The relationship of moisture and temperature to the concentration of O₂ and CO₂ during biodrying in semi-static piles

Biodrying was studied over 46 days in two piles (P1 and P2) composed of orange peel and two structuring materials (mulch: P1; sugarcane bagasse: P2). The oxygen and carbon dioxide levels were recorded at different depths (0.1 to 0.6 m). From the beginning to days 33–35 the drying was carried out by a combination of microbial heat, convection, and solar irradiance; moisture reached 30%, corresponding to a water activity (a_w ) of 0.88–0.9, which was insufficient to maintain microbial activity. Additionally, the O₂ and CO₂ levels (21% and 0%, respectively) evidenced the end of the biological phase of the process. After day 35, the drying occurred only by convection and solar irradiance. At the end, moisture reached 14% (P1) and 12% (P2), showing that the turning frequency, as well as the type and proportion of the structuring materials, were adequate and significantly influenced moisture reduction, allowing the aeration necessary for biodrying. At the end, a material with an average calorific value of 15,500 kJ/kg was obtained, comparable to wood and other fuels obtained from orange peel, making the biodrying process a suitable option for the stabilization and energy recovery of agricultural and agro-industrial waste with high moisture content.