And development trend.
Sewage Treatment Plant 1 Overview of Automatic Control System
The automatic control system of urban sewage treatment plant is an integrated automation system composed of automation technology, computer technology, network technology and graphic technology. It is an information platform that integrates the functions of sewage treatment plant management, scheduling, on-site control, etc. under the network environment, and the sewage treatment process runs with high quality and reliability, and the discharge reaches the standard. Through PLC and network technology, the integration of management and control and comprehensive information processing in sewage treatment process can be realized. According to the actual situation and process requirements of the sewage treatment plant, the automatic control system of the sewage treatment plant adopts the system mode of combining distributed control with field bus, which consists of two levels of control systems, namely management level and field control level, and communicates with each other through 10/ 100M Ethernet, that is, the automatic control system is a two-level distributed control system (DCS) consisting of a central control room computer and various PLC control units at the field level. Distributed control system is a high-tech control device integrating automatic control technology, computer technology, communication technology and CRT display technology. It is a new control system, which is at the forefront of new technologies in production management, data acquisition and various process control. The central monitoring station and a number of field control stations are connected through communication networks to form a centralized and decentralized computer measurement and control management system, which is called distributed control system for short. DCS system overcomes the disadvantages of centralized control system, such as concentrated risk, poor reliability, difficult system expansion and large number of control cables. , and realize the real centralization of information, management and scheduling, and at the same time disperse the functions and risks, such as computer failure in the central control room, so that each field substation can still work independently and stably, which fundamentally improves the reliability of the system. The hierarchical structure of the automatic control system of the sewage treatment plant is shown in Figure 1, and the composition of the automatic control system is shown in Figure 2.
1. 1 field control layer
The field control layer consists of field-level PLC control unit, field measurement and control instruments and control equipment. The control layer consists of four main stations: 1 field PLC station, No.2 field PLC station, No.3 field PLC station and No.4 field PLC station. The management layer adopts industrial computer, and the functional layer realizes all process parameter preset, equipment operation state and electrical parameter data acquisition and equipment control through PIC. And sends data and receives control instructions to the central control layer through industrial Ethernet. The system realizes the control and protection of the process parameters and electrical equipment in the main production links such as coarse/fine grid, lifting pump station, grit chamber, anaerobic tank, oxidation ditch and dewatering machine room, so as to ensure the safe, stable, reasonable and efficient operation of the production process. According to the requirements of process control, the parameters such as pressure difference before and after the grid, pump pool level, dissolved oxygen concentration in anaerobic pool and oxidation ditch, PH value, import and export flow, and mud storage pool level are monitored and controlled at the same time. The functions of each PLC station are as follows:
1) PLC 1, control station of pretreatment section. The PLC workstation is located in the control room of the intake pump house in the plant, and is responsible for monitoring the pretreatment section of the sewage treatment plant. Its main control objects are coarse grids between coarse grids, electric intake gate, sewage lift pump of intake pump house, sand discharge device of grit chamber, sand-water separation equipment, etc. In addition, it is also responsible for on-line detection of influent water quantity and water quality such as COD, pH and SS (turbidity measurement).
The main equipment control methods are as follows:
Coarse grid and fine grid: according to the time interval, PLC automatically controls the grid rake to remove the grid slag, and at the same time, when the water level before and after the grid exceeds a given value, PLC can also automatically control the grid rake to remove the grid slag. Moreover, the grid machine and the screw conveyor should be linked, and the starting sequence of each equipment is to start the screw machine first, and then start the grid machine. It should also be linked when stopping, and the order is opposite to that when starting. When the conveyor fails, the fine grid stops running.
Inlet pump house: the inlet pump house is equipped with three submersible pumps, two for use 1 standby, two liquid level meters and a liquid level switch. PLC automatically controls the number of pumps according to the water level in the pump pool, and automatically rotates the pumps according to the running time of each pump, so that the running time of the pumps is equal. There are upper and lower limit alarms to prevent the pump from running dry. In programming, the operation and scheduling of pumps follow the following principles: ensure that the incoming water quantity is consistent with the pumping quantity, that is, pump as much as you come; Keeping the pump pool running at a high water level can reduce the working head and power consumption of the pump on the premise of ensuring the pumping capacity; Do not start and stop the pump too frequently; Ensure that the operation times and running time of each pump are basically equal.
Cyclone grit chamber: includes two cyclone grit chambers. Blower, sand-water separator and supporting equipment are intermittently linked according to the period set by the operator. When any equipment fails, an alarm should be given and relevant equipment should be shut down. In the automatic working mode, each device controls the start and stop of each motor and the switch of each solenoid valve according to the pre-programmed program of PLC.
