Membrane Fouling Mechanism Of Typical Membrane Separation Process In Water Treatment

Water shortages have always been a major challenge for human survival and development,so water recycling is particularly important.Membrane Separation Technology for Wastewater Treatment,especially membrane bioreactor（MBR）technology has become one of the most promising technologies for wastewater treatment due to its small footprint,high processing efficiency,good effluent quality and low surplus sludge over conventional activated sludge process.Despite the significant advantages of MBR technology,the problem of membrane fouling has been an inevitable problem in this method,which greatly limits and restricts the development of this technology.It is generally believed that the adhesion of foulants on the membrane surface and the formation of a foulant layer are the main causes of membrane fouling in MBR,and these two processes are directly related to the interfacial behavior of membrane foulant.In addition,the foulant layer formed on the membrane surface can be further divided into a gel layer and a cake layer.Although the cake layer is thicker in appearance,the filtration resistance caused by it is much lower than the gel layer.In some cases,the formation of a gel layer directly determines the degree of membrane fouling in MBR.This study explores membrane fouling mechanisms associated with gel contamination by gel fouling and model materials formed by typical membrane separation processes.According to the Extended-Derjaguin-Landau-Verwey-Overbeek（XDLVO）theory,a radial basis function（RBF）artificial neural network（ANN）was developed to quantify the interface interactions associated with membrane fouling in an MBR.In addition,based on the Flory-Huggies theory,the reason for the high specific filtration resistance（SFR）of the gel layer was explained,and the contribution of hydrogen bonding to membrane fouling was quantified by using poly(N-isopropylacrylamide（PNIPAM）as a model material.In order to further explore the foulant behavior of natural organic matter（NOM）,sodium alginate（SA）was used as a model material,and the mechanism of Fe（III）flocculant concentration on membrane fouling was explained from the molecular insight by density functional theory（DFT）calculation.These studies provide new insights into the understanding of membrane fouling.The main results are as follows:（1）Adhesion of foulant on the membrane surface is mainly controlled by short-range interface interactions.Although the advanced XDLVO method can quantify this interaction,the high time-consuming limits the application of this method.In this paper,three different artificial neural networks（ANN）（radial basis（RBF）ANN,Back Propagation（BP）ANN,and Generalized regression neural network（GRNN））using five related factors as input variables were applied to quantify interfacial energy with randomly rough membrane surface.It was found that,ANN could well capture the complex non-linear relationships between the related factors and interfacial energy.Among them,BP ANN shows the best quantization performance.The ANN quantification results showed high regression coefficient and accuracy,suggesting its high capacity to quantify interfacial energy.Compared to at least one-week time consumption of the advanced extensive Derjaguin-Landau-Verwey-Overbeek（XDLVO）approach,quantification by ANN only took several seconds for a same case,indicating the high efficiency of RBF ANN.This research provides a new idea for studying membrane fouling in MBR.（2）Gel layer formation in some cases directly determines membrane fouling extent in MBR.While hydrogen bonding interactions extensively exist in gelling foulants and sludge suspension,their exact roles in fouling remain unveiled.Filtration results in this study showed that,SFR of a gel layer formed in the MBR was as high as2.06?×?10¹⁹?m·kg^-1 at 20?°C,and moreover,SFR of both the real gel and model gel（Poly（N-isopropylacrylamide）（PNIPAM））decreased with temperature.Fourier-transform infrared spectroscopy（FTIR）analysis indicated that gel samples were abundant of good hydrogen bonding donors/acceptors to form hydrogen bonding,and hydrogen bonding strength decreased with temperature.From viewpoint of free energy,mathematical models depicting roles of hydrogen bonding were proposed.For the first time,contribution level of hydrogen bonding effects to total gel SFR was quantified to be around 20%.These results offered in-depth insights into membrane fouling in MBR.（3）Molecular mechanisms responsible for the filtration behaviors of sodium alginate（SA）in presence of different iron（III）ion concentration were explored in this study.It was found that specific filtration resistance（SFR）of alginate mixtures（1.0 g SA/L）firstly increased and then decreased to a trough with iron（III）concentration increase from 0 to2.5?mM.Alginate mixture interacting with 0.1?mM iron（III）possessed an SFR as high as1.65?×?10¹⁴?m·kg^-1,which could be explained by Flory-Huggins lattice theory related with gel filtration.Optical observation showed significant morphology transition（from gel to granular solids）of foulant layers with iron（III）concentration increase.A series of characterizations indicated the change of microstructure,pH and surface charge of alginate mixture with iron（III）concentration.Density functional theory（DFT）simulation suggested that iron（III）ion preferentially forms coordination bonds with three terminal carboxyl groups of alginate chains,facilitating elongation and cross-linking of alginate chains.Such a coordination mode induces formation of a slime and homogeneous gel,corresponding to high SFR.Continuous increase in iron（III）concentration leads to non-terminal coordination,which makes alginate chains more clustered and coiled.This effect,together with effects of the reduced surface charge and electric double layer compression,significantly decrease SFR of alginate mixtures.This study provided deep molecular insights into effects of iron（III）ions on alginate fouling.