Study On Mechanism And Efficiency Of Coagulation-ultrafiltration As A Pretreatment Process Of Seawater Desalination

Reverse osmosis desalination technology is an important strategic means to deal with the global water crisis,which has been widely concerned and studied.In recent years,with the maturity of ultrafiltration technology,the decrease of engineering application cost and the improvement of membrane performance,ultrafiltration has replaced many traditional processes,such as sand filtration,and has become the preferred pretreatment technology for reverse osmosis desalination.However,membrane pollution still seriously affects the operation cost and effluent quality of ultrafiltration process.As a low-cost and effective method to improve water quality and alleviate membrane pollution,chemical coagulation has been widely studied and applied.Therefore,carry out research on the pretreatment technology of reverse osmosis membrane with coagulation-ultrafiltration as the core,optimize the operating parameters of coagulation-ultrafiltration technology,explore the control mechanism of coagulation on membrane pollution,and stabilize the reverse osmosis seawater desalination system Operation is of great significance.In this paper,PACl,which is commonly used in industry,is selected as coagulant to study the control mechanism of coagulation process parameters on floc morphology and the Removal efficiency of conventional pollutants in seawater.On this basis,the coagulation and ultrafiltration process are combined,and the effects of process parameters such as coagulant dosage,hydraulic conditions,transmembrane pressure difference on coagulation and membrane pollution alleviation are mainly studied Finally,the enhanced effect of sedimentation process and coagulant aids on coagulation control membrane pollution was investigated to form a coagulation-ultrafiltration integrated process suitable for pretreatment of reverse osmosis membrane in seawater desalination.The main conclusions are as follows:The results show that the flocs with large volume and dense structure are beneficial to the treatment effect of coagulation and sedimentation,while the flocs with multi-stage agitation can obviously accelerate the growth speed of flocs.The addition of 40 mg/L coagulant can form larger floc size and denser floc.The Removal efficiency of coagulant dosage on turbidity and organic matter is obvious,while the increase of coagulation time can strengthen the Removal efficiency of turbidity,but has little effect on the removal rate of organic matter.The agitation intensity had little effect on turbidity and organic matter removal.When the coagulant dosage is 40 mg/L,the speed of 300 r/min is used for fast stirring for 2 min and 100-50-30 r/min for 5 min respectively,the sedimentation time is30 min,the better turbidity and organic matter Removal efficiency can be obtained.For coagulation-ultrafiltration pretreatment process,the flocs with large particle size and low fractal dimension can form loose and porous filter cake layer,and the pollution of ultrafiltration membrane can be alleviated to the greatest extent by reasonably controlling the operation conditions in the coagulation-ultrafiltration process.The addition of 20 mg/L PACl can form the largest floc particle size and maintain the loose structure of floc,which is caused by the characteristics of coagulation mechanism in the electric neutralization region.In the process of coagulation-ultrafiltration,the dosage of20 mg/L PACl and the flocculation time of 8 min,the filtration velocity of the membrane surface of 0.025 m/s can wash the membrane and exchange the concentrated water in the surface membrane module while maintaining the floc particle size above 200?m,further increases the water flux.However,a 60 k Pa transmembrane pressure difference can maintain a lower membrane fouling rate to obtain a higher water flux and maintain the long-term and effective stable operation of the ultrafiltration membrane.For seawater,the peak value of particle size distribution in ultrafiltration effluent appears at 0.6?2 nm,which effectively removes submicron pollutants in the range of300?1000 nm particle size in seawater.The effluent SDI₁₅ meets the requirements of reverse osmosis influent.Compared with coagulation-ultrafiltration process,coagulation-sedimentation-ultrafiltration process slightly alleviates the problem of membrane fouling,but it does not significantly improve the membrane flux and effluent quality.For the seawater with low pollution degree,the coagulation-ultrafiltration process can maintain low membrane pollution and overcome the defects of large coagulant dosage,long flocculation time and sedimentation time in the coagulation--sedimentation-ultrafiltration process.However,the addition of non-ionic PAM resulted in the decrease of water flux due to the blocking effect of macromolecular structure on membrane pore,which was not suitable for the coagulation-ultrafiltration process of seawater.