A grab sample is most suitable for measuring which of these constituents?

A grab sample is most suitable for measuring residual chlorine because it provides a snapshot measurement of chlorine levels at a specific moment in time. Residual chlorine is typically monitored to ensure that sufficient disinfectant levels are maintained in the water treatment process to effectively kill bacteria and other pathogens. Since residual chlorine can fluctuate due to various factors, including water flow and varying demand, a grab sample captures this instantaneous concentration, allowing operators to assess compliance with health standards and treatment efficacy.

Turbidity, pH levels, and nutrients can also be measured through grab sampling; however, these parameters often benefit from more comprehensive sampling methods that can better account for variations over time or specific processes within the treatment system. For example, turbidity can change quickly due to disturbances, and measuring it over time helps to determine trends or issues in the treatment process. Nutrients might require composite sampling to obtain an average concentration over a period, thus providing a more reliable assessment of their levels. In contrast, residual chlorine's nature makes grab sampling the most effective approach for timely and actionable results.