For many micro pollutants, such as ingredients of household chemicals, substances used in commerce and industry, pharmaceutical pollutants and biocides the municipal sewage system represents the dominant entry path into the aquatic environment. Even if the concentrations are typically very low (especially pharmaceutical pollutants), results for individual agents demonstrate clear environmental risks. Because these substances cannot be eliminated completely during the conventional processes (primary, secondary and tertiary treatment) a fourth stage treatment is needed. Already applied techniques include oxidative (e.g. ozone and UV-light irradiation), absorptive (e.g. activated charcoal) and physical (e.g. nano filter) processes which are costly while others are still being tested. So far, just Switzerland has a regulation about organic trace substances entering the environment after leaving treatment plans under water protection law. (UBA 2014 & Walz & Götz 2014)