0 Introduction
Modern enterprises have increasingly higher requirements for wastewater treatment, demanding not only safety but also real-time performance, precision, and energy efficiency. Automated monitoring and real-time control of wastewater treatment processes are key to improving efficiency and reducing energy consumption. Traditional control methods will gradually be replaced by intelligent bus control, which is becoming a trend.
Water treatment control typically involves configuring necessary level, flow, and water quality analysis instruments according to the process flow. Automatic control and adjustment systems are set up based on the operational requirements of electrical equipment and the control requirements of key process parameters. A three-level computer monitoring system is established according to the principle of centralized monitoring management and decentralized control.
Control equipment used in wastewater and sludge treatment sites should be designed for long-term, reliable, and stable operation in similar projects. It should be functional, stable, reliable, easy to operate and maintain, corrosion-resistant, have a long service life, and be capable of continuous operation. The materials used, installation methods, and power supplies should be adapted to the site environment, and in particular, should have hydrogen sulfide protection capabilities. The PLC controller should have a unified, open fieldbus interface, allowing it to connect to self-contained system control cabinets by configuring different fieldbus interface modules.
1. Introduction to Ecological Recycling Treatment Process
The water ecological recycling project adopts the "sedimentation-hydrolysis acidification-circulating activated sludge process (CAST)" treatment process. The process flow of this project is shown in Figure 1.
The water circulation project treats wastewater from the steel plant and other unconventional water sources to meet environmental emission requirements, while also serving as a safe water source for the enterprise, providing qualified water quality. First, the existing ponds in the steel plant were modified and utilized as regulating ponds to adjust the influent quality and quantity of the wastewater treatment plant, reducing the impact of water quality and quantity fluctuations on system operation and ensuring the normal operation of subsequent biological and advanced treatment systems. Then, a hydrolysis acidification process is used to enhance the treatment process. Utilizing the reaction of hydrolysis and acid-producing bacteria, insoluble organic matter is hydrolyzed into soluble organic matter, and large molecules are broken down into smaller molecules, increasing the BOD/COD ratio of the effluent and enhancing the biodegradability of the wastewater. Finally, the wastewater undergoes a CAST reactor cycle of influent-aeration, sedimentation, decanting, and influent, continuously cycling through this process to remove pollutants from the wastewater. Finally, through the membrane modules of the CMF continuous membrane filtration system and the RO reverse osmosis system, water molecules permeate through the membrane under certain pressure, while suspended solids, colloids, macromolecular organic matter, and microorganisms are blocked, thereby achieving the purpose of purification and separation. The treated wastewater is used as the circulating cooling water source for the company's steelmaking process.
2 Control System Design
2.1 Design Principles
Considering the scale of this project, the automatic control system and equipment should reach an advanced level, and the system composition should adapt to the future trend of computer development to realize the automation of production management and ensure the safety and reliability of the water ecological recycling project.
The automation system should comprehensively consider factors such as production, management, and safety, and all process equipment should be integrated into the automation system network. "Decentralized control and centralized management" will ensure the coordinated operation of the entire water ecological recycling project.
The hardware configuration should conform to national standards, possess high reliability, strong adaptability, flexible expansion, and be easy to operate and maintain. The software should be complete and have an open structure to facilitate future secondary development by users. The human-machine interface should be convenient and intuitive. Based on process requirements and equipment characteristics, the control of major mechanical equipment should employ three operating modes: local manual control, automatic control, and central control room monitoring. The system should meet the process engineering requirements for automatic control, ensuring the integrity and adaptability of the control system configuration.
2.2 Design Scope (1) Based on the operating requirements of electrical equipment and the control requirements of main process parameters, an automatic control and automatic adjustment system shall be set up.
(2) Configure necessary level, flow and water quality analysis instruments according to the process flow.
(3) Transmission and display of operating signals of all testing instruments and electrical equipment.
(4) Establish a three-level computer monitoring system in accordance with the principle of centralized monitoring and management and decentralized control.
