Improvement of the energy efficiency of public buildings appears to be one of the best ways to simultaneously reduce energy consumption as well as the negative impacts on the environment. The work is dedicated to the analysis of modernization process of the energy system in a sports facility in a way leading to design of smart energy system. The proposed solution, being a specific case study, offers optimal use of energy in the facility, significantly reducing the demand for energy derived from fossil fuels (heat providers and conventional power plants). The project, on its first step, consists of recovering energy from sewage that usually is irretrievably lost. This option allows to achieve the assumed goals simultaneously optimizing the investment costs. The proposed solution mitigates air pollution and harmful gas and dust emissions to the atmosphere, and contributes to an increase of both the attractiveness and competitiveness of the area in which the sports facility is located. The next step will be further automation of the system and intelligent synchronization of time-dependencies of the processes.