Mechanism Of Membrane Fouling In Coagulatio-nultrafiltration Water Treatment Process And Typical Membrane Filtration Process

For the treatment of micro-polluted water bodies such as contaminated surface water,multiple processes are often used in combination to achieve better treatment results,the integrated coagulation-ultrafiltration（UF）/microfiltration^[1]process,one of which is considered to be the most promising water treatment technologies.However,various natural organic matters（NOMs）in surface water interacts with the membrane surface would cause membrane fouling,which limits the efficiency of membrane treatment severely.Exploring deeper membrane fouling mechanisms,their influencing factors and regulation methods have become the main research directions in membrane technology.Due to the complexity of NOMs,model substances,such as sodium alginate（SA）and humic acid（HA）,are usually used in surface water contamination studies.However,the effect of a single contaminant on membrane fouling is not sufficiently representative.In fact,flocculants as well as metal ions present in surface water can also largely affect the structure and aggregation of NOMs,which have further effects on their fouling behavior.An in-depth and comprehensive discussion of membrane fouling associated with NOMs is crucial for both the treatment of surface water and the prevention and control of membrane fouling.In this study,SA and HA were used as model substances for polysaccharides and humic substances in NOMs.The study explored the deep fouling mechanisms between flocculants and typical contaminants in surface water that cause membrane fouling.The membrane fouling was assessed by specific filtration resistance（SFR）and filtration fluxes through filtration resistance tests in a laboratory-scale filtration device.The results of the filtration experiments combined with a series of characterizations indicated that the homogeneity of the gel system,the electrostatic bilayer compression effect,the molecular conformation of SA/HA and metal ions were responsible for the variation of SFR in the SA/HA solution system.In this study,the potential mechanism of membrane fouling is explained in depth and comprehensively by density functional theory（DFT）and thermodynamic mechanism based on Flory-Huggins lattice theory.The main findings of this paper can be summarized as follows:（1）To some extent,the increase of PAM can reduce membrane fouling.With the increasing concentration of PAM（0～50 mg/L）,the SFR of alginate showed a V-shaped profile.The specific filtration resistance（SFR）of the SA solution without PAM is extremely high(up to 3.28×10¹⁴m·kg^-1).When the amount of PAM added is20 mg/l,the SFR is significantly reduced.The ultra-high filtration resistance of polymer gels can be explained by the Flory-Huggins lattice theory from a thermodynamic point of view.Experimental characterizations indicated that,the changes of the solution physicochemical properties and the foulant morphology are associated with the SFR change.Low dosage of PAM addition decreases the negative zeta potential and homogeneity of a gel system,causing the reduced SFR.In contrast,further PAM addition increases the negative zeta potential and homogeneity of a gel system,and then increases the SFR of the gel layer.Therefore,the second filtration phenomenon can be reasonably explained.This study offered novel insights into the effects of PAM on the ultrafiltration behaviors.（2）The effects of calcium ions and polyaluminum chloride（PACl）on membrane fouling in coagulation-ultrafiltration（UF）were studied,and three interesting filtration phenomena were found:1)high specific filtration resistance（SFR）of alginate solution with certain CaCl₂or PACl addition;2)unimodal pattern of alginate SFR with PACl or CaCl₂addition alone;3)synergistic effects between CaCl₂and PACl on alginate SFR.Morphological changes of the related foulants driven by the thermodynamic mechanisms were found to be related with these phenomena.Coordination with Ca²⁺and Al³⁺ions preferentially form tetrahedron geometry and geometry of coordinating three terminal carboxyl groups,respectively,which facilitated to elongate the alginate chains and form more stable gel,increasing SFR by density functional theory（DFT）calculations.Improving Ca²⁺and Al³⁺dosages triggered transition to forming other geometries for clustering polymeric network and flocculation,reducing SFR.Due to the higher binding affinity of Ca²⁺than Al³⁺,Ca²⁺and Al³⁺sequentially take roles of enlarging polymeric network and clustering the coordination compounds,and therefore facilitate to form large size flocs,reducing SFR.The newly proposed thermodynamic mechanisms satisfactorily explained these interesting fouling phenomena,allowing to further optimize coagulation-UF process.（3）The effect of calcium ions on the filtration behavior of humic substances in surface water and the corresponding scaling mechanism has been investigated.This study found that the addition of calcium ions can significantly increase the filtration flux of HA solution and significantly reduce its reversible fouling.Interestingly,the research phenomenon shows that the polarization resistance is reduced after calcium is added,while the deposition resistance on the film surface increased slightly.The experimental characteristics show that membrane fouling is mainly caused by the formation of the gel layer,rather than the clogging of pores and the formation of agglomerates/flocs.Meanwhile,analysesbytheextended Derjaguin-Landau-Verwey-Overbeek（XDLVO）theory showed that the addition of calcium ions will increase the mutual repulsion between foulants and membrane.The changes in the physicochemical properties（particle size）and dirt morphology of the solution are related to the changes in the chemical composition,and are related to the above-mentioned interesting filtration behavior.