A starting mixture of liquid and solid organic wastes that is susceptible to at least partial microbiological degradation, with formation of biogas, is treated by the following process. The mixture is mechanically separated into a first sludge phase with enriched dry substance and a first liquid phase containing dilute solids. The first liquid phase is subjected to anaerobic treatment leading to formation of biogas and to the development of a second sludge phase and a second liquid phase. The first sludge phase is subjected to aerobic treatment resulting in the formation of compost, fertilizer, or feed. At least part of the second sludge phase is recirculated into the starting mixture, thereby causing the formation of biogas in the starting mixture and facilitating decomposition of higher compounds contained in the starting mixture.