[Abstract] This paper takes the Class A upgrade and renovation project of the Zoucheng Wastewater Treatment Plant as an example. It designs and implements the networking of 10 PLC stations throughout the wastewater treatment plant and integrates them into a unified monitoring platform. A client/server architecture is adopted to expand the monitoring and management platform and enable remote monitoring. This paper also introduces the key technologies involved in the system integration process.
Keywords : System integration, water treatment, database monitoring
Introduction
To guide the development of urban wastewater treatment and pollution control technologies, accelerate the construction of urban wastewater treatment facilities, and prevent urban environmental pollution, the Ministry of Construction and the State Environmental Protection Administration have set higher policy requirements for wastewater treatment in cities of all sizes across the country. As a pilot unit of the national Huai River Water Diversion Project, the Zoucheng Wastewater Treatment Plant actively responded to the national call and undertook a Class A upgrade project for its wastewater treatment system. The upgrade included repairs to the existing Phase I and Phase II systems, networking of the existing PLC system, integration of the reclaimed water system, design of the anaerobic tank and filter system, design of the ultraviolet disinfection system, integration of the central control system, remote transmission of key video signals, and remote management of the local monitoring system. This article focuses on the PLC station networking, monitoring system integration, and key technologies in the integration process of the entire wastewater treatment plant.
II. Introduction to Wastewater Treatment Process
2.1 Basic Methods of Wastewater Treatment
Wastewater treatment involves using various technologies and methods to separate, remove, recycle, or convert pollutants in wastewater into harmless substances, thereby purifying the water.
Modern wastewater treatment technology, based on the degree of treatment, is generally divided into three levels of treatment:
Primary treatment, also known as physical treatment, mainly removes suspended solids from wastewater. After primary treatment, about 50% of suspended solids can be removed from the wastewater, while BOD can generally only be removed by about 30%, which does not meet the discharge standards.
Secondary treatment, also known as biological treatment, is the core of urban wastewater treatment plants, primarily removing colloidal and dissolved organic pollutants from the water. After secondary treatment, the BOD removal rate can reach over 90%, and the BOD value in the wastewater can be reduced to 20-30 mg/L, ensuring that organic pollutants meet national discharge standards.
Tertiary treatment, also known as advanced treatment, aims to further remove pollutants that were not removed by secondary treatment. These include organic matter that microorganisms have not been able to degrade and soluble inorganic substances such as nitrogen and phosphorus that can lead to eutrophication of water bodies.
Biological methods, with their advantages of high purification capacity, low cost, and high operational reliability, are the main method for wastewater treatment and occupy an important position in the field. Currently, urban wastewater treatment in my country primarily uses biological methods, supplemented by physical and chemical methods. Treatment processes mainly based on biological methods include activated sludge processes, A/O processes, A2/O processes, SBR processes, MSBR processes, UNITANK processes, integrated oxidation ditch processes, and DAT-IAT processes, all of which simultaneously degrade BOD5 and remove nitrogen and phosphorus.
2.2 The process used in this system
(1) Ober oxidation ditch process
The Ober oxidation ditch was invented in the United States in the 1960s. It consists of three elliptical channels. Wastewater from the grit chamber, mixed with returned sludge, first enters the outer channel, then sequentially flows into the middle and inner channels, finally draining into the secondary sedimentation tank via a weir at the central island. The Ober oxidation ditch uses rotating brushes or rotating discs for aeration. The discs are densely covered with raised teeth, which break up the wastewater into fine droplets when in contact with the water, resulting in a high mixing and oxygenation capacity, making them the preferred equipment for the Ober oxidation ditch.
(2) Inflow and outflow indicators
The influent and effluent parameters of the wastewater treatment plant in Zoucheng City are shown in Table 2.1.
Table 2.1 Influent and Effluent Indicators
The main goal of this renovation is to control the COD and ammonia nitrogen content of the effluent to meet the Class A standard for wastewater treatment.
(3) Process flow
Figure 2-1 Overall process flow diagram of Zoucheng Wastewater Treatment Plant
The wastewater treatment process in Zoucheng City mainly includes two systems: wastewater treatment and sludge treatment, as shown in Figure 2-1. The wastewater treatment system includes an inlet well, a coarse screen chamber, a booster pump station, a fine screen chamber, a vortex grit chamber, an anaerobic tank, an oxidation ditch, a secondary sedimentation tank, ultraviolet disinfection, and filters. The sludge treatment system includes a sludge return pump station, a dewatering room, and other components.
Design and Implementation of Three-System Integration Solution
The entire wastewater treatment plant's automatic control system consists of 10 PLC stations, of which 7 are used for the main process flow control, and the remaining 3 (PLC104, PLC105, and PLC106) are used for the reclaimed water reuse system. The functions of each PLC station are shown in Table 3.1.
