Abstract: Hollysys LK series PLCs were applied to the upgrade and renovation project of Tianjin Jizhuangzi Wastewater Treatment Plant. Based on the process requirements of the wastewater treatment plant, a cost-effective, space-saving, and optimized control scheme was provided. This paper introduces the composition of the control system and the software design concept. The complete hardware and software design of the wastewater treatment plant's automated control system was completed by Beijing Hollysys Automation Drive Technology Co., Ltd. The automated control system consists of a central control room and four PLC field control stations.
Keywords : Hollysys LK series PLC; wastewater treatment plant automatic control system
1 Introduction
The Jizhuangzi Wastewater Treatment Plant is located in the area west of the Weijin River, north of the Jingang Canal, and south of the Jizhuangzi Sewage Discharge Canal in Hexi District, Tianjin, covering an area of 30 hectares. The plant employs a multi-stage AO (Anaerobic/Anaerobic) treatment process. The upgrade and renovation project of the Jizhuangzi Wastewater Treatment Plant is one of Tianjin's wastewater treatment projects, primarily involving the upgrading and renovation of the plant's process equipment, electrical equipment, and automatic control instrumentation. The plant serves the Jizhuangzi drainage system in Tianjin, and the design treatment capacity of the upgrade and renovation project is 450,000 tons/day.
The upgrade and renovation project of Jizhuangzi Wastewater Treatment Plant includes the renovation of the old system's aeration sedimentation tank, the renovation of the old system's primary sedimentation tank's distribution well, the renovation of the old system's primary sedimentation tank, the renovation of the old system's primary sedimentation tank into an anaerobic tank, the renovation of the old system's biological treatment tank, the renovation of the old system's sludge pumping station, the construction of a new contact tank for the old system, the renovation of the old system's chlorination room, the renovation and expansion of the system's primary sedimentation tank into an anaerobic tank, the renovation and expansion of the system's biological treatment tank, the construction of a new and expanded system's contact tank, the renovation and expansion of the system's sludge pumping station, the construction of a new chemical dosing and carbon source addition room, and the addition of a deodorization system. The wastewater and sludge treatment process flow is shown in Figure 1.
Figure 1. Flowchart of wastewater and sludge treatment process
2 Overall Design of Wastewater Treatment Automatic Control System
2.1 System Overall Design
To ensure the safety, reliability, and continuity of production, the two supplementary stations continue to use Omron CS1 series PLCs, while the two new stations adopt a centralized and decentralized automation control system based on Hollysys LK series PLCs. This system monitors, controls, interlocks, alarms, and prints reports for all process parameters, electrical parameters, and equipment operating status across the plant. Through communication between the master station and remote stations, it performs the necessary functions for the entire process, including data acquisition, data communication, sequential control, time control, loop regulation, and supervisory control and management. This not only meets the requirements of the process flow but also ensures safe production and improves production management. The entire system consists of one central control room and four field control stations. The field control stations, composed of PLC systems and monitoring instruments, provide decentralized control for each process step, while the central control room provides centralized management for the entire plant.
2.2 System Network Structure
The network communication structure of this system is characterized by its simplicity, efficiency, and openness. It simplifies the previous five-layer communication structure into a three-layer network with excellent communication capabilities: a management layer, a control layer, and a device layer, as shown in Figure 2. The management layer uses an Ethernet network to realize data communication between the PLC and the host computer, and between the PLC and third-party devices. The control layer uses a Profibus-DP network to handle communication between each controller and the I/O modules.
The device layer can use various networks such as serial bus network, Ethernet network or Profibus-DP network.
The device layer network is used to enable communication between field devices such as switches, instruments, and human-machine interfaces and the PLC.
Local control has the highest level of control authority. When the automatic control system fails to operate normally due to force majeure, local control can ensure the continuity of the wastewater treatment process. Switch the "Local/Remote" knob on the local control box to the "Local" position, and use the "Start/Stop" button on the box to achieve local manual control.
Figure 2. Three-layer architecture of Hollysys LK series PLC control system network
2.3 System Structure and Configuration
Based on the control requirements of the wastewater treatment process, the automated control system of the wastewater treatment project is divided into three levels of management: production management level (central control room), field control level (PLC control station), and local control level. Various field data are acquired through the PLC system and transmitted to the monitoring computer in the central control room via the backbone communication network for centralized monitoring and management. Data communication between the central control room and the PLC control station uses high-speed real-time industrial Ethernet, with a ring network structure, optical fiber as the transmission medium, and a communication rate of 100Mbps. Similarly, control commands from the central control room monitoring computer are also transmitted to the PLC through the same network for distributed control of each unit. The topology and functional configuration of the wastewater treatment plant's automated control system are shown in Figure 3.
