Study On Interface Thermodynamic Mechanism Of Membrane Fouling In MBR And Its Application In Membrane Modification

In recent years,the increasing scarcity of water resources and the demand for high-quality drinking water have attracted more and more attention to the recycling of wastewater.To solve the solid-liquid separation problem that has been difficult to solve in the traditional biological wastewater treatment process,membrane bioreactor(MBR)technology has become one of the most effective solutions to this problem.The application of MBR technology can be broadly defined as a system that combines the biodegradation of wastewater with membrane filtration.It turns out that they can effectively remove organic and inorganic pollutants as well as microorganisms,and thus MBR is receiving more and more attention.However,membrane fouling is the biggest bottleneck in the MBR sector and limits the development of this technology.Therefore,a deep understanding of the mechanism of membrane fouling and the search for effective modification measures to reduce membrane fouling are the hot topics of this study.This paper attempts to improve the membrane anti-pollution research.Hydrophobic polyvinylidene fluoride(PVDF)film was modified by radiation grafting with hydrophilic hydroxyethyl acrylate(HEA)as the monomer.A series of performance characterizations of the modified membranes were performed based on the fractal theory and the extended Derjaguin-Landau-verwey-overbeek(XDLVO)theory to demonstrate the thermodynamic analysis of the anti-fouling properties.Then,a laboratory-scale immersed MBR system was used to investigate the influence of fractal roughness(G)and random phase(φ-(m,n))on the interfacial interaction during the process of membrane fouling by combining Fractal theory with XDLVO theory.The main findings are as follows:(1)The grafted membrane with hydrophilic HEA monomers exhibit enhanced hydrophilicity,moisture content,settling ability and wettability.Interestingly,the filtration test found that the water flux of the grafted membrane in the acidic range was clearly sensitive to the pH of the solution.Atomic Force Microscopy(AFM)analysis provides evidence that the surface pore size of the grafted membrane decreases as the pH of the solution changes.This is due to the decrease in pore size of the membrane surface caused by swelling of the graft chain matrix as the pH changes.At the same time,it was found that the grafted membrane had a significantly lower flux decline rate than the control membrane.In addition,the flux of the grafted membrane to bovine serum albumin(BSA)was significantly greater than that of pure water.This is due to the compression of the electric double layer when the grafted polymer chains swell.(2)In this paper,the fractal theory and the XDLVO theory were used to investigate the interfacial interaction between the control membrane and grafted membrane in the two cases of smooth membrane-smooth particle and fractal membrane-smooth particle respectively.From the thermodynamic point of view,the improved antifouling performance of the grafted membrane was further demonstrated.This study not only provided a pH-sensitive PVDF membrane potentially useful for various applications,but also proposed novel mechanisms underlying the enhanced performance of the grafted membrane.(3)We can get the necessary parameters to simulate the real membrane surface by analyzing the AFM images by power spectrum.According to the modified two-variable Weierstrass-Mandelbrot(WM)function,which is related to the fractal theory,we can construct the membrane surface.Comparing the morphology and surface roughness of the two,the results show that this method is feasible.(4)Based on XDLVO theory,the interface interaction between sludge particles and membrane surface was calculated.At the same time,combining the surface element integral method(SEI)and the composite Simpson’s rule simplifies the cumbersome calculation formula and realizes the efficient calculation process.The interfacial interactions(including acid-base,Lifshitz-vander Waals and electrostatic double layer interactions)were systematically evaluated in the two cases of smooth membrane-smooth particle and fractal membrane-smooth particle.(5)Fractal roughness is one of the most important properties of a fractal surface.In this study,it was found that,randomly rough membrane surface was a fractal surface,which could be digitally modeled by a modified two-variable Weierstrass-Mandelbrot(WM)function.Fractal roughness of membrane surfaces has a typical power function relation with the statistical roughness of the modeled surface.Assessment of interfacial interactions showed that an increase in fractal roughness of membrane surfaces will strengthen and prolong the interfacial interactions between membranes and foulants,and under conditions in this study,will significantly increase the adhesion propensity of a foulant particle on membrane surface.This interesting result can be attributed to that increase in fractal roughness simultaneously improves separation distance and interaction surface area for adhesion of a foulant particle.(6)As an important parameter in fractal geometry,random phase plays an important role in the simulation of fractal surface.This article explores the effects of random on membrane surface morphology and interfacial interactions.During the course of the study,it was found that the change of random phase will have a significant effect on the surface morphology of the membrane.Meanwhile,the roughness of simulated surface in a certain range of random number will fluctuate within a relatively small range.In addition,the results of the interface force show that the random number in a certain multiple,the interface will be reduced.In the context of thermodynamics,the effects of fractal roughness and random phase on the interfacial interaction and membrane fouling were studied in this paper,which provided a deep insight into membrane fouling control of membrane bioreactor.