COD water quality automatic analyzer: working principle and technical application research

　　summary

Chemical Oxygen Demand (COD) is a key indicator for measuring the degree of pollution of water bodies by reducing substances, especially reflecting the content of organic matter in water.COD water quality automatic analyzerAs the "eyes and ears" of the environmental monitoring network, it achieves real-time, continuous, and accurate monitoring of water quality conditions. This article systematically explains the COD automatic analyzerCore working principle, with a focus on comparative analysisPotassium dichromate digestion photometric methodandUV absorption methodThe advantages and disadvantages of the two mainstream technological routes were discussed in detail, and their applications in areas such as pollution source monitoring, surface water warning, and sewage treatment process control were exploredtechnology applicationFinally, the article looks forward to the development trend of this technology, emphasizing the development direction of intelligence, miniaturization, and multi parameter integration.

　　1、 Introduction: Why do we need COD automatic monitoring?

Although traditional laboratory COD detection methods (such as the national standard method) have high accuracy, they have drawbacks such as time-consuming (2 hours), cumbersome operation, high reagent consumption, and inability to obtain real-time data. With the increasingly strict requirements for environmental protection and the increasing demand for refined water environment management,Online automatic monitoringBecoming an inevitable choice. COD automatic analyzer can:

　　real-time monitoringTimely detection of sudden pollution incidents.

　　Data provenanceProvide reliable basis for pollution discharge fees and environmental law enforcement.

　　optimal controlGuide real-time adjustment of process parameters in sewage treatment plants to save energy and reduce consumption.

　　early warning and forecastingEstablish a water quality warning system to ensure the safety of drinking water sources.

　　2、 Core working principle

The working principle of COD automatic analyzer can be divided intoSample collection and pretreatment、Core reactions and measurements、Data processing and outputThree stages.

　　1. Sample collection and pretreatment

　　samplingRegularly or continuously extract water samples from monitoring points (such as sewage outlets, river sections) through peristaltic pumps or jet pumps.

　　preprocessing: Usually includesfilter(Remove large particles to protect subsequent pipelines and reaction systems)refrigeration(Inhibit microbial activity to prevent sample spoilage) andquantitative(Accurately extract a certain volume of water sample into the reaction system for measurement).

　　2. Core reactions and measurements (two major technological routes)

This is the key to distinguishing different types of COD analyzers.

　　Technical route one: Potassium dichromate digestion spectrophotometric method

This is currently the most widely used, mature, and accurate method both domestically and internationally, and is also recommended by the Chinese environmental protection industry standard (HJ 377-2019).

　　working principle：

　　Quantitative injectionAccurately measure a certain volume of sample and compare it with a known excessPotassium dichromate standard solutionMix in a strongly acidic environment.

　　Add catalyst: JoinSilver sulfateAs a catalyst, accelerate the oxidation of organic compounds; For water samples with high chloride ion content, addMercuric sulfateTo eliminate the interference of chloride ions.

　　High temperature sealed digestionPlace the mixture in165 ± 2°CRapid digestion is carried out in a high-temperature digestion reactor (usually only 10-15 minutes, much faster than the 2-hour standard method). Under these conditions, the reducing substances (mainly organic matter) in the water sample are oxidized by potassium dichromate:

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

　　Colorimetric measurementAfter digestion, the reaction solution changes from orange (the color of Cr ⁶⁺) to blue-green (the color of Cr ³ ⁺). In600nmMeasure its absorbance using a spectrophotometer at the wavelength.

　　Calculate COD valueBy measuring the change in absorbance and comparing it with the working curve drawn using COD standard solution, the COD concentration of the sample can be calculated. The remaining amount of unreacted potassium dichromate can be accurately determined by titration or Coulomb method, but modern instruments often use photometric methods for direct calculation.

　　feature：

　　advantageHigh accuracy, good correlation with national standards, and is an arbitration method.

　　shortcoming：

　　secondary pollutionThe use of a large amount of toxic and harmful chemical reagents (potassium dichromate, mercury sulfate, silver sulfate) generates waste liquids containing heavy metals, resulting in high treatment costs.

　　High operating costsThe consumption of reagents is high.

　　potential safety hazardHigh temperature and high pressure digestion pose safety risks.

　　Unable to measure low concentrationFor clean surface water, its oxidation efficiency is low and the error is large.

　　Technical route 2: UV absorption method

This method utilizes the principle that most organic compounds have characteristic absorption in the ultraviolet region (UVC, 200-280nm), and indirectly reflects the content of organic compounds by measuring the absorbance at 254nm.

　　working principle：

　　UV light sourceThe instrument emits ultraviolet light of a specific wavelength (usually 254nm).

　　liquidity poolThe water sample flows continuously through a quartz material circulation pool.

　　Absorbance measurementUltraviolet light passes through water samples and is absorbed by organic matter within them. The photodetector measures the intensity of transmitted light and calculates its ratio to the intensity of incident light to obtain the absorbance value A.

