Application of Automation Control System Based on LK PLC in Sewage Treatment Plant
Yangyang Guo, Weiyang LU，Qiang LI，Rong LIU
(1.University of Science and Technology Beijing, Beijing 100083, China；
2. Beijing HollySys Automation and Drive Co. Ltd., Beijing 100083, China；)
Abstract: This paper mainly introduces the automation system structure, 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 four 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 world has entered the 21st century, the population of China has been growing fast. At the same time, there is growing demand for water, which has become one of the main contradictions of sustainable urban development. Therefore, to address the issues of urban water shortage and deteriorating water environment brooks no delay. With automation technology growing fast in variety of fields, there is no expectation to sewage treatment fields. At present, the mode of utilizing PLC to carry out automatic control and upper computer to monitor and set parameters is normally adopted [1]. Basing on Haixing Sewage Treatment Plant, this paper mainly introduces the structure and function of control system.
The waste water treatment capacity of Haixing Sewage Treatment Plant is designed to be up to and the effluent can comply with the Class A of integrated wastewater discharge standard. Cyclic activated sludge system (CASS) and Advanced Treatment Process are mainly used in this plant. The whole plant chiefly consists of rude grille and careful grille system, wastewater pumping house system, CASS tank system, biological aerated filter system, V-type filter as well as sludge thickening and dewatering system.
2. The structure design of sewage treatment automation system
The management of control system is composed of production management layer (Central Control Room), field control layer (PLC stations), and local control layer. All the real-time data is collected by PLC and transmitted to upper computer in CCR for surveillance and management via industrial Ethernet. Similarly, commands from upper computer in CCR can reach PLC terminal and implement distributed control.
(1) Production management layer (Central Control Room)
Production management layer in CCR which carry out the function of management of sewage treatment procedure and realization of office automation is the core of control system. Through liquid crystal display with high resolution and projector which can intuitively and dynamically demonstrate the real-time data and process parameters in every process of the whole plant, operating staff could grasp the factory status opportunely.
(2) Field control (PLC stations)
Field control layer is a key component of automation control system. Depending on the technological requirements for automatic control, the program in the controller accomplishes the data collection of all devices working status, process parameters (for instance: pressure, flow, temperature, PH) and implementation of orders from Management layer.
(3) Local control layer
The “Remote control/Local control” button on the control cabinet of device is switched to Local control and operating staff could get devices switch on or off locally through the operation of “Start/Stop” button.
Structure and configuration of control system is depicted as Fig.1.
Fig.1 structure and configuration of control system
3. Field control level (PLC stations)
3.1 Configuration of PLC control system
LK PLC made by Hollysys Company is adopted as the main controller for monitoring and controlling of whole production. The hardware of PLC control system chiefly consists of CPU module, I/O module, communication module, power module, interface module and so forth. The configuration of hardware is based on the functional complexity and the amount of I/O point. According to calculation, its I/O point number is 829 and there are four PLC stations in all. There is only one master station in No.1 station and No.2 station respectively. No.3 station comprises one master station and one I/O station while No.4 station is made up of one master station and two I/O stations. The function of every PLC station is listed as below:
(1) Sludge thickening and dewatering station-No.1 PLC station
This station is in charge of detecting the data of inlet wastewater quality, monitoring the status of grille, whorl stream grit pool, thickening and dewatering devices.
(2) Air blower room station-No.2 PLC station
As No.2 station, it accomplishes to inspect the data of water quality in CASS pool, and monitor the status of agitation system, decanter system, air blower system devices.
(3) Biological aerated filter station-No.3 PLC station
No.3 station includes one master station and one remote I/O station which mainly collect the information of wastewater quality in the filter, and monitor and the status of secondary pumping station, aeration system, backwashing devices and controls these devices.
(4) V-type filter station
The surveillance of the effluent water quality in V-type filters, the status of backwashing devices and control of these devices are carried out by No.4 station with one master station and two remote I/O stations.
Above all, PLC Control system realizes the functions described below: the data exchange and the data processes between master station and slave station, master station monitors the remote I/O stations, the visibility of person machine alternately.
3.2 The design of PLC program
Prowerpro, released by Hollysys Company, is chosen to design the PLC program which contains the subroutine of grille system, wastewater pumping house system, CASS tank system, biological aerated filter system, V-type filter, depending on the technological requirements for automatic control. The process flow chart of sewage treatment is described as Fig 2.
Fig.2 Flow chart of sewage treatment process
(1) Control of grille system
The devices in grille system are grille devices, screw conveyor and ultrasonic liquid level meter. It can be controlled automatically either by level setting or by time setting. When using level control, the program starts or stops the devices according to the liquid level difference of before and after grille. When the liquid level difference is up to the upper limit, PLC will send commands to start corresponding devices. Similarly, grille devices will stop after receiving stop commands from PLC if the liquid level difference comes to lower limit. Operators can set the liquid level for the start/stop of the devices or the running period.
