Study Of Advanced Treatment For Coking Wastewater

Coking wastewater is a kind of non-biodegradable wastewater with complicated components. The level of chemical oxygen demand (COD) of the coking wastewater which the experiment is used is about 120 mg·L-1, though the biotreated coking wastewater has been treated by the Fenton step. Activated carbon has good effect on coking wastewater treatment, but powdered activated carbon is less applied for the projects because of its difficulty in storage and regeneration. Powdered activated carbon is used to make a further advanced treatment of the coking wastewater in this experiment, and the level of chemical oxygen demand (COD) was used as the index for evaluating the treatment efficiency. In addition to the optimal conditions of adsorption and regeneration were researched, adsorption quilibrium isotherm with calculation of thermodynamic parameters and adsorption kinetics were studied to discuss the adsorption mechanism. The thermal regeneration process was monitored by thermal analysis (TGA/DSC), SEM. Based on the experimental result, advanced treatment of coking wastewater with powdered activated carbon was designed, and the economic feasibility was analyzed.According to the orthogonal experiment, the influences decreased successively in the following sequence:reaction time> the dose of activated carbon> pH value> wastewater temperature. The optimal adsorption conditions of treating 100 ml coking wastewater was that kept the raw wastewater’s pH and reacted 30 minutes with 2g powdered activated carbon. Under this condition, the COD decreased to 57.68 mg·L-1 and the removal rate was 53.73%, which reach the effluent limitation of Emission standard of pollutants for coking chemical industry (GB16171-2012).Langmuir, Freundlich and Tempkin isotherms were used to analyze the adsorption by the powdered activated carbons. The result showed that the adsorption isotherm was fit to the Freundlich with the correlation coefficient of 0.984. Compared with the pseudo-first-order and Elovich model, the pseudo-second-order model was the most suitable in describing the adsorption kinetics with the correlation coefficient of 0.999.Physical adsorption play the main role in the process of treating coking wastewater as the adsorption free energy E was smaller than8 kJ·mol-1.The negative A G value indicated spontaneous nature and feasibility of the adsorption. The negative A H indicated exothermic nature of the adsorption process and higher temperature is not favorable for the adsorption, which was fitted to the adsorption quilibrium isotherm. The entropy change AS is 0.640 J·mol-1·K-1. The positive AS indicated that the degrees of freedom increased at the solid-liquid interface.From the thermal regeneration study, it can be seen that the first regeneration efficiency was 89.98% with PAC regenerated 30 minutes under the temperature of 250℃. Under this condition, the regeneration efficiency was always above 80.23% after 16 repeated regeneration experiments, and the average weight loss was 1.08%.Scanning electron microscopy(SEM) was used to characterize the surface morphology of original activated carbon, saturated activated carbon and regenerated activated carbon. It showed that the micropore fraction was increased and the removal of attachment was obvious. The above results showed that the powdered activated carbon was regenerated effectively. Speculating from the DSC-TG graph, the best regeneration temperature of PAC is between 250℃ to 400℃,which is consistent with the regeneration experiment.Based on the experimental results and the characteristic of powdered activated carbon, one-stage process and countercurrent two-stage process were designed to treat the coking wastewater. Comparing the two processes, the cost of countercurrent two-stage process was lower as the cost of per ton wastewater is 1.86YRMB, just 40.7 percent the cost of one-stage process.