Efficiency And Mechanism Of Process Control And Membrane Modification On Pollution Removal By Ultrafiltration Membrane

Ultrafiltration(UF)technology has been widely used in water treatment industry.Membrane fouling is still the key and difficult problem needing to be comprehensive studied.The model of ultrafiltration operation and membrane chemical cleaning was used to understand organic matters removal better.Meanwhile the cambination of conventional water treatment technology and ultrafiltration was used to enhance pollution removal.In addition,developing new membrane materials was developed to enhance membrane performance and remove membrane pollution.PVC hollow-fiber ultrafiltration membrane were used in the ultrafiltration experiment with humic acids as model pollutants in response surface model of organic matter removal.The evaluation parameters were humic acid(HA)removal rate and water transport coefficient.Solution p H,ion concentration and initial filtration flux were selected as the influence factors.Results of response surface methodology indicated that the significance of the factors on impacting HA removal rate was as following order: solution p H > ion concentration > initial filtration flux;the order for water transport coefficient was: solution p H = initial filt ration flux > ion concentration.The factors in response surface method for chemical cleaning of membrane were sodium hydroxide(Na OH)concentration,sodium h ypochlorite concentration(Na Cl O),citric acid concentration and cleaning duration.HA(20 mg/L)was used as the model foulant,and chemical enhanced back flush was employed to simulate the chemical cleaning efficiency.Among the variables,Na Cl O concentra tion and cleaning duration showed a positive relationship in increasing che mical cleaning efficiency.The efficiency was hardly improved with increasing Na OH concentrations.However,a low concentration of Na OH was necessary.Consideration for membrane life,54 sets of cleaning schemes with cleaning efficiency among 80% to 100% were observed using response surface methodology.The high concentration powdered activated carbon-ultrafiltration membrane(HCPAC-UF)process was employed to remove organic matters and ammonia nitrogen from the slightly polluted water.The effect of PAC particle size on HCPAC-UF process performance and membrane fouling was investigated.The result showed that the PAC size had negligible effect on the removal efficiency of UV254 and DOC,but significantly influenced the removal of ammonia nitrogen.When the PAC size was 0.2?m,microorganisms reproduced and ammonia nitrogen removed slowly in the system.However,the membrane fouling was alleviated most effectively.Combined effects of coagulation and PAC to enhance organic matter removal and alleviate membrane fouling were investigated.The results indicated that coagulation and PAC showed synergistic effect on removing organic matter.Coagulation removed substnce with molecular weight larger than 30 K,while PAC adsorpt the organic component with molecular weight less than 30 k effectively.Coagulation+PAC-UF process removed 85.3% strong hydrophobic substance and alleviated membrane fouling,suggesting that strong hydrophobic substance was the cause of membrane fouling.When PAC dosage was only 20 mg/L,the UV254 and DOC removal efficiency was 93.6% and 69.7%,repectively,in the coagulation+PAC-UF process,and the standard flux ratio was 0.86 at the end of membrane filtration cycle.Finally,a new kind of membrane was prepared from modified membrane material in order to improve the hydrophilicity and enhance organic pollutants removal performance of ultrafiltration membrane.Multi-walled carbon nanotubes(MWCNTs)were modified by sulfuric acid and nitric acid,then Fe3O4 were loaded on the surface by chemical hydrothermal method.The magnetic nano particle(MWCNTs/Fe3O4)were blended with polyvinyl chloride(PVC)to prepare ultrafiltration membranes in magnetic field or without magnetic field to obtain ordermodified MWCNTs or disorder-modified MWCNTs membranes.Meanwhile,pure PVC membrane was also prepared as control.The ordered modification mechanism and the cross-section morphologies of the membranes were analyzed.The membr ane properties such as contact angle,swelling,porosity,average pore size,pure water flux,rejection and adsorption were examined and compared.The results showed that MWCNTs/Fe3O4 could improve the hydrophilic performance of membrane,increase pure water flux and rejection performance significantly.Ordered modified MWCNTs membrane was more compact and had more narrow pore structure and superior diversion channels network.The modified membrane showed more advantages in removing pollution.