Fouling Mechanism And Antifouling Modification Of Ceramic Ultrafitration Membrane For Treating O/W Emulsion

With the increasing concern of environmental protection, the emission standard for oily produce water is more and more stringent. Oil/water(O/W) emulsion widely exists in oily wastewater, which is difficult to be effectively treated because of its stable physicochemical properties. Conventional treatment techniques are not efficient to remove tiny oil droplets and thus difficult to meet the stringent discharge standard. Ceramic membrane ultrafiltration is considered as a reliable process for O/W emulsion treatment, and yet still restricted by severe membrane fouling. Membrane fouling is largely influenced by the properties of O/W emulsion and membrane filtration material, which have not been systematically studied. In addition, there were no clear fouling mechanism for ceramic ultrafiltration membrane in the treatment of O/W and lack of efficient membrane cleaning method. However, the key to a wide application of ceramic membrane in O/W emulsion treatment lies in the solution to these issues.This study investigated the property influences of O/W emulsion on ceramic membrane fouling, revealed the relationship between surface properties of filtration layer and membrane fouling tendency, and proposed membrane fouling mechanism. Hydrophilic Fe2O3 with high antifouling ability could be used as a promising ceramic membrane material. The antifouling performance of ceramic membrane was improved through in-situ synthesis of dynamic Fe2O3 membrane on the surface of ceramic membrane. Meanwhile, an efficient membrane cleaning method for the composited membranes was developed. On the basis of this, the pilot-scale units of dead-end filtration and cross-flow filtration were established, and the filtration condition was optimized. The real O/W emulsion was efficiently treated by the composited membranes under optimized condition, which validated the feasibility of their industrial application.The synthetic O/W emulsion was prepared based on the properties and composition of practical O/W emulsion. The preparation conditions were as follows: 100 mg/L crude oil, 10 mg/L surfactant, 50 min ultrasonic time and 96 μm amplitude intensity. The synthetic emulsion had the average oil droplet size of 356 nm and zeta potential of-45.3 m V, which was stable within 3 days. The influence of the emulsion composition on its properties was investigated: the oil droplet size was largely determined by types of base oil; the larger proportion the heavy components take, the bigger the droplet size is; the electrical property of emulsion was mainly dominated by the types of surfactants. The emulsion prepared with the anionic surfactant was negatively charged, while that prepared with the cationic surfactant was positively charged. In addition, the zeta potential and average droplet size exihibited almost no change in p H 5-10 and increased as the p H decreased to 5 due to the absorption of H+ and thus the coalescence of oil droplets.The influence of the emulsion properties on ceramic membrane fouling was investigated. The smaller the size of oil droplets is, the more irreversible fouling is. This phenomenon can be attributed to that more small oil droplets penetrated into membrane pores leading to more irreversible membrane fouling. Membranes for treating differently charged emulsions suffered different fouling extents. Irreversible fouling was less when the treated emulsion is oppositely charged to the membrane. The comparison of cross section SEM images of fouled and virgin membranes suggests that the membrane for treating similarly charged emulsion had more pore blockage fouling. The possible fouling mechanism was proposed: surfactant molecular plays an important role in membrane fouling process. The steric effect and demulsification effect were responsible for less irreversible fouling. The membrane fouling evolutions during the treatment of O/W emulsions were identified with curve fitting to the four fouling models in terms of correlation coefficients(R2) and fouling index(K). The feeds and permeates of O/W emulsions were characterized with COD, FEEM and GC/MS analysis. Regression results of classical fouling models and analysis of organic components rejected by the membrane confirm our proposed mechanism. Furthermore, this mechanism was further verified by the existence of steric effect and demulsification effect. On the basis of membrane fouling mechanism and characteristics of oil foulants, an efficient membrane cleaning method was provided: the fouled membrane was first washed by 3 g/L SDS for 60 min, and then by 1.2 g/L Na OH for 40 min. The recovery rate of the membrane flux was 98.8%.Ceramic membranes deposited with five metal oxides(i.e., Ti O2, Fe2O3, Mn O2, Cu O and Ce O2) by PLD were tested for O/W emulsion treatment to investigate the relationship between the properties of filtration-layer metal oxide and membrane fouling tendency. Distinct membrane fouling tendency was closely correlated to surface properties of the filtration-layer metal oxides. The hydrophilicity of the filtration-layer metal oxide is a major factor influencing irreversible fouling tendency of ceramic membrane. The surface hydroxyl groups of the metal oxides(Me O–H) largely influences their hydrophilicity. Higher hydrophilicity is related to lower oil adsorption energy on metal oxides and lower irreversible fouling tendency. The opposite charge of metal oxide to that of emulsion can help to alleviate irreversible membrane fouling in ultrafiltration. The irreversible fouling tendency of the deposited membranes follows the order of Fe2O3 < Ti O2 < Cu O < Ce O2 < Mn O2. Highly hydrophilic Fe2O3 with lowest fouling tendency and catalytic property could be a potential material of ceramic membrane for O/W emulsion treatment. In addition, two stages of flux decline were observed during the treatment. This phenomenon can be ascribed to the first stage of the penetration of oil droplets into membrane pores and the following second stage of the coalescence of oil droplets on membrane pores.According to experimental results, two types of Fe2O3 dynamic membranes(i.e., pre-coated and self-forming dynamic membranes) were prepared to mitigate the fouling of ceramic membrane for O/W emulsion treatment. Pre-coated dynamic membrane(DM) significantly reduced the ceramic membrane fouling. Two reasons may account for that 1) the improvement of hydrophilicity of ceramic membrane by Fe2O3 DM, 2) the exposure reduction of ceramic membrane to oil foulant and 3)the easy removal of fouled Fe2O3 particles. In contrast, self-forming DM aggravated the membrane fouling because of the pore blockage of dynamic membrane and difficult removal of fouled Fe2O3 particles. In addition, the cleaning efficiency of the composite membranes was improved by substitution of alkalescent water backwash for deionized water backwash. This enhancement can be attributed to the charge reversal of dynamic membrane. On the basisi, the pilot-scale filtration units of dead-end and cross filtration were established, respectively. The optimum operating conditions for dead-end filtration were that: constant flux = 10 L/h; filtration time = 10 min; pressure of back wash = 0.2 MPa; backwash time = 60 s; air flush time = 10 s. The optimum operating conditions for cross filtration were that: trans-membrane pressure = 0.2 MPa; CFV = 1.5 m/s. The practical O/W emulsion was filtered by the composite membranes under two filtration processes with 100% SS removal rate, oil content < 5 mg/L, turbidity of 0 NTU and COD < 15 mg/L, which meet the emission standard and validate the feasibility of the composite membranes for treating oily wastewater in industrial scale.