Chemical Oxygen Demand (COD) Sensors Explained

Key takeaways

• Chemical oxygen demand (COD) measures how much oxidant is consumed when a strong chemical oxidises the pollutants in a water sample, reported in mg/L.
• Traditional laboratory methods (dichromate reflux and potassium permanganate) take between 30 minutes and several hours per sample.
• Online COD sensors using UV absorption at 254 nm to 600 nm ranges give continuous readings without reagents, suited to river, lake and industrial discharge monitoring.
• MetSensor UK supplies the ZWQ-COD1 COD sensor for UK water quality projects.

What chemical oxygen demand actually measures

Chemical oxygen demand is an indirect measure of the organic and some inorganic pollutants in a water sample. Rather than identifying each pollutant individually, the test measures how much of a strong oxidising chemical is consumed when it reacts with everything oxidisable in the sample. The result is reported as a concentration in milligrams per litre (mg/L), and a higher COD value means more oxidisable material is present, which generally points to a more polluted sample. COD is one of the standard indicators used alongside biochemical oxygen demand (BOD) and total organic carbon (TOC) to characterise water pollution levels.

Water quality classification by COD level

Regulatory frameworks commonly group water bodies into classes based on COD concentration. Water at or below roughly 15 mg/L is generally consistent with drinking water standards. As COD rises through bands of roughly 20 mg/L, 30 mg/L and 40 mg/L, the water is classified as increasingly polluted. These thresholds are used by environmental agencies to decide whether a river, lake or discharge point meets acceptable water quality standards, and repeated exceedance can trigger enforcement action against a discharger.

Traditional laboratory measurement methods

Two methods dominate laboratory COD testing. The dichromate reflux method heats the sample with potassium dichromate at around 148°C for roughly two hours, then measures how much dichromate was consumed by the oxidation reaction. This is the most widely recognised national standard method because of its accuracy, but the long digestion time makes it unsuitable for anything other than periodic sampling. The potassium permanganate method is faster, using a 30-minute boiling water bath, but is generally regarded as less accurate for samples containing more complex organic pollutants.

Modern instrumental methods

Beyond the two classic wet-chemistry methods, several instrumental approaches are used, particularly where continuous or near-continuous monitoring is needed. Spectrophotometric methods estimate COD by measuring how strongly a sample absorbs ultraviolet light, commonly in the region around 254 nm to 600 nm, since many of the organic compounds that contribute to COD absorb UV light in this range. Electrochemical methods oxidise the sample at an electrode and measure the resulting current, offering faster results with less chemical waste than the dichromate method. Atomic absorption methods are also used in some specialist applications.

How an online UV absorption COD sensor works

An online COD sensor built around UV absorption measures the intensity of ultraviolet light after it passes through the water sample and relates the amount of absorption to a COD concentration. Because this method does not require adding reagents or heating the sample, it can run continuously, providing a live data stream rather than a single point-in-time lab result. This is the principle used in the ZWQ-COD1 sensor, which measures across a 0 to 500 mg/L range with 0.01 mg/L resolution and roughly ±7% accuracy, communicating over RS485 using Modbus RTU. The probe body is built from 316L stainless steel, a grade chosen for its resistance to corrosion in wastewater and river environments, and it is designed for continuous submerged operation.

Where COD monitoring is used

COD sensors are deployed wherever organic pollution needs to be tracked over time rather than checked occasionally. Common locations include river and lake monitoring stations run by environment agencies, industrial wastewater outfalls in the petrochemical, pharmaceutical and food processing sectors, and municipal sewage treatment works that need to demonstrate discharge compliance. In smart city water networks, COD readings are often combined with other parameters such as pH, turbidity and dissolved oxygen to build a fuller picture of water quality at a monitoring point, an approach covered in our guide to multiparameter sondes.

Choosing between laboratory testing and an online sensor

Laboratory COD testing remains the reference method for regulatory reporting and for occasional spot checks, since dichromate reflux is the most established and widely accepted standard. An online sensor is better suited to situations where continuous data matters more than laboratory-grade precision on every reading, such as tracking a trend over days or weeks, catching a pollution spike as it happens, or feeding data into an automated alert system. Many operators run both in parallel, using the online sensor for day-to-day monitoring and periodic lab samples to verify and calibrate the sensor readings.

Practical specification points

When specifying a COD sensor for a UK installation, it is worth checking the measuring range against the expected pollution levels at the site, since a sensor calibrated for river monitoring (low COD) may not read accurately at an industrial outfall with much higher concentrations. The communication protocol also matters. RS485 with Modbus RTU is common in this class of instrument and integrates directly with most existing SCADA and telemetry systems, a topic covered in more detail in our RS485 and Modbus explainer. It is also worth confirming probe material and cable length suit the mounting arrangement at the monitoring point.

MetSensor UK is a UK supplier of our manufacturing partner water quality sensors, including COD, pH, dissolved oxygen, turbidity and conductivity probes. Our water quality sensors range and probe comparison page can help match a sensor to your monitoring requirements, and our water quality monitoring applications page covers typical deployment scenarios.