The viability of Mechanical Biological Treatments (MBT) is a widely discussed topic regarding Municipal Solid Waste (MSW) management in the world. Bio-drying is a MBT process that shows its best performances when the waste has a high moisture content. The purpose of bio-drying is the exploitation of the exothermic bacterial reactions for the evaporation of the highest part of the moisture in the waste with the lowest conversion of organic matter. As side effects the waste stream increases its calorific value and its composition is partially stabilized. This paper presents a simple model based on bio-drying mass and energy balances. The model takes into account the heat release from biochemical oxidation of the volatile matter, the evaporation of moisture, the heating of process air and the thermal losses, as well as changes in the chemical composition of the volatile matter that remains in the waste. Some very simple rules are identified to allow a rapid evaluation of the process performances.