Definition
Chemical oxygen demand water quality online monitor is an instrument and equipment used for continuous and automatic determination of chemical oxygen demand parameters in water bodies. Chemical oxygen demand refers to the amount of oxidant consumed when treating a water sample with a strong oxidant under specific conditions, expressed in milligrams per liter of oxygen. This index reflects the degree of reducing substance pollution in the water body, and is one of the key parameters for evaluating the organic pollution load of the water body. Online monitors can achieve real-time and continuous measurement, providing timely data support for water quality management and pollution control.
Measurement principle
The core principle of the Chemical Oxygen Demand Online Monitor is based on redox reactions. The instrument reacts a water sample with an oxidant (usually potassium dichromate) in a high-temperature and strong acid environment by simulating the chemical process of standard laboratory methods. The oxidation dose consumed during the reaction is directly proportional to the concentration of organic matter and oxidizable inorganic substances in the water sample. The instrument calculates the chemical oxygen demand value indirectly by measuring the change in oxidant concentration before and after the reaction, or by measuring the absorbance of the trivalent chromium ions produced by the reaction at a specific wavelength. The basic relationship can be expressed as follows: the amount of oxidation consumed is equivalent to the amount of oxygen consumed. For instruments with colorimetry, their calculations are based on Lambert-Beale's law, which is expressed as:A = εlc, where A is the absorbance, ε is the molar absorbance coefficient, l is the length of the optical path, and c is the trivalent chromium ion concentration.
Main measurement methods
At present, the mainstream online monitoring technology is mainly based on potassium dichromate digestion method, and several different measurement methods have been derived. High-temperature digestion colorimetric is one of the common methods, which involves water sample collection, addition of potassium dichromate solution and silver sulfate catalyst, digestion reaction at high temperature, and subsequent measurement of the absorbance of trivalent chromium by photometer and calculation of concentration. The other is the coulombic titration method, which uses ferrous ions produced by electrolysis as a titrant, titrates the remaining potassium dichromate after digestion, and calculates the chemical oxygen demand value according to the amount of electricity consumed by electrolysis. In addition, UV absorption spectroscopy is used as an auxiliary or alternative technique to quickly correlate chemical oxygen demand by measuring the absorbance of water samples in the UV band, but it often requires a calibration model for specific water quality. These methods must follow national or international standards, such as HJ 377-2019 and other specifications, to ensure data comparability and accuracy.
Influencing factors
The accuracy of online monitoring data is affected by a combination of factors. The characteristics of the water sample body are key factors, such as high chloride ion content can positively interfere with the potassium dichromate method, which usually requires the addition of mercury sulfate for masking. The turbidity and color of the water sample may affect the photometric measurement results. The operating parameters of the instrument, such as digestion temperature, digestion time, reagent addition ratio, etc., must be strictly controlled and consistent with standard methods. Environmental conditions such as ambient temperature fluctuations can affect reagent activity and the stability of the instrument's electronic components. In addition, regular maintenance of the instrument, including flow path cleaning, optical window wiping, reagent replacement, and calibration, is essential to maintain long-term measurement reliability. Deviations from any factor can lead to measurement bias.
Applications
The Chemical Oxygen Demand Online Monitor is widely used in situations where continuous monitoring of effluent water quality is required. In the field of municipal sewage treatment, instruments are often installed at the water inlet and outlet to monitor the treatment load and treatment efficiency, and provide a basis for process regulation. In industrial wastewater discharge monitoring, such as enterprise outlets in industries such as papermaking, printing and dyeing, food processing, etc., online monitors are an effective regulatory tool to ensure compliance with discharge. In the automatic monitoring stations of surface water quality such as rivers and lakes, chemical oxygen demand is continuously monitored as a comprehensive pollution indicator to serve the environmental management of the river basin. Some industrial processes, such as circulating cooling water systems, also use online monitoring for water quality control. These applications help extend from end-of-line monitoring to process management.
Key points to consider in selection
When choosing an online chemical oxygen demand monitor, a comprehensive evaluation of various aspects is required. The monitoring needs should be clearly defined, including the expected range of chemical oxygen demand for the water to be tested, the concentration level of the main interfering substances (e.g., chloride ions), and the required measurement frequency. Secondly, it is necessary to examine the technical performance of the instrument, pay attention to whether the measurement principle adopted is suitable for the target water quality, and verify whether its performance indicators such as detection limit, measurement range, and repeatability meet the requirements of applicable standards. The operating cost of an instrument involves reagent consumption, waste fluid generation, and the frequency and complexity of routine maintenance. The environmental adaptability of the equipment, such as whether the protection level meets the working conditions of the installation site, is also an important consideration. Lastly, the instrument's data output interface, remote control capabilities, and compatibility with existing monitoring platforms contribute to the convenience of data management. It is recommended to refer to relevant industry standards and verify the suitability of the instrument through comparison tests of actual water samples.