There are two types of analyzers used in sewage treatment plants: pH meters and dissolved oxygen analyzers.
First, the working principle 1. pH meter working principle Water pH value with the amount of material dissolved, so the pH value can be sensitive to indicate the change in water quality. The change of pH value has great influence on the reproduction and survival of organisms, and it also seriously affects the biochemical action of activated sludge, that is, it affects the treatment effect. The pH value of sewage is generally controlled between 6.5 and 7.
Water is chemically neutral, and some water molecules spontaneously decompose according to the following formula: H2O = H + + OH-, which decomposes into hydrogen ions and hydroxide ions. In the neutral solution, the concentrations of the hydrogen ion H+ and the hydroxide ion OH- are both 10-7 mol/l, and the pH is the negative number of the logarithm of the hydrogen ion concentration at the base of 10: pH = -log, so neutral The pH of the solution is equal to 7. If there is an excess of hydrogen ions, the pH is less than 7 and the solution is acidic; conversely, if the hydroxide ion is excessive, the solution is alkaline.
The pH is usually measured by the potentiometric method. Usually a constant potential reference electrode and measuring electrode form a primary cell. The size of the electromotive force of the primary cell depends on the concentration of hydrogen ions and on the pH of the solution. The CPS11 pH sensor and CPM151 pH transmitter were used. There are special glass probes sensitive to pH on the measuring electrode. It is made of special glass that can conduct electricity and can penetrate hydrogen ions. It has the characteristics of high measurement accuracy and good anti-interference. When the glass probe is in contact with hydrogen ions, a potential is generated. The potential is measured by a silver control reference electrode suspended in a silver chloride solution. With different pH values, the corresponding potentials are also different. It is converted by the transmitter to a standard 4-20mA output.
2. How Does the Dissolved Oxygen Analyzer Work? The oxygen content of water can fully show the degree of water self-purification. For a biological treatment plant using activated sludge, it is very important to know the oxygen content of the aeration tank and the oxidation ditch. The increase of dissolved oxygen in the sewage will promote biological activities besides anaerobic microorganisms, thus it can remove volatile substances and be easy to use. Naturally oxidized ions purify sewage.
There are three main methods for determining oxygen content: automatic colorimetric analysis and chemical analysis measurement, paramagnetic measurement, and electrochemical measurement. Dissolved oxygen in water is generally measured electrochemically. The plant uses a COS4 dissolved oxygen sensor and a COM252 dissolved oxygen transmitter.
Oxygen is soluble in water and the solubility depends on the temperature, the total pressure on the surface of the water, the partial pressure, and the dissolved salts in the water. The higher the atmospheric pressure, the greater the ability of water to dissolve oxygen. The relationship is determined by Henry's law and Dalton's law. Henry's law states that the solubility of gas is proportional to its partial pressure.
Take the COS4 oxygen measurement sensor as an example. The electrode consists of a cathode (usually made of gold and platinum), a current-carrying counter electrode (silver), and a current-free reference electrode (silver). The electrode is immersed in electrolytes such as KCl and KOH. The sensor is covered with a membrane and the membrane The electrodes and the electrolyte are separated from the liquid being measured, thus protecting the sensor, preventing both the escape of the electrolyte and the ingress of foreign substances, resulting in contamination and poisoning.
A polarizing voltage is applied between the counter electrode and the cathode. If the measuring element is immersed in water with dissolved oxygen, oxygen diffuses through the membrane and oxygen molecules that are present at the cathode (excess electrons) are reduced to hydroxide ions:
Electrochemically equivalent silver chloride precipitates on the counter electrode (electron deficiency): 4Ag+4Cl-®4AgCl+4e-.
For each oxygen molecule, the cathode emits 4 electrons. The counter electrode receives electrons and forms a current. The current is proportional to the oxygen partial pressure of the tested effluent. This signal is sent to the transformer along with the temperature signal measured by the thermal resistance of the sensor. The sender calculates the oxygen content in the water using the relationship curve between oxygen content, oxygen partial pressure, and temperature stored in the sensor, and then converts it into a standard signal output. The function of the reference electrode is to determine the cathode potential.
The response time of the COS4 DO sensor was 90% of the final measurement after 3 minutes and 99% of the final measurement after 9 minutes; the minimum flow rate requirement was 0.5 cm/s.
