TIAN Yu ¹, LIU Fuyao ¹, SHI Mingan ¹, WANG Xuelian ¹, AN Fuqiang ^{1, 2}
(1. School of Chemistry and Chemical Engineering, North University of China, Taiyuan 030051, Shanxi, China; 2. Shanxi Center of Technology Innovation for Coal Mine Wastewater Treatment, Taiyuan 030000, Shanxi, China)

Extended Abstract: [Background and purpose] The improvement in the degree of industrialization, the continuous development of machining, steel, minerals and other industries have led to increasing demand for emulsified oil in the industry. With the expanded application of emulsified oil, the environmental problems caused by wastewater discharge become more and more aggravated. Such wastewaters contain high concentration COD_Cr, with complex composition and difficulty in degradation, which poses a threat to the ecological environment and human health. Conventional treatment processes, such as electrolysis, precipitation and flocculation, have encountered problems, such as low removal efficiency, high cost and secondary pollution. Ceramic membranes have strong anti-pollution performance, long cleaning cycle, easy maintenance and no need of chemical agents, thus being in line with the concept of environmental protection. Therefore, it has been widely used in the treatment of emulsified oil wastewater. In this study, the efficient interception of emulsified oil in wastewater by using flat ceramic membrane was realized, aiming to provide theoretical basis for the application of flat ceramic membrane in the treatment of wastewater containing emulsified oil. [Methods] The rationality of the experimental design was verified by analyzing the characteristics of the emulsified oil and the characterization properties of the flat ceramic membrane. The influences of influent COD_Cr concentration, pH value, aeration and other factors on the treatment effect of the flat ceramic membrane were explored. In order to obtain the optimal process conditions for treating wastewater containing emulsified oil, GPC was used to detect the emulsified oil, so that the molecular weight and distribution of the components were determined. SEM and EDS were used to characterize the ceramic membranes and observe their surface morphology. COD_Cr was determined according to HJ/T 399-2007 Determination of Water Chemical Oxygen Demand by Rapid digestion Spectrophotometry.In order to ensure the accuracy of the experiment, each group of experiments were repeated three times and the arithmetic average of the three experiments was taken as the test result.[Results] According to the gel permeation chromatography (GPC) diagram of the emulsion oil, the size of the emulsified oil molecule is larger than 1/10 of the aperture of the flat membrane (100 nm), so that the emulsion oil can be completely trapped by the flat membrane. As revealed by SEM images, the inner surface of the flat ceramic membrane (i.e., the interception surface) became obviously rough, suggesting that the emulsion oil molecules were trapped on the surface. EDS indicated that, after the interception of emulsified oil, there are obviously a large number of carbon-containing substances on the surface of the plate ceramic membrane, implying that the ceramic membrane can effectively intercept the emulsified oil. As the concentration of COD_Cr in wastewater increases, the removal rate increases gradually. This indicates that the concentration of the emulsified oil wastewater has a great influence on the treatment efficiency. The higher the concentration of the wastewater, the more the organic molecules are trapped by the plate ceramic membrane. As the pH value of the wastewater increases, the removal efficiency of COD_Cr gradually increases. This is because NaOH solution, as a regulator and demulsifier, forms a mixed membrane with the film-forming substances in the emulsified oil of the original wastewater, which has lower strength than the original interface membrane, resulting in the destruction of the interface membrane. In this case, the water was wrapped in the membrane, whereas the water droplets collide one another to form large water droplets, eventually settling down to the bottom. Aeration has little effect on the removal of COD_Cr in effluent. This is mainly because the oil droplets in the waste emulsified oil are stably dispersed in the water by the emulsifier, which not only includes oil substances, but also may contain other organic substances, such as emulsifiers, undecomposed additives, etc., to form a stable emulsion, thus making it difficult for the oil droplets to gather and float or separate through simple aeration. In aerating state, the processing capacity of the equipment is significantly higher than that in non-aerating state. This is because the aeration forms cross-flow on the membrane surface, which would wash the membrane surface, remove the material on the membrane surface and reduce the concentration polarization, thus reducing the occurrence of irreversible membrane pollution, effectively minimizing the probability of membrane pollution and improving the continuous processing capacity.[Conclusions] A new process - short flow plate ceramic membrane process - is proposed for the treatment of emulsified oil wastewater from mine hydraulic supports. The plate ceramic membrane can effectively retain emulsified oil and reduce the COD_Cr in water. Under the conditions of an influent COD_Cr of 120 mg·L⁻¹, a pH of 9 and aeration, the treatment efficiency of the plate ceramic membrane was optimized. Specifically, the effluent COD_Cr is reduced to 57 mg·L⁻¹, corresponding to a removal rate of 52.5%. Meanwhile, aeration can effectively reduce the degree of membrane pollution and extend the service life of the plate ceramic membrane.
Key words: plate ceramic film; interception; emulsified oil; chemical oxygen demand; wastewater