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1, the preamble in recent years as China's rapid economic development, urban and industrial solid waste a substantial increase, the current garbage treatment is the primary means of sanitary landfill, which are open landfill operations, refuse the pressure It later, with the decomposition of garbage in the encounter rain and snow and weather, rain and snow into the landfill, will have leachate. Leachate is a high concentration of organic wastewater, the concentration of big changes, including hydrocarbons, nitrate, sulphate and trace copper, cadmium, lead, heavy metals, bacterial indicators of high, if not handled directly into the water, Seriously polluted the local water environment. To protect the water environment, the need to strengthen the monitoring of the discharge. Detection point in the design and instrumentation (mainly for water quality analysis) the quality of the water environmental monitoring plays a vital role, this combination of a sewage treatment plant design elaborate on this experience.
Second, water quality analysis for the working principle of a sewage treatment plant for use of two: pH meter and dissolved oxygen analyzer.
1, pH of the working principle of water as the pH value of the material dissolved by the number may be, sensitive to the pH value can be directed to the changes in water quality. pH value of the change on biological reproduction and survival are greatly affected, and also seriously affect the role of biological and chemical sludge that affect the treatment effect, sewage control the pH values between 6.5 to 7. Water is neutral in the chemical, some water molecules spontaneously in accordance with the under-decomposition: H2O = H + + OH-, that is broken down into hydrogen ions and hydroxide ions. In the neutral solution, hydrogen ions and H + OH-hydroxide ion concentrations are 10-7 mol / l, pH value is the concentration of hydrogen ions to 10 as of the end of the number of negative: pH =- log, a result of Solution of the pH value is equal to 7. If there is an excessive amount of hydrogen ions, the pH value of less than 7, acid solution; Instead, excessive hydroxide ion, the solution was alkaline. pH value of electricity is usually measured, usually with a constant potential of the reference electrode and measuring the electrode to form a battery, the battery EMF depends on the size of the concentration of hydrogen ions, also depends on the pH solution. The plant used CPS11-pH sensor and CPM151 pH of the transmitter. Concrete structure as shown in Figure 1, with special electrodes measured the pH responsive to the glass probe, which is conductive to, hydrogen ions to infiltrate the special glass made with high accuracy, and good anti-jamming features. When the glass contact with the probe and hydrogen ions, producing potential. Potential through suspension in silver chloride solution Yinsi control of the Senate than the electrodes measured. pH values different from the corresponding potential is not, by its transmitter will be converted into standard 4 ~ 20 mA output.
2, dissolved oxygen analyzer of the principle of the oxygen content in water can fully show the extent of self-purification of water. The use of activated sludge treatment plant, the understanding of aeration tanks and oxidation ditch the oxygen content is very important, sewage increase in dissolved oxygen, will promote the addition of anaerobic microorganisms other than the biological activities, thus removing volatile substances and Easy to natural oxidation of the ions, so that sewage purification. Determination of oxygen content There are three main ways: automatic color analysis and chemical analysis measurement, with magnetic measurement, electrochemical measurement. General dissolved oxygen in water by electrochemical measurement. Mr plant used COS 4-COM252-dissolved oxygen dissolved oxygen sensors and transmitter. Oxygen can dissolve in water, solubility depends on the temperature, the surface of the water pressure, water pressure and dissolved salts. The higher atmospheric pressure, water the greater the ability of dissolved oxygen, their relationship by Henry (Henry) and Dalton's Law (Dalton) determine the law, Henry's Law that the solubility of gases in direct proportion to their pressure. To COS 4 oxygen sensor as an example, the structure shown in Figure 2. One of the electrodes from the cathode (made of commonly used gold and platinum) and with the current anti-electrode (silver), a current reference electrode (silver), with electrodes submerged in electrolyte such as KCl, KOH, the sensor is divide coverage, Divide and electrolytes and electrodes will be measured separately from the liquid and therefore the protection of the sensor, it can prevent the electrolyte escape, and prevent the intrusion of foreign substances caused pollution and poison. To the anti-imposed between the electrode and cathode polarization voltage, if the measuring devices are immersed in the dissolved oxygen in water, oxygen through divide-proliferation, in the cathode (electronic surplus) will be reduced oxygen molecules into hydrogen and oxygen ions: O2 +2 H2O +4 e-® 4OH-. Electrochemical equivalent of silver chloride precipitation in the anti-electrode (less than electronic): 4 Ag +4 Cl-® 4AgCl +4 e-. For each oxygen molecules, cathode released four of electronics, anti-electrode to accept electronic form current, and measured the size of current plans 1 pH electrode measurements (left) and the reference electrode (right), the structure of the two three-electrode COS DO Sensor structure of the sewage is directly proportional to the partial pressure of oxygen, the sensor signals, together with thermal resistance measured the temperature signals are sent to the transmitter, using sensors in the oxygen content and oxygen storage, the relationship between temperature curve Calculate the oxygen content in water and then into a standard signal output. Reference electrode is to determine the function of the cathode potential. COS 4 dissolved oxygen sensor response time: three minutes after a final measurement of 90%, reached the final nine minutes after the measurement of the 99% minimum requirement for the flow rate of 0.5 cm / s.
