| The problem of algal-laden water caused by eutrophication of natural water bodies seriously affects the drinking water safety for residents.How to ensure the biological safety of drinking water is still one of the important issues of concern in China and even the world.Ultrafiltration membrane treatment technology is an effective algae removal technology because of its high performance in retaining and separating contaminants and relatively low energy requirements.However,the inevitable membrane fouling in the process of algae removal will directly reduce the effluent quality and increase the energy consumption.Therefore,it is important to clarify the triggering mechanism of membrane fouling in the process of ultrafiltration membrane treatment for algal-laden water.In addition,it is necessary to establish targeted and optimized control strategies for the further application of ultrafiltration technology in the field of drinking water.The results of this study have important theoretical and engineering guidance significance.In this paper,the ultrafiltration membrane fouling under different operating conditions was explored by comparing different pre-membrane pretreatment methods(direct filtration,ferric chloride pre-coagulation,Fe(II)coupled potassium permanganate pre-oxidation enhanced coagulation).Besides,different backwash conditions(backwashing method,water washing intensity,air washing intensity,membrane pool emptying cycle and air washing cycle)were also investigated.The optimal membrane fouling control strategy were established by comparing membrane fouling under different pretreatment methods and different backwashing conditions.On this basis,the migration and transformation law of pollutants and the characteristics of deposited pollutants at the membrane surface interface were further analyzed.The main factors inducing the membrane fouling were identified and the membrane fouling mechanism was revealed.The main results and conclusions are as follows:1)When direct filtration with untreated high algae water,algal cell deposition could cause serious contamination of the surface of ultrafiltration membrane(transmembrane pressure reaches 25KPa.Water washing or air washing could effectively alleviate membrane pollution by reducing the transmembrane pressure from 25KPa to 15KPa,respectively.The degree of membrane fouling alleviation increased with the increase of water washing intensity or air washing intensity.However,during the process of air washing,the solution in the membrane pool could be easily disturbed,resulting in the resuspension of the deposited sludge at the bottom of the membrane pool and the formation of"secondary pollution".Therefore,the air washing backwash parameters required to be carefully selected.In this study,the air washing intensity was set at 0.6L/min and the emptying cycles was shortened to12h,which could effectively reduce the amount of sludge,and thus effectively alleviate the membrane fouling.2)When polyferric chloride was selected for the coagulation pretreatment,the polymerization of Fe(III)introduced could cause algal cells to agglomerate and formed biological aggregates.The cake layer deposited on the ultrafiltration membrane surface after filtration was loose and porous,and the transmembrane pressure increased slowly,which was more conducive to the shedding of the contaminated layer during backwashing.Increasing the washing intensity(80L/(m~2·h)),air washing intensity(0.8L/min)and shortening the sludge discharge cycle(12h)were conducive to the mitigation of membrane fouling.When regulating the combined backwashing of gas and water(water washing intensity was set as 40L/(m~2·h)and air washing intensity was set as 0.6 L/min)and the emptying cycle was set as12h,the optimized membrane fouling control effect could be obtained.3)When Fe(II)-coupled potassium permanganate pre-oxidation was used to enhance coagulation,the algal cells fed into the membrane pool further aggregate into clusters and formed larger biological agglomerates,with a particle size distribution of about 20μm.At this time,the algal cells in a free state were relatively few,and the particle size distribution was about 8μm.The experimental results of water washing intensity,emptying cycle and air washing cycle showed that increasing the washing intensity from 20 L/(m~2·h)to 80 L/(m~2·h),the transmembrane pressure value of the same cycle was reduced from 90KPa to 60KPa.The backwashing method of water washing could effectively remove the pollutants deposited on the membrane surface.The combination of air washing and emptying strategy could greatly slow down the deposition speed of pollutants on the membrane surface.Air washing has an excellent backwash effect owing to the shear force of the airflow could make the deposits on the surface of the membrane completely fall off.Moreover,the air washing caused turbulence in the water body of the membrane pool,and the deposited sludge changed into suspended state.As a result,the sludge emptying process could be carried out more thoroughly.Therefore,the combination of gas and water backwash could achieve the best ultrafiltration membrane fouling controlIn summary,in the process of treating high algae water by submerged ultrafiltration process,the addition of iron salt coagulant and Fe(II)coupled with potassium permanganate pre-oxidation to enhanced coagulation could significantly improve inlet water quality and further alleviate the membrane fouling induced by algae source pollutants.On this basis,the backwashing method and intensity should be appropriately arranged with the adequate sludge emptying cycle to effectively regulate the membrane fouling and ensure the efficient operation of the ultrafiltration system. |
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