Abstract: This paper mainly introduces the architecture of the automatic control system of Haixing County Wastewater Treatment Plant, the configuration of the LK PLC system, and how to use the PLC to achieve automatic control of the entire plant. Hollysys LK PLC is used as the controller. Fiber optic industrial Ethernet communication is used between the central control room and each master station in the plant, while Profibus-DP communication is used between the master stations and remote I/O stations. The automatic control system of the water plant ensures the safe and reliable operation of wastewater production, improves the level of automation control and management of wastewater treatment, and increases production efficiency.
Keywords: wastewater treatment; programmable logic controller (PLC); automatic control system
Application of Automation Control System Based on LK PLC in Sewage Treatment Plant
GUO Yang-yang, LV Wei-yang , LI Qiang , LIU Rong
(University of Science and Technology Beijing, Beijing 100083, China)
Abstract: This paper mainly introduces the automation system structure, LK PLC system configuration and how to utilize PLC to achieve automation control in sewage treatment plant. LK PLC made by Hollysys Company is adopted as the controller. Central control room comminutes with master stations via optical fiber industrial Ethernet, while Profibus-DP is utilized for the communication between master stations and remote I/O stations. The automation control system guarantees the stability of sewage treatment process and improves the level of automation control and management of waste water treatment and enhances productivity and efficiency.
Key words: Sewage Treatment; PLC; Automation Control System
1 Introduction
Since the beginning of the 21st century, China's population has grown rapidly, and the demand for water has increased significantly. This is one of the main contradictions in the sustainable development of Chinese cities. Therefore, it is urgent to solve the problems of urban water shortage and water environment deterioration. With the continuous development of automation technology in various industries, the automation level of the sewage treatment industry is also rapidly improving. At present, the sewage treatment industry mostly adopts the system control mode of PLC controller for automatic control and upper computer for monitoring and setting process parameters [1] . This article takes Haixing County Sewage Treatment Plant as an example to introduce the system composition and functions.
The Haixing County Wastewater Treatment Plant is designed to treat 20,000 tons of wastewater per day, with an effluent standard of Class A. The plant adopts the CASS+ advanced treatment process, and the plant area mainly consists of a screen and grit removal system, a booster pump station, a CASS biological reactor system, an aerated biological filter system, a V-type filter system, and a sludge thickening system.
2 Hardware Design of Wastewater Treatment Control System
2.1 Overall Structure of the Control System
The wastewater treatment plant's automated control system is divided into three levels of management: production management (central control room), field control (PLC control station), and local control. Various field data are collected by the PLC system and transmitted to the central control room monitoring computer for centralized monitoring and management via the backbone communication network—industrial Ethernet. Similarly, control commands from the central control room monitoring computer are also transmitted to the PLC's measurement and control terminal through the same channels, enabling distributed control of each unit.
(1) Production Management Level (Central Control Room)
The central control room management layer is the core of the system, responsible for managing and controlling all parts of the wastewater treatment process and achieving plant-level office automation. High-resolution LCD displays and projectors provide a clear and dynamic view of the real-time operating status of each process segment and the trends of various process parameters, allowing operators to stay informed about the overall plant operation.
(2) Field control level (PLC station)
The control layer is crucial for achieving automatic system control. According to the requirements of automatic control processes, the PLC in the control layer controls the equipment throughout the wastewater treatment plant through programs, collecting data on the operating status and parameters of the field equipment (such as pressure, flow rate, temperature, and pH value), and executing commands from the management layer.
(3) Local control level (equipment layer)
Switch the "Local/Remote" knob on the field control box to the "Local" position, and use the "Start/Stop" button on the box to control the equipment locally.
