Which of the following would you not use a grab sample on?

In wastewater treatment, grab samples are typically used for parameters that can provide immediate insight into the quality of the water at a specific moment in time. However, for biochemical oxygen demand (BOD), a grab sample would not be the preferred sampling method due to the time-sensitive nature of its analysis.

BOD is a measure of the amount of oxygen that microorganisms will consume while decomposing organic matter in water. This process takes place over a standard incubation period, generally five days at a specific temperature. Therefore, the sample needs to be collected under specific conditions and incubated for accurate results, making grab sampling less suitable. Instead, composite samples or integrated samples might be utilized for BOD testing to adequately represent the variations in organic material concentration over time.

On the other hand, parameters like dissolved oxygen (DO), pH, and temperature can be effectively measured using grab samples, as they can be directly linked to the conditions in the water at the moment of sampling, providing immediate and relevant data for assessing the water quality. This is why BOD is the correct choice when identifying a parameter for which grab sampling is not appropriate.