Biological Oxygen Demand (BOD)

What BOD measures

• Sewage and food scraps contain organic matter.
• Bacteria use oxygen as they break it down.
• BOD estimates that oxygen demand in a water sample.
• It helps assess organic pollution in rivers and wastewater.
• It does not measure oxygen in blood or air.
• Dissolved oxygen (DO) is oxygen in the water now. BOD is oxygen microbes may use during the test. They are different readings.

Follow the oxygen through a river

1. A drain releases untreated sewage.
2. Microbes feed on its organic matter.
3. Their activity uses dissolved oxygen.
4. Fish may then struggle to breathe.

• More biodegradable waste often raises BOD.
• A single BOD result cannot prove a river is safe or unsafe. Flow, temperature, sunlight and fresh oxygen also matter.
• High BOD is a warning of oxygen pressure. It is not a direct measure of poison, germs or every pollutant.

How the five-day test works

1. A laboratory measures dissolved oxygen in a prepared sample. It may dilute or seed the sample.
2. The sealed sample stays at 20°C in the dark for five days.
3. The laboratory measures dissolved oxygen again. The fall gives five-day BOD.

• A result of 6 mg/L means microbes used oxygen equal to 6 milligrams per litre during the test.
• It does not say the river held 6 mg/L of oxygen when sampled.
• Compare results only if the same standard method was used.
• Source: USGS five-day BOD method.

India’s water-use classes

• The Central Pollution Control Board (CPCB) sets designated-best-use criteria.
• A class describes a water use and several needed conditions. BOD alone does not assign a class.

• Classes also use other tests, such as bacteria, pH or ammonia.
• Class E is a use category, not a declaration that water is clean.
• Source: CPCB designated-best-use criteria.

A treatment-plant example

• A town’s sewage enters a plant at 180 mg/L BOD. Treated outflow measures 30 mg/L.
• The measured BOD fell by 150 mg/L, or about 83%.
• This shows treatment can reduce biodegradable waste. It does not prove the discharge meets a legal limit.
• River water then mixes with the outflow. Never treat a wastewater result as a river classification.

What BOD can and cannot tell you

• A high BOD suggests microbes can use much oxygen. Low dissolved oxygen can harm fish.
• Fish tolerance differs by species and life stage. Avoid one universal “safe” oxygen number.
• CPCB lists 4 mg/L or more for its fisheries class.
• Low BOD does not rule out toxic chemicals, disease-causing microbes or salt pollution.
• A sunny river can show high daytime DO despite organic waste. Measure both BOD and DO.
• Untreated sewage is a common source of organic loading. See the river biodiversity guide.

BOD and COD are different tests

• COD is usually higher than five-day BOD in ordinary wastewater.
• “COD is always at least BOD” is too strong. The methods differ.
• Five-day BOD can include oxygen used during nitrification. Some methods block this to measure carbonaceous BOD.
• Do not treat the two numbers as interchangeable.
• Source: US EPA wastewater technical document.