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Abstract: combined with the operation of Nanning Langdong sewage treatment plant, this paper introduces the composition of automatic detection instruments and their application in sewage treatment, and analyzes and summarizes the problems and operating procedures that should be paid attention to in the standard management of automatic control instruments

key words: automatic detection instrument sewage treatment plant automatic control system PLC controller

automatic detection instrument is one of the key subsystems in the automatic control system. General automatic detection instruments are mainly composed of three parts: ① sensors, which use various signals to detect the measured analog quantity; ② The transmitter 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 Display, display the measurement results intuitively and provide the results. These three parts are organically combined. Without any of them, it cannot be called a complete instrument. Automatic detection instruments have been widely used in industrial production because of their accurate measurement, clear display, simple operation and other characteristics. Moreover, automatic detection instruments have an internal interface with computers, which is an important part of automatic control system, which is called "the eye of automatic control system"

with the development of science and technology, automatic detection technology has also been greatly developed. Automatic detection instruments have also been widely used in sewage treatment, so that sewage treatment plants not only save a lot of human and material resources, but also can adjust the process in time. Taking Nanning Langdong sewage treatment plant as an example, this paper introduces the application of automatic detection instruments in sewage treatment

1. Project overview

Nanning Langdong sewage treatment plant project was approved at the end of 1993 and officially commenced on November 27, 1997; On September 28, 1999, the water was put into trial operation, and in February 2000, it operated normally at full load

Nanning Langdong sewage treatment plant, phase I project is designed to have a primary sewage treatment capacity of 240000 m3/D and a secondary sewage treatment capacity of 100000 m3/d. The designed service area is 30.5 km2, and the planned service population is 343000. Some of the clean water purified by Langdong sewage treatment plant is directly discharged into Zhupaichong, and some is used for the reinjection of Nanhu Lake to improve the water pollution of Nanhu lake

2. Treatment workers 3. Configuration of membrane pendulum impact experimental machine

Nanning Langdong sewage treatment plant introduces a full set of the most advanced water treatment process equipment from abroad, adopts the traditional activated sludge method of secondary biological treatment process, and adopts OOC process in its core part - aeration process according to the characteristics of low pollutant concentration of sewage quality in Nanning. The process has the advantages of low energy consumption, low operation 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 shall be pretreated first. Pass through the coarse screen in the intake pump room to remove the large garbage and floating objects in the sewage; Five large sewage pumps are used to lift the sewage to the fine grid to remove the small floating objects; Remove sand and oil in sewage in aerated grit chamber; Then enter the metering tank to measure the sewage treatment capacity. The pretreated sewage is treated in the primary sedimentation tank to remove about 30% of organic matter; The effluent from the primary sedimentation tank enters the secondary treatment. First, the core part of the biological treatment process - aeration tank, is used for biodegradation of organic matter; The mixed liquid in the aeration tank is sent to the secondary sedimentation tank for sedimentation and sludge water separation. The upper clarifier is used as the clean drainage water after purification; Part of the settled sludge is recycled after returning to the aeration tank, and part of it is returned to the primary sedimentation tank as surplus sludge. The sludge from the primary sedimentation tank is pumped to the sludge thickener, which further thickenes the sludge. Through the sludge treatment system, the muddy sludge is dewatered and pressure filtered to form a dry sludge cake. See Figure 1 for the process flow

Figure 1 process flow of sewage treatment plant

3. Introduction to main application instruments

3.1 ultrasonic level gauge, level difference gauge, flowmeter

(1) grid operation control. One ultrasonic liquid level difference meter is installed for the coarse grid and the fine grid respectively, which reflects the blocking degree of the grid through the liquid level difference before and after the grid, and is transmitted to the PLC controller for analysis and calculation. When the liquid level difference exceeds the preset value, control the operation of the grille, remove garbage, ensure normal water flow, and reasonably reduce equipment wear

(2) lift pump operation control. In order to realize the automatic control of the intake lift pump, two ultrasonic liquid level meters are installed at the intake pump well to measure the water level of the pump well and transmit it to the PLC controller in real time. H) curve selection: stress-strain, force time, strength time and other curves can be selected according to needs for display and printing; And the upper computer for system analysis. Automatically control the operation combination of the lift pump according to the control program corresponding to the measured value. In this way, the pump operation state can be adjusted accurately and timely according to the external water volume of the plant, so as to reduce equipment fatigue; At the same time, the human resource consumption of the traditional three shift pump station can be eliminated

(3) real time monitoring of flow and processing capacity. For the operation and management of sewage treatment plant, water volume is an important control parameter. Accurate and timely control of water inflow plays an important role in process control and improving the ability of sewage plant to resist the impact of hydraulic load. The traditional water measurement adopts weir plate or venturi flow tank, which has the shortcomings of real-time monitoring and real-time display. The measurement 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 on the upper computer, so as to accurately measure the treatment water volume and provide real-time flow for operation management

3.2 dissolved oxygen meter, redox potentiometer, sludge concentration meter

(1) dissolved oxygen control of aeration tank. Nanning Langdong sewage treatment plant adopts the OOC improved process of traditional activated sludge method. Dissolved oxygen meters with a measurement range of 0.05 ~ 10 mg/l are installed in the aerobic area of the inner ring of four circular aeration tanks respectively to monitor the concentration of dissolved oxygen in real time and transmit it to PLC and upper computer. When the measured concentration is less than the set concentration, the automatic control system starts the blower to oxygenate the aeration tank; On the contrary, when the oxygen is sufficient, the blower will stop running. Controlling the blower through the dissolved oxygen meter can accurately control the start and stop of the blower according to the demand of aerobic flora for dissolved oxygen, which not only ensures the good biochemical capacity of the flora, but also saves energy consumption, protects the equipment, and enhances the decomposition capacity of aerobic flora

