Thermochemical Precipitation as a Pretreatment Step for the Chemical Oxygen Demand and Color Removal from Pulp and Paper Mill Effluent

In the present work, the removal of chemical oxygen demand (COD) and color of paper mill wastewater due to the thermochemical precipitation of dissolved solids was studied in the temperature range from 20 to 95 (degree) C using different catalysts/chemicals. The homogeneous CuSO4 catalyst was found to be the most active in comparison to the other heterogeneous catalysts under similar operating conditions. The pH value showed a pronounced effect on the precipitation process. At an optimum initial pH of 5.0, a maximum COD reduction of 63.3% was obtained with a catalyst concentration of 5 kg m^-3, although the maximum color removal was 92.5% using a CuSO4 concentration of 2 kg m^-3. The residual copper in the supernatant works as a good catalyst for wet air oxidation of the supernatant. Thermogravimetric analysis showed that the thermal oxidation of the solid residue follows an one-way transport diffusion model with first-order irreversible reaction kinetics. The heating value of the precipitate was found to be comparable (19.72 MJ/kg) to that of the Indian coal (20.90 MJ/kg).