Study On Ultrafiltration Membrane Pollution And Control Based On White Water Characteristics Of Papermaking

As an efficient physical separate processing mode, ultrafiltration has been extensively applied to the closure of whitewater systems in the papermaking industry. Nevertheless, membrane fouling caused by organic pollutant in whitewater always hinders the application of membrane technology and affects the removal efficiency of organic matters. The study on the characteristics and control of membrane fouling caused by organics in whitewater is essential for the promotion of membrane separation efficiency. Therefore, the research on the influence factors of membrane fouling caused by the characteristics of organic matters and the solution chemistry of whitewater was carried out. Moreover, the fixatives（ polyethyleneimine） were used to fix the main fouling substances onto fiber, remove them from the closed white water system and cut down membrane fouling degree.Dissolved organic matter（DOM） is one of the main components in whitewater. In this study, XAD fractionation and grading membrane fil tration were used to characterize the hydrophilicity and molecular weight distribution of DOM in whitewater. Membrane fouling influenced by DOM was evaluated by the flux decline. Moreover, the filtration resistances, the scanning electron microscope and li nearized Herman’s blocking models were used to analyze the membrane fouling mechanisms. The study results indicated that DOM in whitewater was characterized by the low molecular weight and strong hydrophilicity. During the process of ultrafiltration, hydrophilic organics were the main membrane foulants. In the initial stage of ultrafiltration, hydrophilic organics can quickly reach the membrane surface. Low molecular hydrophilic components enter the membrane pores, causing blockage pollution. High molecular hydrophilic components mainly caused concentration polarization and cake layer. While, the hydrophobicity components tend to adsorb on the membrane surface slowly and form cake layer. In comparison, the hydrophilic fraction was the major foulant causing t he membrane flux to decline. Effect of DOM molecular size on membrane fouling was also studied. The scanning electron microscope and linearized Herman ’s blocking models showed that the low molecular weight organics can block the membrane pores and form int ermediate blocking, while the high molecular weight organics can form a gel layer on the membrane surface. M embrane interception is the main mechanism for the removal of high molecular weight organics in whitewater. Electrostatic interactions and adsorption are the main mechanism for the removal of low molecular weight organics in whitewater.Effects of whitewater solution chemistry（p H values, Na+ concentration and polyvalent metal ion） on membrane fouling were also studied in this paper. The study results indicated that acidic condition can weaken the electrostatic repulsion between organic matters and membrane surface. Low p H values can accelerate the deposition of organic matter on the membrane surface, causing serious membrane flux decline. The presence of Na+ can also increase the membrane fouling degree for double-decked compression and e lectrostatic shielding effect. Polyvalent metal ions can accelerate the aggregation of organic matters in whitewater. Meanwhile, polyvalent metal ion s can also accelerate the deposition of organic matters on the membrane surface through ―bridging action‖ between organics and membrane surface. At high metal ions concentration, a more compact cake layer with higher resistance formed on the membrane surface, causing serious membrane flux decline. The impact of Fe3 + on membrane fouling is more serious than Ca²⁺ at the same concentration. The study also found that membrane fouling was more serious when filtering colloidal organic matters at low p H values, higher ionic strength and Ca²⁺ concentration.Polyethyleneimine with high charge density and low molecular weight can fix the main fouling substances onto fiber, remove them from the closed white water system and cut down membrane fouling degree. The effects of polyethyleneimine dosage on the removal of organic matters in whitewater were studied by measuring cationic demand of filtrate, turbidity of whitewater and Zeta potential of organic matters in whitewater. The effect of pretreatment with polyethyleneimine on mitigating membrane fouling was also studied. The study results indicated that, with the increasing of polyethyleneimine dosage, cationic demand of filtrate and turbidity of whitewater obviously decreased, the main organic pollutant in whitewater can be removed effectively by polyethyleneimine. Meanwhile, polyethyleneimine pretreatment can reduce the membrane fouling effectively through reducing the membrane foulant load and improving the structure of filter cake layer.The research on the influence factors of membran e fouling caused by the characteristics of organic matters and the solution chemistry of whitewater was carried out. Based on the results, a reasonable pretreatment to mitigate membrane fouling was constructed. This thesis can provide supports for the stea dy running of the whitewater closure systems in which membrane technology is used.