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What is BOD?
Biochemical oxygen demand — abbreviated BOD — is a measure of how much dissolved oxygen microorganisms need to break down organic matter in a sample of water at a given temperature over a set period of time. In plain terms, it answers one question: how much oxygen will this water consume as bacteria decompose whatever organic pollution is present?
The higher the BOD value, the more organic material is present and the more oxygen is being stripped from the water. For aquatic life that depends on dissolved oxygen to survive, a high BOD reading is an early warning sign of ecological stress.
Key definition: BOD is expressed in milligrams of oxygen consumed per litre of water (mg/L) over a standard incubation period — most commonly five days at 20 °C, giving the internationally recognised BOD₅ value.
Why It Matters
When organic waste — from sewage, food processing, agriculture, or paper mills — enters a body of water, bacteria go to work decomposing it. This biological activity consumes dissolved oxygen faster than the water can replenish it from the atmosphere and often even mechanical methods. Downstream, the water may turn anoxic, producing foul-smelling gases and creating dead zones.
BOD testing was developed in the early twentieth century by British scientists studying the heavily polluted River Thames, and it remains the standard metric used by regulators worldwide to set discharge limits for wastewater treatment plants and to classify river health.
The "Old School" BOD₅ Measuring Method
The classical procedure is straightforward in concept but demands careful technique in practice.
Step 1 — Collect the sample
Water is collected in airtight bottles, minimising agitation to preserve oxygen levels exactly as they exist in the source.
Step 2 — Measure initial dissolved oxygen (DO)
The dissolved oxygen concentration is recorded at the start using a calibrated probe or Winkler titration.
Step 3 — Incubate at 20 °C for five days
Sealed bottles are placed in a dark incubator at 20 °C for exactly five days. Darkness prevents photosynthesis from adding oxygen and confounding the result.
Step 4 — Measure final DO
After five days, the remaining dissolved oxygen is measured again using the same method.
Step 5 — Calculate BOD₅
The result represents the oxygen consumed by microbial activity over the test period.
When the expected organic load is very high — such as in raw sewage — the sample must first be diluted with oxygen-saturated water to ensure enough oxygen remains at the end of the test to register a meaningful difference. Seed bacteria may also be added when the native microbial population of the sample is insufficient to drive decomposition at a consistent rate.
Interpreting the Results
BOD values span many orders of magnitude depending on the source.
Water source | Typical BOD₅ (mg/L) | Quality |
|---|---|---|
Clean river or lake | 1 – 2 | Good |
Moderately polluted river | 3 – 8 | Marginal |
Poorly treated effluent | 20 – 100 | Poor |
Raw domestic sewage | 150 – 300 | Very poor |
Food processing wastewater | 500 – 10,000+ | Severe |
Most regulatory standards for treated sewage discharge into rivers require BOD₅ to be below 20–25 mg/L; many European standards set the limit as low as 5 mg/L for sensitive receiving waters.
Limitations and Alternatives
The five-day wait is the test's biggest practical drawback. Wastewater treatment operators managing real-time processes cannot wait five days for a result. This has driven development of several faster proxies.
Chemical oxygen demand (COD) oxidises all organic matter chemically rather than biologically and returns a result in a few hours. It tends to read higher than BOD because it measures everything oxidisable, not just what bacteria will consume.
Total organic carbon (TOC) analysis, using infrared detection, gives results in minutes and has become the preferred method in many modern laboratories, though it requires a site-specific calibration against BOD to be used in regulatory contexts.
In Waboost we provide Aqualabo's STACSENSE sensor which measures BOD, COD and TOC.
Summary
BOD monitoring is not just a compliance formality. It underpins decisions about how much organic load a receiving water body can safely absorb, how a treatment plant should be operated, and whether industrial dischargers are meeting their permits. It remains one of the most widely used analytical tools in environmental science more than a hundred years after its development.
