Definition
Chemical oxygen demand online water quality analyzer is an instrument and equipment used for continuous and automatic monitoring of chemical oxygen demand parameters in water bodies. Chemical oxygen demand refers to the oxidation dose consumed when treating water samples with specific strong oxidants under certain conditions, converted into the mass concentration of oxygen, expressed in milligrams per liter. This index reflects the degree of water pollution by reducing substances and is one of the key parameters for assessing the water organic pollution load. Online analyzers enable real-time monitoring and provide timely data support for water quality management and process control.
Principle
The Chemical Oxygen Demand Online Analyzer works on the principle of redox reaction. The instrument completely oxidizes the reducing substances in the water sample with a strong oxidant through the digestion process under high temperature and pressure or catalytic conditions. The corresponding oxygen demand is then indirectly calculated by measuring the oxidant consumption or the change in the generated product. Common oxidants include potassium dichromate and potassium permanganate, among which potassium dichromate method is widely used in online monitoring due to its high oxidation rate and good reproducibility. The entire measurement process follows the basic chemical logic of the standard method and realizes automated operation and result calculation.
Measurement method
On-line chemical oxygen demand analysis mainly uses potassium dichromate digestion spectrophotometry or electrochemical measurement method. The potassium dichromate method is carried out under acidic heating conditions, and after the reaction, the absorbance of trivalent chromium ions at a specific wavelength is measured by a spectrophotometer, the concentration is directly proportional to the oxidation dose consumed, and the chemical oxygen demand value is calculated according to Lambert-Beale's law. The basic process of reaction can be expressed as:
Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O
Some instruments also use alternative technologies such as ultraviolet digestion or ozone oxidation to reduce the risk of secondary pollution. The measurement cycle can usually be set within 10 minutes to 2 hours, depending on the demand.
Influencing factors
The results of online chemical oxygen demand measurements are influenced by a variety of factors. When the concentration of chloride ions in the water sample is high, it will react with the oxidant to produce positive interference, and mercury sulfate is usually added for masking. The control of digestion temperature and time directly affects the oxidation efficiency, and temperature fluctuations may lead to biases in results. The uniformity and representativeness of water samples are also crucial, as too much suspended solids can clog the sampling line or affect the optical path measurement. In addition, reagent purity, optical window cleanliness, ambient temperature variations, and instrument calibration cycles all contribute to measurement stability and accuracy. These factors need to be controlled according to the characteristics of water quality during operation.
Application:
Chemical oxygen demand online analyzers are widely used in municipal sewage treatment plant inlet and outlet water monitoring, industrial wastewater discharge outlet compliance monitoring, automatic monitoring stations of surface water bodies such as rivers and lakes, and industrial process water quality control. In the process of sewage treatment, real-time chemical oxygen demand data can be used to optimize aeration and chemical dosing to achieve energy conservation and consumption reduction. In the field of environmental regulation, continuous monitoring data helps to detect abnormal emissions in a timely manner. Some industrial circulating water systems also use online chemical oxygen demand monitoring as an auxiliary index for system corrosion and scaling control. When applying, it is necessary to choose the appropriate measurement method according to the chemical oxygen demand range and matrix complexity of the specific water quality.
Selection
When selecting an on-line chemical oxygen demand analyzer, it is necessary to consider the measurement range, method principle, maintenance needs and environmental adaptability. For industrial wastewater with high chemical oxygen demand concentration, it is advisable to use potassium dichromate method instruments with a wide range and strong anti-interference ability. For clean surface water monitoring, low reagent consumption methods such as UV digestion can be considered. The installation environment needs to be evaluated to see if the protection level meets the requirements of on-site dustproof and waterproof, and cold areas need to pay attention to thermal insulation and anti-freezing measures. The instrument should have automatic cleaning, fault alarm and data storage functions to reduce the frequency of manual maintenance. At the same time, it is necessary to confirm that the instrument complies with the applicable technical specifications in the region, such as the Chinese environmental protection standard or the relevant standards of the International Organization for Standardization, and consider the cost of reagent consumption and waste liquid treatment requirements.