What Is Chemical Oxygen Demand (COD) in Water?

Chemical oxygen demand, or COD, is a measure of how much oxygen would be needed to chemically break down all the organic (and some inorganic) material dissolved or suspended in a water sample. In practical terms, it tells you how polluted a body of water or a wastewater stream is with organic substances: the higher the COD value, the heavier the load of oxidizable material.¹ It is one of the most widely used metrics in environmental monitoring and wastewater treatment, but what the number actually represents, how it is measured, and where it falls short are all worth understanding in more detail.

What COD Actually Tells You

When organic matter enters a river, lake, or wastewater treatment plant, microorganisms begin breaking it down. That breakdown consumes dissolved oxygen. If too much organic material is present, the oxygen in the water gets used up faster than it can be replenished, and aquatic life suffers. COD provides a quick, chemistry-based estimate of the total amount of oxidizable substances in the water, expressed as milligrams of oxygen per liter (mg O₂/L). A pristine mountain stream might have a COD in the single digits. Raw domestic sewage commonly falls in the hundreds, and certain industrial wastewaters can reach tens of thousands.

The key word is “chemical.” COD does not care whether bacteria could actually consume a given compound. It forces oxidation with a powerful chemical reagent under harsh conditions, so it captures everything from easily biodegradable sugars to stubborn, hard-to-break-down molecules that microbes might leave alone. That makes it both useful and imperfect: it gives you the worst-case picture of oxygen demand, but it can overestimate what would happen in a real river.

How COD Is Measured

The standard laboratory method has been around, in various forms, for roughly 150 years.² The dominant modern version uses potassium dichromate as the oxidizing agent. A water sample is mixed with sulfuric acid, potassium dichromate, and a catalyst (silver sulfate), then heated to about 140 °C for two hours. During that time the dichromate oxidizes organic compounds, and the amount of dichromate consumed tells you how much oxygen was equivalent. The leftover dichromate can be measured either by titration or by reading its color with a spectrophotometer.³

Mercury(II) sulfate is also added to the reaction mixture, and its role is to neutralize chloride ions. Without it, chloride would react with the dichromate and inflate the reading, making the water look more polluted than it is. This detail matters because the reagents involved, particularly chromium and mercury compounds, are themselves toxic. Every COD test generates a small volume of hazardous waste that must be carefully handled and disposed of.²

The detection limit for this kind of procedure sits around 3 mg/L, and the reproducibility is good: at a concentration of about 112 mg COD/L, results vary by only about 4% from test to test. When compared head-to-head against the older open-reflux method, simplified dichromate digestion produces statistically equivalent results.³

COD Versus BOD and TOC

If you have spent any time reading about water quality, you have probably seen three acronyms mentioned together: COD, BOD, and TOC. They measure related but different things, and knowing what each one captures helps you interpret the numbers.

BOD, or biochemical oxygen demand, measures how much oxygen living microorganisms actually use when they decompose organic material in a sample, typically over five days (called BOD₅). Because it relies on biology, it only captures the biodegradable fraction. COD captures both the biodegradable and the non-biodegradable portion, so it is almost always equal to or higher than BOD. The ratio between the two gives a rough sense of how treatable a wastewater is: if COD is only a little higher than BOD, most of the organic load is biodegradable and conventional biological treatment should handle it. If COD is much higher, a significant chunk of the pollution resists microbial breakdown.

In domestic sewage, the two track each other closely. One study of a wastewater treatment facility found a correlation of 0.985 between COD and BOD₅ across multiple sampling points.⁴ That tight relationship makes it possible to use a quick COD test as a stand-in for the slower five-day BOD test in routine monitoring.

TOC, or total organic carbon, measures the mass of carbon in organic compounds rather than the oxygen needed to oxidize them. It sidesteps some of the chemical hazards of COD testing because it does not require dichromate or mercury. Research has shown that TOC can reliably replace both COD and BOD₅ in incoming domestic wastewater, but in treated effluent it only correlates well with COD, not with BOD₅.⁵ The practical implication is that if a facility wants to phase out toxic COD reagents and switch to TOC, it works well for tracking day-to-day performance but may not give a clean read on the biodegradability of its final discharge.

