Biochemical oxygen demand, or BOD, is a measure of how much oxygen microorganisms consume when they break down organic matter in water. It’s one of the most widely used indicators of water pollution. A clean, unpolluted river typically has a BOD of 5 mg/L or less, while raw sewage can register between 150 and 300 mg/L.
How BOD Works
Every body of water contains bacteria and other microorganisms that feed on organic material, things like plant debris, animal waste, food scraps, and sewage. These organisms need oxygen to do their work, just like you need oxygen to digest food. When a large amount of organic waste enters a river or lake, the microbial population surges to consume it, pulling dissolved oxygen out of the water in the process.
BOD quantifies that oxygen consumption. A higher number means more organic pollution is present, because more oxygen is being used up by decomposition. The measurement is expressed in milligrams of oxygen consumed per liter of water (mg/L).
There are actually two components of oxygen demand. Carbonaceous BOD comes from microbes breaking down carbon-based organic compounds like sugars, fats, and proteins. Nitrogenous oxygen demand comes from the breakdown of nitrogen-containing compounds like ammonia. Standard BOD tests often capture both, though labs can isolate the carbonaceous portion when needed for wastewater treatment sizing.
How BOD Is Measured
The standard test, called BOD5, is straightforward in concept. A water sample fills a sealed, airtight bottle. The dissolved oxygen in the sample is measured, then the bottle is incubated in the dark at 20°C (68°F) for five days. After incubation, the dissolved oxygen is measured again. The difference between the two readings is the BOD: it tells you how much oxygen the microorganisms consumed over those five days.
Five days at 20°C is a convention, not a complete picture. Microbial decomposition continues well beyond five days, but the test captures enough of the process to be useful for comparison and regulation. Clean water at 68°F can only hold about 9.1 mg/L of dissolved oxygen at saturation, which is why heavily polluted samples with BOD values in the hundreds or thousands of mg/L must be diluted before testing.
What the Numbers Mean
BOD values give you a quick snapshot of water quality:
- 1 to 2 mg/L: Very clean water with minimal organic pollution.
- 3 to 5 mg/L: Moderately clean. This is the upper range for healthy natural waterways.
- 6 to 9 mg/L: Somewhat polluted. Sensitive aquatic species may start to be affected.
- 100+ mg/L: Heavily polluted. Untreated domestic sewage typically falls between 110 and 350 mg/L depending on concentration.
For context, U.S. federal regulations require that treated wastewater discharged from sewage plants meet a 30-day average BOD no higher than 30 mg/L, with at least 85% of the original organic material removed during treatment. Weekly averages must stay below 45 mg/L. These limits exist specifically to protect the rivers, lakes, and coastal waters that receive the discharge.
Why High BOD Harms Aquatic Life
The danger of high BOD isn’t the organic matter itself. It’s what happens to dissolved oxygen. When microbes consume oxygen faster than it can be replenished from the atmosphere or photosynthesis, oxygen levels plummet. This condition, called hypoxia, can suffocate fish and other aquatic organisms.
Research on freshwater fish species shows that mortality rates climb sharply once dissolved oxygen drops below about 2 to 3 mg/L. Some species begin showing distress behaviors, like gulping air at the surface, when oxygen falls to around 3 to 5 mg/L. The lethal threshold varies by species: hardier bottom-dwelling fish can survive oxygen levels as low as 0.25 mg/L for short periods, while others start dying at 1.5 mg/L or higher. Large pollution events that spike BOD can cause mass fish kills when oxygen crashes across a wide area.
Temperature makes the problem worse in two ways. Warmer water holds less dissolved oxygen to begin with, and it also speeds up microbial metabolism, increasing the rate of oxygen consumption. A polluted river in summer faces a double hit: less oxygen available and faster depletion.
BOD Versus COD
You’ll often see BOD mentioned alongside COD, or chemical oxygen demand. Both measure oxygen consumption, but they work differently. BOD measures oxygen used by living microorganisms over five days, capturing only the biodegradable portion of pollution. COD uses a strong chemical oxidizer to break down virtually all organic matter in just a few hours, including compounds that bacteria can’t easily digest.
COD results are always equal to or higher than BOD because the chemical test is more aggressive. COD is faster and more reproducible, which makes it useful for routine monitoring at treatment plants. BOD is slower but more relevant to what actually happens in a natural waterway, where living organisms are doing the work. Treatment facilities often use both: COD for quick operational decisions and BOD for regulatory compliance.
Factors That Affect BOD Readings
Temperature is the biggest variable. The standard test runs at 20°C, but real-world water temperatures vary widely. Microbial decomposition follows a predictable pattern: after an initial lag phase of 10 to 20 hours where oxygen decreases slowly, consumption accelerates exponentially. Warmer temperatures compress and intensify this cycle, while colder water slows it down.
Heavy metals in the water can throw off BOD results significantly. Copper and chromium, even at concentrations of just 1 to 2 mg/L, can inhibit the microorganisms responsible for decomposition, producing artificially low readings. Mercury is particularly toxic to the test organisms, suppressing BOD readings by 76% to 118% depending on concentration. Nickel, cobalt, zinc, and lead also interfere. This means industrial wastewater containing metals may appear cleaner than it actually is when tested with a standard BOD procedure, because the very organisms needed to run the test are being poisoned.
Some metals produce the opposite effect. Aluminum, zinc, copper, and lead at certain concentrations can paradoxically increase observed BOD, possibly by disrupting normal cellular processes in ways that increase oxygen uptake. These interference effects are one reason why labs sometimes use COD alongside BOD for industrial waste streams where metals are likely present.
Common Sources of High BOD
Municipal sewage is the most familiar source, but it’s far from the only one. Agricultural runoff carrying manure and fertilizer, food processing wastewater, paper mill discharge, and stormwater that washes organic debris into waterways all contribute. Leaf litter from autumn tree fall can temporarily raise BOD in small streams. Even lawn clippings and pet waste washing into storm drains add to the organic load in urban waterways.
Wastewater treatment plants are designed specifically to reduce BOD before discharge. The biological treatment stage, where bacteria are deliberately cultivated to consume organic matter in controlled tanks, mirrors the same process that would otherwise happen in the receiving waterway. By consuming the organic material inside the plant, the oxygen demand is satisfied before the water ever reaches a river or lake.