and development trend
1 a sewage treatment plant automatic control system status overview
A certain city of a sewage treatment plant automatic control system is through the use of automation technology, computer technology, network technology, graphic technology, etc. Composition of a comprehensive automation system is to ensure that meet the technical requirements and sewage treatment process quality and reliable operation, discharge standards target under the premise of the sewage treatment plant management, scheduling, on-site control and other functions integrated into the network environment through PLC and network technology. Under the premise of ensuring that the specified technical requirements are met and the quality and reliability of the sewage treatment process are achieved, and that the emission standards are met, it integrates the functions of management, scheduling and on-site control of the sewage treatment plant in a network environment, and builds an information platform for realizing the integration of control and integrated comprehensive information processing of the sewage treatment process by means of PLC and network technology. According to the actual situation of the sewage treatment plant and process requirements, the automatic control system of the sewage plant adopts a system mode combining centralized control and field bus, consisting of management level and field control two-level control system, the management level and control level communicates through 10/100M Ethernet, i.e., the automatic control system is a two-level centralized control system consisting of the computer of the central control room and each PLC control unit at the field level ( DCS). The DCS is a high-tech control device integrating automatic control technology, computer technology, communication technology and CRT display technology, and it is a new control system at the forefront of new technology for production management, data acquisition and various process control. Through the communication network will be the central level monitoring station and a number of field control station connected to form a centralized management, decentralized control of computer measurement and control management system, referred to as the centralized control system. DCS system to overcome the centralized control system danger of centralized, poor reliability, the system is not easy to expand, the control of the cable with a large amount of shortcomings, etc., to achieve the real information, management and scheduling of the centralization of the function and danger of decentralization. If the computer in the control room fails, the field substations can still work independently and stably, which fundamentally improves the reliability of the system. A sewage treatment plant auto-control system hierarchy see Figure 1, auto-control system composition see Figure 2.
1.1 Field control layer
Field control layer by the field level of each PLC control unit and field measurement and control instrumentation and control equipment. The control level consists of four field master stations: No.1 field PLC station, No.2 field PLC station, No.3 field PLC station and No.4 field PLC station. The management level adopts industrial control machine, and this functional level realizes the presetting of all process parameters of each process section of the sewage treatment plant, data acquisition of equipment operation status and electrical parameters, and control of equipment through PIC. And through the industrial Ethernet to the central control layer to transmit data and accept its control instructions. The system realizes the control and protection of process parameters and electrical equipment of the main production segments such as coarse/fine grating, lifting pumping station, sand sedimentation tank, anaerobic tank, oxidation ditch, dewatering machine room, etc. in this layer to ensure the safe, stable, reasonable and efficient operation of the production process. According to the requirements of process control, the parameters such as differential pressure before and after grating, pump pool level, dissolved oxygen concentration in anaerobic pool and oxidation ditch, PH value, inlet and outlet water flow rate, and mud storage pool level are monitored and controlled at the same time. Each PLC station functions as follows:
1) Pre-treatment section control station PLC1. The PLC workstation is located in the control room of the influent water lifting pump room in the plant, and is responsible for monitoring and controlling the pre-treatment section of the sewage treatment plant. Its main control object for the coarse grating and inlet electric gate in the coarse grating room, sewage pumps in the influent pump room, sand sedimentation tank sand discharge device and sand-water separation and other equipment, in addition, is responsible for the influent water quantity, water quality, such as COD, pH, SS (turbidity measurements), and other parameters of the on-line detection.
The main equipment control mode is as follows:
Coarse grating and fine grating: according to the time interval PLC automatically control the grating rake to remove the slag, and at the same time when the water level before and after the grating exceeds the given value, PLC can also automatically control the grating rake to remove the slag. And grating machine, screw conveyor to linkage operation, the starting order of the equipment for the first start screw machine, then start the grating machine. Shutdown should also be linked, the order is the opposite of the start. When the conveyor has a fault, fine grating stop running.
Into the pump room: into the pump room with three submersible pumps two with a backup, level meter two, and set the level switch. PLC according to the pump pool water level automatically control the number of pumps running, and according to the running time of each pump, automatically rotate the running pumps, so that the pump running time is equal. There are upper and lower limit alarms to prevent the pump from running dry. Programming pump operation scheduling to follow the following principles: to ensure that the amount of water and pumping the same amount of water, that is, how much to pump how much; to keep the pumping pool running at high water level, so as to reduce the pump's working head, in order to ensure that the pumping volume under the premise of reducing power consumption; pumps can not be too frequent; to ensure that the number of times the pumps are put into operation and the basic running time is equal.
Cyclonic sand sedimentation tank: including two cyclonic sand sedimentation system, blower, sand-water separator and supporting equipment according to the period set by the operator intermittent linkage operation, any one equipment failure, should be alarmed and shut down its linkage equipment. In the automatic working mode, each equipment controls the starting and stopping of each motor and the switching of each solenoid valve according to the pre-programmed program of PLC.
