The subject of research under this project will be the initial identification of metabolic processes and microorganisms participating in the emission of nitrous oxide during biological treatment of municipal wastewater, with particular emphasis on the role of denitrifying phosphate accumulating organisms DPAO. While the mechanisms of N2O production carried out by nitrifying bacteria and conventional denitrification bacteria are documented by numerous publications, in the case of DPAO potentially involved in the production of N2O has not been the subject of dedicated research. Expanding knowledge about the activity of DPAO can significantly contribute to the development of strategies to reduce greenhouse gas emissions in municipal wastewater treatment plants. As part of this project, it is planned to carry out dedicated technological tests that will allow determining the rates of metabolism of nitrogen and phosphorus compounds by DPAO organisms, depending on the available source of carbon and nitrogen. During the tests, nitrous oxide production will be measured. An additional research aspect will be microbiological analyzes. The next research series will use biomass from three local wastewater treatment plants. The presented research plan and a set of methods will allow to obtain data that will expand the available knowledge regarding the basic metabolic potential of DPAO bacteria in activated sludge. The use of N2O concentration measurement will determine whether DPAO activity is accompanied by nitrous oxide emissions. If confirmed, the DP2's N2O production potential will be compared to conventional nitrifying and denitrifying bacteria. The use of microbiological analyzes based on wide-pass sequencing of 16S rRNA gene fragments will allow for the initial identification of DPAO bacteria in the biomass samples used, and will also allow comparison of the full composition of bacterial consortia between the analyzed objects. The research that is the subject of this project is preliminary, which will allow the design of further experiments aimed at the multiplication of DPAO bacterial biomass in activated sludge systems as part of subsequent competitions financed by the National Science Center.