Study On The Softening Separation Performance And Membrane Fouling Of Brackish Water In Nanofiltration Membrane

Brackish water softening is one of the effective ways to solve the shortage of freshwater resources in China,this thesis simulated brackish water Huang Huai area,through laboratory test equipment,constructing of nanofiltration(NF)membrane softening system,and actively explore the influence factors of NF membrane fouling,as well as reducing membrane fouling,water production cost,promoting the industrialization development of brackish water softening technology.This topic mainly includes the following aspects:(1)Investigated including transmembrane pressure(0.60-2.20MPa),tangential velocity(0.09-0.31 m·s-1)and water temperature(17-37°C)operating conditions and the value of p H(3-10),TDS(1317-5926 mg·L-1)and effect of water properties on the DL and DK type NF membrane softening the separation performance of the membrane surface and inorganic pollution.The experimental results show that the operating conditions for the DK film the optimal operating pressure is 1.60 MPa,DL membrane the optimal operating pressure is 1.50 MPa,the DK membrane tangential flow velocity of 0.17 m·s-1,DL membrane of 0.15 m·s-1when the choice,the feed water temperature is 25?,DK and DL films have a higher retention rate for the main two valence scale ions Ca2+,Mg2+,CO32-and SO42-,indicating that the NF film has a good softening and separation performance.DK and DL nanofiltration membrane specific flux decreased with the increase of p H value were increased first and then decreased and the slow decline of p H is 6,the maximum membrane specific flux and TDS,Mg2+,Ca2+ removal rate is relatively small,we can conclude that p H increased to about 6 when just reached the isoelectric point,the reduction of membrane fouling the membrane specific flux maximum.With the increase of the influent TDS the membrane specificflux decreased,the removal rate of TDS decreased,Mg2+ and Ca2+ removal rate is not changed,because with the increase of water salinity,weaken the Donnan effect and the electrostatic effect of nanofiltration membrane surface and membrane pores,resulting in membrane to ions in solution retention rate decreased,but the solute through membrane force becomes larger,more solutes across the membrane.(2)The observation of membrane surface of organic pollution in the TOC(2.72-12.24 mg·L-1)water conditions,with increasing influent ratio of TOC flux showed a decreasing trend,the average removal rate of UV254 remained above 85%,various functional groups of humic acid contained in the surface complexation with metal ions in water can coexist in the the formation of large molecules,with complex structure leads to serious membrane fouling,membrane specific flux decline and formation of cake layer thickness can improve the retention rate of nanofiltration membrane ion.Changing the concentration of calcium and magnesium ions in the influent,with the increase of ionic concentration,the membrane flux decreased.The roughness of DL film(Rms)changed Mg2+ > Ca2+ > Mg2+ and Ca2+.The organic removal rate increased because of high concentration of calcium and magnesium ions and humic acid and the formation of "salt bridge ",or the formation of Ca2+-HA and Mg2+-HA composite membrane pore blocking the membrane flux decline,the removal rate of organic matter increased.It can be found clearly through the three-dimensional fluorescence.When HA exists alone or coexists with different cations membrane specific flux decline follows HA>HA+Na Cl>HA+Ca Cl2 =HA+Mg SO4>HA+Fe Cl3,synergistic effect of HA with different cations coexisting pollution induced by flux decline is different because of the worst effects of HA molecules on the membrane,the minimum deposition on the membrane molecules leads to membrane pore blocking,the membrane flux decreases most quickly.(3)According to the water physical properties and fluid mechanics experimental results,theoperating conditions are to maintain the best separation effect,long-term stable operation(operation pressure 1.55-1.60MPa;inlet side of the tangential velocity of 0.15 m·s-1;the water temperature of 25?.The interception effect of DL nanofiltration membrane on the main inorganic ions and natural organic matter in brackish water was investigated,and then the softening separation effect of DL membrane on Brackish Water in long-term stable operation was studied.The experimental results show that the water flux,retention rate of inorganic salt and total organic carbon removal rate(UV254 value)increased with time showed a downward trend,and can remove the bitter salty water for 40% Ca2+ and Mg2+,the order of Mg2+>Ca2+>Na+>K+ nanofiltration membrane desalination.Greatly reduce brackish water hardness,good separation performance of nanofiltration membrane softening,run to 267 h,membraneflux decreased by more than 10%,chemical cleaning,pickling after membrane specific flux recovery rate reached 90.97%,after the acid treatment and alkali washing value as high as 94.57%,combined with scanning electron microscopy and atomic force microscopy characterization results that has a good removal effect on membrane fouling in the chemical cleaning.