Treatment with powdered activated carbon (PAC) and ozonation are established methods for advanced wastewater treatment plants to comply with the new Swiss water protection act to remove micropollutants. Recently, granular activated carbon (GAC) filtration was tested and found to be economically competitive under certain conditions. A minimal empty bed contact time (EBCT) of 20 minutes is crucial to achieve a good performance. Moreover, DOC, industrial contribution to wastewater, and rain events have a great impact on the performance and lifetime of GAC filters. Pre-treatment with low ozone doses allows to at least double the life-time of a GAC filter, and also a PAC pre-treatment was successfully tested. To understand better the dynamics of micropollutant concentration and abatement in GAC filters during rain events, we are doing online-LC-HRMS (high-resolution mass spectrometry) measurements of micropollutants with the MS2field instrument, which allows measurements at high temporal resolution. So far, biologically treated wastewater from the Eawag WWTP and pilot-scale GAK columns have been used, with the aim of making future measurements at municipal WWTPs with activated carbon treatment.