Table 3.1 Functional Description of Each PLC Station
3.1 System Overall Structure
The entire system consists of a remote monitoring room, a central control room, distributed PLC stations, field instruments, and control cabinets, forming a four-level monitoring network.
(1) Instrumentation and control cabinet level
Each field control cabinet has two functions: manual operation and connection to various PLC stations to transmit equipment status signals to the relevant acquisition modules of the PLCs. Manual operation has higher priority than distributed control stations and the central control room; remote control only functions when the status selection switch is set to automatic. Therefore, before commissioning automatic control, the field control cabinets must be debugged first, and remote control can only be implemented after ensuring the safety of the field equipment. The instrumentation system continuously measures the main process parameters in the wastewater treatment plant's process flow and sends the measurement data to the computer data acquisition and monitoring system. The process monitoring instruments throughout the wastewater treatment plant are distributed throughout the various process treatment structures and on process pipelines.
(2) Distributed control station
The field PLC completes the acquisition and control of signals from various points, improving the system's reliability and maintainability. The touchscreen can display instrument data and equipment status, and also control the field equipment. The field PLC station connects to the engineering station in the central control room via industrial Ethernet. When the control mode is set to the "remote" position, operators cannot use the start/stop buttons on the electrical control cabinet or control box to run/stop the equipment. In this case, control of the equipment is transferred to the computer, and the PLC will control the equipment's operation "automatically" or "manually" according to the control instructions from the operator station.
(3) Central control room
The system can perform system configuration, debugging, and online modification and setting of control parameters. It also performs data acquisition, monitoring, reporting, and printing functions for the entire wastewater treatment plant. The operator station primarily performs data acquisition, monitoring, reporting, and printing functions for the entire wastewater treatment plant. During normal system operation, the engineer station can also function as an operator station, facilitating monitoring and operation and improving system reliability. The DLP large screen displays the operating status, process parameters, and process flow of all process equipment in the plant, providing a clear understanding of the plant's overall condition and enabling rapid response to any problems. The main monitoring screen in the central control room is shown in Figure 3-1.
Figure 3-1 Main screen of the central control room monitoring
(4) Remote monitoring room
The water supply group can remotely access on-site data of the sewage treatment plant via Internet Explorer and issue commands based on the actual situation.
3.2 Network Composition
Figure 3-2 System Network Structure Diagram
The entire network system (as shown in Figure 3-2) involves communication methods including fiber optic, Ethernet, and ControlNet. Given the advantages of fiber optic communication—wide bandwidth, large transmission capacity, and the stability of fiber optic ring networks—all distributed control stations throughout the plant are connected via fiber optic ring networks. Communication between each substation and the master station uses Rockwell's ControlNet (C-network) to ensure data communication stability. Ethernet fiber optic switches are used to convert optical ports to electrical ports, enabling connections between various PLCs and between the PLC network and the central control room monitoring computer.
Introduction to Four Key Technologies
4.1 Report Creation
Traditional report generation typically relies on scripts (VBA, C) within configuration software. This approach has a fatal flaw: the report program cannot run when the configuration software is not in operation, resulting in missing data. Furthermore, if unexpected issues arise, such as network problems preventing data access when reading variables, the entire configuration software will pause, especially if the report program has periodically running tasks. This system utilizes a separate report generation program that bypasses the configuration software by directly reading data from the OPC server. This offers greater independence and stability, and leverages the interface layout and data processing advantages of visual programming software to create more comprehensive reports.
4.2 Data Transmission
Considering the high cost of web publishing components for configuration software, this system adopts a client/server architecture, and independently develops client and server programs to achieve remote access.
(1) Server side
Visual programming software: Visual Basic
Backend database: SQL Server 2000
The server collects data using ODBC technology and stores it in a SQL Server database. With the help of SQL Server 2000's security management mechanism, the reliable storage of the original data is guaranteed.
(2) Client
Visual programming software: Visual Basic
Backend database: SQL Server 2000
The client uses Microsoft's ADO (ActiveX Data Object) to access the remote database, and leverages Visual Basic's powerful graphical design capabilities to dynamically display the collected data in the client software.
This C/S architecture ensures the stability and ease of data collection while also providing interfaces for enterprise information management systems, such as ERP systems, thus laying the foundation for integrated enterprise management and control.
V. Conclusion
Unification and standardization are requirements for standardized management in enterprises. This system, from a practical perspective, connects all distributed control stations in the wastewater treatment plant and integrates the control platform in the central control room. It also employs a cost-effective client/server architecture to ensure remote data access, providing technical support for integrated enterprise management and control.
About the author:
Zhang Xiao (1987-) Male, currently pursuing a Master's degree. His main research areas are power electronics and electric drives.
