Water Quality Analyzer

There are two types of analyzers used in sewage treatment plants: pH meters and dissolved oxygen analyzers.
1.The Working Principle of the pH Meter
The pH value of water depends on the amount of dissolved substances, making it a sensitive indicator of changes in water quality. Fluctuations in pH can significantly impact the reproduction and survival of organisms and can adversely affect the biochemical activity of activated sludge, ultimately influencing treatment effectiveness. 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−\text{H}_2\text{O} \rightleftharpoons \text{H}^+ + \text{OH}^-H2​O⇌H++OH−
In a neutral solution, the concentration of hydrogen ions (H+\text{H}^+H+) and hydroxide ions (OH−\text{OH}^-OH−) is both 10−7 mol/L10^{-7} \, \text{mol/L}10−7mol/L, leading to a pH value of 7, which is the negative logarithm of the hydrogen ion concentration:
PH=−log⁡[H+]\text{pH} = -\log[\text{H}^+]pH=−log[H+]
If there is an excess of hydrogen ions, the pH value is less than 7, indicating an acidic solution; conversely, if there is an excess of hydroxide ions, the solution is alkaline. The pH value is usually measured by the potentiometric method, which typically involves a galvanic cell composed of a constant potential reference electrode and a measuring electrode. The electromotive force of the galvanic cell depends on the concentration of hydrogen ions and the pH of the solution. The plant uses the CPS11 type pH sensor and the CPM151 type pH transmitter. The specific structure is shown in Figure 1.
The measuring electrode features a special glass probe sensitive to pH. This probe is made of special glass that can conduct electricity and allows penetration by hydrogen ions. It has high measurement accuracy and good anti-interference characteristics. When the glass probe comes into contact with hydrogen ions, a potential is generated. This potential is measured by a silver wire reference electrode suspended in a silver chloride solution. Different pH values correspond to different potentials, which are converted into a standard 4-20 mA output by the transmitter.

2.The Working Principle of the Dissolved Oxygen Analyzer
The oxygen content in water is a crucial indicator of the degree of water self-purification. For biological treatment plants that use activated sludge, understanding the oxygen content in aeration tanks and oxidation ditches is essential. Increased dissolved oxygen in sewage promotes biological activities other than those of anaerobic microorganisms, aiding in the removal of volatile substances and the oxidation of ions to purify sewage.
There are three main methods for determining oxygen content: automatic colorimetric analysis, chemical analysis measurement, paramagnetic measurement, and electrochemical measurement. The amount of dissolved oxygen in water is generally measured using electrochemical methods. The plant uses the COS 4 type dissolved oxygen sensor and the COM252 type dissolved oxygen transmitter. Oxygen is soluble in water, and its solubility depends on temperature, the 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. This relationship is described by Henry's law and Dalton's law. Henry's law states that the solubility of a gas is proportional to its partial pressure.
Taking the COS 4 oxygen measurement sensor as an example, the electrode consists of a cathode (commonly made of gold or platinum), a current-carrying counter electrode (silver), and a current-free reference electrode (silver). The electrode is immersed in an electrolyte such as KCl in KOH, and the sensor is covered with a diaphragm. The diaphragm separates the electrode and electrolyte from the liquid being measured, protecting the sensor from contamination and the escape of the electrolyte.
A polarization voltage is applied between the counter electrode and the cathode. When the measuring element is immersed in water containing dissolved oxygen, the oxygen diffuses through the diaphragm. The oxygen molecules that reach the cathode (which has excess electrons) are reduced to hydroxide ions:
O2+2H2O+4e−→4OH−\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-O2​+2H2​O+4e−→4OH−
Simultaneously, silver chloride precipitates on the counter electrode (which has insufficient electrons):
4Ag+4Cl−→4AgCl+4e−4\text{Ag} + 4\text{Cl}^- \rightarrow 4\text{AgCl} + 4e^-4Ag+4Cl−→4AgCl+4e−
For each oxygen molecule, the cathode emits 4 electrons, while the counter electrode accepts these electrons, generating a current. The magnitude of this current is proportional to the oxygen partial pressure in the sewage. This signal, along with the temperature signal measured by a thermal resistor on the sensor, is sent to the transmitter. The relationship curve between oxygen content, oxygen partial pressure, and temperature is stored in the sensor to calculate the oxygen content in the water, which is then converted into a standard output signal. The reference electrode serves to determine the cathode potential. The response time of the COS 4 dissolved oxygen sensor is 90% of the final measured value after 3 minutes and 99% after 9 minutes. The minimum flow rate requirement is 0.5 cm/s.