Wastewater pollutants are generally classified under the Clean Water Act as conventional, non-conventional, and toxic or priority pollutants. Each of these three categories is discussed below. In most cases, the "pollutant" being regulated corresponds to one of the pollutant categories discussed above. For example, ammonia, nitrogen, temperature, and turbidity are specific conventional pollutants. In other cases, however, the regulated "pollutant" is a parameter that can be correlated with the actual pollutant. For example, siltation potential is measured by total suspended solids or turbidity while salinity is measured by total dissolved solids.

The measurement of pollutant concentrations, or the concentrations of specific chemical species, is performed by the conduct of test procedures that are meticulously described in Standard Methods for the Examination of Water and Wastewater; SW-846 Test Methods for Evaluating Solid Waste, Physical/Chemical Methods; test methods specified in the Code of Federal Regulations and in other standardized publications.