Design and Application of the Automatic Instrument in Sewage Treatment
Time:2022-11-25 Read:684
Reading guide: In the new era, water pollution is becoming more and more serious, especially urban and industrial sewage discharge continues to increase, leading to water pollution gradually serious. Our government pays more and more attention to the protection of water resources and takes a series of measures to strengthen the treatment of urban and industrial sewage, promote the sewage treatment process to achieve the best treatment effect, and the treatment equipment can operate normally and stably, thus putting forward higher requirements for automatic instruments in sewage treatment, and relevant enterprises must pay high attention.

Relevant personnel shall monitor the impurities and content parameters in the sewage and take corresponding measures to deal with the problem. At present, in the actual monitoring, because the impurity content of sewage has various and complex characteristics, the manual measurement method not only increases the work amount but also is difficult to achieve the goal of material composition measurement. The automatic instrument design can automatically measure sewage materials, provide accurate sewage material cost, and has been widely used in sewage treatment.

Importance of automatic instruments in sewage treatment

The continuity of sewage treatment is strong, the process is complex, the equipment is numerous, and the management is difficult. In order to achieve the safe and long-term operation of sewage treatment equipment and treatment process, it is necessary to scientifically manage the entire production process of sewage treatment. The important value of automatic control of sewage treatment in today's society is gradually emerging. In the whole process of modern sewage treatment, no matter what method is used, Many parameters of these processes need to be tested. 

At this stage, many large-scale process sewage and domestic sewage treatment plants are equipped with automatic instruments such as online total phosphorus, online ammonia nitrogen, online COD, online BOD, redox potential, temperature, pH value, and liquid level difference. Through the use of these automatic instruments, various process parameters are detected, and they are transmitted to the DCS system and other upper computers. Through the upper computer, different process equipment in the process construction is scientifically and effectively controlled, Coordinate the relationship between sewage treatment processes, supply, and demand, and promotes the full and reasonable utilization of different facilities and equipment. At the same time, some important process parameters in the design and application of automatic instruments, such as the measurement accuracy of aeration rate and dissolved oxygen value, are the preconditions for ensuring automatic control. 

The measured values of these instruments can automatically control and adjust the process equipment and effectively guide the sewage treatment process. In addition, the automation instrument and DCS system, and other upper computers, can alarm the process in real-time to ensure the safety of equipment and production. It can be seen that the automation instrument plays a very important role in the sewage treatment process.

Importance of automatic instruments in sewage treatment

Specific applications of automatic instruments in sewage treatment

Application of temperature instrument in sewage treatment

Generally, an anaerobic digestion process is required in the process of sewage treatment. This process requires dynamic monitoring with temperature instruments and pressure instruments. The purpose is to facilitate the staff to properly adjust the anaerobic digestion conditions in combination with their dynamics, so as to obtain the best effect. In the application process of temperature instruments, the most commonly used thermal element is a platinum thermistor. However, considering the small number of temperature control points of the platinum thermistor, it is usually necessary to use it together with the temperature transmitter. Through the comprehensive application of the two, the thermal signal can be converted into an electrical signal of 4~20mA, and then connected to the analog module of the electrical signal, so that the data can be extracted and the entire processing process can be simplified.

Application of flow meter in sewage treatment

In sewage treatment, flow meters play a very important role, mainly in the detection of sewage volume, sewage return flow, aeration volume, and other parameters, providing a detailed and reliable basis for the control of sewage treatment operating conditions. At the same time, the flow meter can also reasonably judge the operation of the equipment in sewage treatment. At present, the flow meters in sewage treatment mainly include ultrasonic flowmeters, differential pressure flowmeters, and electromagnetic flowmeters. In the design stage, the most appropriate one should be selected according to the actual needs of the user to ensure the effectiveness of sewage treatment.

The flow meter used by a certain unit is an electromagnetic flowmeter. The working principle of the electromagnetic flowmeter is the electromagnetic induction principle. It measures the flow of the conductive fluid according to the induction electric magnetic field generated by the conductive fluid when the external magnetic field is applied. The characteristics of the flowmeter are that it is not affected by the change in fluid density, temperature, viscosity, and conductivity, and the induced electrical signal of the sensor has a linear relationship with the average flow rate. The measurement has high accuracy, and it is suitable for measuring the fluid with small particles in sewage treatment.

Application of flow meter in sewage treatment

Application of COD analyzer in sewage treatment

Chemical oxygen demand (COD) refers to the amount of strong oxidant required for sewage treatment per unit volume under certain conditions. It is an important part of the indicator system for measuring sewage treatment effects. It can also be used to measure the organic content in sewage. The greater the COD, the higher the organic content in sewage, and the more serious the water pollution. COD measuring instruments are divided into two types: spectrophotometry and electrochemical measurement. 

