Application of dissolved oxygen meter in sewage treatment
The oxygen content in water can fully indicate the degree of self-purification of water. For biological treatment plants that use activated sludge, it is very important to understand the oxygen content of the aeration tank. The increase in dissolved oxygen in the sewage will promote biological activities other than anaerobic microorganisms, thereby removing volatile substances and easily oxidized naturally. ions to purify sewage.
There are three main methods for measuring oxygen content: automatic colorimetric analysis and chemical analysis measurement, paramagnetic method measurement, and electrochemical method measurement. The amount of dissolved oxygen in water is generally measured using electrochemical methods.
Oxygen is soluble in water, and its solubility depends on temperature, total pressure at the water surface, partial pressure, and 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 a gas is proportional to its partial pressure.
The oxygen measurement sensor consists of a cathode (commonly made of gold and platinum), a current-carrying counter electrode (silver), and a non-current reference electrode (silver). The electrodes are immersed in electrolytes such as KCl and KOH, and the sensor is covered by a diaphragm. The coating separates the electrode and electrolyte from the liquid being measured. Only dissolved gases can penetrate the coating, thus protecting the sensor, preventing the electrolyte from escaping and preventing the intrusion of foreign substances from causing 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 will diffuse through the separator, and the oxygen molecules present on the cathode (excess electrons) will be reduced to hydroxide ions. [OH-]. The electrochemical equivalent of silver chloride is precipitated on the counter electrode (short of electrons). For each oxygen molecule, the cathode emits 4 electrons, and the counter electrode accepts electrons, forming a current: 4Ag+4Cl-=4AgCl+4e-.
The size of the current is proportional to the oxygen partial pressure of the measured sewage. This signal, together with the temperature signal measured by the thermal resistor on the sensor, is sent to the transmitter. The oxygen content stored in the sensor is used to calculate the relationship between the oxygen partial pressure and temperature. The oxygen content in the water is calculated using the relationship curve, and then converted into a standard signal output. The function of the reference electrode is to determine the cathode potential.
