The overall aim of the CWPharma project is to reduce the load of active pharmaceutical ingredients (APIs) going into the aquatic environment and especially the Baltic Sea. Municipal wastewater treatment plants (WWTPs) are relevant point sources of APIs, as they treat the wastewater from public households, hospitals and industry of the connected catchment area. However, conventional "state-of-the-art" WWTPs can only remove some APIs, which are either easily biodegradable and/or absorbable to activated sludge, whereas other APIs can pass the WWTP with minor to no reduction. Therefore, reduction of a broad range of APIs can only be achieved by using targeted advanced treatment techniques such as ozonation or powdered and granular activated carbon, respectively, which have already been applied on full-scale for API removal in wastewater treatment in Germany and Switzerland and proven their practical and economical suitability. At the usual applied ozone doses, ozonation of secondary effluent does not mineralize (convert an organic substance into inorganic matter) but transforms organic compounds into smaller and (usually) more biodegradable compounds. Secondary effluent is a complex water matrix consisting of hundreds of different organic substances, and it is not feasible to determine all possible transformation products and oxidation by-products, which might be created by the ozonation process. Thus, utilities and water authorities sometimes struggle with the uncertainties of the ozonation process as they perceive difficulties to judge whether oxidation of the organic matrix is beneficial or if it is creating more problems. As chemical analysis of the water only provides quantitative data for known APIs and transformation products for which chemical standards are available, effect-based ecotoxicological test systems can be used to assess the integrated actual toxicity of the whole water matrix. Based on previous research compiled by Völker et al. (2019), ozonation has a positive impact on several toxicological endpoints. But there are also indications that ozonation can create negative effects for a few toxicological endpoints that can be reduced by a suitable post-treatment. However, only little knowledge is available regarding suitable post-treatments and which ecotoxicological test systems are appropriate to evaluate their impact. In addition, post-treatment options might also have beneficial impacts on water quality parameters, APIs and transformation products. Thus, this report will evaluate different aspects regarding the impact of ozonation and its posttreatment options on (i) water quality parameters, (ii) APIs and transformation products (TPs) and (iii) ecotoxicological effects. The evaluation was conducted at three WWTPs in Linköping (SE), Kalundborg (DK) and Berlin (DE) and different post-treatment options such as moving bed bioreactors (MBBR), deep-bed filters, and a constructed wetland.