In the process of sewage treatment, by increasing the oxygen content in the sewage, pollutants can be decomposed through activated mud to achieve the purpose of sewage purification. Online measurement of oxygen content helps to determine the best purification method and the most economical aeration tank configuration. In the fermentation process of measuring data in oxygen content can guide the process, such as to determine the critical oxygen concentration of oxygen capacity, fermentation process and cell activity and cell growth rate, and according to the changes of oxygen supply and oxygen fermentation to guide feeding operation.
1. Measurement principle of dissolved oxygen analyzer
The solubility of oxygen in water depends on the temperature and pressure and the dissolved salt in the water. The sensing part of the dissolved oxygen analyzer is composed of a gold electrode (cathode) and a silver electrode (anode) and potassium chloride or potassium hydroxide electrolyte. The oxygen diffuses through the membrane to enter the electrolyte and the gold electrode and the silver electrode to form a measuring loop. When adding the polarizing voltage of 0.6 to 0.8V to the dissolved oxygen analyzer electrode, the oxygen diffuses through the membrane, the cathode releases electrons, the anode accepts electrons, and generates the electric current. The whole reaction process is
AgCl+2e-<br / Ag+Cl&rarr anode;
Cathode 4OH-<br / O2+2H2O+4e→
According to Faraday's law, the current of the electrode flowing through the dissolved oxygen analyzer is directly proportional to the oxygen partial pressure. When the temperature is constant, there is a linear relationship between the current and the oxygen concentration.
Two, the expression method of dissolved oxygen content
There are 3 different ways to express dissolved oxygen: oxygen partial pressure (mmHg), percent saturation (%) and oxygen concentration (mg/L or 10-6). These 3 methods are essentially different.
(1) partial pressure representation: oxygen partial pressure representation is the most basic and most essential expression method. According to Henry's law, we can get P= (Po2+P H2O) × 0.209, P is total pressure; Po2 is oxygen partial pressure (mmHg); P H2O is water vapor partial pressure; 0.209 is oxygen content in air.
(2) percentage saturation representation: because the aeration fermentation is very complex, the partial pressure of oxygen can not be calculated. In this case, the expression of percentile saturation is the most suitable. For example, the dissolved oxygen in the calibration is determined to be 100%, and the zero oxygen is 0%. The dissolved oxygen content in the reaction process is the percentage of the standard timing.
(3) oxygen concentration representation: according to Henry's law, oxygen concentration is directly proportional to its partial pressure, that is: 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 is related to the composition of the solution. For a water solution with constant temperature, the A is constant and the concentration of oxygen can be measured. Oxygen concentration representation is not commonly used in the fermentation industry, but in the process of sewage treatment and drinking water, the oxygen concentration is used to indicate the oxygen concentration.
Three. Factors affecting the measurement of dissolved oxygen
The solubility of oxygen depends on the temperature, pressure and dissolved salts in the water, and the diffusion of oxygen is fast through the diffusion ratio of the solution through the membrane, such as the slow velocity of flow, which will cause interference.
1. effect of temperature
Because of the change of temperature, the diffusion coefficient and the solubility of oxygen will change. It will directly affect the output of dissolved oxygen electrode, and thermistor is often used to eliminate the influence of temperature. When the temperature rises, the diffusion coefficient increases and the solubility decreases. The effect of temperature on the solubility coefficient a can be estimated according to the Henry law. The temperature of the film diffusion coefficient &beta can be estimated by Alen Ni Lucius's law.
(1) the solubility coefficient of oxygen: because the solubility coefficient a is not only influenced by the temperature, but also influenced by the composition of the solution. The actual oxygen concentration of different components may be different under the same oxygen partial pressure. According to Henry's law, it is found that the oxygen concentration is proportional to the partial pressure. For the dilute solution, the change of the temperature change coefficient a is about 2%/ C.
(2) the diffusion coefficient of membrane: according to Alen Ni Lucius's law, the solubility coefficient &beta, and the relationship between temperature and T are C=KPo2·, exp (-&beta, T), assuming that K and Po2 are constants, then &beta can be calculated; 2.3%/ temperature is 25 degrees. When the solubility coefficient a is calculated, the diffusion coefficient of the membrane can be calculated through the comparison between the instrument indication and the test analysis value. The diffusion coefficient of the membrane is 1.5%/ C at 25 C.
The effect of the 2. atmospheric pressure is based on the Henry law, and the solubility of the gas is proportional to its partial pressure. The oxygen partial pressure is related to the altitude of the area. The difference between the plateau and the plain area is up to 20%. It must be compensated according to the local atmospheric pressure before use. Some instruments are equipped with barometer, which can be automatically calibrated while calibrating. Some meters are not equipped with barometer. When calibrating, they must be set according to the data provided by the local weather stations. If the data is wrong, it will lead to larger measurement errors.
In 3. solution, the dissolved oxygen in salt water is obviously lower than the dissolved oxygen in tap water. In order to accurately measure the dissolved oxygen, the effect of salt content on dissolved oxygen must be considered. When the temperature is constant, the salt content increases by 100mg/L, and the dissolved oxygen is reduced by about 1%. If the salt content of the solution used in the calibration is low and the salt content of the actual measured solution is high, it can also lead to error. In practical use, the salt content of the measured medium must be analyzed in order to accurately measure and correct the compensation.
4. the flow rate of oxygen in the sample is slow through the diffusion ratio of the film to the sample, and it is necessary to ensure that the electrode membrane is in full contact with the solution. For the flow detection, the oxygen in the solution diffuses into the circulation pool, causing the oxygen loss in the solution near the membrane, causing diffusion interference and affecting the measurement. In order to measure accurately, the flow of the solution flowing through the membrane should be increased to compensate for the diffusion of the lost oxygen.