Programmable controller PLC is the key to realize the automatic control of sewage treatment plant, county-level small and medium-sized cities of sewage treatment plant treatment capacity is generally below 100,000 m3 / d, many are 1 to 50,000 m3 / d, the number of monitoring points is generally not a lot of, and the choice of Siemens S7-300 PlC is a that is, the economy is a realistic solution, which has a high degree of modularity structure, the instruction set is powerful, to meet the requirements of the system hardware configuration, software design, and operational results. It has a highly modular structure and a powerful command set to meet the diverse and individualized needs.
1 sewage treatment process
From the current sewage treatment process and technology research, development and application of the situation, due to the traditional activated sludge treatment process in small and medium-sized sewage treatment plants there are high infrastructure investment, high operating costs and high power consumption, therefore, the treatment of water treatment in the county-level small and medium-sized cities with a water treatment capacity of less than 100,000 m3/d sewage treatment plant treatment processes Oxidation ditch and SBR process are generally used. A county sewage treatment plant in Northwest China treats 40,000 tons of sewage per day, the treatment process used is the Aubert oxidation ditch. The main treatment: section has mechanical treatment section, biological treatment section, and sludge dewatering treatment section. Mechanical treatment section consists of coarse grating room, intake pump room, fine grating room, cyclone sand sedimentation tank. The biological treatment section consists of anaerobic tank, oxidation ditch, reflux and residual sludge pumping room, two sedimentation tanks, contact tanks, etc. The sludge dewatering treatment section consists of sludge storage tanks, sludge dewatering machine room. The flow of the process is shown in Figure 1.
The sewage collected from the city sewage network arrives at the influent pump room, where large solid impurities are filtered out by the grating machine, and then the thinner sewage is lifted and pumped to the cyclone sand sedimentation tank, where the solid particles are removed again due to the action of the mixer and the sand suction machine. The next process is the sewage is first sent to the anaerobic tank, the purpose is to remove phosphorus, denitrogenation, and then flow into the oxidation ditch, where there are appropriate strains of bacteria to decompose, purify sewage, oxidation ditch sludge bacteria ecosystem to maintain the main conditions of survival of a sewage in the main organic impurities, the other is the supply of oxygen, so oxidation ditch on the aeration of the rotating disk's role is the sewage is oxygenated. After oxidation ditch treatment of sewage in the second sedimentation tank after precipitation and then in the contact pool chlorination can be discharged directly after the standard, the excess sludge is discharged by the sludge pump to the dewatering machine room, through the dewatering process, the sludge is made into a cake by pressure drying and used as fertilizer.
2 automatic control system hardware selection
Wastewater treatment is mainly sequential logic control, which is where the advantages of PLC control. In the many PLC manufacturers, Siemens S7-300 PLC series products with its high cost-effective to become the first choice of many users. S7-300 is a modular small and medium-sized PLC system to meet the medium performance requirements of the application, modular, fanless structure, a variety of individual modules can be a wide range of combinations between for expansion. Interface modules (IM) are used to connect the main rack (CR) and the expansion rack (ER) in a multi-rack configuration. The S7-300 can operate up to 32 modules via a distributed main rack (CR) and two expansion racks (ER). The central processing unit (CPU) has integrated PROFIBUS-DP and MPI communication interfaces, and the multi-point interface (MPI) is used for simultaneous connection of programmers, PCs and HMIs. The signal module (SM) is used for digital and analog inputs and outputs.
3 Overall control requirements and functions
The requirements of the automatic control system of the sewage treatment plant is to automatically control and automatically adjust the sewage treatment process, so that the processed water quality indicators to achieve the required range; in the company's central control room to send out the upload command, the current moment of the operation process of the main operating parameters (water quality parameters, flow rate, level, etc.), the operating status and a certain period of time within the The main working parameters (water quality parameters, flow rate, liquid level, etc.), operation status and main process curves in a certain period of time are uploaded to the central control room of the company. Functions are as follows:
1) Control operation: In the central control room, the controlled equipment can be controlled online in real time, such as starting and stopping a certain device, adjusting the size of certain analog outputs, online setting of certain parameters of the PLC, and so on.
2) display function: graphic real-time display of the site of the controlled equipment operating conditions, as well as the site of the state parameters.
3) data management: according to the different operating parameters of the changes in the speed and importance of the establishment of the production history database, storage of raw production data for statistical analysis. Compare and analyze the data in the real-time database and historical database to derive some useful empirical parameters, which is helpful for optimizing the quasi-closed-loop control of the oxidation ditch, and display some necessary parameters and results to the real-time screen and reports.
4) Alarm function: when a certain analog quantity (such as current, pressure, water level, etc.) measured value exceeds the given range or a certain switching quantity (such as motor start and stop, valve switch) valve shift, according to different needs to issue different levels of alarm.
5) Printing function: can realize the report and graphic printing as well as a variety of events and alarms real-time printing. Printing mode can be divided into: timed printing, event-triggered printing.
4 System Composition and Layout
If the conventional PLC centralized control mode, the field signals are connected to the centralized control room through the cable on the PLC, due to the process line is long, the field control points are widely distributed, you need to lay a large number of cables and bridges, and the site environment is harsh, and the construction is very difficult. In view of this, the PROFIBUS fieldbus technology, according to the process division, the system **** set up two master stations, two sub-stations, two operator stations. Adopting Siemens S7-300 series PLC, the master station adopts CPU315-2DP, which comes with a DP communication port and an MPI port. The sub-station adopts the ET200M remote module with good versatility, which is used for on-site data acquisition and control, and facilitates the establishment of the control network system with the help of PROFIBUS (industrial field bus). Its automatic control system is shown in Fig. 2.
