Ethernet, with its wide application, low cost, high communication speed, abundant hardware and software products, and mature application support technology, reduces network construction costs and improves system scalability and compatibility compared to other fieldbus control schemes. Industrial Ethernet is used for data exchange in the automatic control system of wastewater treatment, improving signal transmission speed while ensuring accuracy, resulting in high system reliability and scalability. Keywords: Industrial Ethernet, PROFINET, SBR process, SCADA 1 Introduction In recent years, the SBR (Sequencing Batch Reactor Activated Sludge Process) has been widely used in urban wastewater treatment due to its low investment and flexible operation. Developed as early as 1904, the SBR process achieved good results, but its widespread application was limited by the level of automation and equipment manufacturing processes at the time. However, the rapid development of automation and online monitoring technologies in recent years has provided the preconditions for the development and application of the SBR process. Automated monitoring and real-time control of wastewater treatment processes are key to improving wastewater treatment efficiency and reducing energy consumption; therefore, the SBR process is one of the wastewater treatment processes with the highest requirements for automation systems. The SBR (Sequencing Batch Reactor) process primarily takes place within a biological reactor and consists of five stages: influent, aeration, sedimentation, effluent discharge, and idle. The treatment efficiency of the SBR process depends mainly on its operating parameters, with key parameters including the reaction time and aeration intensity. A PLC-based automated process monitoring system is typically used to control blowers, pumps, electric valves, and other equipment, as well as the reaction time, in real time, ensuring the water quality meets national discharge standards. Therefore, the SBR wastewater treatment process is a complex system with multiple parameters (such as liquid level, flow rate, pressure, and biological indicators), multiple tasks (such as wastewater transport, airflow control, and pump start-up/shutdown), and multiple pieces of equipment (such as bar screens, pumps, blowers, and valves), exhibiting randomness, time-varying characteristics, and coupling. Therefore, it should be automatically managed by a stable and reliable data exchange network and integrated management system to ensure its safe and reliable operation. 2. Application of PROFINET Industrial Ethernet Technology Industrial Ethernet generally refers to technology that is technically compatible with commercial Ethernet (i.e., the IEEE 802.3 standard), but in product design, it meets the needs of industrial sites in terms of material selection, product strength, applicability, real-time performance, interoperability, reliability, interference resistance, and intrinsic safety. PROFINET, short for Process Field Net, is a fully open communication protocol between PROFIBUS and Ethernet, developed by PROFIBUS customers, manufacturers, and the Systems Integration Alliance. PROFINET is an automation solution based on real-time industrial Ethernet. It provides Ethernet migration services for PROFIBUS and various other fieldbus networks in a complete, high-performance, and scalable manner. The openness of the PROFINET standard ensures its long-term compatibility and scalability, thereby protecting users' investments and interests. PROFINET makes engineering, configuration, commissioning, operation, and maintenance more convenient, and can achieve seamless integration and connection with PROFIBUS and other fieldbus networks. This open standard across vendors is built on the foundation of industrial Ethernet and covers all areas of factory automation. Relying on existing IT standards and unconditionally supporting the TCP/IP protocol, PROFINET ensures integrated communication across the company, from the office area to the field level. It primarily comprises three technologies: ① a distributed automation system based on the Common Object Model (COM); ② defined open and transparent communication between PROFIBUS and standard Ethernet; ③ a manufacturer-independent system model including device and system layers. PROFINET uses standard Ethernet as the connection medium and employs the TCP/IP protocol plus application-layer RPC/DCOM to complete communication and network addressing between nodes. It can simultaneously connect to traditional PROFIBUS systems and new intelligent field devices. Existing PROFIBUS network segments can be connected to the PROFINET network through a proxy device, enabling the entire PROFIBUS set of devices and protocols to be used seamlessly within PROFINET. Traditional PROFIBUS devices can communicate with COM objects on PROFINET through the proxy and achieve calls between COM objects through the OLE automation interface. Compared with other fieldbuses, industrial Ethernet applications have the following main characteristics: (1) Real-time communication adopts Ethernet switching technology, full-duplex communication, flow control and other technologies, as well as deterministic data communication scheduling and control strategies, simplified communication stack software layers, and micro-segmentation of field device layer networks, which are all measures for real-time communication in industrial process control, thus improving the real-time performance of Ethernet communication. (2) Bus power supply adopts DC power coupling, power