In Central and Eastern Europe huge wastewater treatment (WWT) developments have been carried out in the last 25 years. These include the building and upgrading of wastewater treatment plants (WWTPs), especially extending their lifetimes through nutrient removal and digester gas production. In Hungary, for example, in 1990, 42% of households were connected to a WWT, and this number had increased to 75% by 2015, an increase of 33%. Currently, medium-sized WWTPs with a capacity of up to around 120,000 population equivalent (PE) are facing further development needs, especially in their technical and energy-efficiency aspects. This paper presents the results of a comprehensive investigation program that included 21 WWTPs with capacities ranging from 10,000 to 120,000 PE. The investigations included an evaluation of all important parameters, such as inflowing wastewater (WW) characteristics, mechanical and biological WWT, and effluent wastewater quality and removal efficiency. These were investigated with reference to their electrical energy consumption. The extension of these WWTPs with anaerobic sludge digestion has led to the possibility of producing electricity from digester gas. This prospect is currently of great interest for medium-sized WWTPs and so was also investigated. The results are also compared to international data, standards, and guidelines obtained for this research. As a basis for a development strategy, the main bottleneck areas of design and operation are considered, and further upgrading requirements are explored. (C) 2017 American Society of Civil Engineers.