2.3 System Composition
Based on the actual conditions and process requirements of the project, this design adopts an advanced and mature distributed control system (DCS) both domestically and internationally. It implements three levels of computer management: information layer, control layer, and field device layer. It integrates computer technology, control technology, communication technology, and display technology, connecting the central monitoring station and several sub-control stations via industrial Ethernet. The field control level uses a fieldbus communication network to achieve centralized monitoring and management and decentralized control. This overcomes the shortcomings of centralized control systems, such as concentrated risk, poor reliability, limited scalability, and large amount of control cabling, achieving centralization in information, scheduling, and management, and decentralization in functionality and control risks. Even if the central control room microcomputer fails, each sub-control station can operate independently and stably, fundamentally improving system reliability. The DCS system, primarily composed of PLCs, has the following main characteristics:
(1) Improve equipment utilization and ensure water treatment quality. Various process parameters are continuously monitored by instruments installed on site. The automatic control system can coordinate the relationship between water treatment processes in each section based on these parameters, ensure full utilization of equipment, and correct deviations in a timely manner based on the data detected by the instruments, thereby ensuring water treatment quality.
(2) Ensure reliable system operation. As corresponding water quality monitoring instruments are installed in each process flow section, and two redundant monitoring and management computers are set up in the central control room, the operating parameters and operating status of all equipment in the plant can be monitored, equipment failures can be detected at any time, and alarms can be triggered in a timely manner.
(3) Due to the implementation of microcomputer optimization control, manpower is saved and the intensity of manual labor is reduced, daily operating costs are saved, and the cost of sewage treatment is reduced.
2.3.1 Network Structure
The automatic control system of the water ecological cycle project adopts a three-layer network structure: information layer, control layer, and field device layer. The three-layer network must be a completely open, mature, and advanced technology, and must be able to be fully integrated with enterprise-level networks and information systems.
A three-layer network should ensure a consistent application layer, and information sharing and access should avoid any special programming or gateway devices. The network should maintain functional integration. At any network level, transparent system browsing, programming configuration, real-time control, data acquisition, and system diagnostics should be possible on the same medium. General information access on the network should not affect the system's real-time control performance.
(1) Information layer
The information layer network between the central control room and each sub-control station must be based on open network technology using 100Mb/s Ethernet, with broad vendor support. All network media and accessories (switches, hubs, cables, connectors, tools, etc.) must be available from third parties, not proprietary to any particular vendor. The information management component consists of communication servers, data and network servers, monitoring computers, industrial Ethernet equipment, and related software, all connected via a TCP/IP-based Ethernet network. This Ethernet network permeates all management departments. Personal terminals connected to the Ethernet network can access data within their authorized access limits via a web browser. Engineering workstations connected to the Ethernet network, after being granted certain permissions, can access the control layer through a gateway to monitor the control system. The information management system must support remote login for data access.
The connection between the sub-control stations and the central control room must be established via a switch, using fiber optic cable as the connection medium. The Ethernet equipment in the central control room uses Category 5 twisted-pair cable to access the switch. The system achieves Ethernet redundancy through a self-healing fiber optic ring network, meaning that if the fiber optic connection at any point on the backbone is unexpectedly lost, the system can provide a backup Ethernet link through a reverse ring, ensuring system availability while maintaining cost-effectiveness. The system should be able to simultaneously support data acquisition, programming upload/download, and I/O control functions on the same medium link.
(2) Control layer: The communication between the sub-control station and the I/O station adopts the field control bus network.
(3) Equipment layer: The communication between the automatic control system and the equipment-supporting control cabinet must adopt the open and internationally recognized Profibus-DP field control bus.
2.3.2 Central Monitoring System
The central control room is located in the comprehensive office building of the water ecological recycling project, and centrally monitors, controls, and manages all production and technological processes. It provides online real-time monitoring of automatic control, alarms, automatic protection, automatic operation, automatic adjustment, and key parameters in each process flow, as well as real-time monitoring of the operating conditions of process equipment.
(1) System structure
Two central operator workstations are installed in the central control room of the integrated office building, forming a 100Mb/s switched local area network centered on these workstations. The central operator workstations are configured with hot standby redundancy to improve data security. The PLC control systems of the central control room and the plant's sub-control stations are connected via a fiber optic ring network, using industrial Ethernet with a transmission rate of 10~100Mb/s. The two central operator workstations are configured with dual-machine hot standby; under normal circumstances, one is used for process monitoring, while the other serves as a backup, ready to replace faulty equipment at any time. The hardware and software configurations of the two operator workstations must be identical, and their functions and monitored objects should be interchangeable.
