Working principle of water quality analyzer

1. Introduction

With the rapid development of my country’s economy in recent years, the city’s industrial and domestic waste has increased substantially. At present, the main method of garbage disposal is sanitary landfill. Decomposition of medium organisms and in rainy and snowy weather, rainwater and snowwater seep into the landfill area and produce landfill leachate. The leachate is a high-concentration organic wastewater with a wide range of concentration values. It contains hydrocarbons, nitrates, sulfates and trace copper, cadmium, lead and other heavy metal ions. The bacteria indicators are high. If it is not treated, it will be directly discharged into the water. Will seriously pollute the local water environment. In order to protect the water environment, monitoring of sewage discharge must be strengthened. The design of detection points and the quality of detection instruments (mainly water quality analyzers) play a vital role in water environment monitoring. This article will discuss this experience in conjunction with the design of a certain sewage treatment plant.

2. Working principle of 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 pH meter The pH value of water depends on the amount of dissolved substances, so the pH value can sensitively indicate the change of water quality. The change of pH value has a great impact on the reproduction and survival of organisms, and also seriously affects the biochemical effect of activated sludge, that is, 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 is decomposed into hydrogen ions and hydroxide ions. In a neutral solution, the concentration 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 based on the base 10: pH=-log, so neutral The pH of the 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 is alkaline. The pH value is usually measured by the potentiometric method. A galvanic cell is usually 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 has a special glass probe that is sensitive to pH. It is made of special glass that can conduct electricity and can penetrate hydrogen ions. It has the characteristics of high measurement accuracy and good anti-interference. When the glass probe is in contact with hydrogen ions, a potential is generated. The potential is measured by a silver wire control reference electrode suspended in a silver chloride solution. The pH value is different, and the corresponding potential is different, which is converted into a standard 4-20mA output by a transmitter.

2. The working principle of the dissolved oxygen analyzer The oxygen content in the water can fully show the degree of water self-purification. For biological treatment plants that use activated sludge, it is very important to understand the oxygen content of aeration tanks and oxidation ditch. The increase of dissolved oxygen in sewage will promote biological activities other than anaerobic microorganisms, so that volatile substances can be removed and easily Naturally oxidized ions purify sewage. There are three main methods for determining oxygen content: automatic colorimetric analysis and chemical analysis measurement, paramagnetic measurement, and electrochemical measurement. The amount of dissolved oxygen in water is generally measured by electrochemical methods. Mai’s plant uses COS 4 type dissolved oxygen sensor and COM252 type dissolved oxygen transmitter. Oxygen is soluble in water, and its solubility depends on the temperature, the total pressure of the water surface, the partial pressure and the 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. Take the COS 4 oxygen measurement sensor as an example. The electrode consists of a cathode (commonly made of gold and 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, the sensor is covered with a diaphragm. The diaphragm separates the electrode and electrolyte from the liquid to be measured, thus protecting the sensor, which can prevent the electrolyte from escaping, and the intrusion of foreign substances that may cause pollution and poisoning. A polarization voltage is applied between the counter electrode and the cathode. If the measuring element is immersed in water with dissolved oxygen, the oxygen will diffuse through the diaphragm, and the oxygen molecules appearing on the cathode (excess electrons) will be reduced to hydroxide ions: O2+2H2O+4e-® 4OH-. Electrochemical equivalent of silver chloride precipitates on the counter electrode (insufficient electrons): 4Ag+4Cl-® 4AgCl+4e-. For each oxygen molecule, the cathode emits 4 electrons, and the counter electrode accepts electrons to form a current. The magnitude of the current is proportional to the oxygen partial pressure of the sewage. This signal and the temperature signal measured by the thermal resistance on the sensor are sent to the transmitter , Use the relationship curve between oxygen content and oxygen partial pressure and temperature stored in the sensor to calculate the oxygen content in the water, and then convert it into a standard signal output. The function of the reference electrode is 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% of the final measured value after 9 minutes; the minimum flow rate requirement is 0.5cm/s….

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