46.What is dissolved oxygen?
Dissolved oxygen DO (abbreviation for Dissolved Oxygen in English) represents the amount of molecular oxygen dissolved in water, and the unit is mg/L. The saturated content of dissolved oxygen in water is related to water temperature, atmospheric pressure and the chemical composition of water. At one atmospheric pressure, the oxygen content when dissolved oxygen in distilled water reaches saturation at 0oC is 14.62mg/L, and at 20oC it is 9.17mg/L. An increase in water temperature, an increase in salt content, or a decrease in atmospheric pressure will cause the dissolved oxygen content in the water to decrease.
Dissolved oxygen is an essential substance for the survival and reproduction of fish and aerobic bacteria. If the dissolved oxygen is lower than 4mg/L, it will be difficult for fish to survive. When water is contaminated by organic matter, the oxidation of organic matter by aerobic microorganisms will consume the dissolved oxygen in the water. If it cannot be replenished from the air in time, the dissolved oxygen in the water will gradually decrease until it is close to 0, causing a large number of anaerobic microorganisms to multiply. Make the water black and smelly.
47. What are the commonly used methods for measuring dissolved oxygen?
There are two commonly used methods for measuring dissolved oxygen, one is the iodometric method and its correction method (GB 7489–87), and the other is the electrochemical probe method (GB11913–89). The iodometric method is suitable for measuring water samples with dissolved oxygen greater than 0.2 mg/L. Generally, the iodometric method is only suitable for measuring dissolved oxygen in clean water. When measuring dissolved oxygen in industrial wastewater or various process steps of sewage treatment plants, corrected iodine must be used. quantitative method or electrochemical method. The lower limit of determination of the electrochemical probe method is related to the instrument used. There are mainly two types: the membrane electrode method and the membraneless electrode method. They are generally suitable for measuring water samples with dissolved oxygen greater than 0.1mg/L. The online DO meter installed and used in aeration tanks and other places in sewage treatment plants uses the membrane electrode method or the membrane-less electrode method.
The basic principle of the iodometric method is to add manganese sulfate and alkaline potassium iodide to the water sample. The dissolved oxygen in the water oxidizes low-valent manganese to high-valent manganese, generating a brown precipitate of tetravalent manganese hydroxide. After adding acid, the brown precipitate dissolves and It reacts with iodide ions to generate free iodine, and then uses starch as an indicator and titrates the free iodine with sodium thiosulfate to calculate the dissolved oxygen content.
When the water sample is colored or contains organic matter that can react with iodine, it is not suitable to use the iodometric method and its correction method to measure the dissolved oxygen in the water. Instead, an oxygen-sensitive film electrode or a membrane-less electrode can be used for measurement. The oxygen-sensitive electrode consists of two metal electrodes in contact with the supporting electrolyte and a selective permeable membrane. The membrane can only pass through oxygen and other gases, but water and soluble substances in it cannot pass. The oxygen passing through the membrane is reduced on the electrode. A weak diffusion current is generated, and the size of the current is proportional to the dissolved oxygen content at a certain temperature. The filmless electrode is composed of a special silver alloy cathode and an iron (or zinc) anode. It does not use a film or electrolyte, and no polarization voltage is added between the two poles. It only communicates with the two poles through the measured aqueous solution to form a primary battery, and the oxygen molecules in the water are Reduction is performed directly on the cathode, and the reduction current generated is proportional to the oxygen content in the solution being measured.
48. Why is the dissolved oxygen indicator one of the key indicators for the normal operation of the wastewater biological treatment system?
Maintaining a certain amount of dissolved oxygen in the water is the basic condition for the survival and reproduction of aerobic aquatic organisms. Therefore, the dissolved oxygen indicator is also one of the key indicators for the normal operation of the sewage biological treatment system.
The aerobic biological treatment device requires the dissolved oxygen in the water to be above 2 mg/L, and the anaerobic biological treatment device requires the dissolved oxygen to be below 0.5 mg/L. If you want to enter the ideal methanogenesis stage, it is best to have no detectable dissolved oxygen (for 0), and when section A of the A/O process is in an anoxic state, the dissolved oxygen is preferably 0.5~1mg/L. When the effluent from the secondary sedimentation tank of the aerobic biological method is qualified, its dissolved oxygen content is generally not less than 1mg/L. If it is too low (<0.5mg/L) or too high (air aeration method >2mg/L), it will cause water effluent. Water quality deteriorates or even exceeds standards. Therefore, full attention should be paid to monitoring the dissolved oxygen content inside the biological treatment device and the effluent of its sedimentation tank.