　　Calculation and correctionAccording to Lambert Beer's law, absorbance is directly proportional to the concentration of organic matter in the water sample within a certain range. By establishing a calibration curve between absorbance and COD concentration, the COD value can be directly read out. To eliminate interference from factors such as turbidity, an additional one is usually added546nmOr other visible light detectors can be used for turbidity compensation.

　　feature：

　　advantage：

　　Strong real-time performanceNo chemical reagents or digestion process are required, and the response time can reach seconds, truly achieving continuous monitoring.

　　green and environmentally friendlyNo secondary pollution and extremely low operating costs.

　　Easy maintenanceNo complex digestion units or reagent pipelines, high reliability.

　　shortcoming：

　　Accuracy is limitedIt can only reflect organic compounds with UV absorption and cannot oxidize all organic compounds (such as straight chain alkanes, benzenesulfonic acid, etc.), and inorganic compounds (such as nitrates, turbidity) can also cause interference.

　　Cannot be used as legal data: Usually used asTrend monitoring and early warningIts absolute value needs to be calibrated and verified through standard methods.

　　3、 Technology Application Research

　　1. Automatic monitoring system for pollution sources

　　Application pointWastewater discharge outlets of key polluting units.

　　Technology Selection：Potassium dichromate methodMainly. Due to its clear legal status and accurate and reliable data, it can serve as a direct basis for environmental law enforcement and pollution discharge fees. Despite high operating costs, it is necessary for regulatory needs.

　　2. Automatic monitoring system for surface water environmental quality

　　Application pointAutomatic water quality stations for rivers, lakes, and reservoirs.

　　Technology Selection：UV absorption methodIt's the main force.

　　reasonThe COD concentration in surface water is relatively low, and the potassium dichromate method has incomplete oxidation and large errors. The UV method can quickly and continuously reflect the changing trend of organic pollution in water bodies, providing early warning for algal outbreaks and sudden pollution events (such as industrial illegal discharge).

　　practice: Usually used“Real time monitoring by UV method+regular comparison and verification by laboratory national standard method”The model balances timeliness and accuracy.

　　3. Control of sewage treatment process

　　Application pointThe biological reaction tank inlet, aeration tank, and outlet of the sewage treatment plant.

　　Technology Selection：UV absorption methodandPotassium dichromate methodAll have applications.

　　UV法: Used forReal time process controlOperators can quickly adjust key parameters such as aeration rate and sludge return flow rate based on real-time changes in inlet COD, optimize the microbial degradation environment, and achieve energy conservation and consumption reduction.

　　Potassium dichromate methodUsed for assessing the quality of effluent waterHigh precision compliance assessmentEnsure the final emission data.

　　4. Other emerging applications

　　Total accounting and management of pollutant discharge permitsCombined with flow meter data, COD automatic monitoring data is the basis for calculating the total pollutant emissions of enterprises.

　　Smart Water Management and River Chief SystemAs a key node in the perception layer of the Internet of Things, it provides massive real-time data for the establishment of watershed water environment models and decision support systems.

　　4、 Technical Challenges and Development Trends

　　1. Current challenges

　　Greening of potassium dichromate methodHow to reduce the amount of toxic reagents and develop low toxicity alternative oxidants (such as ozone and persulfate) is an important research direction.

　　Standardization and accuracy improvement of UV methodWe need to establish more comprehensive calibration methods and interference correction models to improve their applicability in different water bodies.

　　Instrument reliabilityThe ability to withstand complex water samples (high salt, high oil, high suspended solids) and the stability of long-term operation are still tests.

　　2. Future Development Trends

　　Intelligence and NetworkingIntegrate AI algorithms to achieve fault self diagnosis, intelligent data auditing, automatic tagging of outliers, and remote operation and maintenance.

　　Miniaturization and modularizationDevelop miniaturized and portable on-site detectors to meet the needs of emergency monitoring and distributed monitoring.

　　Multi parameter integrationCOD, ammonia nitrogen, total phosphorus, total nitrogen pH、 Sensors such as dissolved oxygen are integrated into one, sharing sampling and pretreatment systems to reduce costs and provide a more comprehensive water quality profile.

　　New sensing technologyExplore reagent free or micro reagent COD detection technologies based on new principles such as biosensors, fluorescence methods, Raman spectroscopy, etc., to fundamentally solve the problem of secondary pollution.

　　Conclusion

COD water quality automatic analyzer is the core technical equipment for water environment management. The potassium dichromate digestion photometric method dominates the field of pollution source supervision due to its accuracy, while the UV absorption method plays an irreplaceable role in surface water and process control due to its rapid and green advantages. The future technological development will no longer be limited to precise measurement of a single indicator, but will move towardsMore intelligent, environmentally friendly, comprehensive, and integratedThe direction of evolution provides stronger technical support for winning the Battle of Blue Water.