(2) Control of lift pumps
The process requirements for pumps automatic control is to control lift pumps with liquid level in the pool. If one of the life pumps breaks down, program of the faulty pump will be cut out from automatic subroutine, while pump in reserve will cut in automatically. This kind of measure can guarantee the roughly equal running time of every lift pump in the long run.
(3) Control of whorl stream grit pool system
Agitators, Roots Blowers and grit-water separators are main objects for control in this system. The main principle of this system: wastewater flows into grit pool tangentially with certain flow velocity, which can exert centrifugal force on grit to make heavier grit sink along pool wall. The rotation of agitator’ bladders cause axial vortex and slight ascendant flow which could drive wastewater to the next station for deeper treatment. Roots Blowers provide air for grit pool and airlift deliver sands to grit-water separator for complete separation of grit and water [2]. Process control for Agitators, Roots Blowers and grit-water separators Operators is that these devices run at certain period and running time can be set in engineer station.
(4) Control of CASS tank system
The cycle of CASS tank system is divided into four stages. Aeration stage:
Blowers supply the pools with oxygen and organic pollutant is decomposed by microorganism; Sediment stage: microorganism carries out further oxidative decomposition and activated sludge gradually sinks to the bottom and the upper layer of water becomes clean. The sludge reflux pumps send some activated sludge back to pre-reaction area and the excess sludge pumps deliver the excessive sludge to thickening and dewatering room. Decanting stage: at the end of sediment stage, the decanters installed at the end of reaction area start to work with rhythm from top to bottom and drain off the upper layer water. Idle stage: the decanters go up to the original position and prepare decanting for the next period. In accordance with the process mentioned above, make subprogram for each stage.
(5) Control of Biological aerated filter system
The control requirement of blowers is listed as below: there are three blowers in all and every blower works for 24 hours one time; stop the running blower at 12 and start another one, which can guarantee the roughly equal running time of every blower in the long run.
Control of backwashing system is mainly to control backwashing blowers, pumps and valves to realize the function of backwash once every two days.
(6) Control of V-type filter system
Control of V-type filter system is mainly to realize the function of automatic backwashing control. Backwashing blowers, pumps, valves and instruments are main objects for control to implement the function of backwash once every two days.
4. Production management level (Central Control Room)
4.1 configuration of engineer and operator station
Engineer and operator station are designed in CCR and consist entirely of two redundant computers, a projector, a printer for printing malfunction articles, a printer for printing diagrams and a set of UPS device.
One of the two upper computers is for surveillance of control system and could display the condition of all the devices and real-time process value while the other is for recording data on time as well as printing reports.
4.2 The realization of upper computer software
FacView made by Hollysys Company was utilized as upper computer software. It provides operators with vivid, intuitional and real-time pages which display every component of the plant. Operators could make corresponding decisions depending on the data displayed in upper computer software. Distributed and single loop control systems are united by friendly HMI for uniform management. Furthermore, there are data alert, storage of historical data, report display, trend display and other kind of functions.
The monitoring pages on computer roughly includes drawing of whole plant process, grille system, sludge thickening and dewatering system, CASS tank system, biological aerated filter system, V-type filter, instrument data, trend, alarm and report. Every page mentioned above can be switched freely. The flow chart of HaiXing County Sewage Treatment Plant is depicted as Fig.3.
Fig.3 The flow chart of HaiXing County Sewage Treatment Plant
5. Conclusion
Basing on centralized control distributed management, this automation control system separates the management layer and control layer. Through monitor and control of whole process in the plant, the system achieves wholly automatic control of sewage treatment. 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. There is no any doubt that PLC plays crucial role in the system. The control system has been working steadily and reliably since its commissioning and the effluent quality meets the standard of discharge. Such kind of control system does not only improve people’s living environment but also exerts active role in sustainable development of society, which achieves social and economic benefits.
References
[1] Chun Luo，Lishi Ma，Jian Wu，Chaoping Zhou. Present Situation of Domestic City Sewage Treatment and Its Development Trend [J] .National Water Treatment Technology Symposium in 2007. 2007：1
[2] Yaogang Li，Xue-dong Liu，Lan-su Shui. Control of PLC in Whorl Stream Grit Pool System [J] .Mechanical and Electrical Engineering Technology.2003，32 (4):58-59
Authors’ introduction:
Yangyang Guo, male, postgraduate of University of Science and Technology Beijing, major in Automation Control;
Weiyang LU, male, the associate professor of University of Science and Technology Beijing, mainly research in industrial control and automation.
Qiang Li, male, electronic engineer of Beijing Hollysys Automation and Drive Co. Ltd. major in industrial control and automation.
Rong Liu, male, electronic engineer of Beijing Hollysys Automation and Drive Co. Ltd. major in industrial control and management.