Biogas, as renewable energy form, can properly substitute conventional sources of energy (fossil fuels, oil, etc.) which are causing ecological–environmental problems as fast increasing of CO 2 emissions. This paper analyse different type of anaerobic processes, which difference depend of the alimentation to anaerobic digester. Several anaerobic plants were analysed with different reactors, different feedstock, agricultural by-products, slaughterhouse wastes conveniently used. For each plant has been analysed yield, biomass used, biogas composition, nitrogen balance and a final comparison of different technologies involved in the processes.