Where the Standard Test Can Mislead You

The dichromate method is robust, but it has blind spots. The biggest is interference from halide ions. Chloride is the most common culprit, and mercury sulfate is added specifically to bind it. That fix works for chloride, but it does not work for bromide. If your wastewater contains significant bromide concentrations, the standard mercury sulfate complexation step will not neutralize the interference, and your COD reading will come back artificially high.⁶ This matters for certain chemical industry effluents and for coastal or estuarine waters where bromide levels may be elevated.

There are other, less dramatic sources of error. Some inorganic compounds like ferrous iron and sulfides are oxidized by dichromate, adding to the measured COD even though they are not organic pollutants. Volatile organic compounds can escape from the sample before the reaction is complete, leading to an undercount. And the test cannot distinguish between a harmless natural organic compound like humic acid from decomposing leaves and a genuinely toxic synthetic chemical: both register as COD. A high reading signals that something is there, but you need additional analysis to know whether it is dangerous.

Getting a Good Sample

Even a perfectly run test is useless if the sample has changed before it reaches the lab. Organic matter in water does not sit still; microorganisms begin consuming it as soon as a sample is collected, which means the COD can drift downward during transport and storage. One approach is chemical preservation. Early research demonstrated that adding mercuric chloride at about 890 ppm can keep sewage samples stable for over six weeks, with no significant change in COD or several other parameters.⁷

Acidification with sulfuric acid to bring the pH below 2 is also common practice, and refrigeration at 4 °C helps further slow microbial activity. A comparative study found that unpreserved wastewater samples yielded the lowest COD readings, confirming that without some preservation step, biological degradation during storage pulls the numbers down and underestimates the true organic load.⁸ The takeaway for anyone collecting samples: get them cold, get them acidified or chemically preserved, and get them to the lab promptly.

Why High COD Is an Environmental Problem

A high COD value in a river or stream is a warning flag for dissolved-oxygen depletion. When microorganisms work through a heavy load of organic material, they strip oxygen from the water column. Once dissolved oxygen drops below about 4 or 5 mg/L, many fish species begin to suffer. Below 2 mg/L, you get hypoxic “dead zones” where only the hardiest organisms survive. High COD is not inherently toxic in itself, but it signals that the water’s self-purification capacity is being overwhelmed, and some of the individual organic compounds contributing to that COD may be directly harmful to aquatic organisms even at low concentrations.⁹

Because of this, most environmental regulations set discharge limits for COD. The specific numbers vary by country and by industry type, but the principle is universal: before wastewater is released into a natural water body, its COD must be reduced to levels the receiving water can handle without oxygen collapse.

Stubborn Organics and Why Treatment Gets Complicated

Not all COD is created equal. Some of the organic matter in wastewater is easily consumed by bacteria in a conventional biological treatment plant. The rest, sometimes called refractory or bio-refractory organics, passes through biological treatment largely intact. In landfill leachate, for example, humic and fulvic acids are prominent refractory compounds that resist standard aerobic and anaerobic digestion.¹⁰ In distillery wastewater, melanoidins and lignin breakdown products are the stubborn culprits, resisting even aerobic polishing steps and contributing both COD and color to the effluent.¹¹

This refractory fraction is the reason a facility can have a well-running biological treatment process and still fail to meet its COD discharge limit. It is also the reason more aggressive treatment technologies exist.

Advanced Oxidation and Industrial Wastewater

When biological treatment cannot bring COD low enough, advanced oxidation processes (AOPs) are the next step. These methods generate highly reactive hydroxyl radicals that can break down organic molecules that resist conventional treatment. The idea is to either mineralize the stubborn compounds completely (turning them into carbon dioxide and water) or at least chop them into smaller molecules that bacteria can then finish off.

Electrochemical oxidation, for instance, generates hydroxyl radicals directly at an electrode surface. Boron-doped diamond electrodes are particularly effective and have been used at smaller scales for removing COD from industrial wastewater and for ballast water disinfection.¹² Ozone-based processes (ozonation, ozone combined with hydrogen peroxide, or ozone with UV light) have shown strong results in industrial settings. In textile wastewater containing reactive dyes, ozone-based AOPs achieved roughly 90% color reduction even in undiluted wastewater, while UV/hydrogen peroxide alone was hindered by the presence of salts and surfactants in the effluent.¹³

For landfill leachate, combined ozone/hydrogen peroxide and microwave/persulfate processes have proven effective at degrading the humic and fulvic acids that biological systems leave behind, reducing the overall complexity and aromaticity of the remaining organic material.¹⁰ The choice of AOP depends on the specific wastewater: what organic compounds are present, what salts or other interferents are in the mix, and what the target discharge limit is.

