A BOD test, short for biochemical oxygen demand test, measures how much dissolved oxygen microorganisms consume when breaking down organic matter in a water sample. It’s the standard method for gauging organic pollution in rivers, lakes, and wastewater. The higher the BOD value, the more polluted the water. Clean, unpolluted water typically has a BOD of 5 mg/L or less, while raw sewage can range from 150 to 300 mg/L.
How the BOD Test Works
The test relies on a simple biological principle: when organic material like food waste, sewage, or plant matter enters water, naturally occurring bacteria begin consuming it. Those bacteria need oxygen to do their work, just like you need oxygen to digest food. The more organic pollution in the water, the more oxygen the bacteria use up.
To run the test, a technician takes a water sample, measures its dissolved oxygen level, then seals it in a dark bottle and incubates it at 20°C (68°F) for five days, plus or minus six hours. Darkness prevents algae from growing and producing oxygen, which would throw off the results. After five days, the dissolved oxygen is measured again. The difference between the starting and ending oxygen levels, adjusted for any dilution, gives you the BOD value in milligrams per liter. This standard five-day version is called BOD5.
Some samples, like industrial waste or water from a very clean source, don’t contain enough bacteria on their own to produce reliable results. In those cases, live bacteria are added to the sample in a process called seeding, and a correction factor is applied to the final calculation to account for the oxygen the added bacteria themselves consume.
What BOD Levels Tell You About Water Quality
BOD is measured in milligrams of oxygen consumed per liter of water (mg/L). Here’s how different levels break down:
- 1 to 2 mg/L: Very clean water, typical of pristine streams and lakes.
- 3 to 5 mg/L: Moderately clean, considered fair quality.
- 6 to 9 mg/L: Somewhat polluted. Organic matter is present and being actively decomposed by microorganisms.
- 150 to 300 mg/L: Raw, untreated sewage.
Wastewater treatment plants use BOD readings to evaluate how well their processes are working. If effluent leaving the plant still has a high BOD, it means too much organic matter is passing through untreated. Regulatory agencies set BOD limits on discharge permits for this reason.
Why High BOD Is Dangerous for Ecosystems
When water with a high BOD enters a river or lake, the bacteria consuming that organic matter pull dissolved oxygen out of the water faster than it can be replenished. This creates oxygen-depleted zones where fish and other aquatic organisms suffocate. The problem intensifies in warm weather because warmer water holds less dissolved oxygen to begin with, and microbial activity speeds up with heat. The result can be large-scale fish kills and long-term degradation of water quality in affected areas.
BOD vs. COD
You’ll often see BOD mentioned alongside COD, or chemical oxygen demand. Both measure pollution, but they work differently and tell you different things.
BOD uses living bacteria to break down organic matter over five days. It only measures the biodegradable fraction of pollution, the stuff microorganisms can actually eat. COD, on the other hand, uses a strong chemical oxidizer to break down everything in the sample, both biodegradable and non-biodegradable compounds. A COD test takes only a few hours rather than five days and produces more consistent, reproducible results.
Because COD captures everything and BOD captures only the biodegradable portion, comparing the two values tells you something useful: how much of the pollution in a sample can actually be broken down by biological treatment. Water with a high COD but low BOD contains a lot of material that bacteria can’t easily decompose, which may require chemical treatment instead.
Limitations of the BOD Test
The BOD test has been a cornerstone of water quality monitoring for over a century, but it comes with real drawbacks. The most obvious is time. Waiting five days for results means you can’t respond quickly to a pollution event or make real-time adjustments at a treatment plant.
Reproducibility is another challenge. Because the test depends on living organisms, results vary from sample to sample even under controlled conditions. Some organic compounds, like simple sugars and starches, break down quickly and reliably. Others resist biological breakdown and may need specially adapted bacteria to show any BOD at all. This biological variability makes it harder to compare results across different labs or sampling events. Labs verify their technique using a standard glucose-glutamic acid solution, which must be seeded with bacteria since the solution itself is sterile.
Nitrification is a subtler issue. Many wastewater treatment plants produce effluent rich in nitrifying bacteria, organisms that convert ammonia and organic nitrogen into nitrates. These bacteria consume oxygen too, but the oxygen they use isn’t related to organic pollution. In a highly nitrified sample, this nitrogen-related oxygen demand can inflate the BOD result by as much as 50%. To get around this, labs can add a nitrification inhibitor to the sample and report the result as CBOD (carbonaceous BOD), which reflects only the oxygen used to break down carbon-based organic matter.
Even light exposure matters. If algae grow in the sample before testing, they produce oxygen through photosynthesis and artificially raise the starting dissolved oxygen level, skewing results downward. Samples need to be kept in the dark and sometimes deaerated to remove excess oxygen before the test begins.