II. Features 1. Characteristics of the pH meter The glass on the pH electrode will gradually age with time, and the gradient (the change value of the electrode output potential caused by the change of the unit pH) will deteriorate, and it will take a long time to reach the stable potential. The service life of general electrodes can reach up to two years. In addition, the temperature has a great influence on the aging, and the degree of aging stored at 100° C. for several weeks is equivalent to the aging degree stored at room temperature for one year.
The pH meter has the advantages of high accuracy, high reliability, convenient installation and maintenance, etc. At the same time, it is also sensitive to pollution and requires frequent calibration. Generally, it is calibrated every other one to one and a half months, and the electrodes are replaced once every two years.
2. Features of Dissolved Oxygen Meter Dissolved Oxygen Meter has the characteristics of convenient installation, long calibration period (3 to 4 months), insensitivity to other substances, and can monitor the use of electrolytes in diaphragms and probes, generally every one to three years. Replace the electrolyte and membrane once.
COM025 type dissolved oxygen transmitter is an intelligent instrument with HART or Profibus communication protocol, and also has self-diagnosis function. When a fault occurs, the fault code will be displayed to indicate the fault of the maintenance personnel. Find out the fault by checking the maintenance manual. The approach greatly reduces maintenance time and workload.
Third, installation and maintenance 1. pH meter installation and maintenance pH meter installation methods are flow-through and immersed two. Sewage treatment plants are generally selected for immersed installations. For example, the pH meter of the wastewater treatment plant is installed in the overflow gutter of the oxidation ditch. The pH value here is more representative, and the water flow is steady, which will not cause any problem to the pH meter. Big impact.
Regular maintenance helps to measure the instrument accurately and extend the life of the instrument. It should be noted that the special cable between the sensor and the transmitter must not be exposed to moisture, otherwise the electrode's high-impedance, low-voltage signal will not be transmitted to the transmitter. If the electrode does not measure, it should be put on the yellow protective sleeve, it can make the electrode in a wet state, which is conducive to prolong the service life of the electrode. Every other month or so, the electrode should be cleaned, and then the attachments should be sprayed with soft water, then the electrode should be soaked in the cleaning solution for a period of time, and then washed with clean water. The sensor bracket should also be cleaned. After each cleaning, the buffer solution is used for calibration. At present, the domestic pH calibration solution may have pH=4, and pH=7 may not be accurate, which may affect the calibration results. Therefore, it is best to use the manufacturer's calibration solution, the manufacturer generally provides two bottles of standard solution, a bottle of pH equal to 7, for the calibration instrument zero; a bottle of pH equal to 4, for calibration of the instrument output signal slope.
2. Dissolution and Installation of Dissolved Oxygen Meters Dissolved oxygen meters are generally immersed and installed. It should be noted here that the original mounting bracket must be used. The mounting bracket provided by the manufacturer is made of stainless steel and has a plastic chain. The length of the sensor can be changed by adjusting the length of the chain. The guide tube on the bracket ensures that the sensor is always in a vertical position. The bracket part is specially designed to transmit the fluctuation of the water surface to the dip tube, causing slight vibration of the dip tube, so that an additional cleaning effect is produced on the surface of the probe through the dip tube. In order to reduce the investment, some users make their own mounting brackets, which often leads to poor sealing between the dip tube and the sensor on the bracket, which causes the sewage to infiltrate. This makes the connection between the special cable and the sensor soaked in the sewage for a long time, which may easily damage the sensor. Some do not even install a bracket and directly put the sensor into the water, so that a large pulling force is generated between the sensor and the cable, and the sensor is more easily damaged.
Dissolved oxygen probes were gently washed with water every week and it was found that the membrane head damage should be promptly replaced, and the electrolyte should be promptly replaced when it is contaminated. When the sewage contains H2S, NH3, benzene or phenol, they are harmful to the membrane head. In this case, the membrane head must be replaced frequently. To judge whether the electrode in the probe is good or bad, just look at the color, the reference electrode should be dark gray, the cathode (gold electrode) should be yellow, and the counter electrode must be bright, otherwise it should be cleaned or regenerated.
With the increasing emphasis on the protection of water resources in China, the purification of sewage becomes more and more important, and the detection instruments necessary for the processing process are indispensable. The water quality analyzer is the most important instrument in the sewage treatment industry. Besides the correct selection and installation, regular maintenance and calibration are also very important, and it is the key to making the instrument work.
