Biogas slurry and biogas residue are good fertilizers, but they cannot be used directly because the biogas residue contains 20% water, which needs to be dehydrated and then fermented. When dehydrating the biogas slurry, the first flocculant should be selected first. The suitable flocculant can increase the water discharge rate, and the moisture content of the extruded mud cake is low. According to our many years of practical experience, it is generally necessary to choose cationic PAM. The flocculation and effluent effect of relatively high ionicity is better. When selecting flocculant, it is best to first test in the laboratory and then on the machine. So as not to waste manpower and material resources.