Dehydrated sludge refers to the solid residue left after dewatering wet sludge by means of drainage or filtration. After various dewatering treatments, the water content is reduced to 65% ~ 85% and has become a wet soil-like sludge. Non-toxic dehydrated sludge can be directly used as fertilizer in farmland. In the process of wastewater treatment, the sediment, moisture content of 99%, in order to reduce the volume of sludge before treatment to facilitate transport and further treatment, part of the water must be removed. The methods to remove moisture from sludge include gravity concentration, vacuum filtration, pressure filtration and stacking and natural air drying.