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NA400 PLC engineering example analysis-sewage treatment system


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NA400 PLC engineering example analysis-sewage treatment system

  • Categories:Municipal
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  • Time of issue:2020-03-31 11:16
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(Summary description)Relevant information shows that the United States has an average sewage treatment plant for every 10,000 people. The United Kingdom and Germany have one sewage treatment plant for every 7000~8000 people. China has a sewage treatment plant for every 1.5 million people. Therefore, sewage treatment in China is urgent, and the sewage treatment market also shows a vigorous scene.

NA400 PLC engineering example analysis-sewage treatment system

(Summary description)Relevant information shows that the United States has an average sewage treatment plant for every 10,000 people. The United Kingdom and Germany have one sewage treatment plant for every 7000~8000 people. China has a sewage treatment plant for every 1.5 million people. Therefore, sewage treatment in China is urgent, and the sewage treatment market also shows a vigorous scene.

  • Categories:Municipal
  • Author:
  • Origin:
  • Time of issue:2020-03-31 11:16
  • Views:


1. Overview

Relevant information shows that the United States has an average sewage treatment plant for every 10,000 people. The United Kingdom and Germany have one sewage treatment plant for every 7000~8000 people. China has a sewage treatment plant for every 1.5 million people. Therefore, sewage treatment in China is urgent, and the sewage treatment market also shows a vigorous scene.

2. Commonly used sewage treatment process

Sewage treatment methods are mainly physical, chemical, physical chemical and biological. According to the actual situation, these methods can be used singly or mixed for different sewage. At present, the methods of sewage treatment are generally based on biological treatment methods supplemented by physical treatment methods and chemical treatment methods. Commonly used processes are as follows:

(1) Traditional activated sludge method. The oldest sewage treatment process, the key components are aeration tank and sedimentation tank. When the organic matter in the sewage stays in the aeration tank, the microorganisms in the aeration tank adsorb most of the organic matter in the sewage. And it is oxidized to inorganic matter in the aeration tank, and then the microbial flocs sink in the sedimentation tank. After a period of time, you can output clean water. At the same time, in order to maintain the concentration of sludge in the aeration tank, a part of the activated sludge in the sedimentation tank after sedimentation needs to be returned to the aeration tank.




The advantages of this process are: high organic removal rate. The sludge load is high. The volume of the pool is small. Low power consumption and low operating cost.

The disadvantages of this process are: the common aeration method occupies a lot of land, so the construction investment is large; it can only meet the relevant indicators in the sewage treatment standard. It is easy to produce sludge swelling. The removal rate of phosphorus and nitrogen is low.

(2) A/O method. A sewage treatment process developed on the basis of the traditional activated sludge method, where A stands for Anocix (anoxic) and O stands for Oxic (aerobic). A/O method is an anoxic-aerobic biological wastewater treatment process. By adding a nitrification-denitrification reaction system formed by the aerobic tank and the anoxic tank, this process can treat the nitrogen content in the sewage well, and has obvious denitrification effect. However, the nitrification-denitrification reaction system needs to be well controlled, which puts forward higher management requirements for the process, which also forms a major disadvantage of the process.

(3) Oxidation ditch method. It is a variant of the activated sludge method, which belongs to the low-load, delayed aeration activated sludge method. The mixed liquid of wastewater and activated sludge circulates continuously in the ring-shaped aeration channel, so it is also called "circulation aeration tank". Oxidation ditch method has simple treatment process and structures, no preliminary sedimentation tank and sludge nitrification tank, high organic matter removal rate, nitrogen removal and phosphorus removal (addition of anaerobic tank before the ditch), less residual sludge and easy dehydration, treatment effect Stability and other advantages. However, it has the disadvantages of low load, large footprint, high power consumption, and high operating costs. It is suitable for small and medium-sized low-load sewage treatment plants. According to different structures, it can be divided into: Carrousel oxidation ditch method, Orbel oxidation method and integrated oxidation ditch method.