Third, characteristics
1, pH of the characteristics of pH electrodes on the glass over time will gradually aging, and a gradient (in pH value caused by changes in the electrode potential changes in output value) deterioration takes longer time to achieve stability potential. General electrode life, up to two years. In addition, the temperature on aging have a greater impact, 100 ℃ store a few weeks under the aging equivalent to one year of storage at room temperature aging. pH of a high precision and high reliability, installation and maintenance of the advantages of convenience, but also more sensitive to pollution, the need for regular calibration, generally every 1-1 calibration and a half months time, once every two years to replace electrodes.
2, DO-DO-the characteristics of ease of installation, calibration long cycle (3-4 months), is not sensitive to other substances, and other characteristics, and to monitor and probe divide within the use of electrolytes, general replacement of every one to three years And an electrolyte membrane. COM252-dissolved oxygen transmitter is intelligent instruments, or Profibus with HART communication protocol, but also a self-diagnostic function, when a fault, fault code will show, suggesting that failure of maintenance personnel, maintenance manual search through the failure to find solutions Approach, greatly reduced the maintenance time and workload.
4, installation and maintenance
1, pH of installation and maintenance of pH installation of a flow and submerged two. Sewage treatment plant is generally optional Immersion installation, such as the sewage treatment plant installed in the pH of the oxidation ditch export overflow tank, where the pH value of more representative, and a smooth flow, not on the pH meter A big impact. Regularly contribute to the maintenance of accurate measurement instruments and meters to extend the service life. It should be noted between the transmitter and sensor cables for not exposed to moisture, the high resistance or low-voltage electrodes will not be able to send signals to the transmitter. If the electrodes do not measure, should be put yellow casing protection, it can electrode in a moist state, to extend the life of electrodes. Every month or so, should electrode cleaning, to use gentle spray of water washing fixtures, and then soaked in cleaning fluid electrodes in a period of time, then wash with water. Support should also be cleaned sensor. After each washing, use a buffer solution calibration, the current domestic pH calibration solution, pH = 4 of the can, pH = 7 of the inaccurate, the results will affect the calibration. So the best use of the calibration of the manufacturer, manufacturers generally provide two bottles of standard solution, a bottle of pH value is equal to 7, for the demarcation of 0.1 meters, equivalent to a bottle of pH 4, for the instrument calibration signal output slope.
2, dissolved oxygen for the installation and maintenance of dissolved oxygen for the general use of submerged installation, it should be noted that the manufacturer must choose to install stents. Manufacturers with the mounting bracket for the band made of stainless steel with a plastic chain, by adjusting the length of the chain can change the immersion depth sensor, to guide the stent on the guarantee of the sensor will always be in vertical position. Some stents have been specially designed, it will spread to the surface of the volatility of ingress, thus leading to the slight vibration of ingress, making through immersion in the probe of the surface of an additional cleaning effect. Some users in order to reduce investment and produce their own mounting brackets, often on the ingress of the stent and sensors between Mifengbuyan, sewage infiltration, making for the connecting cables and sensors, long-term immersion in the sewage, likely to cause damage to the sensor ; Some even do mounting brackets, sensors directly into the water, so the cables between sensors and will form the larger rally, sensors more easily damaged. Application of water per week dissolved oxygen probe gently cleansing, the first film that should be a timely replacement of damaged and contaminated electrolyte replacement should also be timely. When the sewage containing H2S, NH3, benzene or phenol these elements, the first film is harmful. In such situations, the first film to be changed frequently. Probe judgement in the performance of electrodes can only see colors, and the reference electrode is gray, the cathode (the electrodes) should be yellow, while the anti-electrode to be lit, clean or otherwise, should be renewable. 5, with the concluding remarks on China's increasing emphasis on the protection of water resources, sewage purification treatment become increasingly important, and supporting the process with the necessary instrumentation is essential. Analysis of water-treatment industry as the most important instrument, in addition to the correct selection and installation, regular maintenance and calibration are also very important, but so is the instrument can really play the key role.