This paper includes an analysis of the biological treatment process existing within the water supply and sewerage of Iași City. The main objective of biological treatment is the removal of solid organic substances from wastewater, the stabilization of sludge, the reduction of nutrients loads etc. The Iași City Wastewater Treatment Plant was developed in several stages since year 1968. Nowadays, the facility operates at a design flow rate of 4 m3/s during dry weather and 8 m3/s during heavy rainfalls. This study is focused on the following aspects: wastewater treatment plant’s diagram, the wastewater parameters inside the treatment plant, the biological treatment process analysis and a few conclusions.