In addition, there are A2/O method, A/B method, SBR method and UNITANK method, which will not be introduced in detail here. For details, please refer to relevant professional information.

3. Control form of sewage treatment system

(1) DCS system. Is the abbreviation of distributed control system, also known as distributed computer control system. It is formed by the mutual penetration of computer technology, signal processing technology, measurement control technology, communication network technology and so on. It is composed of a computer and a field terminal, and connects the field control station, detection station and operation station, control station, etc. through the network to complete the functions of decentralized control and centralized operation and management. It is mainly used in various production processes and can improve the level of production automation and management. Its main features are as follows:

① The use of hierarchical distributed control reduces the amount of information transmission of the system and makes the system application program relatively simple.

② Realized decentralized control, which makes the danger of the system dispersed and the reliability improved.

③Strong expansion ability.

④Software and hardware resources are abundant, which can adapt to various requirements.

⑤Good real-time performance and quick response.

(2) Fieldbus control system. It is developed by DCS and PLC and is an automatic control system based on fieldbus. The system performs data transmission and exchange between intelligent devices and computers in accordance with an open and standardized communication protocol, thereby realizing an automatic control system that integrates control and management. The advantages are as follows:

①The rich software and hardware resources of the computer can be used.

②Fast response and good real-time performance.

③ The communication protocol is open and different products can be interconnected.

(3) PLC system. The programmable logic controller is used to process the controller of the system to achieve the functional requirements of the control system. It can also use the computer as a host computer and connect to the PLC through the network to monitor the production process in real time. Its characteristics are as follows:

① Convenient programming, short development cycle and easy maintenance.

② Strong versatility and easy to use.

③ Strong control ability.

④Modular structure, strong expansion ability。

System design

1. Working principle of sewage treatment system

(1) Overall framework of the control system:

As shown in Figure 2, the NA400 PLC is the core controller, which completes the operation, stop and speed control of related equipment by detecting the input of the operation panel button, the input of various sensors and the input of related analog quantities.




  Figure 2

(2) Working process:

In the manual state, the control of various types of equipment is controlled according to the button input of the operation panel. Without logic restrictions, it can be controlled without depending on the state of the sensor. In the automatic mode, closed-loop control is performed, and the system controls the device based on the status of the detected external sensor. Its working process is shown in Figure 3

① Turn on the power, start the automatic control mode, start the submersible mixer and scraper.

② Operate the coarse and fine grid machines to perform intermittent operation, that is, run for a period of time and then stop for a period of time, and circulate.

③ Control the operation and stop of the cleaning machine according to the liquid level difference returned by the feedback.

④ The submersible pumps in the inlet pump room operate, stop and control the number of operations according to the level of the liquid level.

⑤The rotary disc aerator is controlled by PLC calculation according to the analog quantity fed back by the dissolved oxygen meter, and controls the operation and stop of the separator at the same time.

⑥ The operation and stop of the sludge return pump are controlled according to the level of the liquid level.

⑦ In the sludge dewatering system, the centrifugal dewatering machine is started in sequential control mode, and its equipment is started in sequence.



  Figure 3

2. Hardware system configuration

(1) PLC selection. According to the functional requirements of the electrical control system of the sewage treatment system and its complexity, from the aspects of economy and reliability, NA-400PLC is selected as the control host of the electrical control system of the sewage treatment system. The system involves many digital input/output points, and it also needs to collect analog and use the functional requirements of analog control, so it is also necessary to expand the analog input/output module. The specific configuration is shown in Table 1:




(2) PLC I/O point resource configuration

① Digital input part. In this control system, the required input is basically a digital input. Mainly includes various control buttons, rotary buttons and digital inputs, a total of 31 digital inputs. As shown in table 2.



Table 2

②Digital output part. In this control system, the main output control devices are various contactors, valves, etc., with a total of 19 output points. The specific allocation is shown in Table 3.



