Total dissolved solids (TDS) are referred to as what by the EPA?

Total dissolved solids (TDS) are referred to as "filterable residue" by the EPA because TDS encompasses all the soluble ions and molecules present in water that pass through a filter. This includes salts, minerals, and organic matter that can affect water quality.

In the context of analytical procedures, TDS is determined by filtering a water sample to remove larger particulate matter, and then evaporating the filtered water to leave behind the dissolved solids. This method is consistent with how the EPA categorizes TDS, emphasizing that these substances are not visible and can pass through a standard filter used in laboratory analysis.

The term "filterable residue" specifically highlights that the measurement is focused on the substances that remain in solution and are not part of the larger, suspended particles that would be retained by the filter, hence distinguishing TDS from other forms of solids in the water. This understanding is crucial for water quality assessment, as TDS levels can impact taste, color, and the overall usability of water for drinking and other purposes.