In this paper, an existing cogeneration system driven by biogas internal combustion engines (ICE) is a subject of an investigation by energy and exergy analyses. The system is installed in the Varna Wastewater Treatment Plant (Varna WWTP), Bulgaria and its purpose is to utilize the methane produced as a byproduct of the solids stabilization process at Varna WWTP. Otherwise, the produced methane would pollute the environment. The presented paper has been organised in the following way: first, in order to define the basic thermodynamic parameters on the stations of the cogeneration system streams, the energy balance equations for each component of the system are formulated. Then, the rate of exergy destruction within the the kth system component is calculated using the exergy balance equations. Moreover, according to the methodology introduced in (The European Education Tool on Cogeneration System (EDUCOGEN), 2001), energy efficiency, power to heat ratio, energy saving ratio, energy efficiency used under the Public Utilities Regulatory Policy Act (PURPA efficiency) are defined for the cogeneration system. The same thermodynamicperformance assessment parameters (without PURPA efficiency) are determined on exergy base. In addition to the thermodynamic performance assessment parameters of the cogeneration system, a detailed exergy analysis on the component level is performed. To our knowledge, there has been little discussion about exergy efficiency of ICE based cogeneration systems for use with biogas from wastewater treatment plants. To address this niche in the global research work, in this investigation is suggested detailed exergy analysis permitting us to assess thermodynamic performance of similar class energy conversion systems.