On-line dissolved oxygen (DO) analyzer measurement principle and maintenance

In the process of sewage treatment, the pollutants are decomposed through the activated sludge by increasing the oxygen content in the sewage, so as to achieve the purpose of sewage purification. On-line measurement of oxygen content helps determine the best purification method and the most economical aeration tank configuration. . The oxygen content measurement data in the biological fermentation process can guide the process, such as determining the critical oxygen concentration in the fermentation process, the oxygen supply capacity of the fermentor, the activity of the bacteria and the growth of the bacteria, etc., and according to the fermentation time Oxygen and aerobic changes guide the feeding operation.

First, the dissolved oxygen analyzer measurement principle The solubility of oxygen in water depends on temperature, pressure and dissolved salt in water. Dissolved oxygen analyzer sensing part is composed of gold electrode (cathode) and silver electrode (anode) and potassium chloride or potassium hydroxide electrolyte, oxygen diffusion through the membrane into the electrolyte and the gold electrode and silver electrode constitute a measurement loop. When a polarization voltage of 0.6 to 0.8 V is applied to the electrodes of the dissolved oxygen analyzer, oxygen diffuses through the membrane, the cathode releases electrons, and the anode accepts electrons to generate an electric current. The entire reaction process is: anode Ag Cl→AgCl 2 e−cathode O 2 2H 2 O 4e→4OH—According to Faraday's law: The current flowing through the electrode of the dissolved oxygen analyzer is proportional to the partial pressure of oxygen, and the linear relationship between the current and the oxygen concentration is obtained when the temperature is constant.

Second, the expression of dissolved oxygen content There are 3 different ways to express dissolved oxygen: oxygen partial pressure (mmHg); percent saturation (%); oxygen concentration (mg/L or 10-6), these three methods Essentially no different.

(1) Partial Pressure Representation: Oxygen partial pressure representation is the most basic and most essential representation. According to Henry's law, P=(Po2 PH2O)×0.209, where P is the total pressure; Po2 is the partial pressure of oxygen (mmHg); PH2O is the partial pressure of water vapor; 0.209 is the content of oxygen in the air.

(2) Percent saturation notation: Since the aeration fermentation is very complicated, the partial pressure of oxygen cannot be calculated. In this case, the expression of percent saturation is most appropriate. For example, when the dissolved oxygen is set at 100% during calibration and 0% at zero oxygen, the dissolved oxygen content during the reaction is the percentage of the calibration.

(3) Oxygen Concentration Representation: According to Henry's law, the oxygen concentration is proportional to its partial pressure, ie: C=Po2×a, where C is oxygen concentration (mg/L); Po2 is oxygen partial pressure (mmHg); a is Solubility coefficient (mg/mmHg·L). The solubility coefficient a is not only related to the temperature but also to the composition of the solution. For a constant temperature aqueous solution, a is a constant, then the oxygen concentration can be measured. The oxygen concentration notation is not commonly used in the fermentation industry, but it is expressed in terms of oxygen concentration in sewage treatment, drinking water, and other processes.

Third, the factors that affect the measurement of dissolved oxygen The solubility of oxygen depends on the temperature, pressure and salt dissolved in water. In addition, diffusion of oxygen through the solution is faster than diffusion through the membrane. If the flow rate is too slow, interference will occur.

1. Influence of temperature As the temperature changes, the diffusion coefficient of the film and the solubility of oxygen will change, directly affecting the current output of the dissolved oxygen electrode. The thermistor is often used to eliminate the influence of temperature. As the temperature rises, the diffusion coefficient increases and the solubility decreases. The influence of temperature on the solubility coefficient a can be estimated according to Henry's law, and the temperature on the membrane diffusion coefficient β can be estimated by Arrhenius' law.

(1) Oxygen solubility coefficient: Since the solubility coefficient a is not only influenced by the temperature, but also by the composition of the solution. The actual oxygen concentration of different components may also be different at the same oxygen partial pressure. According to Henry's Law, it is known that the oxygen concentration is proportional to the partial pressure thereof, and for dilute solutions, the temperature change solubility coefficient a changes by about 2%/°C.

(2) Diffusion coefficient of the membrane: According to Arrhenius' law, the relationship between solubility coefficient β and temperature T is: C=KPo2·exp(-β/T), where K and Po2 are assumed to be constant and can be calculated β is 2.3%/°C at 25°C. When the solubility coefficient a is calculated, the diffusion coefficient of the membrane can be calculated by comparison between the instrument indication and the assay analysis value (the calculation process is omitted here). The diffusion coefficient of the membrane is 1.5%/°C at 25°C.

