A composite sample is based on:

A composite sample is designed to provide a representative measurement of a wastewater flow over a specified period. This method involves collecting multiple individual samples at designated intervals and then combining them into a single sample for analysis. The primary goal is to account for variations in flow and concentration that may occur over time, ensuring that the analysis reflects the average characteristics of the wastewater during that period.

By focusing on flow and/or time, a composite sample allows operators to monitor the influent or effluent more effectively, capturing fluctuations that may be linked to specific operational conditions or variations in wastewater contributions. This method contrasts with grab samples, which only represent a snapshot in time and may not accurately reflect the overall characteristics of the wastewater being treated.

Other options, while relevant in their contexts, do not encompass the primary rationale behind composite sampling. For instance, time and temperature or contaminant type might relate to other sampling considerations but are not the fundamental basis for compositing samples. Similarly, location and depth can be important when defining sample sites in certain scenarios but do not apply to the concept of composite sampling.