In recent years, many research and development activities have focussed on endocrine disrupting compounds (EDCs) in rivers, lakes and surface waters as the potential cause of reproductive disturbances in different aquatic organisms e.g. fish and mollusk. The effluent of wastewater treatment plants was identified as main source for EDCs entering the aquatic environment. The purpose of the present study was to determine the estrogenic activity of wastewater and the elimination efficiency of various WWTPs in the different purification steps using the E-screen assay, an in-vitro test system based on the increasing proliferation of human breast cancer cells (MCF-7) in response to the presence of hormonal active substances. In contrast to expensive and time-intensive targeted instrumental single substance analysis an effect-related biological testing provides a sum parameter for the entirety of compounds contributing to the total estrogenic activity (agonists and antagonists, anti-estrogenic and also toxic compounds) in concentration units of the reference substance 17b-estradiol. The current standard purification methods of biological wastewater treatment in particular the activated sludge process significantly reduce estrogenicity (average 95%). Sorption on activated carbon and subsequent precipitation leads to a further reduction of the overall estrogenic activity up to 99%.
 C. E. Purdom, P. A. Hardiman, V. V. J. Bye, N. C. Eno, C. R. Tyler and J. P. Sumpter, “Estrogenic Effects of Effluents from Sewage Treatment Works,” Chemistry and Ecology, Vol. 8, No. 4, 1994, pp. 275-285.
 L. C. Folmar, N. D. Denslow, V. Rao, M. Chow, D. A. Crain and J. Enblom, “Vitellogenin Induction and Reduced Serum Testosterone Concentrations in Feral Male Carp (Cyprinus carpio) Captured near a Major Metropolitan Sewage Treatment Plant,” Environmental Health Perspective, Vol. 104, No. 10, 1996, pp. 1096-1101.
 J. E. Harries, D. A. Sheahan, S. Jobling, P. Matthiessen, P. Neall and E. J. Routledge, “A Survey of Estrogenic Activity in United Kingdom Inland Waters,” Environmental Toxicology and Chemistry, Vol. 15, No. 11, 1996, pp. 1993-2002. http://dx.doi.org/10.1002/etc.5620151118
 E. J. Routledge, D. Sheahan, C. Desbrow, G. C. Brighty, M. Waldock and J. P. Sumpter, “Identification of Estrogenic Chemicals in STW Effluent. 2. In Vivo Responses in Trout and Roach,” Environmental Science & Technology, Vol. 32, No. 11, 1998, pp. 1559-1565.
 C. D. Metcalfe, T. L. Metcalfe, Y. Kiparissis, B. G. Koenig, C. Khan and R. J. Hughes, “Estrogenic Potency of Chemicals Detected in Sewage Treatment Plant Effluents as Determined by in Vivo Assays with Japanese Medaka (Oryzias latipes),” Environmental Toxicology and Chemistry, Vol. 20, No. 2, 2001, pp. 297-308.
 L. J. Mills and C. Chichester, “Review of Evidence: Are Endocrine-Disrupting Chemicals in the Aquatic Environment Impacting Fish Populations,” Science of the Total Environment, Vol. 343, No. 1-3, 2005, pp. 1-34.
 A. M. Soto, C. Sonnenschein, K. L. Chung, M. F. Fernandez, N. Olea and F. O. Serrano, “The E-SCREEN Assay as a Tool to Identify Estrogens: An Update on Estrogenic Environmental Pollutants,” Environmental Health Perspective, Vol. 103, 1995, pp. 113-122.
 W. Korner, W. Schuller, H. Hagenmaier and V. Hanf, “Development and Testing of a Simple Screening System for Estrogen-Like Acting Environmental Chemicals, Project Report,” Environment and Health Project (PUGU 95 004), University of Tübingen and University Women’s Hospital Ulm, 1999.
 T. Schultis, “Detection of Estrogenic Activity of Environmental Samples and Pure Compounds Using Biological Test Systems—Development and Comparison of in Vitro Assays,” PhD Thesis, University of Stuttgart, Stuttgart, 2005.
 P. Skehan, R. Storeng, D. Scudiero, A. Monks, J. McMahon and D. Vistica, “New Colorimetric Cytotoxicity Assay for Anticancer-Drug Screening,” Journal of the National Cancer Institute, Vol. 82, No. 13, 1990, pp. 11071112. http://dx.doi.org/10.1093/jnci/82.13.1107
 A. M. Soto, K. L. Chung and C. Sonnenschein, “The Pesticides Endosulfan, Toxaphene, and Dieldrin Have Estrogenic Effects on Human Estrogen-Sensitive Cells,” Environmental Health Perspective, Vol. 102, 1994, pp. 380-383. http://dx.doi.org/10.1289/ehp.94102380
 W. Korner, “Detection of Estrogenand Androgen-Like Active Substances in the Environment by Combination of Chemical and Biological Analysis,” Habilitation Treatise, Eberhard-Karls-University of Tübingen, Tübingen, 2000.
 P. Spengler, W. Korner and J. W. Metzger, “Substances with Estrogenic Activity in Effluents of Sewage Treatment Plants in Southwestern Germany. 1. Chemical Analysis,” Environmental Toxicology and Chemistry, Vol. 20, No. 10, 2001, pp. 2133-2141.
 W. Korner, P. Spengler, U. Bolz, W. Schuller, V. Hanf and J. W. Metzger, “Substances with Estrogenic Activity in Effluents of Sewage Treatment Plants in Southwestern Germany. 2. Biological Analysis,” Environmental Toxicology and Chemistry, Vol. 20, No. 10, 2001, pp. 21422151.