Application of PLC-based DCS control system in sewage treatment plants

With the rapid development of national economic construction, governments at all levels have paid more attention to environmental protection, and a large number of new sewage treatment plants have been built in various places. These newly built sewage treatment plants have put forward new requirements for the performance and price of the control system according to their own characteristics.
Chongqing Fengjie Sewage Treatment Plant uses CASS process to treat domestic sewage, with a daily treatment capacity of 30,000 tons. Its DCS system uses cost-effective PLC as the control unit and commercial computers as the monitoring station. The entire system effectively reduces the system cost while realizing production automation, and has been operating well since it was put into production in 2004.
1 Introduction to production process
Sewage treatment is a continuous biochemical reaction process, with many different treatment processes such as oxidation ditch process, AO, SBR, CASS, etc. The sewage treatment process adopted by Fengjie Sewage Treatment Plant is a new improved CASS (Cyclic Activated Sludge System, cyclic activated sludge method) process, which is a kind of improved SBR process, consisting of a screen well, CASS tank, blower room, dosing room, sludge storage tank, dewatering room, and contact disinfection tank. The process has the following characteristics:
(1) The liftable decanter minimizes the disturbance of the drainage water flow to the bottom sediment sludge.
(2) It has strong impact resistance, high removal efficiency for difficult-to-degrade organic matter, and has the function of nitrogen and phosphorus removal.
(3) The process flow is short, the footprint is small, the construction cost is low, and the operating cost is low.
(4) The management is simple, the operation is reliable, the effluent water quality is good, and there is no odor.
(5) The sludge output is low, the sludge properties are stable, and the sludge does not expand.
2 DCS control system selection and hardware configuration
2.1 DCS system selection
Chongqing Fengjie Kouqian Sewage Treatment Plant is the first batch of sewage treatment plants built by the Three Gorges Reservoir National Debt Project. According to the actual process requirements and construction characteristics, the project puts forward the following requirements for the control system:
(1) The number of control points of the controlled equipment is within 800 points. The control process is relatively complex, but the algorithm we need is not complex, and it is basically based on time control and position control.
(2) The control system must be mature and reliable, and easy to debug and maintain.
(3) Considering that there will be third-party products, network communication and its protocols must be open and standardized.
(4) Both the measuring equipment and the controlled equipment are traditional equipment, and basically do not have fieldbus communication capabilities, so the input and output control is still based on traditional I/O points.
(5) As a national debt investment project, the cost should be reduced as much as possible while meeting the requirements.
After comparison, the factory's automation control system adopts a DCS control system based on PLC. The system has achieved a good balance between performance and price on the basis of meeting production requirements.
2.2 PLC-based DCS system
The traditional DCS was developed for the instrument control system of the process industry, and its main function is to realize the monitoring and adjustment of continuous physical quantities. PLC was developed for the traditional relay control system, and its main function is to realize the logical control of the switching quantity.
Generally speaking, PLC is a local controller, but with the expansion of the application scale and the emergence of industrial Ethernet, multiple PLCs can be interconnected to form a larger control system. Compared with a single PLC, there are several major changes:
(1) An online general-purpose computer is connected to the network, which is used to configure, program and download the system and monitor the status of the controlled process online. In this way, a DCS prototype with a field control layer and a coordinated control layer has emerged.
(2) Analog I/O interfaces and numerical calculation functions are added to the PLC, so that the PLC can not only complete logical control, but also complete analog monitoring and control and mixed control functions.
(3) More and more PLC manufacturers have changed their dedicated networks into general networks, which makes it possible for PLCs to be integrated with various other computer systems and devices to form a large control system.
The above changes make the system composed of PLCs have the form of a DCS. Since PLC products have been on the market for many years, their I/O interfaces, programming methods, and network communications have tended to be standardized and adapted to the requirements of open systems. At the same time, field bus communication functions can be added through expansion. Coupled with the price advantage of PLC, PLC has a very important position in the field of distributed control systems and has considerable competitive advantages in many application fields.
2.3 Configuration and composition of DCS system in Fengjie sewage treatment plant
The DCS control system is based on the open network structure of industrial Ethernet, which consists of three lower control stations PLC0, PLC1, and PLC2, one upper engineer station, and one upper operator station. Its topology is shown in Figure 1.

The PLC0 control station is installed in the large-screen mosaic simulation display screen of the central management center. It collects the status signals of each device through Ethernet and communicates with the mosaic screen using RS232 communication to display the entire production process and equipment status.

