In this research we focused on biological phosphorus removal in Wastewater treatment plant Domzale-Kamnik (WWTPDK). In 2016 WWTPDK was upgraded with four sequence batch reactors (SBR), which enable tertiary treatment of wastewater. The aim of our research was to find out, if the upgrade of WWTPDK reduces the amounts of phosphorous in wastewaters below the values established in a new regulatory limits. With appropriate functioning, SBR reactors should reduce the total phosphorus concentrations in WWTPDK effluent below 1 mg/L. In second part of the research, we tried to detect phosphate accumulating organisms (PAO) in the active sludge. PAO presence in active sludge from different parts of WWTPDK was confirmed via the presence of functional genetic marker ppk1. The latter encodes the enzyme crucial for synthesis of polyphosphate from ATP - polyphosphate kinase 1. Specific 16S rDNA primers were also used to identify important yet unisolated representative of PAO Candidatus Accumulibacter phosphatis (CAP). Chemical analysis of wastewater proved that the upgraded WWTPDK can satisfactory reduce the amounts of phosphorous below the limiting values established in a new regulation. Microbiological phosphorus removal efficiency in wastewater was 35 %. We also detected PAO in the active sludge from different sampling points.