2) biological treatment system/distribution center station PLC2. The workstation is generally located in the plant-wide distribution center control room, responsible for monitoring the sewage biological treatment section. Its main control objects are underwater agitators, underwater propellers and aeration equipment in the biological tank, sludge return pumps, residual sludge pumps in the sludge return pump room, and scraper suction machine in the secondary sedimentation tank. In addition, it is also responsible for the online detection of DO, ORP, MLSS of the biological tank; pH, MLSS of the sludge pumping room, and electrical parameters such as current, voltage, active power, reactive power, active power, reactive power and other parameters of the distribution center.
The main equipment control is as follows:
Control of return sludge pump and residual sludge pump: The task of return sludge volume regulation is to ensure that the concentration of mixed liquid in the biochemical treatment system is maintained within a certain range. The regulated volume is the amount of activated sludge flowing back into the anaerobic tank. The electromagnetic flow meter is installed on the return sludge official channel. The return sludge volume adjustment is realized by running the number of return sludge pumps, which is adjusted according to the proportion of influent flow, and the return ratio can be preset on the PLC or set on the computer in the central control room. The residual sludge pumps are operated according to the following principles: A. Automatically operate at time intervals; B. The residual sludge pumps operate when the sludge buffer tank is at low level; C. The pumps are stopped when the sludge buffer tank is at high level; D. The pumps and valves are controlled by the intermodal operation.
Oxidation ditch: two anaerobic tanks are equipped with six stirrers, stirrers run continuously. Two oxidation ditch were installed in the outer ditch 8 aerators, the ditch and the inner ditch installed 4 aerators. At the same time, respectively, in the outer, middle and inner ditch with a dissolved oxygen meter, an ORP meter, the middle ditch with a turbidity meter. According to the oxidation ditch in the dissolved oxygen meter to monitor the oxygen content of sewage, control the number of aerator operation to change the amount of oxygen, which can save energy.
3) sewage disinfection system / pumping station PLC 3. The PLC workstation is located in the pumping room control room. Its main control object for the water lifting pumps, switching well electric valve and chlorination disinfection equipment, in addition to its water quality is also responsible for such as: residual chlorine, COD, flow and other parameters of the on-line detection.
4) Sludge treatment system / dewatering workshop PLC4, the PLC workstation is generally located in the dewatering workshop distribution room control room, responsible for monitoring the sludge treatment section. Its main control objects are the agitators and electric valves of the sludge storage tank, and the equipment of the dewatering workshop such as the sludge feed pump, dewatering machine, thickener and dosing system.
The main equipments are controlled as follows:
Sludge storage tank: the agitator of the sludge storage tank operates continuously and can be operated remotely, and it is equipped with high and low level alarms (0.5 meters can be set), and the level alarm limit can be set on the upper computer (4.5 meters can be set).
Sludge dewatering machine room: the dosing system is mainly manually braked, and the dosing metering pump can be turned on at any time when there is no alarm for the low level of the dosing tank. The operation signal of the dosing system is sent to PLC, the dewatering machine system internal correction, flushing is completed by the on-site control box, PLC only gives the dewatering machine start, stop command, and complete the linkage with other related equipment. The start-up sequence of the dewatering system is as follows: start the belt conveyor first, then start the dewatering system, then start the dosing system, and finally turn on the sludge screw pump, and the shutdown sequence is reversed. When a device fails during operation or the buffer level reaches the set low level, the device will shut down according to the shutdown sequence, and the monitoring and management computer can remotely control the above devices.
Additionally, there is a communication module in the layer, also called communication management unit. Communication management unit is the middle layer of the automatic control system, responsible for the collection and forwarding of information of the whole control system; the communication management machine will collect and organize the data of PLC, meter and other automatic control system, and then transmit them to the backstage system through fiber optic, and at the same time, it can forward all kinds of control commands issued by the backstage to the corresponding units.
At present, most of the communication management unit network system of the DCS system of the sewage plant is optical brazing as a transmission medium, that is, between the central control room and a number of on-site control stations in the plant with a redundant 100Mbps fiber-optic industrial Ethernet to form a wired data communication network.
1.2 Central Control Layer
1) This layer is also called the backstage monitoring system layer, is the information display and control center of the system, which consists of two monitoring and management computers, color CRTs and two printers and other equipment hooked up to the industrial Ethernet as the operating station. Monitoring and management computer system through the l0/100M network to collect the sewage treatment plant process parameters, electrical parameters and the main equipment operating status information, analysis of various data, processing and storage, various types of process parameters to make the trend curve, to complete the centralized control of the sewage treatment plant process segments, detection functions, through a simple operation, can be carried out in the system functions of configuration, monitoring, alarms, control parameters, online modification and record of the plant's processes. On-line modification and recording of each process flow in the whole plant.
