What does biological oxygen demand (BOD) measure?

Biological oxygen demand (BOD) is a critical parameter in assessing water quality, specifically the amount of organic matter present in the water that can be biologically degraded. It measures the quantity of oxygen that microorganisms will consume while decomposing organic matter under aerobic conditions over a specified period, typically five days at a controlled temperature.

This measurement provides insights into the organic pollution level of the water, as higher BOD values indicate a greater presence of biodegradable material, which in turn reflects the potential for depletion of dissolved oxygen in the water body. Oxygen depletion can be detrimental to aquatic life, highlighting the importance of monitoring BOD in wastewater treatment processes.

The other options refer to different aspects of water quality measurement. For instance, the level of harmful chemicals in water is generally monitored through different indices such as chemical oxygen demand (COD) or specific contaminant testing. Clarity of water is assessed through turbidity, while temperature is a physical parameter that affects the solubility of gases and the rate of biological reactions but is not related to BOD itself. Hence, the focus of BOD directly correlates with the oxygen consumed by microorganisms, establishing it as the correct answer.