Chemical Oxygen Demand (COD) is one of the most important aggregate indicators for evaluating the level of oxidizable organic pollution in water, wastewater, and industrial effluents. Accurate COD measurement relies on standardized analytical procedures such as the dichromate reflux method, sealed-tube colorimetric digestion, the permanganate index method, and rapid spectrophotometric testing. This technical article explains the principles, applicable standards, sample handling requirements, interferences, and quality control considerations involved in professional COD testing.
Technical Overview
Chemical Oxygen Demand (COD) is defined as the mass of oxygen, expressed in milligrams per liter (mg/L), that is equivalent to the amount of a strong chemical oxidant consumed when a water sample is digested under controlled acidic and thermal conditions. Because most organic compounds in water are oxidized to carbon dioxide and water during the digestion, COD serves as a practical measure of the total reducible organic load, including substances that are not readily biodegradable.
COD is a bulk parameter rather than a compound-specific measurement. It is widely used to characterize the pollution strength of raw sewage, industrial discharges, and receiving waters; to design and optimize biological treatment systems; and to demonstrate compliance with discharge permits and environmental regulations. In routine practice, COD is frequently interpreted alongside Biochemical Oxygen Demand (BOD), and the ratio of BOD to COD is commonly used to judge the biodegradability of a wastewater stream.
Two families of oxidants dominate COD testing worldwide. The potassium dichromate system (CODCr), performed in hot concentrated sulfuric acid with a silver sulfate catalyst, provides high and reproducible oxidation efficiency and is the basis of most regulatory reference methods. The potassium permanganate system (CODMn, or permanganate index) offers milder oxidation and is preferred for relatively clean waters such as drinking water sources, where trace-level organic content must be monitored.
Testing Samples
Surface Water and Groundwater
Rivers, lakes, reservoirs, and groundwater typically exhibit low to moderate COD values. The permanganate index or low-range dichromate methods are generally appropriate. Sampling should account for spatial and temporal variability, and composite samples are often collected for trend monitoring programs.
Municipal Wastewater
Raw sewage, primary effluent, secondary biological effluent, and disinfected final effluent are routinely tested for COD at treatment facilities. Municipal wastewater contains a complex mixture of biodegradable organics, suspended solids, and refractory compounds, so samples must be homogenized before sub-sampling to ensure representative results.
Industrial Wastewater and Process Effluents
Effluents from the chemical, petrochemical, pharmaceutical, food and beverage, brewing, pulp and paper, textile, dyeing, and electroplating sectors can span COD concentrations from tens to tens of thousands of mg/L. High-strength samples usually require dilution, and matrices rich in chloride, ammonia, oil, or suspended solids demand special interference control.
Treated Effluent and Reclaimed Water
Recycled water, cooling make-up water, and discharge-compliant effluents are verified against stringent COD limits, often below 50 or 100 mg/L. Low-range sealed-tube spectrophotometric procedures or the reference titrimetric method with careful blank control are used for such applications.
Sampling and Preservation Requirements
COD samples are collected in clean glass or chemically resistant plastic containers. To suppress microbial degradation of organic matter, samples are acidified with concentrated sulfuric acid to pH 2 or below, cooled to approximately 4 degrees Celsius, and transported in the dark. Governing standards specify maximum holding times; many regulatory frameworks permit storage of properly acidified samples for up to 28 days, although shorter internal targets of 7 days or less are common good laboratory practice. Unpreserved samples must be analyzed as soon as possible, ideally within 24 hours.
Testing Items
Depending on the monitoring objective, laboratories report one or more of the following COD-related parameters:
- Total COD (CODCr): the oxygen demand of the whole, unfiltered and homogenized sample, the standard compliance parameter for most discharge permits.
- Soluble COD (sCOD): the oxygen demand of the filtrate passing through a 0.45 micrometer membrane, used to separate dissolved organics from particulate matter.
- Particulate COD: calculated as the difference between total COD and soluble COD, useful for evaluating solids removal performance.