The topology and functional configuration of the control system of Haixing County Wastewater Treatment Plant are shown in Figure 1:
Figure 1. Topology and configuration of the control system
Fig.1 structure and configuration of control system
2.2 Lower-level PLC system configuration
The automatic control system of Haixing County Wastewater Treatment Plant uses LK series PLCs from Beijing Hollysys as the main controller to monitor and control the production process. The hardware of the modular PLC control system mainly includes a CPU module, I/O modules, communication modules, power supply modules, and interface modules, configured according to the complexity of the control functions and the number of control points. Statistics show that the entire wastewater treatment plant requires 829 I/O points for PLC control, divided into four PLC control stations: Station 1 has only one master station; Station 2 has only one master station; Station 3 includes a master station and one remote slave station; and Station 4 includes a master station and two remote slave stations. The functions of each control station are as follows:
(1) Sludge dewatering machine room workstation - PLC control station #1
Responsible for collecting water quality data of the water plant's influent, as well as the status data and control of the equipment in the bar screen system, vortex grit removal system, and sludge thickening system.
(2) Blower Room Workstation - PLC Control Station #2
Responsible for collecting water quality data from the CASS pool, as well as collecting and controlling the status of the mixing system, decanting system, and aeration system.
(3) Aerated biological filter workstation - PLC control station #3
Responsible for collecting water quality data from aerated biological filters, as well as collecting and controlling the status of equipment in the secondary lifting system, aeration system, and backwashing system.
(4) V-type filter workstation - PLC control station #4
Responsible for collecting water quality data from the V-type filter, as well as collecting and controlling the status of the backwashing system and effluent system equipment.
In summary, the PLC control system enables data exchange and processing between the master station and slave stations, and provides the master station with visibility for monitoring each slave station and human-machine interaction.
2.3 Hardware Configuration of the Supervisory Control System
The engineer and operator stations are located in the central control room, which mainly consists of two redundant operator stations, a projector, a faulty printer, a chart printer, and a UPS power supply.
The central control room has two monitoring and operation stations. One is a system monitoring and management computer, which can detect and monitor the status of various equipment and process parameters in the sewage treatment plant in real time, and provide a human-machine interface for operators and managers to manage the operation. The other is an information monitoring and management computer, which is responsible for real-time and timed recording, as well as the generation and printing of reports.
3. Software Design of Wastewater Treatment Control System
3.1 Development of Lower-Level PLC Control Program
The PLC program was designed using Powerpro lower-level machine software from Beijing Hollysys Co., Ltd. Subroutines were written according to process requirements for tasks such as bar screen, booster pump station, vortex grit chamber, CASS tank, secondary booster pump, aerated biological filter, and V-type filter. The wastewater treatment process flow diagram is shown in Figure 2.
Figure 2 Wastewater treatment process flow diagram
Fig.2 Flow chart of sewage treatment process
(1) Grille system control
The main control components of the bar screen system are the bar screen unit, screw conveyor, and ultrasonic level gauge. Control can be configured on a monitoring computer using level control and timer control. When level control is used, the start and stop of the bar screen are controlled by the level difference before and after the screen. When the level difference reaches the set upper limit, the PLC controller sends a command to start the bar screen; when the level difference is below the set lower limit, the bar screen unit receives a stop signal from the PLC controller. Operators can set the start/stop levels or operating cycles of the equipment on the host computer.
(2) Booster Pump Control
The control process for the booster pumps requires automatic start-up and shutdown based on the liquid level. The project site employs a two-in-one-standby configuration. When one pump fails, it automatically switches out of the automatic control program, and the standby pump automatically switches in. This ensures that the pumps' operating time is approximately the same over the long term.
(3) Control of vortex grit chamber system
The main control objects of the cyclone grit settling system are the agitator, the Roots blower and the sand-water separator. The working principle of the system is as follows: the sewage enters the grit settling tank tangentially and has a certain flow velocity, which generates centrifugal force on the sand and gravel, causing the heavier sand and gravel to settle along the tank wall to the bottom sand collection trough. The blades of the agitator rotate to form an axial vortex, which generates a slight upward flow, thereby driving the sewage out and flowing into the next process flow for treatment. The Roots blower provides air to the cyclone grit settling tank to achieve the function of air lifting. In addition, the air lifting directly transports the settled sand to the sand-water separator to achieve complete separation of sand and gravel from sewage [2] . Its control process requirements are as follows: the agitator, blower and sand-water separator operate at a certain cycle, and the operating time can be set through the engineering station.