Iodometric titration is not suitable for on-site testing, nor can it be used for continuous monitoring or on-site determination of dissolved oxygen. In the continuous monitoring of dissolved oxygen in sewage treatment systems, the membrane electrode method in the electrochemical method is used. In order to continuously grasp the changes in DO of the mixed liquid in the aeration tank during the sewage treatment process in real time, an online electrochemical probe DO meter is generally used. At the same time, the DO meter is also an important part of the automatic control and adjustment system of dissolved oxygen in the aeration tank. For the adjustment and control system plays an important role in its normal operation. At the same time, it is also an important basis for process operators to adjust and control the normal operation of sewage biological treatment.
49. What are the precautions for measuring dissolved oxygen by iodometric titration?
Special care should be taken when collecting water samples for measuring dissolved oxygen. The water samples should not be in contact with air for a long time and should not be stirred. When sampling in the water collection tank, use a 300 ml glass-equipped narrow-mouth dissolved oxygen bottle, and measure and record the water temperature at the same time. Furthermore, when using iodometric titration, in addition to selecting a specific method to eliminate interference after sampling, the storage time must be shortened as much as possible, and it is best to analyze immediately.
Through improvements in technology and equipment and with the help of instrumentation, iodometric titration remains the most precise and reliable titration method for the analysis of dissolved oxygen. In order to eliminate the influence of various interfering substances in water samples, there are several specific methods for correction of iodometric titration.
Oxides, reductants, organic matter, etc. present in water samples will interfere with iodometric titration. Some oxidants can dissociate iodide into iodine (positive interference), and some reducing agents can reduce iodine to iodide (negative interference). interference), when the oxidized manganese precipitate is acidified, most organic matter can be partially oxidized, producing negative errors. The azide correction method can effectively eliminate the interference of nitrite, and when the water sample contains low-valent iron, the potassium permanganate correction method can be used to eliminate the interference. When the water sample contains color, algae, and suspended solids, the alum flocculation correction method should be used, and the copper sulfate-sulfamic acid flocculation correction method is used to determine the dissolved oxygen of the activated sludge mixture.
50. What are the precautions for measuring dissolved oxygen using the thin film electrode method?
The membrane electrode consists of a cathode, anode, electrolyte and membrane. The electrode cavity is filled with KCl solution. The membrane separates the electrolyte from the water sample to be measured, and the dissolved oxygen penetrates and diffuses through the membrane. After a DC fixed polarization voltage of 0.5 to 1.0V is applied between the two poles, the dissolved oxygen in the measured water passes through the film and is reduced on the cathode, generating a diffusion current proportional to the oxygen concentration.
Commonly used films are polyethylene and fluorocarbon films that can allow oxygen molecules to pass through and have relatively stable properties. Because the film can permeate a variety of gases, some gases (such as H2S, SO2, CO2, NH3, etc.) are on the indicating electrode. It is not easy to depolarize, which will reduce the sensitivity of the electrode and lead to deviation in the measurement results. Oil and grease in the measured water and microorganisms in the aeration tank often adhere to the membrane, seriously affecting the measurement accuracy, so regular cleaning and calibration are required.
Therefore, membrane electrode dissolved oxygen analyzers used in sewage treatment systems must be operated in strict accordance with the manufacturer’s calibration methods, and regular cleaning, calibration, electrolyte replenishment, and electrode membrane replacement are required. When replacing the film, you must do it carefully. First, you must prevent contamination of sensitive components. Second, be careful not to leave tiny bubbles under the film. Otherwise, the residual current will increase and affect the measurement results. In order to ensure accurate data, the water flow at the membrane electrode measurement point must have a certain degree of turbulence, that is, the test solution passing through the membrane surface must have a sufficient flow rate.
In general, air or samples with known DO concentration and samples without DO can be used for control calibration. Of course, it is best to use the water sample under inspection for calibration. In addition, one or two points should be checked frequently to verify the temperature correction data.
Post time: Nov-14-2023
