Membrane Filtration Performance And Fabrication Of Cellulose Acetate Membrane In Forward Osmosis Technology

Compared with reverse osmosis, ultrafiltration, nanofiltration and other pressuredriven membrane separation technology, forward osmosis(FO) is a new type of membrane separation technology which because of its no external pressure and has a strong anti-pollution properties of concern by scholars. In recent years, the forward osmosis is more and more widely applied in wastewater treatment, desalination, food processing, drugs control and other fields. But it is the problem of FO itself also restricts its development, such as the draw solution was selected, the internal concentration polarization, membrane preparation and etc. So it is necessary for forward osmosis to research its filtration performance and preparation for forward osmosis membranes.The topic selected HTI’s two commercial FO membrane(TFC-ES, CTA-ES), then compared the performance of two membranes and found TFC-ES better than CTA-ES. The TFC-ES membrane used in brackish water and municipal secondary effluent and researched forward osmosis filtration characteristics(membrane orientation, the type and concentration of draw solution, crossflow rate), the best conditions for membrane cleaning, optimization of operating conditions, screening excellent draw solution to reduce the concentration polarization, reduce membrane pollution and improve the performance of forward osmosis membrane. At the same time, preparation cellulose diacetate FO membrane by phase inversion and characterizing the structure and properties. Explored the effection of different heat treatment temperature, organic solvents and additives on the FO membrane structure and properties. The CA FO membrane and commercial membrane was used in salt water for comparison, providing a reference for subsequent membrane modification. The results are as follows:(1) In FO mode(the active layer toward the feed solution, AL-FS), 7.25cm/s crossflow rate, 1mol/L Mg Cl2 as draw solution, the water flux was greater than 9L/m2·h by TFC-ES membrane, and the reverse salt rejection remained at 99.90%. When Na Cl simulated seawater, NH4HCO3 and glucose were draw solution, the flux of FO membrane relatively low, around 8L/(m2·h), but the salt rejection rate could reach 99% and 88%, respectively.(2) Reversible membrane fouling caused by the secondary effluent could be used for hydraulic cleaning, the flux recovery rate was more than 90% after washing 30 min with DI water at 17.4cm/s crossflow rate. In addition, the dissolved solids(TDS) of the secondary effluent removed more than 98% by TFC-ES membrane; the rejection rate of Al3+ and Fe3+ could reach to 97% and 100%, respectively; the rejection rate of organic matter, TN and TP were more than 82%. This study may provide a reference for the development of wastewater treatment processes.(3) During membrane preparation, the aperture of FO membrane by 80℃ heat treatment significantly higher than 60℃ heat treatment of pore size was small, and uneven distribution. The latter water flux and rejection rate than the former. Dioxane mixed with DMAc as a solvent, a large proportion of DMAc could obtain big pore size and the membrane more brittle, easily broken; while a large proportion of Dioxane could form denser membrane, smaller aperture, and high mechanical strength. With the increase of the Dioxane, water flux decreased, the rejection rate was rising, while ensuring rejection of certain circumstances, when the ratio of Dioxane and DMAc was 70/30 could obtain higher water flux.(4) TEP could improve the water flux and rejection. With the increase of TEP content, membrane pore structure became more open and emerged macroporous structure, while the water flux increased obviously, but it was severely attenuated. Three percent of TEP could obtain the most stable membrane performance, the flux was maintained at 16L/(m2·h) and the rejection rate reached 99%. Methanol could obtain big membrane pore structure, although little change in water flux, but flux decline and severe rejection rate decreased. Formamide had poor effect, the internal structure of the membrane is too loose, water flux dropped 10L/(m2·h) or less, and the rejection rate decreased. Therefore formamide unsuitable for additives. PEG is a good porogen. The lower molecular weight and additive contents of PEG, the better performance of dense membrane.(5) Using a laser scanning confocal microscope to compare the structure of CA membrane and CTA-ES membrane found that the pores of its active layer surface(Z=0) was usually not obvious, when Z≥3μ m, many holes were clearly visible. In addition, with the scanning depth gradually deepened(from 3-9μm), the pore size and porosity increased. But CTA-ES membrane phased more uniform and had better pore structure than CA membrane. The TFC-ES, CTA-ES and CA membranes used in brackish found that the best performance was TFC-ES membrane. Water flux could be maintained at about 10L/(m2·h) and the rejection rate could reach 99%. The CTA-ES membrane’s flux was close to CA membrane, about 6L/(m2·h), but its rejection was still higher up to 99%. The rejection of CA membrane was slightly lower, more than 98%, subjected to post-modification and optimization.