1. https://ec.europa.eu/environment/circular-economy/index_en.htm , Consulted on March 18, 2018
2. The maximum concentration allowed for the silver in the drinking water by the US EPA are: 50 μg/L. Ambient Water Quality Criteria for Silver , Environmental Protection Agency, Office of Water , Washington, DC , 1980 , EPA 440/5-80-071
3. The maximum concentration allowed for the mercury in the drinking water by the US EPA are: 0.002 μg/L. Ambient Water Quality Criteria for Mercury , U.S. Environmental Protection Agency, Office of Water , Washington, DC , 1985 , EPA 440/5-94-026
4. The maximum concentration allowed for the lead in the drinking water by the US EPA are: 0.0015 μg/L. Ambient Water Quality Criteria for Lead , U.S. Environmental Protection Agency, Office of Water , Washington, DC , 1985 , EPA 440/5-84-027
5. A.Rubo , R.Kellens , J.Reddy , N.Steier and W.Hasenpusch , Ullmann's Encyclopedia of Industrial Chemistry , Wiley-VCH Verlag GmbH , 2000