What Is Biochemical Oxygen Demand (BOD) and How Does It Measure Water Quality?

Biochemical oxygen demand (BOD) is the amount of dissolved oxygen required by microorganisms to decompose organic material in water. A high BOD generally indicates greater organic pollution and poorer water quality because less oxygen remains available for aquatic organisms.

This is core SL and HL content in IB ESS subtopic 4.4: Water pollution.

How BOD Measures Water Quality

1. Organic matter enters water from sources such as untreated sewage, manure or food-processing waste.
2. Aerobic microorganisms decompose this material through respiration.
3. Their respiration consumes dissolved oxygen (DO).
4. The greater the quantity of biodegradable organic matter, the more microbial respiration occurs and the higher the BOD.

BOD is commonly determined by measuring dissolved oxygen before and after a water sample is incubated under controlled conditions. For example, if the initial dissolved oxygen is $9.0\ \text{mg L}^{-1}$ and the final concentration is $4.0\ \text{mg L}^{-1}$, then:

$$\text{BOD} = 9.0 - 4.0 = 5.0\ \text{mg L}^{-1}$$

BOD is an indirect indicator of water quality, not a complete measurement. It mainly detects biodegradable organic pollution and may not reveal toxic metals, plastics or pathogens. The historical sewage pollution of the River Thames illustrates how high organic inputs can cause severe oxygen depletion, while improved sewage treatment can reduce BOD and support ecosystem recovery.

A common misconception is that high BOD means the water contains more oxygen. The opposite is generally true: high BOD means microorganisms require and consume more oxygen. In an IB response, explain the full causal chain: organic matter increases $\rightarrow$ microbial decomposition increases $\rightarrow$ dissolved oxygen decreases $\rightarrow$ aquatic organisms may die. For evaluation, add that unequal access to sewage treatment creates an environmental justice issue.