First, the working principle 1. pH meter working principle Water pH value with the amount of material dissolved, so the pH value can be sensitive to indicate the change in water quality. The change of pH value has great influence on the reproduction and survival of organisms, and it also seriously affects the biochemical action of activated sludge, that is, it affects the treatment effect. The pH value of sewage is generally controlled between 6.5 and 7.
Water is chemically neutral, and some water molecules spontaneously decompose according to the following formula: H2O = H + + OH-, which decomposes into hydrogen ions and hydroxide ions. In the neutral solution, the concentrations of the hydrogen ion H+ and the hydroxide ion OH- are both 10-7 mol/l, and the pH is the negative number of the logarithm of the hydrogen ion concentration at the base of 10: pH = -log, so neutral The pH of the solution is equal to 7. If there is an excess of hydrogen ions, the pH is less than 7 and the solution is acidic; conversely, if the hydroxide ion is excessive, the solution is alkaline.
The pH is usually measured by the potentiometric method. Usually a constant potential reference electrode and measuring electrode form a primary cell. The size of the electromotive force of the primary cell depends on the concentration of hydrogen ions and on the pH of the solution. The CPS11 pH sensor and CPM151 pH transmitter were used. There are special glass probes sensitive to pH on the measuring electrode. It is made of special glass that can conduct electricity and can penetrate hydrogen ions. It has the characteristics of high measurement accuracy and good anti-interference. When the glass probe is in contact with hydrogen ions, a potential is generated. The potential is measured by a silver control reference electrode suspended in a silver chloride solution. With different pH values, the corresponding potentials are also different. It is converted by the transmitter to a standard 4-20mA output.
2. How Does the Dissolved Oxygen Analyzer Work? The oxygen content of water can fully show the degree of water self-purification. For a biological treatment plant using activated sludge, it is very important to know the oxygen content of the aeration tank and the oxidation ditch. The increase of dissolved oxygen in the sewage will promote biological activities besides anaerobic microorganisms, thus it can remove volatile substances and be easy to use. Naturally oxidized ions purify sewage.
There are three main methods for determining oxygen content: automatic colorimetric analysis and chemical analysis measurement, paramagnetic measurement, and electrochemical measurement. Dissolved oxygen in water is generally measured electrochemically. The plant uses a COS4 dissolved oxygen sensor and a COM252 dissolved oxygen transmitter.
Oxygen is soluble in water and the solubility depends on the temperature, the total pressure on the surface of the water, the partial pressure, and the dissolved salts in the water. The higher the atmospheric pressure, the greater the ability of water to dissolve oxygen. The relationship is determined by Henry's law and Dalton's law. Henry's law states that the solubility of gas is proportional to its partial pressure.
Take the COS4 oxygen measurement sensor as an example. The electrode consists of a cathode (usually made of gold and platinum), a current-carrying counter electrode (silver), and a current-free reference electrode (silver). The electrode is immersed in electrolytes such as KCl and KOH. The sensor is covered with a membrane and the membrane The electrodes and the electrolyte are separated from the liquid being measured, thus protecting the sensor, preventing both the escape of the electrolyte and the ingress of foreign substances, resulting in contamination and poisoning.
A polarizing voltage is applied between the counter electrode and the cathode. If the measuring element is immersed in water with dissolved oxygen, oxygen diffuses through the membrane and oxygen molecules that are present at the cathode (excess electrons) are reduced to hydroxide ions:
Electrochemically equivalent silver chloride precipitates on the counter electrode (electron deficiency): 4Ag+4Cl-®4AgCl+4e-.
For each oxygen molecule, the cathode emits 4 electrons. The counter electrode receives electrons and forms a current. The current is proportional to the oxygen partial pressure of the tested effluent. This signal is sent to the transformer along with the temperature signal measured by the thermal resistance of the sensor. The sender calculates the oxygen content in the water using the relationship curve between oxygen content, oxygen partial pressure, and temperature stored in the sensor, and then converts it into a standard signal output. The function of the reference electrode is to determine the cathode potential.
The response time of the COS4 DO sensor was 90% of the final measurement after 3 minutes and 99% of the final measurement after 9 minutes; the minimum flow rate requirement was 0.5 cm/s.
