump starting frequently is often encountered. Long-term operation or frequent start of pumps will cause electrical and mechanical equipment impact, affecting the equipment life. Therefore, the working time of the pump is limited, and the pump is forced to switch if the working time exceeds 1 hour. At the same time, the pump is allowed to be restarted only after 10 minutes of stopping. The above programming method is implemented by PLC. Since the interlocking control method of liquid level control and load average distribution has been put into operation, the water level change is relatively smooth, the control effect of liquid level change is strengthened, and the frequent start and stop of pumps is overcome, and the effect is satisfactory. (3) Flotation dosing self-tuning PID control technology Flotation dosing is the key to the process control of sewage treatment plants. It directly affects the sewage treatment effect and treatment cost. Insufficient reagent dosage will affect the water purification effect, while excessive dosage will not only waste reagents and increase production costs, but also cause turbid effluent and increased sludge volume in the sedimentation tank. Therefore, it is necessary to analyze the flotation dosing process in the system design and propose an economical and efficient reagent dosing control method. Based on the actual situation of the wastewater treatment plant, and considering the many factors affecting flotation indicators, such as solution concentration, pH value, water quality, aeration and stirring, type and quantity of flotation reagents, dosing location and dosing method, the flotation inlet flow rate and influent turbidity are used as feedforward variables, and the flotation outlet turbidity is used as feedback variables. PID control is used to regulate reagent dosing. To make the frequency converter control more precise, the reagent flow rate is used as the frequency converter control feedback, which can form a feedforward feedback dual-loop control system as shown in Figure 4. From the perspective of the controlled system, this is a complex system integrating water, electricity, and machinery. In the actual system operation, the characteristics of the object will change accordingly due to factors such as the different number of flotation machines, seasonal changes in water temperature, and changes in reagent percentage. The self-tuning PID technology based on performance index ISE is used to automatically determine the control parameters of the PID controller to stabilize the system characteristics and quickly track changes in turbidity and flow rate. (4) Optimization setting control technology for flotation dosing According to the operating characteristics and control requirements of the flotation dosing process, and combined with the actual situation of the equipment and the site, the control system decomposes the complex controlled process into simple subsystems that work in parallel according to the sewage concentration, pH value and sewage flow rate. Based on advanced control ideas, the major key technical problems in process control are solved, and the dosage of reagents affecting the flotation effect and the ratio of A1 and A2 reagents are adjusted in real time. The loop settings of sewage turbidity, pH value and reagent quantity of the distributed computing level control system of sewage treatment process are optimized to ensure that the key water quality parameters BOD and COD meet the discharge standards and minimize the operating cost of the sewage treatment plant. The process optimization setting design technology is adopted, and the optimization control model based on the production target of sewage treatment water quality is used to realize the optimization setting of the flotation dosing production process control. The structural block diagram of the optimization setting control method is shown in Figure 5. The setpoint optimization control strategy divides the entire control system into two layers. The upper layer is used for optimizing the setpoint values ​​of process variables to achieve the optimal operating state of the entire system, while the lower layer is used for local routine control of subsystems to keep the system in the optimal state. Principal component analysis and fuzzy clustering analysis are commonly used methods for setpoint optimization control[[iii]]. In addition, the number of flotation machines in operation is controlled according to the flow rate entering the flotation chamber, and human-machine combination and coordination optimization setting technology are adopted to realize the optimized management and operation of the flotation dosing process. [align=center] Figure 4 Block diagram of flotation dosing control system[/align] [align=center] Figure 5 Structure diagram of optimization setting control method[/align] 4. Conclusion A wastewater treatment demonstration plant in Shenyang City treats 100,000 tons of wastewater per day and is the first wastewater treatment plant in China to be independently developed and manufactured. The plant adopts a first-stage enhanced flotation process and an advanced graded distributed integrated automation system, which has the characteristics of small initial investment, small footprint, low operating cost, and low cost per ton of water treated. Since its implementation, the system has been operating safely and reliably, greatly reducing the labor intensity of workers. The innovation of this paper lies in the close relationship between the overall level of wastewater treatment and the degree of application of ICA (instrumentation, control, and automation) in wastewater treatment plants [[iv]]. This paper adopts the "EIC three-electric integration" distributed control technology to design a distributed computer control system for the entire wastewater treatment production process. On this basis, by adopting advanced technologies such as optimized setting control technology and adaptive self-tuning PI control technology, key technical problems such as large lag, nonlinearity, and coupling in the control of urban wastewater production processes are solved, realizing automatic control of wastewater lifting, flotation dosing, sludge dewatering, and other wastewater treatment production processes. References [1] Garrett MT, Instrumentation, control and automation progress in the United States in the 24 years, Wat. Sci. Tech., 1998, 37 (12): 21-25. [2] Mao Huiou, Chai Tianyou, Qiao Jinhua. Application research of computer control system for urban sewage treatment. Environmental pollution control technology and equipment. 2001. 2 (5): 32-36. [3] Rosen C. and Z. 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