Introduction
With the improvement of people's living standards and the need for sustainable development strategies, wastewater treatment is receiving increasing attention from society. Wastewater treatment not only plays a role in environmental protection but also enables water reuse, conserving water resources. To improve the quality and efficiency of wastewater treatment while saving manpower and resources, designing an automated monitoring system is essential. This paper takes the Yuncheng County Wastewater Treatment Plant as an example to introduce the overall design scheme of an automated monitoring system for small and medium-sized wastewater treatment plants.
Introduction to the Wastewater Treatment Process of the Second-Level System
The Yuncheng County Wastewater Treatment Plant employs an inverted A2/O treatment process (as shown in Figure 1). The A2/O process is a popular and representative biological nitrogen and phosphorus removal technology for wastewater. The biochemical section of this process consists of three zones: anaerobic (A1), anoxic (A2), and aerobic (O). Zone A1 is located at the very beginning of the process, followed by zones A2 and O, forming the so-called A1/A2/O arrangement. In 1996, Dr. Zhang Bo first proposed the inverted anaerobic/anoxic environment A2/O process, i.e., the A2/A1/O arrangement, and provided a theoretical analysis of the process. After years of systematic research, it has been confirmed that this process is not only superior to the conventional A2/O process in terms of nitrogen and phosphorus removal efficiency, but also has a simpler process flow. Dr. Bi Xuejun further refined the existing theory based on this, explored the feasibility of large-scale production application, determined appropriate process operating parameters, and indicated that the effective volume ratio of each zone, A2/A1/O, is 1:1:2. Practical experience shows that the nitrogen and phosphorus removal function of the inverted A2/O process is significantly better than that of the conventional A2/O process, while its CODcr removal capacity is comparable to that of the conventional A2/O process.
The overall process flow of wastewater treatment is shown in Figure 2.
Three-system overall design
This system uses OMRON's CJ1M and SIEMENS' S7-300 and S7-200 PLCs as data acquisition and control units. It works with various monitoring instruments and control equipment to collect data on control parameters such as wastewater level, flow rate, pH value, temperature, and dissolved oxygen concentration. Simultaneously, it controls various field devices such as coarse and fine screens, lift pumps, and blowers according to control requirements. The system establishes five distribution stations based on the process flow and control requirements, each connected to a host computer via Ethernet. The host computer exchanges data with each station through KingSCADA (Yacon Technology Co., Ltd.), thereby achieving centralized control.
The entire monitoring system is divided into three parts: first, the field data acquisition part, which completes the data acquisition function; second, the field control unit, which consists of a field control cabinet and a PLC station. The field control cabinet mainly completes manual control, while the PLC station mainly completes data processing, automatic control, and communication functions; and third, the upper-level monitoring part, which mainly monitors the operation status of each process flow on site and implements remote control functions.
Four-system hardware structure
Based on the process requirements and site environment of wastewater treatment, this system primarily adopts a monitoring mode of Industrial PC (IPC) + Programmable Logic Controller (PLC) + field monitoring instruments. This control system collects process parameters through field monitoring instruments. The PLC, as the main field control unit, reads field instrument signals and status signals of each controlled device according to the wastewater treatment process flow, and simultaneously transmits control signals to each control device. The IPC acts as the host computer for complex data processing. An industrial Ethernet network connects each control station and the engineering station in the central control room, forming a complete communication network for monitoring. The entire automated monitoring system constitutes a SCADA (Supervisory Control and Data Acquisition) system, completing the functions of data acquisition, processing, monitoring, and control of field equipment.
The entire system consists of a central control room, distributed PLC control stations, and field instruments and electrical control cabinets forming a three-level monitoring network. The system structure is shown in Figure 3.
Figure 3 System Structure Diagram
In this system, KingSCADA communicates with three different PLC models—OMRON CJ1M, SIEMENS S7-300, and S7-200—via network interface cards (NICs) using the TCP/IP protocol. The PLCs' Ethernet ports are connected to an Ethernet switch via fiber optic cables, and the industrial control computer is connected to the switch via a standard NIC. This enables data communication between the PLCs at each substation and the monitoring computer. The two monitoring computers can also exchange data and print reports via the network. The network communication structure diagram is shown in Figure 4.
Figure 4 Network communication structure diagram
Five-system software design
The software design of this monitoring system mainly includes the development of PLC programs and the development of the upper-level monitoring interface.
1 PLC programming software
The PLC programming software uses Omron's CX-Programmer 7.0 programming configuration software, suitable for all OMRON PLC series. It can complete the creation, editing, checking, debugging, and monitoring of user programs, and also has comprehensive maintenance functions, making program development and system maintenance simpler and faster. It can run on Windows 2000 or Windows XP platforms and has basic functions such as hardware configuration and parameter settings, communication definition, programming, testing, startup and service, documentation/archiving, and operation/diagnostic functions. All these functions have detailed online help for user mastery. It adopts a structured, multi-tasking programming approach, supporting IEC-compliant programming methods such as ladder diagrams (LAD), statement lists (STL), structured text language, and function blocks (FB). Figure 5 shows the system hardware configuration diagram.
Figure 5 Hardware configuration
2. Supervisory control and control configuration software
There are many popular configuration software programs available, such as INTOUCH, iFIX, WINCC, and KingSCADA. Among them, KingSCADA 6.52 offers high cost-effectiveness and has relatively complete network communication functions, which can well meet the requirements of this system. Therefore, it is used to develop the upper-level monitoring interface (as shown in Figure 6).
Figure 6. Supervisory control screen of the process flow
VI. Conclusion
This paper designs a control scheme based on the analysis of the basic principles of wastewater treatment and applies advanced intelligent control strategies to the wastewater treatment system, thereby improving the production efficiency of the wastewater treatment plant and reducing energy consumption. Research on automated monitoring systems for wastewater treatment can provide scientific methods and theoretical basis for the technological transformation of wastewater treatment plants, and also provide a theoretical foundation and research methods for the development and innovation of wastewater treatment equipment.