PLC1 is installed in the substation, responsible for controlling the grid well, sewage lifting pump, sand settling tank, No. 1 CASS pool, No. 2 CASS pool, No. 3 CASS pool, blower room, mud storage tank and flow detection of flow well, partial detection and control of contact disinfection tank, and partial power parameter monitoring of the distribution room.
PLC2 is installed in the dosing room. It is used to control the full set of dosing equipment including solenoid valves, stirrers, and diaphragm pumps in the dosing room.
The two monitoring computers in the central management center use DELL's OptiPlex series desktop computers. The OptiPlex series desktop computers are positioned for primary server applications. Under the premise of high performance, their specially designed steel wire shielding layer structure and heat dissipation system ensure the high reliability and stability of the system. The monitoring software used is based on Microsoft's Windows 2000 platform. The compatibility of commercial machines with the operating system is slightly better than that of industrial computers.
The system information exchange layer and control layer of the entire control of the plant are integrated. It adopts advanced industrial Ethernet, which has the characteristics of high speed and reliability. Industrial Ethernet is based on traditional Ethernet. It is an improved information and control network for industrial control requirements. It has the characteristics of one network to the end and reduced network cost.
3 Automatic control system functions
3.1 Monitoring software
The monitoring software uses Kaiwu 2000 software, which is a general monitoring software developed by Beijing Huafu Huitong. The software has good support for OPC communication. OPC (OLE for ProcessControl1) is an object-oriented software protocol based on COM (Component Object Mode1) and DCOM (Distributed Component Object Mode1) technology. OPC provides an open and consistent interface specification between field equipment, automatic control application software and enterprise management application software, and provides "plug and play" connection for software and hardware from different suppliers.
The communication protocol between the monitoring software and DCS hardware of this system adopts OPC mode. ROCKWELL's RSLINX software is the OPC SEVER end and the monitoring software reads data as the OPC CLIENT. The communication method avoids the situation where the monitoring software does not support the DCS system driver program.
3.2 Control mode X
In order to facilitate operation and debugging and emergency handling of accidents, the system control mode is divided into three types:
(1) Local manual mode. That is, the device is operated by the buttons and switches on the local control box or MCC.
(2) Remote control mode. The equipment is controlled in manual mode through two computers in the central management center.
(3) Automatic mode. The system automatically completes the start, stop and adjustment control of the equipment according to the working conditions.
The switching between remote control and local control mode is completed by the MCC cabinet or local control box; the switching between automatic mode and remote control mode is completed by two monitoring computers.
3.3 Monitoring function
The two computers are used as the operation station and the engineer station for mutual backup. They are set up in the central management center to centrally monitor the operating status and operating parameters of the process equipment in the whole plant and remotely control the field equipment. The monitoring computer exchanges data with the PLC system through the industrial Ethernet through the integrated 1000M network card.
Main functions:
(1) Process flow monitoring function. The system can monitor the parameters and equipment status of each link of sewage treatment according to the process requirements, and select automatic, sequential, timing and other control methods according to the process requirements.
(2) Alarm and alarm recording function. When an accident occurs in the equipment, the system will give an alarm indication on the computer, mosaic large screen and local control box. At the same time, the computer will record the alarm content, time and confirmation time. Voice alarm can also be performed on the computer.
(3) Interlock protection function. When the system detects a local fault, the corresponding interlock protection program is started.
(4) Parameter setting function. The upper and lower limits of alarms, adjustment parameters, operating time and other parameters can be set on any monitoring computer in the central management center or the human-machine interface of the field control station.
(5) Data recording and storage function. The system can record and store important data such as process parameters, working conditions, equipment operating time, etc. for future use.
(6) Operation record function. The system automatically saves important operation records, such as parameter changes, operator code, time, content, etc. of the equipment operation.
(7) Real-time data curve and historical data curve. Real-time curve display of important process parameters can be performed, and historical data curves can be recorded.
(8) Multi-level password protection function. Different operation permissions can be set on any monitoring computer in the central management center. Only operators with corresponding operation permissions can access the screen at that level after entering the correct password.
(9) Printing function. Report printing, curve printing and graphic printing can be performed.
4 Project Effects
The project was implemented in May 2003, and the equipment was handed over to the factory for operation in April 2004. Its DCS system operates stably and reliably without damage. After the system was put into use, the operators in the central management center can fully understand the operation of the entire factory. The DCS system can automatically control the main equipment such as pumps and decanters according to pre-set parameters. The entire system only requires operators from the central management center to ensure the normal operation of the system, which greatly reduces the labor intensity of the duty personnel. The commissioning of this system solved the problems of low automation in the previous sewage treatment plants, manual operation of all equipment, high labor intensity of duty personnel, easy operation errors, unstable effluent quality, and inability to centrally monitor the production process. As the first batch of sewage treatment plants built in the Three Gorges Reservoir area, the plant has played a positive role in protecting the water quality of the Three Gorges Reservoir area. Government leaders at all levels have inspected and guided the plant many times, and gave good evaluations on the production and process level of the plant, achieving good social and economic benefits.
5 Conclusion
As a mature controller, PLC has a unified programming language and system structure, which is convenient for debugging and maintenance of the control system. Although the PLC control algorithm is relatively simple, it meets the requirements of sewage production process very well, and reduces the system cost while avoiding functional waste. The use of industrial Ethernet technology ensures good compatibility and stability of the communication network, and at the same time achieves high-speed communication. The DCS control system built with industrial Ethernet based on PLC is mature and reliable in the automation control of sewage treatment plants. It not only meets the production process requirements, but also reduces the project cost, and has a good application prospect.