COD Measurement: Online COD Analyzers
Online chemical oxygen demand (COD) measurement in wastewater and process waters: process COD analyzer for monitoring treatment efficiency and discharge quality.
What is COD and why is it measured?
Chemical oxygen demand (COD) is the amount of oxygen needed to chemically oxidize the organic and reducible substances in water. It is the main indicator of the organic pollution load in wastewater. In wastewater treatment plants, COD is measured to monitor the inlet load, assess treatment efficiency and check discharge quality. Discharge legislation defines plant-specific monitoring obligations for organic pollution parameters, so online measurement is valuable for both compliance and process optimization; it allows rapid response to load changes and spills without waiting for a laboratory result.
Where is COD measured?
- Inlet and outlet of wastewater treatment plants
- Industrial process wastewaters: Food, chemical, textile, refinery, paper and similar plants
- Between treatment units: Before and after biological treatment
How is COD measured?
The laboratory COD method is based on heating the sample to react with a strong oxidant and measuring the oxidant consumed. Online COD analyzers automate this process: the sample and reagents are dosed automatically, react under controlled conditions, and the result is updated periodically. The analyzer operates at measurement cycle intervals; the result is transferred to the monitoring system through a current output or communication. Reagents, tubing and pumps require periodic maintenance and waste management.
Choosing a COD analyzer
- Sample type, suspended solids load and required pre-filtration
- Measurement cycle time and expected COD range
- Reagent consumption, waste reagent management and maintenance kit
- The plant's discharge monitoring and data transfer requirements
COD measurement with Tetra
Tetra supplies the Thermo Orion 3106 COD process analyzer in Türkiye and provides installation, supply of reagents and consumables, commissioning, calibration and service.
Frequently asked questions
What is the unit of COD?
COD is expressed as mg/L O₂, in terms of the oxygen equivalent consumed. Reports generally give only mg/L; the value shows the amount of oxygen needed to oxidize the organic and reducible substances in the sample.
What is the difference between COD and BOD?
BOD measures the organic matter that microorganisms can biologically break down, and the standard analysis takes 5 days. Because COD uses a chemical oxidant, it also covers substances that cannot be biologically degraded and gives a result much faster. COD is therefore generally higher than BOD, and the BOD/COD ratio gives an idea of how biologically treatable the wastewater is.
What is chloride interference in COD measurement?
In the dichromate-based method, chloride ions are also oxidized and make the COD result appear higher than it is. This interference is reduced by using a masking agent that binds chloride, but correction may be limited in wastewaters with very high chloride. When selecting an analyzer for saline process wastewaters, the sample's chloride content must be specified.
Can COD be measured by UV absorbance?
Some online systems estimate COD indirectly by measuring UV absorbance at around 254 nm; these systems are reagent-free and fast. However, this approach is not a direct COD measurement and requires correlation with laboratory results for each sample type. The correlation needs rechecking when the wastewater composition changes.
Products measuring this parameter
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