Article presents the results of long-term testing of prototype of 64 kVA biogas cogeneration set (CHP – Combined Heat and Power), build with support of financial means from the MCP R&D program 1 2 1. Between others, the impact of fueling with biogas from the sewage treatment plant on the engine oil has been investigated. Used oil analyses unveiled adverse presence of silicon compounds, i.e. siloxanes, permissible level of which (according to MAN standard) was exceeded several times. In order to reduce these compounds, prototype biogas filter based on activated carbon has been developed. Based on initial calculations, overall efficiency of this cogeneration set is circa 70%. For the consumer needs, the entire installation is enclosed in a compact soundproof canopy. Besides environmental merits, the cogeneration set shows also economical advantages. If operating continuously, the gen-set CHP would produce electrical and heat power in an amount that its purchasing cost would be returned after about 12 months, under condition of having biogas.