Direct vs. Indirect Thermal Drying; the Similarities and the Differences

Thermal drying of biosolids has been utilized since the early 20th century. The initial implementation was a methodology to reduce the volume of biosolids for disposal by removing the excess water entrained within the biosolids either in the liquid phase or in the cake phase after mechanical dewatering was employed. Prior to the implementation of thermal drying a system of solar drying by means of open air drying beds was used, but the increasing odor issues, the labor intensity, the area of land required, the seasonality and the time element proved to be too difficult to manage effectively.