Q.What is Biochemical Oxygen Demand (BOD)? Explain why a water sample with a high BOD value is considered more polluted than one with a low BOD value.

Biochemical Oxygen Demand (BOD) is defined as the amount of dissolved oxygen, in milligrams per litre, that aerobic micro-organisms consume while breaking down the biodegradable organic matter present in a water sample, conventionally measured over five days at $20^{\circ}\text{C}$. A water sample with a high BOD value contains a large amount of biodegradable organic matter, so decomposing it draws down a correspondingly large amount of dissolved oxygen from the water -- signalling heavily polluted water that is at risk of becoming oxygen-depleted and hostile to fish, exactly the outcome seen in eutrophication. A sample with a low BOD value contains little organic matter, so decomposing it draws down little oxygen, signalling comparatively clean water. Because BOD directly measures a pollutant …

Define BOD by what it actually measures (oxygen consumed during decomposition), then connect 'high value' and 'low value' dire …

Treating BOD as if it directly measures a specific toxic chemical, rather than the total oxygen-depleting potenti …