Biological Oxygen Demand (BOD) of waste water is determined by measuring the amount of:

Question

Accepted Answer

The correct option is 2: Oxygen consumption

Biological Oxygen Demand (BOD) is fundamentally a measure of the amount of dissolved oxygen that aerobic microorganisms need to break down organic matter present in a water sample. Therefore, the BOD test is a bioassay designed to directly measure the **oxygen consumption** that occurs during this microbial decomposition process. In the standard BOD test (typically $\text{BOD}\_5$), a sample of wastewater is incubated in the dark at $\text{20°C}$ for five days. The methodology involves measuring the initial concentration of dissolved oxygen (DO) and then measuring the final DO concentration after the incubation period. The difference between the initial and final dissolved oxygen measurements directly reflects the amount of oxygen that was consumed by the bacteria to stabilize the organic matter. This consumption, expressed usually in $\text{mg}$ of oxygen per liter, is the value reported as the BOD. It serves as a crucial, indirect indicator of the organic pollution level in the water, as a higher consumption of oxygen signifies a greater amount of biologically active organic waste.

The incorrect option is 1: Total organic matter

Total organic matter is a broad term that includes all carbon-containing compounds in the water, whether they are biodegradable or not. The standard BOD test does **not** measure the total amount of this matter; it only quantifies the portion that can be broken down by microorganisms within the five-day period. To measure the *total* carbon content, a different analytical procedure known as Total Organic Carbon (TOC) analysis is used, which involves chemical oxidation of all organic compounds. Since the standard BOD test is a measure of biological activity, it cannot account for non-biodegradable or slowly degrading organic components, making it distinct from a total organic matter measurement.

The incorrect option is 3: Biodegradable organic matter

While it is the presence of biodegradable organic matter that creates the demand for oxygen, the BOD test does not directly measure the mass or concentration of this matter. Instead, BOD is an **indirect measure** of the *effect* this material has on the water body—specifically, the oxygen that must be consumed to neutralize it. The result of the test is expressed in $\text{mg/L}$ of oxygen, not $\text{mg/L}$ of organic matter. Although a higher concentration of biodegradable organic matter will lead to a higher BOD value, the result reported is the quantity of oxygen consumed, which acts as a surrogate for the pollution load.

The incorrect option is 4: Ratio of Biodegradable organic matter to the total organic matter

This option describes a mathematical calculation derived from other analytical results, not the fundamental value measured by the BOD test itself. This ratio would require measuring both the biodegradable organic matter (perhaps inferred from $\text{BOD}$ or another test) and the total organic matter ($\text{TOC}$), neither of which is the direct output of the BOD procedure. The BOD test provides a single, absolute number representing oxygen demand, not a comparative ratio. Therefore, while this ratio could be an index of the *character* of the organic pollution, it is not the process or value used to *determine* the Biological Oxygen Demand.