2) biological treatment system/distribution center PLC2. This workstation is generally located in the control room of the power distribution center of the whole plant, and is responsible for monitoring the sewage biological treatment section. The main control objects are the underwater agitator, underwater propeller and aeration equipment of the biological tank, the sludge return pump and surplus sludge pump of the sludge return pump house, and the sludge scraper of the secondary sedimentation tank. In addition, it is also responsible for biological pools DO, ORP and ALSO; On-line detection of pH and MLSS of sludge pump house and electrical parameters of distribution center, such as current, voltage, active power, reactive power, active power and reactive power.
The main equipment control methods are as follows:
Control of return sludge pump and surplus sludge pump: The task of adjusting return sludge quantity is to ensure that the concentration of mixed liquor in biochemical treatment system is maintained within a certain range. The adjust amount is that amount of activated sludge return to the anaerobic tank. The electromagnetic flowmeter is installed on the sludge return path. The adjustment of the return sludge quantity is realized by the number of return sludge pumps, and the return ratio can be preset on PLC or set on the computer in the central control room according to the adjustment of the inlet flow ratio. ; The operation of the residual sludge pump follows the following principles: a. Automatic operation at regular intervals. B. When the sludge buffer tank is at a low level, the residual sludge pump runs. C. When the liquid level of the sludge buffer tank is high, stop the pump. D pump valve realizes intermodal control.
Oxidation ditch: two anaerobic tanks are equipped with 6 agitators, which run continuously. The two oxidation ditches are respectively provided with 8 aerators in the outer ditch and 4 aerators in the middle ditch and the inner ditch. At the same time, there are 1 dissolved oxygen meter, 1 ORP meter in the outer, middle and inner ditches, and 1 turbidity meter in the middle ditch. According to the oxygen content in the sewage monitored by the dissolved oxygen meter in the oxidation ditch, it can save energy by controlling the number of running aerators to change the oxygen charging.
3) Sewage disinfection system of outlet pump station /PLC3. The PLC workstation is located in the control room of the export pump house. Its main control objects are effluent lift pump, electric valve of switching well, chlorination disinfection and other equipment. In addition, it is also responsible for on-line detection of effluent quality parameters such as residual chlorine, COD and flow.
4) Sludge treatment system/dehydration workshop PLC4. The PLC workstation is generally located in the control room of the power distribution room of the dehydration workshop, and is responsible for monitoring the sludge treatment section. The main control objects are the agitator and electric valve in the mud storage tank, the mud pump, dehydrator, thickener, dosing system and other equipment in the dehydration workshop.
The main equipment control methods are as follows:
Mud storage tank: The agitator of mud storage tank can be operated continuously and remotely, with high and low liquid level alarm (0.5m can be set) and upper computer liquid level alarm limit (4.5m can be set).
Sludge dewatering machine room: the dosing system is mainly braked manually, and the dosing metering pump can be started at any time when there is no alarm at the low liquid level of the dosing tank. The running signal of dosing system is sent to PLC. The field control box is used to calibrate and flush the inside of the dehydrator system. PLC only gives the start and stop commands of the dehydrator, and completes the linkage with other related equipment. The starting sequence of the dehydrator system is as follows: start the belt conveyor first, then start the dehydrator system, then start the dosing system, and finally start the screw pump to send mud. Stop in reverse order. When a certain equipment fails in operation or the liquid level of the buffer tank reaches the set low level, the equipment will be shut down according to the shutdown sequence, and the monitoring and management computer can remotely control the above equipment.
In addition, there is a communication module on this floor, also called communication management unit. Communication management unit is the middle layer of automatic control system, which is responsible for information collection and forwarding of the whole control system; The communication management machine collects and sorts the data of PLC, instrument and other automatic control systems, and then transmits them to the background system through optical fiber, and at the same time, it can forward various control commands issued by the background to the corresponding units.
At present, the vast majority of communication management unit network systems in the DCS system of sewage plants use optical brazing as the transmission medium, that is, a redundant 100Mbps optical fiber industrial Ethernet forms a wired data communication network between the central control room and multiple field control stations in the plant.
1.2 central control layer
1) This layer, also known as the background monitoring system layer, is the information display and control center in the system, which is composed of two monitoring and management computers, color CRT, two printers and other devices as operating stations connected to industrial Ethernet. The monitoring and management computer system collects all kinds of process parameters, electrical parameters and running status information of main equipment of the sewage treatment plant through the l0/ 100M network, analyzes and stores all kinds of data, makes trend curves of all kinds of process parameters, and completes the centralized control and detection function of each process section of the sewage treatment plant. Through simple operation, the system function configuration, monitoring, alarm, online modification of control parameters and records of all technological processes in the whole plant can be carried out.
