The production of veneer in Latvia and many countries of Eastern Europe is accomplished by the hydrothermal treatment of hardwood in special water basins. As a result, formed effluents contain wood-originated pollutants which are responsible for the enhanced chemical oxygen demand and the intensive color of the wastewater. Keeping in mind the zero waste policy for rational use of bioresources and the possible usage of waste wood originated matter in practice, it is very important to extract the formed biomass from the basin’s wastewater. In this work, for imitating woodworking wastewater, birch sawdust was hydrothermally treated in mild alkaline conditions at 90°C. The yield of the solid biomass did not exceed 7% and contained, mainly, hemicelluloses in the polysaccharide form and lignin. The applied instrumental analysis methods (FTIR and UV-spectroscopy, HLPC, 13 C-NMR) testify the dominant content of hemicelluloses in the obtained biomass. The results of fractionation of the biomass have indicated that the content of lignin, hemicelluloses and water- soluble degraded wood products in the solid biomass corresponds to the following mass ratio: 1.2/6.7/1.0, respectively. The effectiveness of the isolation of wood biomass from the hydrolysate with polyethyleneimine (PEI) was studied. In coagulation– flocculation process using single coagulant, coagulant dosage and pH plays an important role in determining the coagulation efficiency. At the optimum PEI dosage of 35 mg/L and optimum pH of 6.0, total biomass removal was determined to be 93% (1302 mg/L), lignin removal is 64%, PI and color reduction is 57% and 90%, respectively. Compared with the widely used polyaluminum chloride (PACl) coagulant, PEI is characterized by higher indices of the removal of total biomass and lignin, as well as color and PI at the application dose 2.8 times lower than that for the PACl.