Table 3

③ Analog input part. Since the data fed back by a dissolved oxygen meter needs to be collected, an analog input and output module is expanded. The details are shown in Table 4.



Table 4

④Analog output part. In this control system, the collected analog quantity needs to be processed, and then the inverter is controlled through the analog output port, and then the operation of other equipment is controlled. As shown in Table 5.



Table 5

(3) Other resource allocation

①Contactor This system needs 17 contactors in total: grille contactor, cleaning machine contactor, submersible pump contactor, separator contactor, rotary disc aerator contactor, submersible mixer contactor, scraper Contactors, etc.

② Frequency converter Use SIEMENS MM430 frequency converter, which is a special frequency converter for fan and pump load. In this control system, the inverter needs to be controlled by communication, so the parameters of the inverter need to be set first. Mainly adjust the following parameters, as shown in Table 6.




For the inverter of the secondary system, communication control is adopted during manual debugging, so the parameters of the inverter need to be changed.

③ Various buttons, man-machine interface, liquid level difference meter, dissolved oxygen meter, etc.

3. Software system design

In the process of designing a program, many intermediate relays, registers, timers, and other soft components are used. In order to facilitate programming and modification, list the possible soft elements before programming, as shown in Table 7.




(1) Manual control program:After the system is powered on and the manual mode is selected as the control mode, the operation of each device can be controlled by pressing the button on the panel. The manual setting is mainly for debugging during the initial installation of the production line, and checking whether each device can operate normally. The manual control procedure is as follows.







(2) Automatic control program:In production, most of them are controlled by an automatic process. The system controls the start and stop of the equipment and the speed control through the feedback signal of the sensor. The automatic control program is shown below.




(3) Function program:In this system, modular programming is used, and the function blocks involve coarse and fine grid machines, submersible pumps, decontamination systems, sludge return systems, and inverter communication. Here are only a few examples and focus on the function blocks that communicate with the inverter.

① Coarse grid machine system program to complete the control of coarse grid machine and cleaning machine, the system ladder diagram is as follows.




② Aeration and grit system program, to complete the control of the operation, speed and stop of the rotary aerator, the ladder diagram of aeration and grit system is as follows.







③ The inverter reads the parameter program, which is used for the output frequency of the inverter. The ladder program for the inverter read parameters is shown below.




④PID instruction, used for analog quantity processing, output data AOUT to %QW0001, used to output analog quantity to control the operation of other equipment.



1. Hardware issues

In the sewage treatment system, the main hardware issues include the mechanical structure and PLC peripheral circuit design and wiring.

The problem of mechanical structure is mainly the installation position of the turntable aerator. It needs to be placed in a suitable position according to the structure of the oxidation tank. If the location is appropriate, a small amount of aerator can be used to complete the oxygen supply function. In the peripheral hardware connection of PLC, it is mainly to add some protection equipment. Because the output is connected to the contactor and other components, the sudden opening and closing of the contactor will form a sudden waveform and damage the PLC output terminal. Therefore, it is necessary to install some protection devices, such as freewheeling diodes, to increase the life of the contacts.

2. Software issues

After the program is written, you need to perform software simulation on the computer first. The main purpose is to check whether there are various errors. You can use the simulation software that comes with NAPro. Then check the program for logic errors by simulating the hardware. When debugging, you can debug separately according to the functional module classification, and finally carry out overall debugging.

Design summary

This control system uses NA400 series PLC of Nanda Auto Technology Co., Ltd. as the core control equipment. In this system, a combination of timers and counters is used to achieve long-term timing. The control method of the frequency converter adopts analog quantity control, and the closed loop control is realized through the PID instruction, which meets the control requirements of the sewage treatment system. I would like to thank Zhang Yunfeng, an electrical engineer of Shandong Feiyang Environmental Engineering Co., Ltd., with his great help, the content of the article has been greatly enriched.

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Time of issue:2020-06-24 00:00:00

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