Chemical oxygen demand is one of the most universally reported parameters in industrial water quality management. It appears on discharge consent conditions, in process control targets, and in environmental compliance reports across virtually every sector that generates wastewater. Yet the number that appears on a COD report is not as unambiguous as it appears. Two facilities can measure COD from the same sample using different methods and obtain different results. The reason is that there is no single way to measure COD — there are multiple methods, each measuring something slightly different, and the results they produce cannot be directly compared without additional site-specific information.
Wet Chemistry COD: The Reference Method
The wet chemistry method for COD measurement — formally APHA Standard Method 5220D, or the equivalent EPA 410.4 — is the reference method on which most regulatory discharge standards are based. It involves digesting a water sample under heat and acid conditions in the presence of a strong oxidant, typically potassium dichromate. The dichromate oxidises the organic compounds in the sample. The amount of dichromate consumed in the reaction is measured photometrically, and this is used to calculate the oxygen demand of the organic matter present.
In practice, the method is typically implemented using sealed reagent vials containing the necessary reagents, a digestion block or reactor that heats the vials to the required temperature for a specified time (usually 150 degrees Celsius for two hours for standard-range measurements), and a photometer that reads the colour change of the digested sample. This is a laboratory method, adapted for field and portable use.
The Pyxis SP-800 and SP-910 handheld analysers use this wet chemistry reactor digestion approach. The operator takes a water sample, prepares and digests the reagent vial, and reads the result with the handheld photometer. The result is a true COD value as defined by the reference method — the number that correlates directly to the regulatory standard against which the effluent discharge is assessed. It is the number that a regulator will accept as the basis for compliance demonstration.
The practical limitation of wet chemistry is that it is a batch process. Each measurement takes approximately two hours from sample collection to result, requires reagent vials and their associated handling and disposal requirements, and produces one data point at a time rather than a continuous signal. It cannot provide the real-time process visibility that continuous monitoring delivers.
UV254 Absorbance: The Surrogate Method
The UV254 approach is fundamentally different. At 254 nanometres, dissolved organic compounds in water absorb ultraviolet light in proportion to their concentration. An optical sensor emitting UV light at 254nm can measure this absorbance continuously, without reagents, without sample preparation, and without the two-hour wait of a digestion cycle. From the UV254 absorbance reading, the sensor derives a COD-equivalent value in real time.
At a second reference wavelength — typically around 365nm — the same sensor measures the background absorbance of turbidity, allowing the 254nm signal to be corrected for turbidity interference. Because the UV-Vis spectral fingerprint of organic matter contains information about multiple parameters, the same optical measurement can simultaneously derive estimates for BOD, TOC, and nitrate. This is why the Pyxis ST-800 inline sensor provides all four parameters from a single installation.
The critical point about UV254-derived COD is that it is a surrogate, not a direct measurement of chemical oxygen demand. The sensor does not perform a chemical oxidation reaction. It measures optical absorbance and uses a mathematical model to predict what the COD result from a wet chemistry measurement would be. That model is based on the correlation between UV254 absorbance and wet chemistry COD in the specific water matrix where the sensor is deployed.
This correlation is water-matrix-dependent. The organic compounds present in an industrial effluent stream determine how much UV254 absorbance they produce per unit of COD. If the organic composition of the water changes — for example, if a new product line is introduced to the facility, or if the nature of the incoming raw materials changes — the correlation between UV254 and wet chemistry COD changes with it. Without periodic recalibration against wet chemistry reference measurements, the UV254-derived COD value can drift away from the true wet chemistry value over time.
Why the Two Values Cannot Be Directly Compared
Wet chemistry COD measures the total oxidisable organic content of the sample under the specific conditions of the dichromate digestion method. UV254-derived COD predicts what that number would be, based on optical absorbance. In a stable water matrix with a well-established site-specific correlation, the two numbers will agree closely. In a changing matrix, or in a matrix that has not been correlated, they will not.
The practical implication is that a UV254-derived COD reading cannot be used directly as a discharge compliance number without first establishing and maintaining the site-specific correlation to wet chemistry reference measurements. A discharge consent condition specifying a COD limit is based on the reference wet chemistry method. Substituting a UV254 reading without establishing its relationship to the reference method is not acceptable for regulatory purposes in most jurisdictions.
Using Both Methods Together
The correct approach is to use the two methods for what each does best, and to use them together rather than as alternatives.
The Pyxis ST-800 inline sensor provides continuous real-time COD-equivalent values that enable process control, early warning of organic loading events, and trend monitoring across the full operating cycle. It sees everything that happens between lab samples — the overnight discharge event, the production peak, the process upset — and logs it all. This is information that weekly or daily grab sampling cannot provide.
The Pyxis SP-800 or SP-910 handheld provides the wet chemistry reference measurements needed to maintain the site-specific correlation for the ST-800, verify its readings, and generate the regulatory-compliant COD values required for discharge reporting.
Together, they provide both the real-time process visibility that the inline sensor delivers and the regulatory-grade reference measurement that the handheld delivers. Used in isolation, each covers only half of what a complete COD monitoring programme requires.
To discuss how to configure a COD monitoring programme using Pyxis inline and handheld instrumentation for your wastewater application, contact us at info@autoflotechnology.com.