ESEL Paper Review_20140309
By Hong Guo
Phone: (+82) (0)10 82276568
1, Title and Author
Title: Sustainable water recovery from oily wastewater via forward osmosis-membrane distillation (FO-MD)
Journal: Water research
Sui Zhang, Peng Wang, Xiuzhu Fu, Tai-Shung Chung
Department of Chemical and Bimolecular Engineering, National University of Singapore, 117576 Singapore
2. Summary of Paper
? This study proposed and investigated a hybrid forward osmosis e membrane distillation (FO-MD) system for sustainable water recovery from oily wastewater by employing lab-fabricated FO and MD hollow fiber membranes
? The water flux in FO undergoes three-stage decline due to fouling and reduction in osmotic driving force, but is quite stable in MD regardless of salt concentration.
? Oily wastewater with relatively high salinity could be effectively recovered by the FO-MD hybrid system while maintaining large water flux, at least 90% feed water recovery could be readily attained with only trace amounts of oil and salts, and the draw solution was re-generated for the next rounds of FO-MD
? Significant amount of acetic acid was also retained in the permeate for further reuse as a chemical additive during the production of crude oil
? FO for oil/water separation: the effects of oil concentration and operation time
? To observe their stability, the oil emulsions are kept for one week and tested again. The variation of relative viscosity is almost negligible for oil concentration up to 4000 ppm in the emulsions. The relative viscosity is similar to that of DI water. When the oil concentration increases to 100000 ppm, the relative viscosity rises to 1.26
? In the FO mode, both 0.58 M (i.e., 3.5 wt%) and 2 M NaCl are employed as draw solutions for comparison and the initial water flux is around 13 LMH when 0.58 M NaCl is the draw solution and DI water is the feed
? A slight drop in water flux is immediately observed when 200 ppm petroleum is added to the solution, most likely due to pore blockage by the attachment of oil
droplets on the polyamide surface. Further increment in petroleum content does not exhibit a significant reduction in water flux, implying that the surface oil attachment is probably saturated at a low oil concentration.
? More than 90% of the initial water flux is retained even in the presence of 100,000 ppm petroleum. Interestingly, when the draw solution is replaced by 2M NaCl(0.58M for previous results), a larger initial water flux is resulted but it gradually decreases with an increase in petroleum concentration.
? A significant drop in water flux is observed when the oil concentration reaches 100,000 ppm. This phenomenon is most likely resulted from the high emulsion viscosity.
? The different trends of flux drop for the two draw solutions indicate that the oil-included flux decline is dependent on the initial water flux.
? From the results of comparison of the performance in PRO and FO modes suggests that the FO mode is a better choice for oil/water separation in terms of transport stability and oil fouling despite of its lower initial water flux.
? The long-term stability of water recovery from oil/water emulsions via FO is investigated in the next step. DI water was chosen as the reference feed and 4000 ppm petroleum was usd to represent the oil emulsions for water flux as a function of time over 24 h experiment by using of 2M concentration of draw solution.
? The oil emulsions result in slight reduction of water flux in the first 7-10 h due to increasing fouling, but no further flux drop is observed in the rest time. The oil fouling on membrane surfaces seems to be fast and stable, and a high water flux can be maintain in the long-term FO process for oil/water separation.
? Characterization of water flux and solute rejection in FO and MD process
? The water flux is largely enhanced fro 26.7+_ 3.5 LMH to 40.2+_ 2.1 LMH when the operation temperature is elevated from 23 to 60 Celsius degree using DI water as the feed. The reasons are (1) the osmotic pressure is higher based on the van’t Hoff equation, (2) similar to that of reverse osmosis membranes the reduced water viscosity facilitates its diffusion rate across the membrane, and (3) the increased diffusion coefficient of NaCl lowers the solute resistivity in the support layer and hence reduces the impact of internal concentration polarization.
? Even though the salt reverse flux is increased as well, the ratio of reverse salt to water fluxes is still kept at around 0.18 g L1, making the membrane quite suitable for salt e water separation at 60 C. and the oil rejection at this level of temperature is again higher than 99.9% in the FO process, while the rejection for acetic acid is lowered down to 74+_2.5%.
? Operating the FO process at elevated temperature not only draws water from oily wastewater at a higher permeation rate, but also enhance the amount of ac. etic acid
in the permeate stream while maintaining a very low oil concentration in the permeate.
? For MD, the PVDF membranes have dual macrovoid layers in the cross-section which offer a high porosity of 86+_6%, a micro porous selective layer on the outer surface and a fully open-channeled inner surface. The membrane is relatively hydrophobic with a contact angle of 88+-2 and effective mean pore size of 0.161+-0.004 um.
? Clean water can be collected at the rate of 5.8 +_ 0.5 LMH in the MD process where 2M NaCl flows through the shell side of the fibers as the feed at 60 Celsius degree
? Water recovery from the hybrid FO-MD process
? The flux pattern is characterized by three stages: a significant flux drop appears in the first 7-10 h, follows by mild flux decay until 20-25h, and then a fast flux decline again in the third stage.
? Consistent with the long-term studies, the water flux drops in the first several hours partially due to fouling. The feed salinity is continuously increased because water permeates from the feed to the draw solution.
? The salt accumulation in the feed becomes serious at the later stage when the recovery is high. And, the draw solution is continuously dilute, leading to a decreased effective osmotic driving force across the membranes. The increasing concentration of oil and acetic acid in the feed also contributes to the flux decline.
? As a result, the combined effects of increasing fouling and decreasing driving force cause a large flux drop in the 7-10 h stage. The flux decline becomes milder in the 2nd stage since fouling is gradually stabilized. Since the feed salinity is substantially increased when higher recovery is reached in the 3rd stage, a sharper decrease in flux is observed in this stage.
? Water permeation rate in MD is not significantly affected by the salt concentration. Therefore, a high draw solution concentration is preferred for FO-MD hybrid systems in pursuit of large water flux in FO.
? The experiments not only demonstrate efficient water recovery from oily wastewater by the proposed FO-MD process with at least 90% water recovery rate, but also partially recover acetic acid and almost completely reject oil droplets whose sizes are below 10 mm.
This research would be fully studied and applied to the research of the shale gas produced water treatment under the application of FO and MD process
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