The pH value of water depends on the amount of dissolved substances, so pH value can sensitively indicate changes in water quality. The change in pH value has a significant impact on the reproduction and survival of organisms, and also seriously affects the biochemical process of activated sludge, which affects the treatment effect. The pH value of sewage is generally controlled between 6.5 and 7.

Water is chemically neutral, and some water molecules spontaneously decompose according to the following formula: H2O=H++OH -, which decomposes into hydrogen ions and hydroxide ions. In a neutral solution, the concentrations of hydrogen ion H+and hydroxide ion OH - are both 10-7mol/l, and the pH value is the negative logarithm of the hydrogen ion concentration with a base of 10: pH=- log. Therefore, the pH value of a neutral solution is equal to 7. If there is an excess of hydrogen ions, the pH value is less than 7 and the solution is acidic; On the contrary, if there is an excess of hydroxide ions, the solution becomes alkaline.

The pH value is usually measured by potentiometry, using a constant potential reference electrode and a measuring electrode to form a primary cell. The magnitude of the electromotive force of the primary cell depends on the concentration of hydrogen ions and the acidity or alkalinity of the solution. The factory adopts CPS11 pH sensor and CPM151 pH transmitter. The specific structure is shown in Figure 1. There is a special pH sensitive glass probe on the measuring electrode, which is made of a special glass that can conduct electricity and penetrate hydrogen ions. It has the characteristics of high measurement accuracy and good anti-interference. When the glass probe comes into contact with hydrogen ions, a potential is generated. The potential was 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 standard 4-20mA outputs through a transmitter.