The hydrothermal carbonization was investigated to convert anaerobically digested sludge to clean solid fuels. The effects of hydrothermal carbonization were evaluated by varying the reaction temperatures in the range of 180-250 degrees C. Hydrothermal carbonization increased the heating value though the reduction of the hydrogen and oxygen content of solid fuel in addition to investigating drying performance, and it can do energy saving on treatment processes. Therefore, after the hydrothermal carbonization, the H/C and O/C ratios decreased because of the chemical conversion. These results suggest that the hydrothermal carbonization process is advantageous technology in improving the properties of sewage sludge as an alternative solid fuel product as clean energy by converting the physical and chemical structure of the sludge in addition to also providing other benefits to treat organic and biomass waste. (C) 2014 Elsevier Ltd. All rights reserved.