Biological oxygen demand (BOD) measures the total organic content of water based on the consumption of oxygen in a sample at 20deg C Celsius over five days. A consumption of 10 to 20 mg of O2 per liter most likely represents a sample from
**Core Concept**
The biological oxygen demand (BOD) test measures the total organic content of water, reflecting the amount of oxygen consumed by microorganisms in breaking down organic matter. This process is a key indicator of water quality, with higher BOD values indicating more organic pollution.
**Why the Correct Answer is Right**
The BOD test results between 10 to 20 mg/L of O2 consumption per liter are typical of moderately polluted water bodies. This range is often associated with waters receiving domestic sewage or agricultural runoff. The oxygen consumption rate is a result of microbial degradation of organic matter, such as proteins, carbohydrates, and fats, present in the water. This process involves the action of various enzymes, including proteases, lipases, and amylases, which break down complex organic molecules into simpler compounds that can be metabolized by microorganisms.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because BOD values can vary greatly depending on factors like temperature, pH, and the presence of nutrients. A single value does not necessarily categorize a water body as pristine or severely polluted.
**Option B:** This option is incorrect because BOD values below 2 mg/L typically indicate very low levels of organic pollution, often seen in well-maintained industrial or municipal wastewater treatment systems.
**Option C:** This option is incorrect because BOD values above 30 mg/L usually indicate severe organic pollution, often associated with raw sewage or industrial effluent.
**Clinical Pearl / High-Yield Fact**
The BOD test is a useful indicator of water quality, but it has limitations. For example, it does not account for the presence of toxic substances that can inhibit microbial growth or other factors that may affect oxygen consumption rates.
**Correct Answer:** D.