The generation of waste and its disposal in landfills results in the formation of landfill gas as a source of renewable energy. Its use for energy in cogeneration units is the optimal use of the chemical energy of this fuel. The aim of the study was to test the used coolant operated in a cogeneration plant (CHP) located at a landfill site and powered by landfill gas. The tests carried out in the study were aimed at verifying whether the used coolant from the heat recovery system of the cogeneration unit should be treated as safe waste and thus requiring special management. The tests showed how the parameters in the coolant change after its use and what potential problems they may cause in the operation of the plant. The research showed significant variability in parameters such as pH, alkaline reserve, coolant density, and freezing point. The research and its results indicate elevated levels of heavy metals for zinc (Zn) and copper (Cu), as well as high levels of sodium (Na), boron (B), and potassium (K). As a result, used coolant should be classified as hazardous waste and managed in accordance with the rules for the management of such waste. A review of the literature and the results of the authors' own research confirmed that a cogeneration (CHP) plant that generates energy is also a producer of hazardous waste.