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Wastewater treatment plants (WWTPs) have become crucial elements of the regional bioeconomy—mainly through energy ( waste to energy ) and matter ( nutrients–energy–water ) recovery as an element of sustainable development of a smart city. In Poland, a decentralized system functioned with 3278 municipal wastewater treatment plants, and less than 80% of the population were connected to public urban wastewater treatment systems. The paper presents the technology selection and implementation of circular economy at WWTPs and the practical approach to challenge of energy-positive and environmentally friendly wastewater and sewage sludge sector. The modernization of technological lines of wastewater treatment has led to a significant improvement in the quality of treated sewage, at the largest WWTPs in Warsaw, Cracow, or Gdansk. Eleven WWTPs produce 34% of the total volume of approximately 337 GWh of electricity from biogas in Poland. The potential of producing electricity from biogas in WWTPs in Poland can be estimated at approximately 700–850 GWh per year. According to the data of the Statistics Poland in Poland, approximately 25% of sewage sludge was used directly in agriculture and for land reclamation. Simultaneously, more than 100 WWTPs produce compost at a high quality. However, only few produce organic/organic-mineral fertilizers, mainly with addition of calcium. Decentralization leads to different technologies used in wastewater treatment plants, and environmental, economic, and social conditions determine the direction of changes and introduction of a circular economy paradigm in the Polish wastewater sector. Graphic abstract

The specificities of the circular economy (CE) in the municipal wastewater and sewage sludge sector—local circumstances in Poland