- Permanganate index (CODMn): the oxygen demand measured with acidic or alkaline potassium permanganate, applied to cleaner waters such as source and drinking waters.
- Biodegradability assessment: the BOD-to-COD ratio, typically interpreted together with nitrogen and phosphorus data for treatment process design.
All results are reported in mg/L of oxygen consumed, together with the analytical method, any dilution factor, and the applicable detection range.
Testing Methods
Several internationally and nationally standardized methods exist. The choice depends on the expected concentration range, the matrix, the required turnaround time, and whether the result must serve as a regulatory reference value.
Potassium Dichromate Open Reflux Method (Titrimetric Reference Method)
This classical procedure is specified in standards such as ISO 6060 and the Chinese national and environmental standards for the dichromate determination of COD, and it corresponds to the macro titrimetric approach in ASTM D1252. A measured sample portion is mixed with a known excess of potassium dichromate in strongly acidic medium, with silver sulfate dissolved in concentrated sulfuric acid acting as the oxidation catalyst. Mercury(II) sulfate is added to complex chloride ions and prevent their interference. The mixture is refluxed at the boiling point of the acidic medium (approximately 148 degrees Celsius) for two hours. After cooling, the remaining unreacted dichromate is titrated with a standardized ferrous ammonium sulfate solution using a ferroin indicator, and the endpoint color change from blue-green to reddish-brown marks the completion. The consumed oxidant is converted to the equivalent oxygen demand through stoichiometric calculation. The method is applicable over a wide range, commonly cited as approximately 30 to 700 mg/L without dilution, and can be extended substantially through appropriate dilution. It remains the arbitration and reference technique for complex or disputed samples.
Closed Reflux Sealed-Tube Methods (Titrimetric and Colorimetric)
ISO 15705 and the closed-reflux variants in ASTM D1252 describe small-scale sealed-tube digestion. The sample, dichromate reagent, sulfuric acid, catalyst, and chloride masking agent are combined in a pre-dosed glass vial and heated, typically at about 150 degrees Celsius for two hours or under accelerated programs. Determination may be titrimetric, as in the open reflux method, or colorimetric: the digestion converts orange hexavalent chromium to green trivalent chromium, and the absorbance is measured spectrophotometrically, either at approximately 600 nm for the trivalent chromium formed or at approximately 440 nm for the residual hexavalent chromium, depending on the calibration range. Sealed-tube methods reduce reagent consumption, improve laboratory safety and throughput, and are available in low-range and high-range formats covering from a few mg/L to several thousand mg/L.
Rapid Digestion Spectrophotometric Method
The rapid digestion spectrophotometric method, standardized in the Chinese environmental standard HJ/T 399, is a high-throughput adaptation widely adopted by accredited third-party laboratories and industrial facilities. Pre-packaged reagent vials are digested in a heating block at approximately 165 degrees Celsius for about 20 minutes, after which the vial is cooled and read directly in a dedicated spectrophotometer with built-in calibration curves. The method delivers results in well under an hour, offers excellent precision within its validated range, and is particularly suited to routine process control and screening, although results for unusual matrices should be verified periodically against the reference reflux method.
Potassium Permanganate Index Method
For waters with low organic content, such as raw drinking water sources, the permanganate index method described in ISO 8467 and corresponding national standards is preferred. The sample is heated with a known excess of potassium permanganate in acidic medium for a fixed time, after which the remaining permanganate is determined by back-titration with sodium oxalate or an equivalent procedure. Because permanganate oxidizes many organic compounds incompletely, the permanganate index is not directly interchangeable with dichromate COD; it is a milder, relative indicator well suited to long-term trend monitoring of clean waters. An alkaline digestion variant exists for chloride-rich waters such as seawater.
Automated and Alternative Techniques
Online COD analyzers based on automated dichromate digestion or on correlation techniques such as ultraviolet absorption at 254 nm and TOC-to-COD conversion are extensively deployed at treatment plants and discharge outfalls for continuous monitoring. Electrochemical and advanced oxidation-based sensors are also emerging. Because these techniques are secondary or correlational, they must be validated against the applicable reference method at defined intervals, and their calibration must be matrix-specific.