(4) CASS Pool System Control
The CASS (Continuous Adsorption and Retention) system operation cycle consists of four steps: Aeration stage: Blowers supply oxygen to the reaction tank, where organic pollutants are oxidized and decomposed by microorganisms; Sedimentation stage: Microorganisms utilize the remaining dissolved oxygen (DO) in the water for further oxidation and decomposition, activated sludge gradually settles to the bottom, the supernatant becomes clear, and a sludge return pump sends some activated sludge back to the pre-reaction zone, while the excess sludge pump pumps the excess sludge from the reaction tank to the sludge dewatering room; Decanting stage: After sedimentation, the decanter at the end of the reaction tank begins operation, gradually discharging the supernatant from top to bottom; Idle stage: The decanter rises to its original position, awaiting the next decanting cycle. Based on the above process requirements, control subroutines are written for each stage of the CASS process.
(5) Control of aerated biological filter system
The control process requirements for aeration blowers are as follows: the aeration blowers should operate 24 hours a day, with one blower being replaced at noon every day, so that the three blowers can operate for roughly the same amount of time.
The backwashing system control mainly involves controlling the backwash fan, backwash pump, and valves to achieve the backwashing function, which is performed every two days.
(6) Control of V-type filter system
The automatic control of the V-type filter system mainly involves the automatic backwashing function of the filter. The main equipment controlled by the subroutine includes the backwash pump, backwash fan, valves, and instrumentation process parameters, and backwashing is performed every two days.
3.2 Implementation of the host computer monitoring system
The supervisory control system (SCADA) of this system utilizes FacView software from Beijing Hollysys. This software displays the operational status of each subsystem in a clear, intuitive, and real-time manner on the industrial computer in the central control room, enabling operators to obtain real-time field data and information and manage the wastewater treatment plant's operation. The user-friendly interface unifies the management of the distributed, single-loop measurement and control systems, and also includes functions such as data alarms, historical data storage, report display, and trend display.
The computer monitoring screen mainly includes the overall plant process diagram, bar screen and grit removal system, CASS process, aerated biological filter, V-type filter, instrument data charts, trend charts, alarm charts, and reports. Users can freely switch between these screens. The overall plant process diagram is shown in Figure 3.
Figure 3. Process diagram of the entire wastewater treatment plant in Haixing County
Fig.3 The flow chart of HaiXing County Sewage Treatment Plant
4 Conclusion
This automated control system employs a centralized control and decentralized management approach, separating the management and control layers. Through monitoring the entire process, it achieves fully automated operation of the wastewater treatment process, ensuring the safety and reliability of wastewater production and operation. This significantly improves the level of automation and management in wastewater treatment, reduces labor intensity, thereby increasing production efficiency and lowering energy consumption at the water plant. The PLC controller plays a crucial role in this process. Since its commissioning, the control system has operated smoothly, and the treated water quality meets discharge standards. This has not only improved people's living environment but also played a positive role in sustainable social development, achieving both social and economic benefits.
References
[1] Luo Chun, Ma Lishi, Wu Jian, Zhou Chaoping. On the current status and development trend of urban sewage treatment in China. Proceedings of the 2007 National Water Treatment Technology Symposium. 2007: 1
[2] Li Yaogang, Liu Xuedong, Shui Lansu. PLC control of cyclone grit chamber [J]. Mechanical and Electrical Engineering Technology, 2003, 32(4):58-59.
About the authors: Guo Yangyang (1987-), male, postgraduate student at Beijing University of Science and Technology, specializing in automation control; Lü Weiyang (1971-), male, associate professor at the School of Mechanical Engineering, Beijing University of Science and Technology, whose main research areas are industrial control and automation; Li Qiang (1982-), male, intermediate engineer at Beijing Hollysys Automation Drive Technology Co., Ltd., whose main work involves industrial control and instrumentation automation; Liu Rong, male, intermediate engineer at Beijing Hollysys Automation Drive Technology Co., Ltd., whose main work involves industrial automation and departmental management.
