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This article explains the working principles of pH meters and dissolved oxygen analyzers and their applications in wastewater treatment systems. It also covers their features as well as installation and maintenance methods. There are two types of analyzers used in wastewater treatment plants: pH meters and dissolved oxygen analyzers. I. Working Principle 1. Working principle of a pH meter: The pH value of water depends on the amount of substances dissolved in it; therefore, the pH value can sensitively indicate any changes in water quality. Changes in pH have a significant impact on the reproduction and survival of organisms; they also greatly affect the biochemical processes in activated sludge, thereby influencing the efficiency of wastewater treatment. The pH level of wastewater is generally maintained between 6.5 and 7. Water is chemically neutral, and certain water molecules spontaneously decompose according to the equation H2O = H+ + OH-, that is, they break down into hydrogen ions and hydroxide ions. In a neutral solution, the concentrations of hydrogen ions H+ and hydroxide ions OH- are both 10-7 mol/L. The pH value is the negative of the logarithm to the base 10 of the hydrogen ion concentration: pH = -log, so the pH value of a neutral solution is 7. If there is an excess of hydrogen ions, the pH value is less than 7, and the solution is acidic ； Conversely, if hydroxide ions are in excess, the solution becomes basic. The pH value is usually measured by potentiometry, which involves using a reference electrode with a constant potential and a measuring electrode to form a galvanic cell; the electromotive force of this cell depends on the concentration of hydrogen ions as well as on the acidity or alkalinity of the solution. A CPS11 pH sensor and a CPM151 pH transmitter were used. The measuring electrode is equipped with a special glass probe sensitive to pH changes; it is made from a special glass that can conduct electricity and allow hydrogen ions to pass through, offering high measurement accuracy and good resistance to interference. A potential is generated when the glass probe comes into contact with hydrogen ions. The potential is measured using a silver wire suspended in a silver chloride solution as a reference electrode. Different pH values result in different potentials, which are converted into a standard 4–20 mA output via a transmitter. 2. Working principle of the dissolved oxygen analyzer: The oxygen content in water can fully indicate the degree of self-purification of the water. For biological treatment plants that use activated sludge, it is very important to know the oxygen levels in the aeration tanks and oxidation ditches. An increase in dissolved oxygen in the wastewater promotes biological activity other than that of anaerobic microorganisms, thereby enabling the removal of volatile substances and ions that are easily oxidized naturally, thus purifying the wastewater. There are mainly three methods for determining oxygen content: automatic colorimetric analysis and chemical analysis measurement, paramagnetic method measurement, and electrochemical method measurement. The dissolved oxygen level in water is generally measured using electrochemical methods. The McGee plant uses COS 4 type oxygen concentration sensors and COM252 type oxygen concentration transmitters. Oxygen can dissolve in water, and its solubility depends on temperature, the total pressure at the water surface, the partial pressures, and the salts dissolved in the water. The higher the atmospheric pressure, the greater the ability of water to dissolve oxygen; this relationship is determined by Henry’s law and Dalton’s law. Henry’s law states that the solubility of a gas is proportional to its partial pressure. Take the COS 4 oxygen level sensor as an example. The electrodes consist of a cathode (usually made of gold and platinum) and a counter electrode that carries an electric current (silver), as well as a reference electrode that does not carry a current (silver). These electrodes are immersed in an electrolyte such as KCl or KOH. The sensor is covered by a diaphragm, which separates the electrodes and the electrolyte from the liquid to be measured; this protects the sensor by preventing the electrolyte from escaping and by stopping foreign substances from entering and causing contamination or poisoning. A polarization voltage is applied between the counter electrode and the cathode. If the measuring element is immersed in water containing dissolved oxygen, oxygen will diffuse through the membrane and appear at the cathode; the oxygen molecules there, which have an excess of electrons, will be reduced to hydroxide ions. Silver chloride, in its electrochemical equivalent form, precipitates on the counter electrode (where there is a shortage of electrons): 4Ag + 4Cl- → 4AgCl + 4e-. For each oxygen molecule, 4 electrons are released at the cathode, and the anode accepts these electrons, thereby generating an electric current. The magnitude of this current is proportional to the oxygen partial pressure in the wastewater being measured. This signal, along with the temperature signal obtained from the thermistor in the sensor, is sent to a transmitter. Using the relationship curve stored in the sensor between oxygen content, oxygen partial pressure, and temperature, the oxygen content in the water is calculated, and it is then converted into a standard signal for output. The function of the reference electrode is to determine the cathode potential. The response time of the COS 4 oxygen sensor is: 90% of the final measurement value is reached after 3 minutes, and 99% of the final measurement value is reached after 9 minutes ； The minimum flow rate requirement is 0.5 cm/s. II. Features 1. Features of pH meters: The glass on the pH electrode gradually ages over time, causing the gradient (the change in the electrode’s output potential per unit change in pH value) to deteriorate, and it takes longer for a stable potential to be reached. The typical service life of an electrode is up to two years. Furthermore, temperature also has a significant impact on aging; the degree of aging after several weeks of storage at 100°C is equivalent to that after one year of storage at room temperature. pH meters offer advantages such as high precision, high reliability, and easy installation and maintenance. However, they are also sensitive to contamination and require regular calibration, usually every one to one and a half months, with the electrodes needing to be replaced every two years. 