Abstract: The present invention relates to a multistage separation reaction tank for treating sewage sludge by forming a collection layer, a diffusion layer, a primary buffer layer, a secondary buffer layer, and an inorganic precipitate layer when treating an organic material by adding a liquid solvent having a specific gravity less than water to the sewage sludge. In addition, the present invention relates to a method for treating sewage sludge by using an alkane-based solvent, wherein the method significantly lowers the water content of the sewage sludge by using an alkane-based solvent which is in a liquid state and non-polar at atmospheric pressure and room temperature.