redundancy management and other technologies, and designs an Ethernet hub that can realize network power supply or bus power supply, thus solving the power supply problem of Ethernet bus. (3) Long-distance transmission adopts network layering, control area micro-segmentation, network time delay relay and fiber optic technologies to realize long-distance transmission of Ethernet. (4) Network security adopts control area micro-segmentation, and each control area is connected to the system backbone through a field controller with network isolation and security filtering to realize logical network isolation between each control area and other areas. (5) Reliability adopts distributed structured design, EMC design, redundancy, self-diagnosis and other reliability design technologies to improve the reliability of field devices based on Ethernet technology. (6) One network can extend all the way to the enterprise's field equipment control layer, so it is generally considered to be the best solution for future control networks. Industrial Ethernet has become the mainstream technology in fieldbus. Engineering practice has proven that using PROFINET communication technology when building an enterprise industrial control network can save nearly 15% of hardware investment. 3 Control System Composition and Functional Design Modern sewage treatment systems need to achieve integrated management and control and realize office automation. The control system not only has a good interface with the lower-level control equipment, but also has an interface for integration with the upper-level management system, and is also scalable. Therefore, the industrial Ethernet PRIFINET or fieldbus PROFIBUS technologies adopted in this system are suitable for this current need. In order to enhance the reliability of the system and enable the entire system to operate without failure for a long time, the upper-level monitoring system also adopts fault-tolerant (redundancy) technology. 3.1 System Composition and Characteristics According to the concept of Totally Integrated Automation, the sewage treatment plant control system is divided into management level, control level and field level. The system diagram is shown on the next page: (1) Management level The management level is the core part of the system, which completes the management and control of each part of the sewage treatment process and realizes the office automation of the plant. The management level provides a human-machine interface and is the interface for the entire control system to exchange information with the outside world. The industrial computers of the management level have the function of mutual communication to realize data exchange or sharing. Considering the hierarchical and divisible nature of the functional structure of the management level, a client/server architecture is adopted. The server uses a large network relational database to meet the requirements of open and distributed database management. The server has remote control operation function, status display function, data processing function, alarm function, report function, communication function and redundancy function. The equipment in the plant central control room includes: two redundant servers with Siemens WinCC monitoring configuration software installed as host computers, with the full version of WinCC configuration software and redundant software packages installed. The two servers are backups for each other to achieve redundancy and improve the reliability of the system. A PC with WinCC running version installed is used as the monitoring engineer's workstation. The main advantage of this configuration is to ensure the integrity of the data and the continuity of the monitoring operation. If a WinCC server fails, the client machines (workstations) of that server automatically switch from the failed server to the backup server, so that all clients can always monitor the production process. After the repaired server returns to the system, it automatically matches the archived data. The data exchange between the management level and the control level adopts industrial Ethernet, which truly reflects the integrated communication from the office area to the field level within the company. (2) Control level The control level is the key to realizing the system functions and is also the hub layer between the management level and the field level. Its main function is to accept the parameters or commands set by the management level, control the sewage treatment production process, and transmit the field status to the management level. The controller is the core of the entire system. Therefore, in the control level, three Siemens S7_300 (CPU315_2PN/DP) PLCs are mainly used. A data exchange network is constructed using optical fiber and switches. The CPU315_2PN/DP is equipped with an integrated PROFINET interface, through which I/O field devices are connected. STEP7 contains the required drivers and can also be extended to devices on low-end PROFIBUS without reconnecting the programmer. Field devices can also be connected using an additional integrated PROFIBUS interface. Moreover, the PROFINET interface can be used for unit-level communication. The PROFINET CPU can be programmed on the industrial Ethernet via a programmer/PC using STEP7. (3) Field Level The field level is the foundation for realizing system functions. The field level is mainly composed of primary instruments (such as level gauges, DO sensors, etc.) and control devices. Its main functions are to monitor the status of system devices and sensor