Recovery of lignin and extractive substances from the hydrolysate of model birch wood hydrolysis with aluminium

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 a high contamination level. One of the main contaminants is birch lignin which accounts for the color of water and high chemical oxygen demand (COD). Insufficient efficiency of purification from high molecular weight components requires application and development of physical-chemical methods (coagulation method). The aim of this work was to find the optimal conditions for coagulation of lignin compounds in hydrolysate of a model birch wood hydrolysis in the presence of aluminium salts. The obtained model hydrolysate imitated wastewater from the plywood hydrothermal basin. Optimal conditions of lignin compounds recovery by aluminium salts were studied by varying the dose of the coagulant and the pH of the systems, which are important technological parameters. The coagulation process was produced by mixing equal volumes of the coagulant and the model hydrolysate. The aggregation stability and coagulation of lignin compounds in the presence of aluminium salts was studied by spectrophotometry. The efficiency of coagulation in a wide range of pH and coagulant dose was determined after 2 h of settling and filtration of the system. It was established that using aluminium sulfate at the optimal dose of 230 mg/l at pH 5.0 and at a temperature of model of 22 °C, the COD reduction was 40.1%, the removal of dissolved solids 80.2%, and colour reduction about 87.3%. The optimum dose and pH for aluminium chloride were 130 mg/l and 6.0, respectively, the decrease of COD being 45.1%, the dissolved solids removal 82.1%, and colour reduction 82.7%. The experiments give the possibility to get the complete information about the efficiency of lignin compounds coagulation and to define the more optimal parameters for the purification of wastewater from hydrothermal basins in plywood industry.