In order to protect the water environment, monitoring of sewage discharge must be strengthened. The design of the detection point and the quality of the detection instrument (mainly the water quality analyzer) play a crucial role in the monitoring of the water environment. This article describes the experience of the design of a sewage treatment plant.

How Water Quality Analyzers Work

There are two types of analyzers used in sewage treatment plants: pH meters and dissolved oxygen analyzers.

The working principle of the pH meter The pH of the water is determined by the amount of dissolved material, so the pH value can be a sensitive indicator of the change in water quality. The change of pH value has great influence on the reproduction and survival of organisms, and it also seriously affects the biochemical action of activated sludge, that is, it 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 the neutral solution, the concentrations of the hydrogen ion H+ and the hydroxide ion OH- are both 10-7 mol/l, and the pH is the negative number of the logarithm of the hydrogen ion concentration at the base of 10: pH = -log, so neutral The pH of the solution is equal to 7. If there is an excess of hydrogen ions, the pH is less than 7 and the solution is acidic; conversely, if the hydroxide ion is excessive, the solution is alkaline. The pH value is usually measured by the potentiometric method. Usually, a constant potential reference electrode and a measuring electrode are used to form a primary battery. The electromotive force of the primary battery depends on the concentration of hydrogen ions and also depends on the pH of the solution. The plant uses a CPS11 pH sensor and a CPM151 pH transmitter. The specific structure is shown in Fig. 1. There is a special glass probe with sensitive pH response on the measuring electrode. It is made of special glass that can conduct electricity and permeate 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 control reference electrode suspended in a silver chloride solution. With different pH values, the corresponding potentials are also different. It is converted by the transmitter to a standard 4-20mA output.

How the Dissolved Oxygen Analyzer Works The oxygen content of the water fully demonstrates the degree of water self-purification. For a biological treatment plant using activated sludge, it is very important to know the oxygen content of the aeration tank and the oxidation ditch. The increase of dissolved oxygen in the sewage will promote biological activities other than anaerobic microorganisms, thus enabling the removal of volatile substances and ease of use. 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. Dissolved oxygen in water is generally measured electrochemically. The plant uses a COS 4 dissolved oxygen sensor and a COM 252 dissolved oxygen transmitter. Oxygen is soluble in water and the solubility depends on the temperature, the total pressure on the surface of the water, 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 gas is proportional to its partial pressure. Taking the COS 4 oxygen measurement sensor as an example, the structure is shown in FIG. 2 . The electrode consists of a cathode (usually made of gold and platinum), a current-carrying counter electrode (silver), and a current-free reference electrode (silver). The electrode is immersed in electrolytes such as KCl and KOH. The sensor is covered with a separator. The electrodes and the electrolyte are separated from the liquid being measured, thus protecting the sensor, preventing both the escape of the electrolyte and the ingress of foreign substances, resulting in 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 diffuses through the membrane and oxygen molecules that are present at the cathode (excess electrons) are reduced to hydroxide ions: O2+2H2O+4e-®4OH-. Electrochemically equivalent silver chloride precipitates on the counter electrode (electron deficiency): 4AG+4Cl-®4AgCl+4e-. For each oxygen molecule, the cathode emits 4 electrons, the counter electrode receives electrons, forms a current, and the size of the current is measured with the structure of the pH measuring electrode (left) and the reference electrode (right) of Figure 1 Figure 2 Three-electrode COS Dissolved Oxygen The sensor structure diagram is proportional to the oxygen partial pressure of the sewage. This signal is sent to the transmitter together with the temperature signal measured by the thermal resistance of the sensor. The relationship between oxygen content, oxygen partial pressure and temperature stored in the sensor is used to calculate the signal. Oxygen content in the water is then converted to a standard signal output. The function of the reference electrode is to determine the cathode potential. The response time of the COS 4 DO sensor was 90% of the final measurement after 3 minutes and 99% of the final measurement after 9 minutes; the minimum flow rate requirement was 0.5 cm/s.

Features pH meter characteristics The glass on the pH electrode will gradually age over time, the gradient (the change in electrode output potential caused by a change in the unit pH value) deteriorates, and it takes a long time to reach the stable potential. The service life of general electrodes can reach up to two years. In addition, the temperature has a great influence on the aging, and the degree of aging stored at 100° C. for several weeks is equivalent to the degree of aging stored at room temperature for one year. The pH meter has the advantages of high accuracy, high reliability, convenient installation and maintenance, etc. At the same time, it is also sensitive to pollution and requires frequent calibration. Generally, it is calibrated every other one to one and a half months, and the electrodes are replaced once every two years.

Features of Dissolved Oxygen Meter Dissolved Oxygen Meter has the characteristics of convenient installation, long calibration period (3 to 4 months), insensitivity to other substances, etc., and can monitor the use of electrolyte in diaphragm and probe. It is usually replaced every three years. Electrolyte and diaphragm. COM025 type dissolved oxygen transmitter is an intelligent instrument with HART or Profibus communication protocol, and also has self-diagnosis function. When a fault occurs, the fault code will be displayed to indicate the fault of the maintenance personnel. Find out the fault by checking the maintenance manual. The approach greatly reduces maintenance time and workload.