One of the most significant sectors of forest industry in Latvia is the production of wood boards, especially plywood. Latvia is the biggest producer of plywood in Eastern Europe. One of the plywood production steps is a hydrothermal treatment of birch wood. Wastewater from plywood hydrothermal pools is characterized by the high contamination level. One of the main contaminants is birch lignin, which are accountable for a color of water and high chemical oxyden demand (COD). The existent methods of biological purification of wastewater do not remove it completely. Insufficient efficiency of purification from high molecular components requires application and development of physical-chemical methods (coagulation method). Coagulation processes are used in wastewater purification and removal of colloidal admixtures and discoloration of waters containing high-molecular-weight colored compounds.