The working principle of spectrophotometry for COD measurement is that when the mixed solution of the water sample, potassium dichromate, catalyst, and concentrated sulfuric acid is put into the digestion tank and heated to a certain temperature, the chromium ion is reduced from tetravalent to trivalent and the color of the liquid changes. The degree of discoloration is opposite to the organic content of the water sample, The instrument changes the COD value by comparing colors and changing colors. In addition, the COD analyzer is a kind of analytical instrument with high precision. Foreign countries are more mature in research, production, production, and technology. To ensure the accuracy of measurement, especially in the case of high process requirements, foreign products should be preferred.

When sewage treatment is carried out, it is necessary to strengthen the determination of the COD index, so as to continuously optimize the sewage treatment process and continuously improve the sewage treatment effect.

Application of ammonia nitrogen online monitor

At present, the methods commonly used for ammonia nitrogen online monitor monitoring in China are spectrophotometry and ammonia gas sensitive electrode method. The spectrophotometry is to mix ammonia in a water sample with potassium iodide and mercury iodide to form an alkaline mixed solution, produce a reddish brown colloidal solution through a chemical reaction, and measure it within the wavelength range of 410~424nm. At the same time, the main interference factor in the process of monitoring is the chemical substances that produce chromaticity and turbidity. In application, most ammonia nitrogen monitors use the ammonia gas-sensitive electrode method.

Application of dissolved oxygen meter in sewage treatment

Dissolved oxygen is closely related to the partial pressure of oxygen in the air, atmospheric pressure, and water temperature. If the dissolved oxygen in pure water is about 9mg/L at 20 ℃ and 100kPa, under the action of aerobic bacteria, the biodegradation of some organic compounds requires the consumption of dissolved oxygen in the water. It is calculated by organic carbon. According to the formula of C+O2=CO2, 12g carbon requires 32g oxygen. In addition, if the dissolved oxygen value in the water drops to 5mg/L, some fish will have difficulty breathing. Generally, there are two sources of dissolved oxygen: first, when the dissolved oxygen in water is not saturated, the oxygen in the atmosphere will seep into the water; Second, plants in water release oxygen through photosynthesis, and the dissolved oxygen in the water will be continuously supplemented with the oxygen dissolved in the air and the photosynthesis of green aquatic plants. However, if the water body is polluted by organic matter, the oxygen consumption is serious, and the dissolved oxygen cannot be supplemented in time, the anaerobic bacteria in the water body will be rapidly propagated, and the organic matter will cause the water body to become black and stinky due to corruption.

At the current stage, the three most widely used methods in sewage treatment are aerobic anaerobic microbial treatment, activated sludge treatment, and biofilm treatment. The content of dissolved oxygen in water plays a decisive role in whether microorganisms can survive in water. Therefore, it is necessary to ensure that there is enough dissolved oxygen in the water to support the activity of microorganisms. The dissolved oxygen meter plays a very important role in dissolution monitoring, The aeration rate can be well adjusted, and the problem of sludge filament bulking in the application of the activated sludge method can also be effectively avoided. Generally, a dissolved oxygen meter is composed of a sensor and a transmitter. Dissolved oxygen sensor includes two types of sensors: coated electrode and filmless electrode, which are both composed of a cathode, anode, and electrolyte. In the specific application, the relevant staff should scientifically and effectively set the installation position of the dissolved oxygen meter according to the actual situation of the sewage and the construction site to effectively avoid electrode scaling and pollution, so as to give full play to the use effect of the dissolved oxygen meter.

Application of dissolved oxygen meter in sewage treatment

Matters need attention in the design of the sewage treatment automation instrument

In the process of applying automatic instruments to sewage treatment, it is very easy to attach some impurities contained in sewage to the surface of the automatic instruments. If this is not handled in the long run, it will have a serious impact on the operation stability of the automatic instruments, thus damaging the accuracy of the instrument detection and measurement data. In the process of automatic instrument design, the problem of instrument cleaning must be solved. First of all, set the automation instrument at a position convenient for disassembly, and the staff can adjust the instrument according to the actual situation. Secondly, scientific and reasonable maintenance measures shall be formulated in combination with the working place and working environment of the automation instrument to make it operate stably for a long time.


To sum up, the automatic instrument plays a crucial role in the sewage treatment process. It is the basic part of the computer and automatic control, which can greatly reduce the workload and improve production efficiency. Relevant enterprises shall select appropriate automation instruments based on the actual situation and design automation instrument systems more perfectly to ensure their reasonable application.