Figure 3. Topology and configuration of the automated control system for a wastewater treatment plant.
3. Structure and Configuration of Wastewater Treatment Automatic Control System
3.1 Production Management Level (Central Control Room)
The central control room is equipped with two monitoring workstations, one engineering workstation, two data servers, one video monitoring server, one DLP video wall, one fault printer, one chart printer, a UPS power supply, and one network cabinet. The central control room primarily manages, schedules, centrally operates, monitors, configures system functions, modifies and sets control parameters online, records data, generates and prints reports, generates and prints fault alarms, processes real-time acquired data, performs control operations, and conducts statistical analysis. Through high-resolution LCD displays and the DLP video wall, the real-time operating status of each process segment and the trend of various process parameters can be displayed intuitively and dynamically, allowing operators to keep abreast of the overall plant operation. The central control room is also equipped with the professional industrial database software King Historian. This project requires the storage of over 8,000 historical data points, with a high data storage frequency requirement: ordinary variables require recording data every 30 seconds, while variables with high precision requirements, such as "instantaneous flow rate," require recording every 10 seconds. The stored data includes water quality data, instantaneous flow rate, and cumulative flow rate at each monitoring point; voltage, current, power, and power consumption of various equipment such as booster pumps, gates, sludge pumps, scrapers, mixers, and blowers; start-up and shutdown times, number of start-ups and shutdowns, cumulative operating time, and number of failures for each piece of equipment.
3.2 Field control level (PLC control station or control layer)
The control layer is the key to realizing the automatic control of the system. The PLC in the control layer controls the entire sewage treatment plant equipment to operate automatically according to the process requirements through the program, and realizes the collection of the operating status of the field equipment. Field parameters include pressure, flow rate, temperature, pH value, etc. The collected data is uploaded to the management layer and finally stored through the industrial database software King Historian.
Based on the technological process and the distribution of structures, the plant currently has four sub-control stations (PLC1~4) of different sizes. According to the scope of the engineering renovation, the functions and equipment quantities of the processes and controlled objects, as well as the technological process, layout, and existing control station layout, existing sub-control stations PLC2 and PLC3 will be updated, and existing sub-control stations PLC1 and PLC4 will be modified. Each PLC will be responsible for acquiring process parameters and controlling equipment operation within its respective scope. Details are shown in Table 1.
Table 1 Process control range of each PLC control station
Station Number | Process control range | PLC requirements |
1# Sub-control Station | It is responsible for the automatic control of the sewage pretreatment area, located in the No. 1 substation. The monitoring scope includes the coarse screen and influent pump room, fine screen station, influent metering channel, vortex grit chamber, aerated grit chamber, influent analysis room, No. 1 substation, biological deodorization in the pretreatment area, etc. | PLC1 supplement: 100M Ethernet module, bus module, I/O module—DI=80, DO=24, AI=8, AO=4. |
2# Sub-control Station | The monitoring unit is responsible for the automation of the old series of biological wastewater treatment areas and is located in the blower room control room. The monitoring scope has been adjusted to include the primary sedimentation tank, the newly built old system wastewater lift pump station, the multi-stage AO biological tank with segmented influent, the secondary sedimentation tank, the secondary sedimentation tank collection and distribution well, the return sludge pump station, the newly built old system contact tank, the blower room, and the No. 1 effluent analysis room, etc. | PLC2 Update: CPU, power supply, 100M Ethernet module, bus module, I/O module—DI=1360, DO=392, AI=228, AO=42, adopts Hollysys LK series PLC, supports CPU redundancy, power supply redundancy, fieldbus redundancy and Ethernet redundancy. |
3# Sub-control Station | The automatic control system responsible for expanding the series of biological wastewater treatment areas is located within the No. 3 substation. The monitoring scope has been adjusted to include the primary sedimentation, the newly built and expanded wastewater lift pump station, the multi-stage AO biological tank with segmented influent, the secondary sedimentation tank, the secondary sedimentation tank distribution well, the return sludge pump station, the newly built contact tank, the effluent flow meter well, the No. 2 effluent analysis room, the No. 3 substation, the newly built substation, the newly built chemical dosing and chlorination room, and the newly built carbon source dosing room, etc. | PLC3 Update: CPU, power supply, 100M Ethernet module, bus module, I/O module—DI=880, DO=280, AI=208, AO=48, adopts Hollysys LK series PLC, supports CPU redundancy, power supply redundancy, fieldbus redundancy and Ethernet redundancy. |
4# Sub-control Station | Responsible for the automatic control of the sludge treatment area, located within the No. 2 substation, the monitoring scope includes sludge storage tank, sludge thickening tank, sludge thickening machine room, sludge digestion tank, sludge control room, sludge dewatering machine room, desulfurization tower, biogas holder, biogas combustion flare, boiler room, No. 2 substation, biological deodorization of the sludge treatment area, newly built sludge homogenization tank, newly built sludge dewatering machine room, etc. | PLC4 Supplement: 100M Ethernet module, I/O module—DI=96, DO=8, AI=36, AO=6. |
4. Main functions of wastewater treatment automatic control system
The wastewater treatment automatic control system based on LK redundancy has a high degree of automation, complete system functions, and good operation and performance.
