| Chemical oxygen demand?COD?,which indicates the content of reductive contaminants in water,is an important indicator to gauge the pollution degree of reductive matter in water.In order to obtain the content of COD and improve the quality of the water environment in time,advanced COD online monitoring instrument is necessary.This paper aims to develop a COD online monitor with low cost,high accuracy and good stability.The main research work of this paper is as follows:?1?A design of COD online monitor based on sealed digestion method and spectrophotometry is proposed,after analyzing the research situation of oxidation method,analysis method,and the development status of COD monitor at local and abroad.Then,some basic knowledge related to the COD online monitor designed by this paper is introduced.Such as the catalytic mechanism of catalyst,influence factors of instrument,theoretical foundation of spectrophotometry,selection rules of light wavelength and foundations of theory calculation for COD determination by spectrophotometry.?2?Main process control unit is designed and optimized in this paper.Pretreatment device is designed to remove inorganic chemicals and oil on the basis of general pretreatment unit which can filter larger impurities and suspended solids.The new pretreatment device makes the measurement results more reliable.In order to solve the problem of large measuring deviation in the original metering unit,this paper adopts a method of combining photoelectric liquid level sensor with new control algorithm,which makes the measurement more precise.Double beam spectrophotometry is used to eliminate the measurement error caused by the light wavelength which is shifted with time and temperature.?3?The new embedded microcontroller STM32 and embedded operating system Linux greatly reduces the cost,compared to the original PLC control.This paper studies on the noise of photoelectric detection circuit,designs the photoelectric detection circuit with low noise,uses software method to filter and reduce noise,corrects the absorbance by a certain algorithm and obtains a more accurate COD value.The method of fuzzy PID temperature control based on pulse width modulation can ensure the precise control of temperature.A scientific control algorithm is applied to control the fast and accurate stepper motor to drive peristaltic pump work,control the operation process of instrument and make the COD online monitor operate reliably and stably.?4?Through orthogonal experimental design method,this paper discusses the main influencing factors on determination result of COD such as oxidant dosage,catalyst dosage,solution acidity,digestion temperature and digestion time.Through direct analysis method and variance analysis method,the results show that digestion temperature is the main influencing factor,the second is digestion time,the third is catalyst dosage,the fourth is solution acidity,and the fifth is oxidant dosage.At the same time,the parameters of COD online monitor were optimized,and the results are as follows:digestion temperature is 170?,digestion time is 15 min,catalyst dosage is 0.04g,solution acidity is 8mol/L,oxidant dosage is 0.025g.Ag2SO4 is selected as the best catalyst for COD online monitoring instrument in industrial wastewater and MnSO4 is used as the best catalyst in domestic wastewater by analyzing the different catalysts through the experiment.The experiment results of instrument performance show that the COD online monitor has good performance with catalyst Ag2SO4 or MnSO4 under the optimal conditions.This instrument can measure COD rapidly,continuously,accurately and automatically. |
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