parameters, and upload the monitored data; and to receive instructions from the control level to control the actuators. Because control equipment is often distributed, in traditional factories, input/output devices are connected to a centralized rack. This leads to a large amount of wiring work, high costs, and low flexibility when equipment is changed or the system is expanded. Connecting field control components (some control devices may be provided by third-party vendors) through the open and standardized PROFIBUS fieldbus system is the best solution to these problems. Distributed configuration means that programmable controllers, I/O modules, and field devices are connected via signal cables from the PROFIBUS_DP fieldbus. Input/output modules are converted into local monitors and actuators, enabling local conversion and processing of process signals. This ensures the independence and compatibility of control-level and field-level control devices, while also solving communication problems with third-party devices (skimmers, dewatering machines, etc.). The third-party devices in this system all use the cost-effective SIEMENS S7_200 small PLC, which can be connected to the PROFIBUS fieldbus via its additional communication module EM277, facilitating system maintenance and repair during operation. In wastewater treatment, it is necessary to monitor various water quality parameters online in real time to ensure accurate process operating parameters and timely display of treatment results. This system uses a large number and variety of sensors, including pH, SS (suspended solids), DO (dissolved oxygen), COD, level sensors, as well as electromagnetic flow meters and pressure gauges. All sensors are products of Endress+Hauser, Germany, and are equipped with PROFIBUS_DP interfaces. Utilizing these intelligent interfaces, these instruments can be integrated with automated process control systems, allowing for centralized display of all process parameters, which can then be used as process control parameters. 3.2 Process Control Laws The main basis for software programming is the control laws provided by the production process. The same treatment process may have different control strategies. Based on current research, SBR process control can be divided into three types: The first is the biological concentration method, which refers to closed-loop control based on online measured water quality parameters and set parameters; the second is reaction time control, which automatically controls the time required for the five stages of the SBR reaction; and the third is flow program control, which automatically adjusts the time required for each stage based on changes in wastewater flow rate. The latter two control methods do not adaptively control certain operating parameters based on changes in wastewater quality. The basic idea of ​​biological concentration control is to dynamically control the reaction time of the SBR (Sequencing Batch Reactor) so that the organic matter concentration (expressed as COD or BOD) reaches the allowable discharge standard, at which point aeration is stopped. Online BOD or COD sensors for measuring organic matter concentration are expensive, generally costing hundreds of thousands of yuan, and there are currently no reported applications. Furthermore, their real-time performance is poor, and the reaction time is slow. The time control program automatically controls the SBR reactor based on the time required for the five operating stages. This method does not adaptively control certain operating parameters based on changes in wastewater quality. The influent time, aeration time, sedimentation time, discharge time, and idle time can all be set by the host computer. Each process stage of the SBR reactor and its execution time strictly follow the time sequence. The status of any equipment in each reactor can be changed via a manual/automatic switch on the electrical cabinet, but the operating sequence set by the PLC cannot be altered. Once switched to automatic mode, the PLC-set sequence is entered. In automatic mode, operators can remotely control the equipment from the central control room via a human-machine interface. The operating status of the field equipment is displayed on the host computer. 3.3 Control Program Features Based on the above, a variety of control modes are designed, which can be selected by the host computer according to the actual situation. (1) Biological Concentration Method Control Program The biological concentration method adopts feedback control, using the influent water quality parameters measured online as input, and performing calculations according to the pre-determined control model. Then, the calculation results are used as outputs to control the equipment on site and dynamically control the reaction time to achieve the purpose of controlling the reaction. (2) Time Control Program ① This control system strictly follows the time sequence and works in sequence. ② It allows for arbitrary switching between remote control and automatic control during the operation without affecting the working sequence. (3) Group Operation Program Due to the uneven influent volume of the sewage treatment plant, the load of the biological treatment tank cannot be balanced, and the energy consumption per ton is large, making the operation extremely uneconomical. Therefore, we adopted the group operation method, with each pair of biological treatment tanks as a group. Both groups can operate simultaneously, or one group can be put into automatic operation while the other group is kept in reserve for