Combined with the operation of Langdong Sewage Treatment Plant in Nanning, this paper introduces the composition of automatic online detection instrument and its application in sewage treatment, and analyzes and summarizes the problems that should be paid attention to in the management and operation procedures of automatic control instrument.
Keywords: automatic on-line detection instrument
Automatic testing instrument is one of the key subsystems in automatic control system. The general automatic detection instrument mainly consists of three parts: ① sensor, which uses various signals to detect the measured analog quantity; ② transmitter, which converts the analog signal measured by the sensor into a current signal of 4 ~ 20 Ma and sends it to the programmable controller (PLC); (3) a display which intuitively displays the measurement results and provides the results. These three parts are organically combined, and it can't be called a complete instrument without any one. Automatic testing instruments have been widely used in industrial production because of their accurate measurement, clear display and simple operation. Moreover, it has an interface with the microcomputer, and is an important part of the automatic control system, which is called the eye of the automatic control system.
? With the development of science and technology, automatic detection technology has also been greatly developed, and automatic detection instruments have also been widely used in sewage treatment, which not only saves a lot of manpower and material resources, but also can adjust the process in time. Taking Langdong Sewage Treatment Plant in Nanning as an example, this paper introduces the application of automatic detection instrument in sewage treatment.
1 project overview
The project of Langdong Sewage Treatment Plant in Nanning was established at the end of 1993, and construction started on127. 65438+ water was put into trial operation on September 28, 1999, and it ran normally at full load in February 2000.
The first-stage sewage treatment capacity of Langdong Sewage Treatment Plant in Nanning is designed to be 240,000 m3/d, and the second-stage sewage treatment capacity is 65.38+100,000 m3/d. The designed service area is 30.5 square kilometers, and the planned service population is 343,000. ? Part of the purified clean water from Langdong Sewage Treatment Plant is directly discharged into bamboo rafts for toilet flushing, and part of it is used for reinjection water in South Lake to improve the water pollution problem in South Lake.
2 treatment process
Langdong Sewage Treatment Plant in Nanning introduced the most advanced foreign water treatment technology and equipment, and adopted the traditional activated sludge process with two-stage biological treatment process. In view of the low concentration of pollutants in the sewage quality in Nanning, OOC process was adopted in its core part-aeration process. The process has the advantages of low energy consumption, low operating cost, good effluent quality, simple management and stable operation.
The sewage collected from the main sewage pipe outside the plant to Langdong Sewage Treatment Plant is pretreated first. Coarse grids are used in the intake pump house to remove large garbage and floating objects in sewage; Sewage is lifted from five large sewage pumps to a fine grid to remove smaller floating objects; Removing sand grains and oil from sewage in an aerated grit chamber; Then enter the metering box to measure the sewage treatment capacity. The pretreated sewage is first treated in the primary sedimentation tank to remove about 30% organic matter; The effluent from the primary sedimentation tank enters the secondary treatment, and the organic matter is biodegraded in the aeration tank, the core part of the biological treatment process; The mixed liquid in the aeration tank is transported to the secondary sedimentation tank for sedimentation and mud-water separation. The supernatant is used as purified clean discharge water; Part of the precipitated sludge is returned to the aeration tank for recycling, and part of it is returned to the primary sedimentation tank as surplus sludge. The sludge in the primary sedimentation tank is pumped to the sludge concentration tank, further concentrated in the tank, dehydrated and filtered by the sludge treatment system to form a dry sludge cake.
3 Introduction of main application tools
3. 1 ultrasonic level gauge, liquid level differential gauge and flowmeter?
(1) Power grid operation control. The coarse grille and the fine grille are respectively equipped with 1 ultrasonic level gauge. The blocking degree of the grille is reflected by the liquid level difference before and after the grille, and transmitted to the PLC controller for analysis and calculation. When the liquid level difference exceeds the preset value, the grid is controlled to run, the garbage is removed, the normal water flow is ensured, and the equipment wear is reasonably reduced. ?
(2) Operation control of the lift pump. In order to realize the automatic control of the inlet lift pump, two ultrasonic level meters are installed at the inlet pump well to measure the water level of the pump well and transmit it to the PLC controller and the upper computer for system analysis. Accord to that control program corresponding to the measure value. In this way, the running state of the pump can be adjusted accurately and timely according to the external water quantity of the factory, and the fatigue of the equipment can be reduced; At the same time, it can cancel the human resource consumption of traditional pumping stations with three shifts.
(3) Real-time monitoring of traffic and throughput. Water quantity is an important control parameter for the operation and management of sewage treatment plants. Grasping the water inflow accurately and timely plays an important role in process control and improving the ability of sewage plants to resist the impact of hydraulic load. The traditional water quantity measurement adopts weir plate or venturi chute, but the disadvantage is that it can not be monitored and displayed in real time. The metering tank of Langdong Sewage Treatment Plant adopts ultrasonic flowmeter combined with Venturi tank, which can display the flow and cumulative treatment capacity in real time on site and in the upper computer, so as to accurately measure the treated water and provide real-time flow for operation management.
3.2 dissolved oxygen analyzer, oxidation? Reduction potentiometer, sludge concentration meter?
(1) dissolved oxygen control in aeration tank. The traditional activated sludge OOC improvement process was adopted in Langdong Sewage Treatment Plant in Nanning. Dissolved oxygen measuring instruments with the measuring range of 0.05 ~ 10 mg/L were installed in the inner aerobic zone of four circular aeration tanks, and the dissolved oxygen concentration was monitored in real time and transmitted to PLC and upper computer. When the measured concentration is less than the set concentration, the automatic control system starts the blower to fill the aeration tank with oxygen; Conversely, when the oxygen is sufficient, the blower will stop running. By controlling the blower with the dissolved oxygen meter, the start and stop of the blower can be accurately controlled according to the demand of aerobic flora for dissolved oxygen, which not only ensures the good biochemical ability of flora, but also saves energy consumption, protects equipment and enhances the decomposition ability of aerobic flora. ?
