The conferences on Large Wastewater Treatment Plants (LWWTP) are the most traditional specialised conferences organized by IWA specialist groups. The first conference (at that time called “workshop”) was held in Vienna and then was repeated in four-year periods in Vienna and since 1987 also in Budapest and Prague. This conference is the 13th conference in a series and celebrates in 2020 the 50 years anniversary. The first conference of the specialist group was in Vienna and the anniversary conference will come back to this venue during a perfect time of the year – not only with respect the expected weather in Vienna but also because of the finalisation of the expansion of Europe’s largest wastewater treatment plant.