The total phosphorus concentration (TP) in Berlin’s surface waters has to be decreased to 6090 µg/L to achieve good ecological conditions. Furthermore, securing the bathing water quality of surface waters gains more attention. The wastewater treatment plant (WWTP) in Ruhleben should be upgraded by tertiary treatment to reduce the TP concentration (< 80 µg/L) and the pathogen germ concentration in the WWTP effluent. The project OXERAM compares different filtration technologies with regard to their efficiency and applicability as tertiary treatment. In this thesis the combination of microsieve filtration and UV-disinfection has been investigated. Before this trial the microsieve was rebuilt after a one year operation period. The volume of the coagulation tank was reduced to 26 % (0.56 m³) of its initial volume. The coagulation stirrer was exchanged by a Turbomix TM, applying a constant G·t-value of 25 000. Additionally, the angle of the blades of the flocculation stirrer was changed, almost doubling the G-value (154 1/s.) Long term trials have shown that an average TP effluent concentration of 60 µg/L can be achieved through microsieve filtration (10 µm) with chemical pretreatment (2.0 mg Al/L PACl and 0.61 mg/L cationic polymer). 80 % of the grab samples had a TP concentration = 73 µg/L. An average effluent suspended solid concentration of 2.2 mg/L was achieved. The average residual Aluminum concentration was 0.35 mg/L. The increase of the G-value during coagulation and flocculation after the rebuild led to an increased energy demand of both stirrers. However, through the new hydraulic conditions, the average polymer dose was reduced by 65 % in comparison to the dynamic operation in summer 2011 and the hydraulic retention time during coagulation could be reduced to 1 minute at peak flow. Furthermore, the impact of applied energy during coagulation and flocculation was investigated. A reduction of the G·t-value during coagulation led to a higher SS effluent concentration of 25 % (2.8 mg/L). During flocculation a high G-value (153 1/s) was favorable for the microsieve performance. A lower backwash time and effluent turbidity were observed. As a result, the increased energy demand of the stirrers can be justified, alongside to the polymer and hydraulic retention time reduction, with an improved effluent water quality and a lower energy demand for the backwash. The UV-disinfection operated reliably after the microsieve filtration. The UV effluent concentration of Enterococci and E.Coli were always under the limit of quantification (15-38 MPN/100 mL), even at a fluence of 361 J/m². Coliphages (= 7 PFU/100 mL) were detected in the effluent of the UVdisinfection, when the calculated fluence was lower than 549 J/m². In this study it was demonstrated that the microsieve filtration with chemical pretreatment and a subsequent UV-disinfection represents an alternative as tertiary treatment. TP effluent values lower than 80 µg/L were reliably achieved. An excellent water quality accordingly to the EG Bathing Water Quality Framework Directive was attained through the subsequent UV-disinfection.