4.4.4—Biochemical oxygen demand (BOD)
- Syllabus
- First assessment 2026
- Objective
- 4.4.4
- Level
- SL
Read BOD as Oxygen Used
BOD is dissolved oxygen consumed by microbes decomposing biodegradable organic matter under a specified test condition.
In the five-day, 20°C test, BOD = initial DO − final DO. More biodegradable material means more microbial respiration and less oxygen left for aquatic life.
If DO falls from 8 to 2 mg/L, BOD is 6 mg O₂/L.
High BOD does not mean the water contains abundant oxygen: it means microbes used oxygen rapidly, leaving aquatic organisms under greater oxygen stress.
BOD is oxygen consumed, not oxygen concentration left in the river.