II. Features 1. Characteristics of the pH meter The glass on the pH electrode will gradually age with time, and the gradient (the change value of the electrode output potential caused by the change of the unit pH) will deteriorate, and it will take a long time to reach the stable potential. The service life of general electrodes can reach up to two years. In addition, the temperature has a great influence on the aging, and the degree of aging stored at 100° C. for several weeks is equivalent to the aging degree stored at room temperature for one year.
The pH meter has the advantages of high accuracy, high reliability, convenient installation and maintenance, etc. At the same time, it is also sensitive to pollution and requires frequent calibration. Generally, it is calibrated every other one to one and a half months, and the electrodes are replaced once every two years.
2. Features of Dissolved Oxygen Meter Dissolved Oxygen Meter has the characteristics of convenient installation, long calibration period (3 to 4 months), insensitivity to other substances, and can monitor the use of electrolytes in diaphragms and probes, generally every one to three years. Replace the electrolyte and membrane once.
COM025 type dissolved oxygen transmitter is an intelligent instrument with HART or Profibus communication protocol, and also has self-diagnosis function. When a fault occurs, the fault code will be displayed to indicate the fault of the maintenance personnel. Find out the fault by checking the maintenance manual. The approach greatly reduces maintenance time and workload.
Third, installation and maintenance 1. pH meter installation and maintenance pH meter installation methods are flow-through and immersed two. Sewage treatment plants are generally selected for immersed installations. For example, the pH meter of the wastewater treatment plant is installed in the overflow gutter of the oxidation ditch. The pH value here is more representative, and the water flow is steady, which will not cause any problem to the pH meter. Big impact.
Regular maintenance helps to measure the instrument accurately and extend the life of the instrument. It should be noted that the special cable between the sensor and the transmitter must not be exposed to moisture, otherwise the electrode's high-impedance, low-voltage signal will not be transmitted to the transmitter. If the electrode does not measure, it should be put on the yellow protective sleeve, it can make the electrode in a wet state, which is conducive to prolong the service life of the electrode. Every other month or so, the electrode should be cleaned, and then the attachments should be sprayed with soft water, then the electrode should be soaked in the cleaning solution for a period of time, and then washed with clean water. The sensor bracket should also be cleaned. After each cleaning, the buffer solution is used for calibration. At present, the domestic pH calibration solution may have pH=4, and pH=7 may not be accurate, which may affect the calibration results. Therefore, it is best to use the manufacturer's calibration solution, the manufacturer generally provides two bottles of standard solution, a bottle of pH equal to 7, for the calibration instrument zero; a bottle of pH equal to 4, for calibration of the instrument output signal slope.
2. Dissolution and Installation of Dissolved Oxygen Meters Dissolved oxygen meters are generally immersed and installed. It should be noted here that the original mounting bracket must be used. The mounting bracket provided by the manufacturer is made of stainless steel and has a plastic chain. The length of the sensor can be changed by adjusting the length of the chain. The guide tube on the bracket ensures that the sensor is always in a vertical position. The bracket part is specially designed to transmit the fluctuation of the water surface to the dip tube, causing slight vibration of the dip tube, so that an additional cleaning effect is produced on the surface of the probe through the dip tube. In order to reduce the investment, some users make their own mounting brackets, which often leads to poor sealing between the dip tube and the sensor on the bracket, which causes the sewage to infiltrate. This makes the connection between the special cable and the sensor soaked in the sewage for a long time, which may easily damage the sensor. Some do not even install a bracket and directly put the sensor into the water, so that a large pulling force is generated between the sensor and the cable, and the sensor is more easily damaged.
Dissolved oxygen probes were gently washed with water every week and it was found that the membrane head damage should be promptly replaced, and the electrolyte should be promptly replaced when it is contaminated. When the sewage contains H2S, NH3, benzene or phenol, they are harmful to the membrane head. In this case, the membrane head must be replaced frequently. To judge whether the electrode in the probe is good or bad, just look at the color, the reference electrode should be dark gray, the cathode (gold electrode) should be yellow, and the counter electrode must be bright, otherwise it should be cleaned or regenerated.
With the increasing emphasis on the protection of water resources in China, the purification of sewage becomes more and more important, and the detection instruments necessary for the processing process are indispensable. The water quality analyzer is the most important instrument in the sewage treatment industry. Besides the correct selection and installation, regular maintenance and calibration are also very important, and it is the key to making the instrument work.
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