Abstract: To raise the rate of separation of the water from biogas residues is very important in reducing the amount work which will be done in post-treatment of biogas engineering.The separation technology is close related to the kind of raw material to be used in producing the biogas. For instance, if the raw material is the dung of pig or chicken,the water can be separated from the residue by sedimentation or centrifugal force. The reason is that the feed for pig and chicken contains a large proportion of grains. The specific gravity of grain is larger than one,so the solid residue will be precipitated to the bottom and the water will be floated on the top. While the raw material is cow's dung,the only way to draw the water from residues is with sieve. As the cow's main feed is grasses, which makes the specific gravity of its dung less than one,so the residue and water mixture is in a colloidal state.Then all the water from biogas residues may be drained into the trench or the pit to let the water to be oxidized by the air in order to make the water under the aerobic fermentation. The last step is to drain the water into the field to let the soil,microorganisms,algae,aquatic plants to decompose all the organic matters within the biogas water to be purified step by step to form a noncontaminated liquid fertilizer. If the condition is permitted, the best way is to use the underground permeated irrigation technology for vegetables or other dry crops to purify the biogas water by the adhesion function of soil particles.