This layer realizes the centralized display and management of measurement data of the whole process of sewage treatment, control configuration of each control unit on site, graphic configuration of data display, real-time data processing, real-time control commands and other functions by means of configuration tools and special monitoring software.
2) The background monitoring system mainly includes workstations and printers and other equipment. For example, the most basic equipment configuration of a central control room includes two monitoring mainframes, monitors, projectors, UPS systems, printers, alarm devices, etc. The functions of each equipment are as follows:
3) The background monitoring system mainly includes workstations and printers. The function of each device is as follows:
Monitoring host: monitoring computer through the communication management unit to collect the sewage treatment plant process parameters, electrical parameters, several major equipment operating status information, and then through the background monitoring system software to analyze the data, processing, storage, all types of process parameters to make the trend curve, complete the sewage treatment plant process sections of the centralized control, detection function, through simple operation, the system can be configured to function, the system can be configured to function. Through simple operation, system function configuration, monitoring, alarm, control parameter on-line modification and setting can be carried out.
CRT, projector: visualization of the whole plant processes.
UPS system: uninterruptible power supply system, automatic control system must be 24h continuous operation, so the UPS system includes at least one set of batteries and a rectifier. Guarantee that the computer system continues to work for a period of time after a power outage to enable users to save the disk in an emergency or take timely measures so that the computer does not affect the work or loss of data due to power failure.
Alarm devices: alarm speakers and so on.
2 Advantages of the DCS system:
1) Overcome the centralized control system danger concentration, poor reliability, the system is not easy to expand, the control cable is large and other shortcomings
2) the realization of the real information, management and scheduling centralized, while the function and control of decentralized
3 Problems
1) low level of networking, its automatic control system Only a single central control and monitoring network, can not realize the unit LAN users and remote network users access and control.
2) The low level of self-control, only to complete a simple mechanical operation of the equipment, from the intelligent self-control there is a big gap.
4 sewage treatment plant automatic control system development trend
With the development of computer technology, network technology, database technology and penetration into the field of automation, so that the automation system architecture is undergoing a profound change, this change directly on the automation of sewage industry has a significant impact. The automation system can be upgraded from the original single process monitoring to a secondary network - sewage treatment operations LAN and process monitoring industrial Ethernet constitutes a secondary network, using the "centralized management, decentralized control" principle, constituting a "vertical layering, horizontal distribution of the network," the "vertical layering, horizontal distribution of the network. The principle of "centralized management and decentralized control" is adopted, constituting a network system structure of "vertical layering and horizontal substation". Under the two-level network structure, the real-time historical database and relational database are used as the center to realize the four functional layers of the control system, i.e. field control layer, process monitoring layer, operation management layer, and remote access control layer. The hierarchical structure of the self-control system is shown in Fig. 3, and the composition of the self-control system is shown in Fig. 4.
4.l Field Control Layer
It is the same as the above.
4.2 Process monitoring layer
The same as the above central monitoring layer, in addition, in the layer can be installed through the installation of professional intelligent control software, so that it can be given to the production process of a variety of data to calculate, analyze, and come to the conclusion of the current state of normal operation as a leading production scheduling, process adjustment, and other reference basis.
4.3 Management Layer
This layer is built on a LAN consisting of management computers and database servers. System administrators can assign different permissions to different users of the enterprise LAN through the authority settings, the leadership can be established in the layer of the relational database, view and access to a variety of data in the sewage plant, and through the installation of professional intelligent control software, so that it can be given to a variety of data in the production process to calculate, analyze, and draw conclusions on the current state of normal operation, as the leadership of production scheduling, Provide the basis for production scheduling, process adjustment, etc., to realize the comprehensive management of sewage plants and other functions; the general user of the plant only have access to some of the data permissions. Remote database user access interface with network security protection is left in the layer to realize the authorized users to access the database remotely.
4.4 Remote Access Control Layer
With the development and continuous improvement of INTERNET, remote access and remote control has been increasingly applied to all walks of life, remote access and management of the water treatment industry was born --- remote access control layer. This layer uses remote access servers, remote monitoring software and other tools to provide services for remote users with privileges, realizing managers' off-site access, maintenance and real-time supervision by higher authorities. According to the division of authority can provide remote users with the following services: remote server-side relational database access, remote server-side real-time database access, sewage treatment process parameters, real-time data, historical data, a variety of graphic client display, real-time operation of the working conditions of the screen remote access, remote monitoring of the water quality parameters online records, remote maintenance of the database and so on.
5 Conclusion
The future of the sewage treatment plant automation network system is a set of computer technology, information technology, automation technology, network technology, intelligent technology in one system, the water treatment industry automation control network operation, intelligent operation will become the dominant trend in the future development.