Research On The Reverse Permeation Of Draw Solute In Forward Osmosis

Forward osmosis (FO) technology has low energy consumption, anti-pollution andmany other advantages compared to reverse osmosi（sRO）, which is a very practical andenvironmentally friendly technology. While selection of appropriate draw solution andthe concentration polarization hamper the development of FO. Ideal semipermeablemembranes should allow only water to pass through and prevent all draw soluteparticles from reverse permeation, while currently FO membranes always encountersolute reverse permeation, not only caused membrane pore blockage can also cause adecline in water flux. In the sewage treatment system, reverse cation ion wouldendanger the microorganisms in activated sludge system, resulting in paralysis of theentire reactor, so it is essential to develop the law of reverse draw solute permeation.The object of this study is to built a reverse draw solute permeation model whichwas developed from previous study on the concentration polarization, a set ofsequencing batch experiments to validate the model was conducted. First select fourtypical draw solutions, namely NaCl (1-1type), Na2SO4(1-2type), MgCl2(2-1type),MgSO4(2-2type), which have different anion and cation ions. According to themathematical models, the process and law of reverse draw solute permeation werestudied. The results show that, when the concentration of draw solution is low,temperature has little influence on the reverse flux, the flux increase slightly with therise of temperature. When FO have a high concentration of draw solution, the reverseflux first increases and then decline with the temperature rise. Running the FO modeand the PRO simultaneously, set the initial osmotic pressure4.2Mpa and temperature at25℃, and finally found that no matter what kind of draw solution, the reverse fulx ofPRO mode is greater than that of FO mode. Another critical factor influencing reverse isthe initial osmostic pressure, water flux and reverse flux increase with the initialosmotic pressure increasing. The hydraulic radius of cation and anion ion also infuencethe process of reverse solute flux. Generally speaking, the greater the hydraulic radiusion the smaller reverse solute flux, but the result was not consistant. The major reasonattributes to the Donnan equilibrium, the system overall is electrically neutral, thenreverse solute was interaction of cation and anion ions. If the anion reverse flux wasgreat, even if the hydraulic radius of the cation ion is large, its reverse flux willaccordingly become larger. And vice versa, the reverse cation ion flux increase willmake the anion ion flux increase. So the reverse solute fux of four kinds of drawsolution follows the order of NaCl> Na2SO4> MgCl2> MgSO4.Finally, a forwad osmosis membrane bioreactor (FO-MBR) platform with NaCl asdraw solution was established, operating the FO mode and PRO mode t simultaneously.The results show that both the water flux and reverse flux of FO mode larger than PRO mode,which contrary to the results of previous model. We speculate that the cation ionsuch as Ca2+, Mg2+in FS combined with soluble microbial product (SMP) and formcake layer at the membrane surface, resulting in membrane fouling and a sharp drop ofwater flux. This conclusion indicating that membrane fouling change the law of reversedraw solute permeation.