Interferences and Quality Control
Chloride is the principal positive interference because it is oxidized by dichromate and inflates the result; it is controlled by complexation with mercury(II) sulfate, typically at a mass ratio of approximately 10 to 1 relative to chloride, and very saline samples may require dilution or dedicated high-chloride procedures. Certain refractory compounds such as pyridine and some aromatic hydrocarbons are only partially oxidized and may yield low recoveries. Residual chlorine, sulfite, and other reducing inorganic species must be considered when interpreting results. Quality assurance measures include reagent blanks, duplicate analyses, calibration verification with certified reference solutions of potassium hydrogen phthalate, which has a well-defined theoretical oxygen demand, matrix spike recoveries typically within 90 to 110 percent, control charts, and participation in interlaboratory proficiency testing.
Applications
COD testing underpins a broad range of environmental, industrial, and regulatory activities:
- Compliance monitoring of municipal and industrial discharge against permit limits and national effluent standards.
- Process control at wastewater treatment plants, including influent load balancing, aeration optimization, and evaluation of biological treatment efficiency.
- Sector-specific effluent management in the chemical, petrochemical, pharmaceutical, food and beverage, brewing, pulp and paper, textile and dyeing, and surface treatment industries.
- Environmental quality assessment of rivers, lakes, reservoirs, and groundwater within routine and event-driven monitoring programs.
- Pollution source tracing, environmental impact assessment, and investigation of water pollution incidents.
- Treatability studies, biodegradability evaluation, and the design basis for new treatment facilities.
- Management of cooling water, boiler feed water, and reclaimed water systems in industrial utilities.
- Research applications in analytical chemistry, ecotoxicology, and water treatment technology development.
FAQ
What is the difference between COD and BOD?
COD measures the oxygen equivalent of all chemically oxidizable matter, including non-biodegradable compounds, and is obtained in about two to three hours. BOD measures the oxygen consumed by microorganisms during aerobic degradation, typically over five days. COD is therefore usually equal to or higher than BOD, and the BOD-to-COD ratio is a practical indicator of biodegradability, with values above roughly 0.3 to 0.4 generally considered amenable to biological treatment.
Which COD method should be selected?
For regulatory reference values, disputed results, or complex matrices, the dichromate open reflux titrimetric method is preferred. For routine monitoring, process control, and high sample volumes, the closed-reflux colorimetric or rapid digestion spectrophotometric methods offer speed and precision. For clean source waters and drinking water applications, the permanganate index is the customary parameter. Accredited third-party laboratories can advise on the most appropriate method for a specific matrix and regulatory context.
How is chloride interference handled?
Mercury(II) sulfate is added before digestion to complex chloride ions, typically at a mass ratio of about 10 to 1 relative to the expected chloride content. Samples with very high salinity, such as certain industrial effluents or marine waters, may require dilution to bring chloride within the masked range, or the use of methods specifically validated for high-chloride matrices.
What is the holding time and preservation requirement for COD samples?
Samples should be acidified with sulfuric acid to pH 2 or below, stored at approximately 4 degrees Celsius in the dark, and analyzed within the holding time specified by the governing standard. Many regulatory frameworks allow up to 28 days for properly acidified samples, while unpreserved samples should be analyzed as soon as possible, ideally within 24 hours.
Why can rapid spectrophotometric results differ slightly from reflux titration results?
Differences arise from digestion conditions, reagent formulations, calibration ranges, and matrix effects such as high chloride, turbidity, or color. Well-maintained rapid methods are formally validated as equivalent within defined concentration ranges, but for unusual or highly variable matrices it is good practice to periodically verify photometric results against the reference reflux method and to document the correlation.
Can COD be monitored continuously online?
Yes. Online analyzers using automated dichromate digestion, or correlational techniques based on ultraviolet absorption at 254 nm or total organic carbon, provide near real-time COD estimation. These systems require periodic calibration, cleaning, and validation against laboratory reference measurements to ensure that reported values remain defensible for compliance purposes.