The invention relates to a coagulation-flocculation method for the separation of biomass containing lignin and hemicelluloses from the wastewater from wood processing factories.The method is characterized by polyaluminium chloride composite coagulant (KPAH) and polyethyleneimine (PEI) with the mass ratio KPAH:PEI = 3/1–2/1 used for coagulation. An optimal dose of coagulant is 50–70 mg/l and optimal pH value of the process is 6.0–7.0. The offered method comprises mixing of a coagulant with the wastewater and the further separation of the coagulated biomass by settling and ultracentrifugation.This method increases the output of waste wood biomass and lignin from the wood processing wastewater as well as significantly decreases aluminium concentration in the treated water that will significantly reduce the load on the waste treatment facilities.