Chemical Oxygen Demand (COD): the number that saves rivers
Chemical Oxygen Demand (COD) is one of the most important measures of water quality. It tells us how much oxygen is needed to break down organic and inorganic substances present in a sample. When COD values rise, rivers and aquatic life are at risk, and treatment plants face higher costs. Understanding what COD measures, where high results come from, and how to safely reduce them helps protect ecosystems, ensure cleaner tap water, and keep industrial processes under control.
The night the river stopped breathing
Rapid Chemical Oxygen Demand (COD) measurement on the riverbank
The night is quiet, and the river looks calm. Then, a sample goes into the cuvette. Soon after, the digester hisses, and the colors in the cuvette change quickly. Consequently, the result climbs. Chemical Oxygen Demand (COD) exceeds the safe threshold, and tension fills the air. To verify, the analyst adds a reference sample and checks the blank to rule out error. Meanwhile, the thermometer shows warm water and low flow. At the same time, the team notes the time, location, temperature, and sky conditions. In addition, a comparison with last week’s reading shows an upward trend. Therefore, a short note goes to the on-call team. Afterwards, they load more cuvettes and repeat the digestion. Finally, the second reading confirms the first…
One number, three effects: DO↓, fish↓, environmental alarm
High Chemical Oxygen Demand (COD) means a greater “appetite” for oxygen. Consequently, Dissolved Oxygen (DO) drops, and therefore fish start gasping. As a result, the environmental alarm goes off, and minutes matter. To begin with, fish gather at the surface. After that, their movements become chaotic and weaken. What’s more, warm water further accelerates any nighttime oxygen deficit. For this reason, operators limit discharges if they can. At the same time, field crews head out to check the river reach. All in all, each passing hour can, in fact, make things dramatically worse. In conclusion, high COD is often a harbinger of fish kill conditions.
Chemical Oxygen Demand (COD) in simple words
What COD measures and in what units
Chemical Oxygen Demand (COD) is a simple measure of the water sample’s “oxygen appetite.” The test uses a strong oxidant and the reagent “burns” what can be oxidized—without real fire. The result shows how much oxygen would be required to break it down. It’s reported in milligrams of oxygen per liter (mg O₂/L). A milligram is one-thousandth of a gram. A liter is a large bottle of water. The higher the COD, the bigger the oxygen appetite and the more oxidizable matter present.
COD doesn’t name compounds. It’s a single number summing everything that reacts with the oxidant. Most often these are organic compounds, but sometimes certain inorganics as well. That’s why COD is a composite indicator. A low value means a small oxygen appetite. A high value signals a risk of oxygen depletion and trouble for fish.
Test variants: COD-Cr and COD-Mn
COD-Cr uses potassium dichromate (Cr). It is a “strong” oxidant, and as such, it covers most organic compounds and some inorganics. Therefore, it is the reference method in laboratories and permits. Typically, digestion time is about two hours. Moreover, when samples contain chlorides, mercuric sulfate is added so they do not inflate the result. In addition, silver sulfate is used as a catalyst. Safety rules certainly apply: fume hood, gloves, goggles, and proper waste disposal.
By contrast, COD-Mn uses potassium permanganate (Mn). This oxidant is milder. Consequently, it “sees” simple compounds better, such as sugars or alcohols. However, it oxidizes persistent and aromatic compounds less effectively. Results are therefore often distinctly lower than COD-Cr. For this reason, this variant is sometimes used as an “oxidizability” indicator, and it works well in waters with low concentrations.
Importantly, COD-Cr and COD-Mn numbers should not be compared directly. After all, these are different tests with different oxidative “strength.” For example, the same sample may show COD-Cr = 120 mg O₂/L and COD-Mn = 30 mg O₂/L. In this case, the method is chosen based on purpose and sample type. It could be a river, COD in wastewater, or drinking water. Furthermore, “mercury-free” COD-Cr kits exist and work when chlorides are low. At high chloride levels, however, correction is needed—or chlorides must be removed before analysis.
Other wastewater pollution indicators alongside COD: BOD₅, DO, TOC, UV254
BOD₅ (Biochemical Oxygen Demand over 5 days) – shows how much oxygen bacteria will consume while degrading matter in the sample over five days. It increases where there’s “fresh,” readily degradable load. BOD₅ is usually lower than COD because it covers only the biodegradable fraction. Example: high BOD₅ with low DO is a fast track to fish kill risk.