The control system is divided into three levels, i.e. field level, control level and management level.
(1) Management level
The management level centrally monitors the operation status of each substation equipment. The PROFIBUS-FMS fieldbus is selected for the management level, and two redundant servers with configuration software are installed in the control room as the operator stations of the PROFIBUS-FMS fieldbus to collect field data at the same time. The servers are equipped with WinCC configuration software with server package option.
(2) Control level and field level
The main function of the control level is to receive parameters or commands set by the management level, control the wastewater treatment production process, and convey the field status to the management level. According to the process flow and general layout of the plant, combined with the location of the motor control center MCC and the scope of power supply and distribution, according to the area of the control object, the number of equipment, the principle of division of the area in order to close to the collection and unit control, in the influent pump room, substation to set up two on-site master station PLC1, PLC2. two on-site sub-stations are set up in the chlorine room, dewatering room, in which chlorine room PLC1-1 for the influent pump room PLC1-1 in the chlorination room is the DP slave of PLC1 in the water intake pump room, and PLC2-1 in the dewatering machine room is the DP slave of PLC2 in the power substation. PROFIBUS-DP fieldbus is used between the field master and the field slaves. The field master PLC1 and PLC2 are composed of power supply module PS 307, CPU315-2 DP module and communication module CP343-5 and corresponding signal module SM respectively. The field sub-station adopts ET200M remote unit, and each ET200M unit is composed of one IMl53-2 bus interface module and several other digital and analog input and output modules.
The monitoring scope of PLC1, the on-site master station of the influent pump house, is coarse grating, influent pump house, fine grating, sand sedimentation tank, etc. Its main control objects are coarse and fine grating between coarse and fine grating and inlet electric gates, sewage lifting pumps in the influent pump house, sand discharge device and sand-water separation in the sand sedimentation tank and other equipment, and is also responsible for the on-line detection of the influent water quality such as pH, SS (turbidity measurement) and other parameters. Its I/O configuration: DI = 144, DO = 48, A1 = 16.
Chlorination room on-site sub-station PLC1-1 monitoring range of chlorination room, contact pool, the main control object for the chlorination and disinfection equipment, in addition to its also responsible for the quality of water such as: residual chlorine, COD, flow and other parameters of the on-line detection. Its I/O configuration is: D1 = 48, DO = 16, AI = 8.
The monitoring scope of PLC2, the main station of the substation, is anaerobic tank, oxidation ditch, reflux and residual sludge pumping station, and the second sedimentation tank, which is mainly used to complete the monitoring and controlling of the treatment parameters, such as the measurement and controlling of the parameters of dissolved oxygen, sludge concentration, pH value, ORP value, etc. The aeration equipment, stirring equipment, and the mixing equipment, as well as the water quality, such as COD, flow, etc., are also responsible for the online detection of water quality. Operation and control of aeration equipment, mixing equipment, drainage equipment, sludge reflux and residual pumps, scraper, etc., to meet the requirements of water quality of treated effluent. The I/O configurations are: DI=224, DO=8O, A1=24.
Dewatering plant room site sub-station PLC2-1 monitors and controls the dewatering plant room and sludge storage tank, and the main control objects are the agitators and electric valves of the sludge storage tank, and the sludge feed pumps, conveyor, thickening machine, dosage system and other equipments in the dewatering plant room. Its I/O configuration is: DI = 48, DO = 16, Al = 8.
5 System Software
The computer operating system adopts Microsoft Windows 2000 Professional Chinese version operating system. The history database adopts Microsoft SQLServer 2000 Chinese version software. The upper computer monitoring software adopts GE's iFIX3.5 configuration software to realize.
1) Operating system software Windows 2000 Professional Chinese version of the operating system provides a fast, efficient multi-user, multi-tasking operating system environment, is currently widely used industrial control operating system. The database server uses Windows 2000 server Chinese version with Microsoft SQL2000 to save historical data. Each workstation uses Windows 2000 Professional Chinese version operating system.
2) iFIX3.5 monitoring software realizes the collection and processing of switching quantity, status quantity, power quantity and analog quantity of the whole system, and displays them on the interface of the main workstation; some important physical quantities of the sewage treatment plant, such as currents, frequencies and outlet pressures of each water intake pump, are displayed in real time on the main interface of the main station, which makes it easy for the dispatcher to keep abreast of the system's operation in time.
3) Microsoft SQL2000 Chinese version database software is installed on the database server, which is used to store the important historical data of the whole sewage plant, and the historical data can be read through the communication between iFIX3.5 and Microsoft SQL 2000. A separate design of a database server can be avoided because of local system failures resulting in the loss of historical data phenomenon.
4) STEP7 can utilize six of the eight programming languages in the IEC-1131 standard (STL, LAD, FBD, CFC, SFC, and SCL) for programming, and this system utilizes STEP7 to configure and program Siemens programmable controllers.
6 Conclusion
With Siemens S-300 PLC designed automatic control system in the sewage treatment plant put into use since the operation so far the system is stable and reliable. The equipment has a simple debugging, easy to operate, safe to use, high efficiency, low failure rate, good sewage treatment results, improve labor productivity, and at the same time, due to the modular structure of both hardware and software, convenient for engineers and technicians to install, debugging and maintenance.