United States Environmental Protection Agency (EPA) classifies Total Dissolved Solids (TDS) as a secondary contaminant. It is measured in milligrams per unit volume of water (mg/L) and also referred to as parts per million (ppm).For drinking water, the maximum concentration level set by EPA is 500 mg/L.t is measured in milligrams per unit volume of water (mg/L) and also referred to as parts per million (ppm). For drinking water, the maximum concentration level set by EPA is 500 mg/L.