(2) Control of aerobic section and anoxic section of aeration tank. The outer ring of each aeration tank, the interface between aerobic zone and anoxic zone is equipped with an oxidation device, and the measuring range is -500 ~ 500 mv. Reduction potentiometer, by measuring oxidation? The reduction potential can control the high-speed operation of the blower, supply oxygen to the outer ring, and form the alternation of strong aerobic aeration stage and anoxic stage, thus improving the ability of phosphorus and nitrogen removal in the treatment process. If oxidation is not installed? Decelerating potentiometer. Then the operation of the blower can only be controlled by time, which will obviously reduce the effect of phosphorus and nitrogen removal. ?
(3) Control of sludge concentration in aeration tank. Sludge concentration in aeration tank is an important process parameter. In the traditional sewage treatment plant, the old laboratory test method is used to monitor the sludge concentration, which has great defects in the timeliness and accuracy of data provision. It is difficult to adjust the process of returning sludge and surplus sludge in time, which leads to errors in time and accuracy. On-line sludge concentration meters with a measuring range of 0.5 ~ 10 g/L were installed in each aeration tank of Langdong Sewage Treatment Plant in Nanning, which solved this problem well. The installation of sludge concentration meter can adjust the process of aeration tank at any time according to the accurately measured sludge concentration, thus reducing the labor intensity of laboratory personnel.
3.3 Electromagnetic Flowmeters and Gas Flowmeters
Five electromagnetic flowmeters were installed in Langdong Sewage Treatment Plant in Nanning, with the measuring range of 0 ~1200m3/h, which were used to measure the flow of returned sludge and surplus sludge. ? After installing the flowmeter, the attendant can judge whether the return sludge pump and the surplus sludge pump work normally according to whether the displayed flow rate is correct, which solves the problem that the submersible pump can not simply judge whether it works normally. The electromagnetic flowmeter also has the characteristics of convenient installation and simple maintenance.
Four gas flowmeters are directly installed on the air pipeline between the blower and the aeration tank, with the range of 0 ~ 4000m3/h (standard state). The installation of gas flowmeter can make the personnel on duty know the amount of gas provided by the blower to the aeration tank at any time.
4 Operation and management experience
4. 1 Experience in operation and management
Nanning Langdong Sewage Treatment Plant was put into operation in February 2000. In more than two years of operation management, some experiences to ensure the normal operation of automatic testing instruments are summarized:
(1) Keep the sensor of automatic test instrument clean. Assign special personnel to clean the probe regularly to ensure the accuracy of data acquisition. Because the instrument works in sewage environment, it is particularly important to clean the instrument, especially the dissolved oxygen instrument and oxidation instrument which are in direct contact with sewage. Reduction potentiometers, sludge concentration meters and other analytical instruments, in order to ensure the normal work of the instruments, we have a special person to clean them regularly, and clean them completely every 7 days 1 time. In order to avoid damaging the instrument, it is necessary to use soft materials.
? (2) Calibrate various instruments regularly. Measurement errors will inevitably occur during the long-term operation of the instrument, which requires regular calibration to ensure the accuracy of instrument measurement. For analytical instruments, we have established a regular calibration 1 time every two months; Moreover, laboratory personnel are required to analyze the corresponding detection items by analytical methods and compare them with the monitoring results of field instruments. If the deviation is too large, the instrument should be corrected in time to ensure the accuracy. ? (3) Ensure the stability of the instrument power supply voltage and prolong the service life of the instrument. Instantaneous high-voltage electric shock is often easy to burn the instrument. During the operation of Langdong Sewage Treatment Plant in Nanning, the transmitters of ultrasonic liquid level differential indicator and ultrasonic liquid level meter were damaged many times due to the unstable power supply voltage, which affected the normal work of the automatic control system. Nanning Langdong Sewage Treatment Plant is undergoing technical transformation to avoid instrument damage caused by unstable power supply voltage, reduce operating costs and improve economic benefits.
4.2 Some experience
During the operation, we also have the following experiences:
(1) In order to improve the economic benefit and management level of the sewage treatment plant, these instruments are not enough. It is necessary to increase the monitoring instruments such as the outlet pressure gauge of the blower and the outlet flowmeter of the sludge pump in the primary sedimentation tank to monitor and adjust the parameters such as the outlet pressure of the large blower and the inlet flow rate of the thickening tank.
(2) The spare parts of imported instruments are expensive and difficult to purchase, which affects the use and maintenance of instruments. For example, the maintenance cost of a ultrasonic level meter transmitter damaged by unstable power supply voltage is more than 8,000 yuan, and it takes more than15,000 yuan to replace a new transmitter; Replace an oxidation? Reduction potentiometer electrode needs more than 2000 yuan, and oxidation? The service life of reduction potentiometer electrode is generally two years; It is difficult for ordinary enterprises to pay this expensive fee for a long time. It is difficult to standardize the proofreading of imported instruments, and the General Administration of Quality Supervision, Inspection and Quarantine does not accept the testing of sewage treatment instruments. The popularity of domestic instruments can not keep up, and the price advantage can not be well reflected, which affects the application of automatic detection instruments in sewage treatment plants to some extent.
Generally speaking, automatic detection instruments have played a great role in the application of sewage treatment plants, but there are still some problems in practical application. We believe that in the future, automatic testing instruments will play a greater role in environmental protection in China.
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