(2) control of aerobic section and anoxic section of aeration tank. An oxidation-reduction potentiometer with a measurement range of -500 ~ 500 MV is installed at the critical interface between the aerobic zone and the anoxic zone of the outer ring of each aeration tank. Through the measured oxidation-reduction potential, the high-speed operation of the blower can be controlled to supply oxygen to the outer ring, forming the alternation of strong aerobic aeration stage and oxygen deficiency stage, so as to improve the ability of phosphorus and nitrogen removal in the treatment process. If redox potentiometer is not installed. Then the operation of the blower can only be controlled by time. In this way, the clamping unit can be adjusted to significantly reduce the effect of phosphorus and nitrogen removal

(3) sludge concentration control in aeration tank. Sludge concentration in aeration tank is an important process parameter. In the traditional sewage treatment plant, the sludge concentration depends on the old test method used in the laboratory for monitoring, which has great defects in the timeliness and accuracy of data provision. It is difficult to adjust the process of reflux sludge and excess sludge in time, resulting in errors in time and accuracy. Nanning Langdong sewage treatment plant has installed a sludge concentration meter with a measurement range of 0.5 ~ 10 g/l on each aeration tank, which solves this problem well. The installation of sludge concentration meter can timely adjust the process of aeration tank according to the accurately measured sludge concentration at any time, and reduce the labor intensity of laboratory staff at the same time

3.3 electromagnetic flowmeter and gas flowmeter

in the return sludge pipeline and excess sludge pipeline, Nanning Langdong sewage treatment plant has installed five electromagnetic flowmeters with a measurement range of 0 ~ 1200 m3/h to measure the flow of return sludge and excess sludge. After installing the flowmeter, the personnel on duty can judge whether the reflux sludge pump and the residual sludge pump work normally according to whether the displayed flow is correct, which solves the problem that the submersible pump cannot simply judge whether the operation is normal. In addition, the electromagnetic flowmeter also has the characteristics of convenient installation and simple maintenance

four gas flow meters with a measurement range of 0 ~ 4000 m3/h (standard condition) are directly installed on the air pipe between the blower and the aeration tank. The installation of gas flowmeter can make the personnel on duty know the amount of gas supplied by the blower to the aeration tank at any time

4. Operation and management experience

4.1 operation and management experience

Nanning Langdong sewage treatment plant has been officially put into operation since February 2000. During the operation and management for more than two years, several experiences to ensure the normal operation of automatic detection instruments are summarized as follows:

(1) keep the sensors of automatic detection instruments clean. Regularly assign special personnel to clean the probe to ensure the accuracy of data collection. Because the instrument works in the sewage environment, the cleaning of the instrument is particularly important, especially the dissolved oxygen meter, redox potentiometer, sludge concentration meter and other analytical instruments that are directly in contact with the sewage. In order to ensure the normal operation of the instrument, we regularly clean it by professionals, and comprehensively clean it every 7 days. Soft materials are required to be used during cleaning to avoid damaging the instrument

(2) calibrate various instruments regularly. The instrument will inevitably produce measurement errors in the long-term operation process, which requires regular correction to ensure the accuracy of instrument measurement. For the analytical instrument, we have formulated a regular correction once every two months; Moreover, the laboratory staff are required to analyze the corresponding test items by using the analysis method and compare with the monitoring results of the field instruments. If the deviation is too large, the instruments should be corrected in time to ensure accuracy

(3) ensure the stability of the power supply voltage of the instrument and prolong the service life of the instrument. Instantaneous high voltage impact often makes the instrument easy to burn out. During the operation of Nanning Langdong sewage treatment plant, the ultrasonic level difference meter and the transmitter of the ultrasonic level meter were damaged many times due to the unstable power supply voltage, which affected the normal operation of the automatic control system. Nanning Langdong sewage treatment plant is undergoing technical transformation to avoid damage to instruments caused by unstable power supply voltage, reduce operating costs and improve economic benefits

4.2 some experience

in the process of continuous maturity of process and automation equipment, we also have the following experience:

(1) to improve the economic benefits and management level of sewage treatment plant, it is not enough to only have the above instruments, but also add monitoring instruments such as blower outlet pressure gauge and primary sedimentation tank sludge pump outlet flowmeter, Monitor and adjust the outlet pressure of the large blower fan and the inlet flow of the concentration tank

(2) the spare parts and components of imported instruments are expensive and difficult to purchase, which affects the use and maintenance of instruments. For example, the maintenance cost of an ultrasonic level difference meter transmitter damaged due to unstable power supply voltage is more than 8000 yuan, and the replacement cost of a new transmitter is more than 15000 yuan; It takes more than 2000 yuan to replace an oxidation-reduction potentiometer electrode, and the service life of the oxidation-reduction potentiometer electrode is generally two years; It is difficult for ordinary enterprises to pay this expensive cost for a long time. The standardized calibration of imported instruments is difficult, and the general quality inspection

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