2. Features of dissolved oxygen meters: Dissolved oxygen meters are easy to install, have a long calibration interval (3–4 months), and are not sensitive to other substances. They can also monitor the usage level of electrolytes in the diaphragm and probe; generally, the electrolytes and diaphragm need to be replaced every one to three years. The COM252 oxygen concentration transmitter is an intelligent instrument that supports HART or Profibus communication protocols. It also features self-diagnosis capabilities; in the event of a fault, it displays a fault code to indicate to maintenance personnel where the problem lies, allowing them to find solutions by referring to the maintenance manual. This **reduces maintenance time and effort. III. Installation and Maintenance 1. Installation and Maintenance of pH Meters There are two methods for installing pH meters: flow-through and immersion types. Wastewater treatment plants generally adopt an immersion-type installation; for example, the pH meter in this wastewater treatment plant is installed in the outlet overflow tank of the oxidation ditch. The pH value here is quite representative, and the water flow is steady, which prevents any significant stress on the pH meter. Regular maintenance helps ensure accurate measurements by the instrument and extends its service life. It should be noted that the dedicated cable between the sensor and the transmitter must not get wet, otherwise the high-impedance, low-voltage signals from the electrode cannot be transmitted to the transmitter. When the electrode is not in use, a yellow protective sleeve should be placed over it; this keeps the electrode moist, which helps to extend its lifespan. The electrodes should be cleaned every month or so: first, rinse off the deposits with gentle water flow, then soak the electrodes in a cleaning solution for a while, and finally wash them with clean water. The sensor bracket should also be cleaned. After each cleaning, calibration must be performed using a buffer solution. Among the pH calibration solutions available domestically, those with a pH of 4 are acceptable, while those with a pH of 7 are not accurate enough and can affect the calibration results. Therefore, it is best to use the calibration solutions provided by the manufacturer. The manufacturer usually supplies two bottles of standard solutions, one with a pH value of 7, which is used to calibrate the zero point of the instrument ； A bottle with a pH of 4, used to calibrate the signal output slope of instruments. 2. Installation and maintenance of dissolved oxygen meters: Dissolved oxygen meters are generally installed in an immersed manner; it is important to use the original manufacturer’s mounting brackets for this purpose. The installation bracket provided by the manufacturer is made of stainless steel and features a plastic chain; by adjusting the length of this chain, it is possible to change the depth to which the sensor is submerged. The guide tube on the bracket ensures that the sensor remains in a vertical position at all times. The bracket section is specially designed to transmit water surface fluctuations to the immersion tube, thereby causing slight vibrations in the immersion tube and generating an additional cleaning effect on the surface of the probe through the immersion tube. Some users, in an effort to reduce costs, fabricate their own mounting brackets, which often results in poor sealing between the immersion tubes and sensors on those brackets. As a result, sewage seeps in, causing the connections between the specialized cables and sensors to remain submerged in sewage for extended periods, which can easily lead to damage to the sensors ； Some even do not use mounting brackets and simply place the sensor in water; this results in a large pulling force between the sensor and the cable, making the sensor more prone to damage. The dissolved oxygen probe should be gently cleaned with water once a week; if the membrane tip is damaged, it must be replaced promptly, and the electrolyte should also be replaced if it becomes contaminated. When the wastewater contains components such as H2S, NH3, benzene, or phenol, it is harmful to the membrane head. In such situations, the membrane tip must be replaced frequently. To determine the quality of the electrodes in the probe, one only needs to look at their color: the reference electrode should be black-gray, the cathode (gold electrode) should be yellow, and the counter electrode must be shiny; otherwise, it should be cleaned or regenerated. As China places increasing emphasis on water resource protection, the purification of wastewater has become increasingly important, and the testing instruments required for the associated treatment processes are essential. As the most important instrument in the wastewater treatment industry, in addition to proper selection and installation, regular maintenance and calibration of water quality analyzers are also crucial; they are key to enabling these instruments to function effectively.