(2) Monitoring of the central control room
Aside from equipment directly controlled by the central control room that is crucial to the overall plant operation and scheduling, the central control room generally does not directly participate in equipment control. Instead, it primarily performs centralized operation, monitoring, and control functions for each process flow segment of the water plant. Through simple operations, it can configure system functions, monitor alarms, and modify and adjust control parameters online; it can transmit and communicate with lower-level terminal microcomputers, collecting monitoring information, operating status, and water quality parameters from each terminal microcomputer; and it performs aggregation, calculation, processing, alarm, and fault analysis to ultimately optimize the control of field equipment. The central control room assigns operational control targets to the sub-control stations for their respective individual units or nodes, controls the activation or deactivation of process equipment, and monitors the entire plant's production process. For equipment or equipment groups that the central control room permits to be put into operation, the specific control process is managed by the sub-control station; for equipment or equipment groups that the central control room prohibits from being put into operation, the relevant sub-control station controls their deactivation and marks them as unusable equipment, preventing their restart.
The central control room is equipped with a large-screen projection system, which can intuitively display the process flow, the real-time status of each section, and the trend of each process parameter. It can also be modified online at any time, enabling operators to keep abreast of the operation status and issue instructions to each sub-control station through the computer network to coordinate the normal operation of each link and ensure safe and economical operation.
(3) SCADA system software
The central control room computer is responsible for monitoring each process. The software running on these computers must be mature and have been widely used both domestically and internationally. In addition to graphical monitoring functions, the monitoring software should also have alarm, trend, and report functions.
The control room's operation and monitoring program is designed using graphical control software, featuring a Chinese interface, operation prompts, and a help system. The interface primarily uses flowcharts, ranging from the overall flowchart to detailed flowcharts of each individual unit. Devices displayed on the flowcharts can be clicked to access further details or control. Process parameters, operating parameters, and equipment status are all visually represented graphically. Operating parameters and target control parameters can be clicked to view their attributes or make settings modifications. Operation control in the control room is achieved through operating terminals, primarily for adjusting the production process and controlling product quality. The central control room mainly handles operation scheduling, parameter allocation, and information management, and can also control the operation of all major equipment in the plant.
The central control room assigns operational control targets to each individual control system, and commands a group of process equipment to be put into or taken out of operation based on the actual water volume and quality. For equipment or equipment groups that the central control room permits to be put into operation, the specific control process is managed by the individual control system; for equipment or equipment groups that the central control room prohibits from being put into operation, the individual control system controls their take-off and will not restart them.
(4) Communication software
The system provides a complete and mature software suite for network browsing, network diagnostics, communication configuration, software drivers, and data communication services, as well as database transaction processing software based on the network architecture. For each network layer, the system provides ready-made communication driver software for interface configuration, real-time transparent network browsing, programming upload/download, and online diagnostics. For software such as upper-level HMIs, batch processing, and enterprise MIS system database interfaces, the software provides mature data communication server functionality, offering high-throughput, stable bidirectional data transmission services between the PC system and the control system. The software must support data transmission methods such as OPC and DDE, and provide optimized communication services for the control system.
(5) Database transaction processing software
To interface with user MIS system databases and other enterprise application software, this system provides mature, optional database transaction processing software, offering high-throughput, stable, and real-time bidirectional data transmission services between the enterprise database system and control system. For the control system, it supports data transmission methods such as OPC and DDE and can interface with HMI software. For the MIS system and enterprise application software, it provides interfaces with mainstream data software such as Microsoft MS SQL Server's OLE-DB and Oracle's OCI database, and also has the capability to support general ODBC database interfaces. It is required to have interface capabilities with future ERP systems and possess certified interface capabilities.
First, complete daily data management. The collected data is calculated, processed, and classified to automatically generate various databases and reports for real-time monitoring, querying, modification, printing, and modification or reorganization of generated report files. The software system's reliability should guarantee absolute data security, preventing unauthorized access, especially modification of original data. Management should be based on operation levels; generally, at least three operation levels should be set up: observation level, control operation level, and maintenance level. Each level must have access control, daily network management functions, maintain the operation of the entire local network, periodically perform self-checks on each interface device, and issue alarm signals when abnormalities occur.
Secondly, it completes equipment management functions. It can perform online monitoring and self-diagnosis of all hardware devices and their operating status that make up the system, monitor and self-diagnose the operating status of all objects under real-time monitoring, monitor the operating status of various equipment (such as cumulative working time, last maintenance date, etc.) online and save them into corresponding documents for maintenance and upkeep, and provide handling opinions for equipment failures for reference.