2. Influence of atmospheric pressure According to Henry's law, the solubility of gas is proportional to its partial pressure. The partial pressure of oxygen is related to the altitude of the area. The difference between the plateau area and the plain area can reach 20%. Before use, compensation must be made according to the local atmospheric pressure. Some instruments have a barometer inside, which can be automatically calibrated at the time of calibration. Some instruments are not equipped with a barometer. The calibration time must be set according to the data provided by the local weather station. If the data is incorrect, it will result in a large measurement error.

3. Salt content in solution The dissolved oxygen in salt water is significantly lower than that in tap water. In order to measure accurately, the effect of salt content on dissolved oxygen must be considered. When the temperature is constant, the dissolved oxygen decreases by about 1% for every 100 mg/L increase in salt content. If the salt content of the solution used by the meter at the time of calibration is low, and the salt content of the actually measured solution is high, it will also cause errors. In actual use, the salt content of the measuring medium must be analyzed in order to accurately measure and correctly compensate.

4. The flow rate of oxygen through the membrane diffusion is slower than diffusion through the sample, and it must be ensured that the electrode membrane is in complete contact with the solution. For the flow-through detection method, oxygen in the solution will diffuse into the flow cell, causing loss of oxygen in the solution near the membrane, causing diffusion interference and affecting the measurement. For accurate measurements, the flow of the solution through the membrane should be increased to compensate for the oxygen lost by diffusion, and the minimum sample flow rate is 0.3 m/s. Four attention to the problem of dissolved oxygen analyzer, as long as the selection, setting, proper maintenance, generally can meet the measurement requirements of the process.

Disadvantages of the use of the dissolved oxygen analyzer are as follows: incorrect use and maintenance; abnormal temperature compensation due to internal leakage of the electrode; and reduced input impedance of the electrode.

1. Daily maintenance The daily maintenance of the instrument mainly includes the periodic cleaning, calibration, and regeneration of the electrodes.

(1) The electrode should be cleaned every 1 to 2 weeks. If there is any contamination on the diaphragm, it will cause measurement error. Care should be taken when cleaning, taking care not to damage the diaphragm. Rinse the electrode in clean water. If the dirt cannot be washed out, use a soft cloth or cotton cloth to scrub it carefully.

(2) The zero and span should be recalibrated from February to March.

(3) The regeneration of the electrodes takes place approximately once every year. When the measurement range is not adjusted, the dissolved oxygen electrode needs to be regenerated. Electrode regeneration includes changing the internal electrolyte, replacing the diaphragm, and cleaning the silver electrode. If the silver electrode is observed to be oxidized, it can be polished with fine sandpaper.

(4) If an electrode leak is found during use, the electrolyte must be replaced.

2. The calibration method of instrument calibration instrument can be calibrated by standard liquid or field sampling.

(1) Standard solution calibration method: The standard solution calibration generally adopts two-point calibration, namely zero calibration and span calibration. The zero calibration solution can use 2% Na2SO3 solution. The range calibration solution can be selected according to the instrument measurement range 4M KCl solution (2mg/L); 50% methanol solution (21.9mg/L).

(2) On-site sampling calibration method (Winkler method): In actual use, the Winkler method is used to perform on-site calibration of the dissolved oxygen analyzer. There are two situations when using this method: when sampling, the meter reading is M1, the assay analysis value is A, and when the meter is calibrated, the meter reading is still M1. At this time, only the meter reading should be adjusted to A; the meter reading is M1, the assay analysis value is A. When the meter is calibrated, the meter reading is changed to M2. At this time, the adjustment meter reading cannot be equal to A, and the meter reading should be adjusted to 1 MA×M2.

3. Problems to be Noted in Use The following problems should be noted during use: Since the signal impedance of the dissolved oxygen electrode is high (about 20 MΩ), the distance between the dissolved oxygen electrode and the converter is a maximum of 50 m; the dissolved oxygen electrode should also be at work when it is not used. State, can be connected to the dissolved oxygen converter. Electrodes with long or renewed regeneration (replacement of electrolytes or membranes) should be placed in an anaerobic environment for 1–2 h before use; due to temperature changes affecting the diffusion of the electrode membrane and the oxygen solubility, the calibration time needs to be higher For a long time (about 10 minutes) to balance the temperature compensation resistance; the oxygen partial pressure is related to the altitude of the area; the meter must be compensated according to the local atmospheric pressure before use; when the salt content of the measurement solution is high, the instrument calibration time should be Use a solution with a similar salt content; for the flow measurement method, the minimum flow rate through the electrode is required to be 0.3 m/s.