Moving Away From Toxic Test Reagents

A recurring irony of COD testing is that a measurement designed to protect the environment generates hazardous waste in the process. Potassium dichromate is a known carcinogen, and mercury compounds are broadly toxic. As the number of COD tests performed worldwide has grown alongside increasing industrial discharge, the volume of toxic reagent waste has grown too.² In some European jurisdictions, the dichromate COD test is being phased out precisely because of these concerns, with TOC increasingly recommended as a replacement.

Electrochemical sensor methods offer another path. Researchers have developed sensors using copper nanoparticles deposited on a glassy carbon electrode that can measure COD without the dichromate reagent entirely. These sensors use glycine as a standard substance and avoid the toxic waste stream of the traditional method.¹⁴ UV-visible spectroscopy approaches have also been explored, where the absorption of light at specific wavelengths is used to estimate COD without adding any reagents at all.¹⁵ These approaches are still being refined, and many labs continue to use the dichromate method because it remains the legally mandated standard in much of the world, but the trend is clearly toward greener alternatives.

Real-Time Monitoring in Treatment Plants

One of the practical limitations of the traditional COD test is the two-hour digestion time. In a treatment plant running around the clock, operators cannot wait two hours to find out whether something has gone wrong. Real-time monitoring uses surrogate measurements instead. Dissolved oxygen, pH, and oxidation-reduction potential (ORP) sensors are already installed in most modern plants, and changes in these readings can signal shifts in organic loading long before lab results come back.

In sequencing batch reactors treating domestic sewage, for instance, monitoring the dissolved oxygen profile during a treatment cycle provides a reliable indicator of whether COD and nitrogen compounds have been fully degraded. When bacterial respiration consumes the organic load, dissolved oxygen levels rise; operators can use that rise as a signal that the batch is done.¹⁶ Pairing these real-time proxies with periodic lab COD tests gives plant operators both immediate feedback and periodic ground-truth calibration.

UV-visible spectroscopy systems have entered this space too, installed directly in pipe runs or channels to continuously estimate COD from the water’s light-absorption signature. The algorithms behind these systems require calibration against traditional COD measurements for the specific wastewater being monitored, and they can drift when the wastewater composition changes, but they offer something the dichromate test never can: a continuous data stream rather than a snapshot taken every few hours.

COD in Recycled Reagent Waste

Given how many COD tests are performed globally, several research groups have explored whether the spent reagent solutions can be recycled rather than sent to hazardous waste disposal. One approach recovers potassium from the dichromate waste to produce potassium-based fertilizer while simultaneously extracting silver as nanoparticles from the silver sulfate catalyst residues.² This is still at the research stage, but it illustrates a broader push in analytical chemistry to close the loop on test waste rather than simply burying it. The ultimate solution likely lies in moving away from dichromate-based methods altogether, but in the interim, minimizing the environmental footprint of existing methods is a pragmatic step.

1
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3
Elsevier. An improved determination of chemical oxygen demand in water and wastes by a simplified acid dichromate digestion
4
PubMed. Use of COD, TOC, and Fluorescence Spectroscopy to Estimate BOD in Wastewater
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PubMed. An investigation of refractory organics in membrane bioreactor effluent following the treatment of landfill leachate by the O(3)/H(2)O(2) and MW/PS processes
11
PubMed. Bio-refractory dissolved organic matter and colorants in cassava distillery wastewater: Characterization, coagulation treatment and mechanisms
12
Heliyon. Advanced oxidation processes for water and wastewater treatment – Guidance for systematic future research
13
Chemical Engineering Journal. Comparison between industrial and simulated textile wastewater treatment by AOPs – Biodegradability, toxicity and cost assessment
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Electroanalysis. Sensitive and Green Method for Determination of Chemical Oxygen Demand Using a Nano‐copper Based Electrochemical Sensor
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