This layer realizes centralized display and management of metering data in the whole process of sewage treatment, control configuration of various control units on site, graphic configuration of data display, real-time data processing, real-time control instructions and other functions through configuration tools and special monitoring software.
2) The background monitoring system mainly includes workstations and printers. For example, the basic equipment configuration of a central control room is: 2 monitoring hosts, monitors, projectors, UPS systems, printers, alarm devices, etc. The functions of each device are as follows:
Monitoring host: The monitoring computer collects the operation status information of the main equipment such as process parameters and electrical parameters of the sewage treatment plant through the communication management unit, and then analyzes, processes and stores the data through the background monitoring system software, and makes trend curves of various process parameters, thus completing the centralized control and detection function of each process section of the sewage treatment plant. Through simple operation, the system function configuration, monitoring, alarm and online modification and setting of control parameters can be carried out.
CRT and projector: display the process flow of the whole factory directly.
UPS system: Uninterruptible power supply system and automatic control system must run continuously for 24 hours, so UPS system at least includes a battery and a rectifier. Ensure that the computer system can continue to work for a period of time after power failure, so that users can save the disk urgently or take measures in time, so that the computer will not affect its work or lose data due to power failure.
Alarm device: alarm speaker, etc.
2 Advantages of DCS system:
1) overcomes the defects of centralized control system, such as concentrated danger, poor reliability, difficult system expansion and large control cable.
2) Realize the real centralization of information, management and scheduling, but the functions and control are scattered.
3 there is a problem
1) has a low degree of networking, and its automatic control system is only a single central control monitoring network, which cannot realize the access and control of local area network users and remote network users.
2) The level of automatic control is low, and only simple mechanical operation of equipment is completed, which is far from intelligent automatic control.
4 sewage treatment plant automatic control system development trend
With the development of computer technology, network technology and database technology and the penetration into automation field, the architecture of automation system is undergoing profound changes, which has a great impact on the automation of sewage treatment industry directly. The automatic control system can be upgraded from the original single process monitoring to a secondary network-a secondary network composed of sewage treatment operation local area network and process monitoring industrial Ethernet. The principle of "centralized management and decentralized control" is adopted to form a "vertical layered and horizontal substation" network architecture. Under the two-level network architecture, four functional layers of the control system, namely, field control layer, process monitoring layer, operation management layer and remote access control layer, are realized centering on real-time historical database and relational database. The hierarchical structure of the automatic control system is shown in Figure 3, and the composition of the automatic control system is shown in Figure 4.
4.l field control layer
Ditto.
4.2 Process Monitoring Layer
Like the central monitoring layer mentioned above, in addition, professional intelligent control software can be installed on this layer to calculate and analyze all kinds of data appearing in the production process, and draw the conclusion whether the current running state is normal or not, which can be used as a reference for leaders to schedule production and adjust technology.
4.3 management
This layer is built on a local area network consisting of a management computer and a database server. System administrators can assign different permissions to different users of enterprise LAN through permission setting, leaders can view and read all kinds of data of sewage plants through the relational database established at the same level, and can calculate and analyze all kinds of data appearing in the production process by installing professional intelligent control software, and draw the conclusion that the current operation state is normal, which can provide basis for leaders to schedule production and adjust technology, and realize comprehensive management of sewage plants. Only ordinary users in the factory can access some data. In this layer, there is a remote database user access interface with network security protection to realize the remote access of authorized users to the database.
4.4 Remote Access Control Layer
With the development and continuous improvement of the Internet, remote access and remote control are increasingly applied to all walks of life, and the remote access and management of water treatment industry is also born-remote access control layer. This layer uses remote access server, remote monitoring software and other tools to provide services for authorized remote users, and realizes remote access, maintenance and real-time supervision of higher authorities. According to the division of authority, the following services can be provided for remote users: remote server-side relational database access, remote server-side real-time database access, sewage treatment process parameters, real-time data, historical data, display of various graphical clients, remote access to real-time operating conditions images, remote monitoring of online water quality parameters, remote database maintenance, etc.
5 conclusion
The future automatic control network system of sewage treatment plant is a system integrating computer technology, information technology, automation technology, network technology and intelligent technology. The networked operation and intelligent operation of automatic control in water treatment industry will become the leading trend in the future.