What factor primarily affects the BOD of a wastewater sample?

The biochemical oxygen demand (BOD) of a wastewater sample is significantly influenced by the temperature of the sample. This is primarily because temperature affects the metabolic activity of microorganisms that decompose organic matter in wastewater. As the temperature increases, the rate of microbial activity generally increases as well, leading to a higher demand for oxygen as these organisms break down organic substances. Consequently, warmer temperatures can result in higher BOD values, indicating more organic material available for microbial consumption.

In contrast, the flow rate of wastewater primarily impacts the dilution of the sample and may not directly correlate with the organic content that microorganisms can degrade over a given period. The presence of heavy metals can inhibit microbial growth and activity, potentially lowering BOD but is not a primary factor in typical organic load measurements. Similarly, while the volume of the sample is important for analytical accuracy, it does not inherently change the BOD of the wastewater itself. Thus, temperature stands out as the primary factor affecting BOD due to its direct relationship with microbial activity.