4.1 Access Control
Operators use passwords to access the system: The system provides a tiered user access password system, so lower-level operators can only access basic operation functions, while higher-level operators can access different system configuration functions with different passwords.
It provides 12 levels of passwords: Once an operator provides the correct password, they can change it, but their permissions cannot be changed. The permissions management window is shown in Figure 4.
Figure 4. Permission Management Window
4.2 Flowchart Display
The LCD screen provides an interface for operators to understand the production process status and supports the following types of screens:
Overview screen: Displays the operating status of each device, apparatus, and area in the system, as well as the status of all process parameters and variables, measured values, set values, control modes (manual/automatic), high and low alarms, and other information. Other screens can be accessed from each display block.
Grouping screen: Displays information from several loops simultaneously in a pre-defined grouped format, such as measured values of process parameters, controller setpoints, output control methods, etc., in the form of analog instrument dials. Variables are updated every second, and grouping can be arbitrary. Operators can retrieve detailed information for any variable (analog or discrete) from the grouping screen.
Single-point screen (adjustment screen): Displays all information about a single parameter and control point, as well as real-time and historical trends. The analog loop can also be set and adjusted directly from the adjustment screen.
4.3 Trend Function
Operators can access the real-time curve screen or historical curve screen via menu or button, select the required process parameters to view the real-time curve or historical curve, display multiple real-time curves simultaneously, zoom in or out on the curves, and arbitrarily select the time period to view.
Based on the collected information, various information databases are established, and trend curves (including historical data) are generated for various process parameter values for dispatchers to analyze and compare, so as to find the optimal operating rules of the wastewater treatment plant, analyze the causes of accidents, improve management methods, ensure effluent quality, and improve economic benefits.
Record historical data for all parameters required for production, with a recording period of no less than three years, and this can be arbitrarily set according to requirements. Important data is stored online. Historical data can be retrieved through historical reports or historical trend curves.
The historical trend monitoring screen is shown in Figure 5:
Figure 5 Historical Trend Monitoring
4.4 Alarm Function
Alarms are prioritized, and all prioritized alarms are displayed on the screen, showing a list of all alarms and their detailed information. An alarm bar should be placed at the top or bottom of the monitoring computer's display screen. This bar displays the details of the three most recent, non-disappearing, and unacknowledged alarms. Operators can select alarm tables via screen switching; these tables display a list of all alarms configured in the central control station computer, along with their detailed information. The detailed alarm information includes:
The current status of the alarm;
The time of occurrence of each "not disappeared" alarm;
The time of occurrence and the time of disappearance for each "disappeared" alarm;
The confirmation time and user for each "confirmed" alarm;
When an alarm occurs, the alarm printer immediately prints the alarm information. The system alarm log is shown in Figure 6.
Figure 6 System alarm log
4.5 Operation Log
Significant system incidents (such as equipment failures, over-limit alarms, start-up and shutdown of large equipment, etc.) or operator actions on equipment and parameter adjustments can be recorded through the event logging function. These incidents and operational processes serve as the basis for future accident analysis. The event log includes various event information, the user at the time of the event, and the time of the event.
4.6 Report Printing
The system automatically records various process operation data, summarizes all data into reports, and reports can be printed on a schedule or by default. Operators can also access the report screen through menus or buttons to view historical reports.
Automatically generated production reports (shift/day/month) are used for production management, with six months' worth of information stored internally. Utilizing DDE technology, users can directly create reports using Excel. Leveraging Excel's powerful features, users can create a wide variety of reports, including both real-time and historical data reports.
5. Application characteristics of wastewater treatment automatic control systems
The wastewater treatment system based on the LK series PLC has the following characteristics:
Availability and reliability
• The system components are designed to meet true industrial standards, and the control system can operate stably and reliably in harsh industrial environments for extended periods, minimizing the risk of control system failure, ensuring energy efficiency, and guaranteeing uninterrupted 24/7 water supply to the water treatment plant.