DO (Dissolved Oxygen) – tells how much oxygen is currently available in the water. It’s the first, very sensitive signal for fish and invertebrates. When COD and BOD₅ are high, DO usually falls. Additionally, in summer at high temperatures, DO falls faster because warm water “holds” oxygen worse.
TOC (Total Organic Carbon) – sums the carbon contained in all organic compounds. It’s independent of bacteria and the oxidant used, so it’s good for load calculations and time-series comparisons. For example, a decline in TOC at constant flow means less organic matter reaching the receiver.
UV254 (absorbance at 254 nm) – increases when there are more aromatic compounds in the sample (e.g., lignin derivatives, phenols). Such compounds are harder to remove and more often require activated carbon or ozonation. Conversely, a drop in UV254 after carbon filtration usually confirms adsorption worked.
Together these indicators form a simple compass. COD and BOD₅ speak to the “oxygen appetite,” DO to the here-and-now, and TOC/UV254 to the total amount and “character” of the matter. It pays to read them together and as a series, not just a single number.
What affects the result: flow, temperature, nutrients
Flow matters most. When there’s little water, the same amount of pollution yields a higher concentration. Conversely, when there’s a lot of water, the same pollution is diluted and numbers drop. Therefore, the same facility can show different results in drought versus after rain. Remember, concentration is mg per liter, and the impact on the river also depends on load—i.e., amount per hour.
Temperature also matters. Specifically, warm water holds oxygen worse, so DO drops occur faster during heat waves. By contrast, winter can be safer because cold water holds more oxygen and degradation is slower. Additionally, at night plants and algae respire, so DO is typically lowest at dawn.
Nutrients—nitrogen and phosphorus—are important, too. These compounds “feed” algae. As a result, when algae abound, they produce oxygen by day. However, once they die off, their decay consumes oxygen at night, and COD then rises. In general, nutrients mainly come from agriculture, wastewater, and runoff from paved surfaces during downpours.
That’s why results should always be read together with weather and river conditions. For instance, droughts, low flows, cloudbursts, and snowmelt can change the picture overnight. In short, the same monitoring point can appear “different” in summer and winter, at low and high water.
Concentration vs load: a simple conversion
Concentration is mg O₂ per liter. Load tells how much matter reaches the river over time. Calculate it as:
load = COD (mg/L) × flow (m³/h) × 0.001 = kg/h.
This helps assess the actual impact on the receiver, not just the water’s “density.”
If COD = 80 mg O₂/L and the flow is 20 m³/h, the load is 1.6 kg O₂/h. If COD drops to 60 mg O₂/L but flow rises to 40 m³/h, the load increases to 2.4 kg O₂/h. Always analyze concentration and flow together.
Why this matters: rivers and tap water
High Chemical Oxygen Demand (COD) means oxygen is consumed quickly. Oxygen disappears first in slow reaches and backwaters. In summer and at low river levels this happens even faster. Bacteria and algae continue to consume oxygen at night, so dawn is often worst. The effect is sluggish fish, fish kills, musty smell, and blooms.
For drinking water plants, high COD means tougher operation. More coagulant and oxidant are needed. Ozone or activated carbon is used more often. Filters clog faster, so backwashing is more frequent. More sludge is produced for disposal. Aeration and pumps consume more energy. As a result, treatment and maintenance costs rise.
That’s why COD measurement—and COD in wastewater at the source—acts like an early alarm. It shows when the load is rising and where to look for the cause. It allows retention, stream separation, and smoothing of inflow. The river gets less “oxygen appetite,” and the plant runs more stably. In short: low COD means a calmer river and easier tap water. High values mean greater fish kill risk and higher cleanup and treatment bills.
COD vs BOD₅ vs DO vs TOC – a short compass
COD is a quick test. It shows how much oxygen a sample would “eat” upon oxidation. Results come in a few hours. It sums all oxidizable compounds.
BOD₅ is a five-day biological test. It shows how much oxygen bacteria consume while degrading matter. It covers only the biodegradable fraction and is usually lower than COD.
DO is dissolved oxygen here and now—the river’s “breath thermometer.” When COD is high and water is warm, DO falls faster. It’s the first warning signal for fish.
TOC is total organic carbon. It shows how much carbon is in organic compounds in the sample. It’s independent of bacteria and oxidant, so it’s good for load balancing and trend tracking.
Using them together: day-to-day, COD is used for quick process control and early alarms. BOD₅ confirms biological treatment performance. DO warns of fish kill conditions. TOC helps with mass balances, comparisons, and chemical dosing. Simpl