2.3.3 PLC sub-control station, remote I/O station
Based on the process characteristics, the layout of the structures, and the distribution of on-site control, five PLC substations were set up. Programmable logic controllers (PLCs) were selected for the PLC field substations. The PLCs have a modular structure, offering flexible hardware configuration and convenient software programming. Furthermore, the PLC substations and their corresponding MCCs are located in the same place, saving on cabling. The substations are divided as follows:
(1) Control station for vortex sedimentation tank, hydrolysis acidification tank, and water collection pump room: PLC1
PLC1 is located in the water collection pumping station: responsible for the automatic control and data acquisition of equipment in the water collection pumping station, vortex sedimentation tank, Parshall flume, and hydrolysis acidification tank. A field control PLC substation is set up in the vortex sedimentation tank, and a WAGO remote I/O station is set up on the hydrolysis acidification tank.
(2) Blower room and CAST pool sub-control station: PLC2
PLC2 is located in the blower room control room: responsible for the automatic control and data acquisition of equipment in the blower room, power distribution room, and CAST pool. Each CAST pool has a separate control PLC substation, and WAGO remote I/O stations are set up on the CAST pool.
(3) Control stations for chemical dosing workshop, chlorination workshop, and intermediate water tank: PLC3
The PLC3 is located in the control room of the chemical dosing workshop: it is responsible for the automatic control and data acquisition of equipment in the chemical dosing workshop, chlorination workshop, and intermediate water tank. Separate control PLCs are set up for the chlorination workshop and the intermediate water tank.
(4) Sludge dewatering workshop control station: PLC4
The PLC4 is located in the control room of the sludge dewatering workshop: it is responsible for the automatic control and data acquisition of the equipment in the sludge dewatering workshop and sludge thickening tank sub-control stations. Each dewatering machine has its own dedicated control PLC substation.
(5) Membrane filtration workshop control station: PLC5
The PLC5 is located in the central control room and is responsible for the automatic control and data acquisition of the membrane filtration workshop, RO reclaimed water pump room, heat exchange workshop, and CMF circulating water supply pump.
Based on the requirements for selecting remote I/O slave stations, the WAGO-I/O-SYSTEM 750 series distributed I/O system from WAGO GmbH, Germany, was chosen. This system features a modular structure, small size, and independence from the fieldbus. The fieldbus node design, which allows for the free combination of digital, analog, and special-function I/O modules, saves both cost and space.
WAGO's remote I/O is designed based on independent fieldbus nodes, with each module having 1, 2, 4, or 8 channels. Following the IEC 61131-3 standard, fieldbus adapters with PLC functionality (controllers) are distributed across the control network. Digital/analog inputs/outputs, as well as special function modules with different voltage levels and signal types, can be combined within the same fieldbus node.
Figure 2 shows the block diagram of the automatic control system for the water ecological recycling project.
2.3.4 Instruments of the automatic control system
Field monitoring instruments are an indispensable part of the computer-controlled system. Considering the adaptability to working environment conditions, especially since sensors are in direct contact with sewage and sludge media, making them highly susceptible to corrosion and scaling, diaphragm-type, non-contact, and easy-to-clean sensors were selected. For ease of maintenance and management, instruments with uninterrupted power supply and long maintenance intervals were chosen whenever possible. All instruments are intelligent instruments with field display transmitters and 4-20mA DC output. Signals are transmitted to the central monitoring computer via field terminals and a communication network, and displayed on the computer's CRT and analog screen.
The basic types of various instruments are as follows:
(1) Flow measurement instruments: Electromagnetic flow meters should be used in narrow pipes, and ultrasonic flow meters should be used in wide pipes. Calorific value flow meters should be used for gas flow meters.
(2) Liquid level detection instrument: In the process where a continuous measurement signal is required, an ultrasonic liquid level gauge or an integrated ultrasonic liquid level gauge is used. In general process, the water level measurement needs to provide an in-situ signal, so a float liquid level switch is used.
(3) Temperature measuring instrument: A temperature measuring instrument integrating sensor and transmitter is used. The thermistor is a platinum resistance thermometer (Pt100).
(4) Dissolved oxygen analyzer: a diaphragmless solid electrode sensor with an automatic cleaning device is used.
(5) Acidity meter: A glass electrode acidity meter and a suspended matter meter are used, and a photoelectric sensor is selected.
(6) Online COD analyzer: The COD is measured using a patented optical method.
3. Conclusion
With the advancement of electronic information technology and the reduction in the cost of automated control products, intelligent automated control systems for water ecological recycling projects have become increasingly mature, demonstrating significant economic and social benefits in this field. Practice has proven that automatic monitoring and control in water ecological recycling projects can ensure effluent quality while liberating productivity, improving production efficiency, and reducing energy consumption. Therefore, an economical and rational automated control system plays a crucial role in the safe, reliable, and scientific operation of the entire water ecological recycling project.
