Influence Of Coexisting Cations On Water Purification Efficiency And Membrane Fouling Mechanisms In Combined Process Of BF-UF

Ultrafiltration（UF）is an important drinking water safety technology,which is more and more widely used in water treatment.However,the problem of membrane fouling limits the development of ultrafiltration technology.Adding a pretreatment process before the ultrafiltration membrane can improve the treatment efficiency of ultrafiltration technology and mitigate membrane fouling.In this paper,ballasted flocculation（BF）is used as one pretreatment process before UF,under the condition that the dosage of coagulant aid CPAM is 1 mg/L,the dosage of micro-sand is 3 mg/L,and the dosage of coagulant AS is 2.5 mg/L,and the common cations including Na⁺,Ca²⁺and Fe³⁺in water were selected,and the concentrations are set to 0,1,5,10,20 mmol/L respectively.The simulated water samples of humic acid-kaolin were treated by the combined process of BF-UF,and the influence of coexisting cations on the morphological characteristic evolution of ballasted flocs and the water purification efficiency of BF-UF was systematically analyzed.Besides,the influence of coexisting cations on the fouling mechanism of ultrafiltration membranes was analyzed from the perspective of floc morphological characteristics.The influence of coexisting cations on the water purification efficiency of BF-UF was studied.The results showed that in the presence of Na⁺and Ca²⁺,the turbidity removal of BF firstly increased and then decreased with increasing of ion concentrations,and the highest turbidity removal rate was as high as 91%when the concentration at 1 mmol/L.However,the presence of Na⁺and Ca²⁺had all most negiligible influence on the turbidity removal rate of the ballasted flocculation-sedimentation（BF-S）.The associated turbidity of the effluent water of BF-S was 2.0 NTU and 2.5 NTU,respectively,corresponding to the turbidity removal rate higher than 98%and 97%.The influence law of Fe³⁺on turbidity removal was slightly different.The turbidity removal rate of the BF and BF-S decreased with the increase of Fe³⁺concentration.Na⁺,Ca²⁺and Fe³⁺all had negligible influence on the turbidity removal rate of BF-UF.After BF-UF,the turbidity of the effluent was less than 1.0 NTU,and the turbidity removal rate was more than 99%.Ultrafiltration can effectively retain turbidity particles and improve the quality of effluent.However,Na⁺,Ca²⁺and Fe³⁺had a great influence on the removal of humic acid,and the removal rate of humic acid increased with the adding of Na⁺and Ca²⁺concentrations.When the ion concentration was 20mmol/L,the humic acid removal rate was 92.26%and 92.22%,respectively.The removal rate of humic acid by Fe³⁺showed different rule.With the increase of ion concentration,the removal rate of humic acid firstly increased,and then decreased.When the concentration of Fe³⁺was 1mmol/L,the removal rate reached 93.41%.The study found that coexistence of different valence cations impacted morphological characteristics of ballasted flocs.The presence of Na⁺and Ca²⁺promoted the growth of flocs.With the increase of the ion concentration,the average particle size of the ballasted flocs first increased,and then decreased,the fractal dimension first decreased and then increased,and the floc structure changed from loose to dense.While the existence of Fe³⁺was not conducive to the growth of flocs.With the increase of Fe³⁺concentration,the average particle size of ballasted flocs continued to decrease.With the increasing fractal dimension,the flocs becomed more and more dense.At the same time,it was also found that coexisting cations with different valence states and concentrations could directly affect the morphological characteristics of ballasted flocs,and indirectly affected the structure of the filter cake layer on the membrane surface,thereby affected the membrane fouling:a)The valence state of Na⁺was less than that of Ca²⁺and Fe³⁺,which had little influence on membrane fouling.The morphological characteristics of flocs could directly characterized the membrane fouling.When the concentration of Na⁺was low,the electrostatic repulsion between suspended particles was small.The ballasted flocs were mainly formed by bridging.The flocs had large particle size and loose structure.The filter cake layer deposited on the membrane surface had loose structure and large porosity.The pollutants were difficult to reach the membrane surface,which could effectively mitigate the membrane fouling.When the concentration of Na⁺was high,the electrostatic repulsion between suspended particles was large,the floc size was small,the floc structure was compact,the structure of the cake layer formed on the membrane surface was compact,the porosity was low,the membrane resistance increased,and the flux attenuation rate of the membrane increased.The high concentration of Na⁺weakened the electrostatic repulsion of the pollutants and the membrane surface,the pollutants formed a dense fouling layer on the membrane,which increased the filtration resistance and aggravated the membrane fouling.b)The morphological characteristics of flocs in the presence of Ca²⁺could not directly characterize the membrane fouling.With the increase of Ca²⁺concentration,the electrostatic repulsion between suspended particles increased,the particle size of ballasted flocs decreased,the structure was dense,but the membrane fouling was alleviated.The reason was that Ca²⁺could directly affect the membrane filtration performance.The hydration radius of Ca²⁺was smaller than that of Na⁺,which was easy to produce hydration repulsion,which shielded the electrostatic repulsion between pollutants and the membrane surface.The intermolecular force was constantly weakened,the pollution layer formed on the membrane surface had certain pores,and there was also a synergistic effect between Ca²⁺and aluminum salts.The higher the concentration of Ca²⁺was,the stronger the synergistic effect was,which could also reduce the membrane resistance,make the cake layer more loose,and effectively alleviate membrane fouling.c)The morphological characteristics of flocs under the condition of Fe³⁺were not the key factors affecting membrane fouling.The existing morphology of Fe³⁺in water could directly affect the membrane filtration performance.When the concentration of Fe³⁺was low,Fe³⁺could complexate and bridge with humic acid,change the deposition of humic acid and other pollutants on the surface of ultrafiltration membrane,and the generated Fe（OH）₃sediment was less,formed large pores between pollutants,reduced the adsorption and deposition of pollutants on the membrane surface and in the membrane pores,reduced the membrane resistance and effectively mitigated the membrane fouling.When the concentration of Fe³⁺was high,the excess Fe³⁺was continuously complexed in the filtration process to generate a large amount of Fe（OH）₃sediment,which deposited on the surface of the ultrafiltration membrane,so that the ultrafiltration membrane hole was basically blocked,the membrane flux decreased rapidly,resulting in serious membrane fouling.