The LK redundancy scheme ensures highly reliable operation of the process. If the main CPU module fails, the system will automatically, quickly, stably, and seamlessly switch to the backup CPU module, avoiding negative effects and production interruptions caused by control layer problems.
PLC control stations are typically located in environments with high electromagnetic interference, such as booster pump rooms, blower rooms, and substations. Our control system has strong electromagnetic compatibility and employs various isolation and anti-interference designs to ensure stable operation in environments with strong electromagnetic interference.
The distance between the PLC control station and the remote I/O stations is relatively long. In this system, the PLC blower room control station and the four remote I/O stations in the biological treatment tanks communicate via a DP fiber optic network. The use of fiber optic communication significantly improves communication reliability. LK's DP fiber optic communication module enables four levels of redundant communication between the PLC control station and the remote I/O stations, with each level providing a communication distance of up to 5km, fully meeting the long-distance communication requirements of large-scale wastewater treatment plants.
• Water treatment environments can become contaminated, accelerating the aging of electronic components in equipment. We can provide modules with special protective processes (protective coatings) to extend the service life of components in corrosive chemicals and environments, prevent unexpected downtime, and reduce maintenance costs.
Energy conservation and consumption reduction plan
• Electricity costs account for one-third of the total operating costs of water treatment facilities. The main power-consuming equipment are water pumps and fans. We can provide optimized frequency conversion control solutions, which can reduce the power loss to 50% to 80% of the rated value.
· For non-frequency conversion motor equipment, such as bar screens, screw conveyors, and agitators, we can provide a process optimization control library, such as control that combines timing control with set parameters, so that these devices can achieve maximum efficiency and minimize power loss under the corresponding process conditions.
² At the Supervisory Control and Data Acquisition (SCADA) layer, tools such as data analysis can be used to create reports and analyze process data, thereby optimizing and improving the process and ultimately reducing energy consumption.
Openness and compatibility
'Communication networks based on standard protocols can easily connect third-party devices, such as sludge dewatering machines, chlorination and dosing systems, and power distribution systems. This can significantly improve the control capabilities for complex processes, allow for the transmission of a large amount of process feedback information, support equipment parameter access functions, and improve the system's performance and diagnostic capabilities.'
• It supports communication with various upper-level monitoring software (such as iFIX, INTOUCH, KingSCADA, MCGS, ForceControl, etc.) and mainstream brand touch screens (such as Pro-Face, HITECH, eView, Weinview, nTouch). As long as the HMI supports the standard communication protocol, it can be easily connected.
• Powerful expansion capabilities reserve interfaces and capacity for future expansion, upgrades, and modifications.
Process diagnostics and online maintenance
The powerful diagnostic functions provided by '²' enable convenient and efficient access to relevant information, identification of the root causes of malfunctions, and process parameters that need to be corrected. This allows for the early detection of problems in the production process, preventing unexpected equipment downtime and reducing operating costs.
• All modules support hot-swapping, allowing faulty modules to be replaced while the system is running, preventing the entire system from shutting down due to the replacement of a single module, thus greatly reducing system maintenance costs.
• The anti-misfit design prevents unnecessary damage caused by incorrect module insertion, ensuring smooth system operation. When replacing a module, simply replace the module directly without rewiring, making maintenance more convenient.
• Supports SD card functionality, allowing for system recovery and upgrades when a computer is unavailable, making maintenance more convenient and faster.
• The operator station is equipped with the host computer monitoring configuration software and the LK programming software PowerPro V4, which has powerful functions and can perform host computer configuration and PLC programming in a convenient and intuitive way. It also supports online download, online modification and offline simulation debugging.
6. Conclusion
This paper discusses the design and implementation of an automatic control system for a wastewater treatment plant based on the LK series PLC. It fully leverages the advantages of the LK programmable controller, such as flexible configuration, reliable control, strong openness, and convenient online maintenance, providing a reliable guarantee for the safe and stable operation of the entire system. The LK-based automatic control system has been practically applied in several wastewater treatment plants, bringing considerable economic and social benefits to the enterprises.
References
[1] Tao Junjie, Urban Wastewater Treatment Technology and Engineering Examples [M], Chemical Industry Press, 2005
[2] Xia Changbin, Luo Bin, Yin Qide, Wastewater Treatment Mechanization and Automation [M], Chemical Industry Press, 2008
[3] Hu Shousong, Principles of Automatic Control [M], Science Press, 2009