manual control. On the one hand, this solves the load problem of the biological treatment tank, and on the other hand, it also solves the maintenance problem of the related equipment of the treatment tank. 4 Upper computer configuration software Requirements for SCADA (data acquisition and monitoring system): It has the ability to communicate with multiple lower computer systems through industrial Ethernet PROFINET, monitor the working status of multiple lower computer systems in real time, and display the working curves in the production process; it has remote control capabilities; it collects data from lower computer systems and stores, queries, displays, prints, etc. on historical data. Therefore, in an automatic monitoring system, the running monitoring configuration software is the data collection and processing center, remote monitoring center and data forwarding center of the system, and together with various control and detection equipment (such as PLC, intelligent instruments, etc.) it constitutes a control center that can respond quickly. This system uses Siemens WinCC (Windows Control Center) 6.0 configuration software. WinCC has powerful functions for controlling automated processes. It is a SCADA-level operation monitoring system based on personal computers and has a very high cost performance. The server version and redundant software package are installed on two servers, and the running version is installed on the workstation. Functions and features of the monitoring system: (1) The interface display has full Chinese display, is user-friendly and easy to operate. The operation screen adopts the form of a main menu, and switches between each screen by buttons. In the central control room, the entire system's controlled equipment can be controlled in real time, such as starting and stopping equipment, and setting certain process parameters in the PLC program online. Mechanical processing monitoring interface (2) Real-time screen display function The operating status of each field controlled equipment and the field status parameters are displayed in real time using graphics. The real-time changes of parameters are dynamically displayed using analog instruments, trend charts, curves, and bar charts. Based on the opening and closing of valves and the start and stop status of water pumps, production management personnel can quickly and clearly understand the production operation status of the entire system. (3) Data management Database stores production process data for statistical analysis. Staff can regularly back up the historical database to other storage media for easy querying of historical data. By comparing and analyzing the data in the database, some useful empirical parameters can be obtained, which is conducive to optimizing the closed-loop control of the SBR process. (4) Alarm function When the parameters exceed the set range or the equipment malfunctions, different levels of audible and visual alarms can be issued according to the configuration. There is an alarm box at the bottom of each screen, which displays the most recently occurring alarm event. All alarm information can be displayed on the alarm integrated monitoring screen. All alarm information is recorded in the alarm database. Through this screen, you can view all alarm information of the current system, as well as alarm historical data for up to several months, which is convenient for future accident analysis. (5) Report printing function can realize report and graphic printing, as well as real-time printing of various events and alarms. The printing methods are divided into timed printing and event-triggered printing. (6) Communication function WinCC is a SCADA system software developed based on the standard Windows platform. It fully considers the necessity of exchanging information with other systems and supports standards such as DDE, OLE, ODBC, OPC and SQL. It can provide multiple ways to exchange data with the upper-level system. (7) Redundancy function Two servers equipped with WinCC configuration software are redundant to each other to ensure data integrity and improve system reliability. 5. Conclusion The adoption of Industrial Ethernet (PROFINET) in the wastewater treatment control system has changed the traditional data exchange method of fieldbus and all-analog transmission. It realizes Ethernet digital transmission at the fieldbus level, achieving the goals of increasing information transmission distance, data volume, and transmission accuracy. This enhances the flexibility of field control, reduces the initial installation cost and design and construction costs of the data exchange network, and achieves the goals of reducing cable laying and facilitating maintenance and expansion. This control system was put into normal operation at the Zhongke Guoyi Zhuozhou Wastewater Treatment Plant at the end of 2006, realizing the automatic control of the SBR process in wastewater treatment. The network data exchange and control functions are very stable, and the efficiency and effect of wastewater treatment are ideal. At the same time, energy consumption has been reduced, which has been recognized by users, thus achieving good economic and social benefits. References: 1. Siemens ProfiNET System Manual. A5E00879148_01 2006/10 2. Yin Chaozhu Li Electrical Era. End or Rebirth_Fieldbus E-Road Forward, 2004/7 3. Yang Xianhui. Fieldbus Technology and Its Application. Beijing: Tsinghua University Press, 1999 4. Gao Yanling Motor et al. New Technologies for Biological Wastewater Treatment Beijing: Building Materials Industry Press, 2006