Latvia is the biggest producer of plywood in Eastern Europe. One of the stages of raw material preparation for plywood producing is hydrothermal treatment of birch wood. Wastewater from plywood hydrothermal basin is characterized by the high degree of contamination. The basic contaminating component of this wastewater is lignin, which is the main reason of a high level of chemical oxygen demand (COD) and color of water. Insufficient efficiency of purification of lignin wastewater requires the development of advanced coagulation methods. It is known that the coagulation process is affected by its conditions, among which a temperature of wastewater plays a significant role. It has been assumed that the correct choice of this parameter can considerably improve the process of purification of lignin wastewater from hydrothermal basin, using chemical coagulants.
The main objective of the present study is to investigate the influence of temperature on birch lignin coagulation in the model solution, which imitated the wastewater of hydrothermal basin of plywood industry. The wastewater samples at different temperatures (3, 22 and 70oC) were prepared by refrigeration and by warming in a water bath, respectively. The coagulation process was produced by mixing wastewater and water solutions of aluminum chloride with different concentrations. The efficiency of the coagulation within a pH range 3-10 was determined after 2 h of settling and filtration of the system. The method of spectrophotometry was used for determination of a remaining concentration of dissolved solids in a filtrate. COD was measured by total chemical oxidation of the received filtrate by adding potassium dichromate (LVS ISO 6060:1989). The color of the filtrate was determined by the method of photometry at the wavelength 436 nm. The results were related to the Pt – Co reference solutions (ISO 7887:1994).
The optimum dosage of aluminum chloride and pH value for the model solution treatment at a temperature 3°C are 400 mg/l and 6.0, respectively, at which 37.2% reduction of COD, 80.0% of dissolved solids removal and 32.8% of color reduction are obtained. The conducted experiments at varying temperatures have demonstrated that, with increasing a treatment temperature, contaminant removal is enhanced. For the model solution with a temperature 22°C, the optimum aluminum chloride dosage is 130 mg/l and optimum pH is 6.0. Applying these parameters, 45.1% of COD decrease, 82.1% of dissolved solids removal and 62.7% of color reduction are obtained. It should be noted that, for the model solution treatment at a temperature 70°C, the optimum aluminum chloride dosage decreases fourfold in comparison with the treatment at 3oC.The system showed the highest efficiency in terms of decrease of COD (54.2%), removal of the dissolved solids (85.7%) and reduction of color (69.0%).
Keywords: Lignin Coagulation,Temperature,Chemical Oxygen Demand (COD)
Authors
Brovkina J.
Laboratory of Lignin Chemistry, Latvian State Institute of Wood Chemistry, Riga, Latvia
Shulga G.
Laboratory of Lignin Chemistry, Latvian State Institute of Wood Chemistry, Riga,Latvia
Ozolins J.
Faculty of Material Science and Applied Chemistry, Riga Technical University, Riga, Latvia
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