The Concentration Of High-salinity Seawater By Nanofiltration Technology

With the mounting of population and the expansion of industry scale, the world is facing severe shortage of freshwater nowadays. To solve this problem, researchers have come up with the methods of seawater desalination for decades. There are several kinds of desalination methods among which reverse osmosis (RO) is the most widely used and the most promising method. For a successful RO process, usually pretreatment is critical. There are several kinds of pretreatment method, among which nanofiltration (NF) is most promising. NF membrane can remove most of divalent ions of seawater. NF softening seawater as feed water of RO system can reduce the energy and the pollution of RO system effectively. Similarly, NF is also promising for softening high salinity brine.Based on the softening experiments of seawater from Jiaozhou Bay by NF technology, this paper made a further investigation on the application of NF technology to process NF retentate. A 100 m/d pilot-scale plant designed according to the concept of integrated membrane system (IMS) was used to provide sufficient amount of high salinity seawater, and a 5m3/d lab-scale setup and a kind of high permeation flux NF membrane (named as NF2) were used to investigate the feasibility of further processing high-salinity seawater by NF membrane.Firstly, the influences of operating conditions, such as the pressure, the water flow and the circulation, were investigated in the brine treatment by the NF2 membrane, and two kinds of operation processes were proposed to lower the tendency of membrane fouling. After that, the influence of operating conditions, such as pressure and salinity was investigated when dealing with the high-salinity simulating brine. At the same time, operating process was used to reduce the concentration of divalent ions by inducing crystals seeds into the feed water. Finally, simulation software and Pitzer’theory of the electrolyte solution were used to predict the scaling tendency of sparely soluble salt on the membrane surface.The results showed that fouling did not occur in the experiments of processing concentrated seawater by the NF2 membrane, and the membrane flux increased linearly with the increasing pressure. According to the experiment, the retention rates of the mono-valence ions were generally as low as 20%-30%, while that of Ca2+ remained at around 55% and that of Mg2+ maintained between 75%-80%. However, the retention rate of SO42- had been stable at more than 95%. The water flux of NF2 membrane increases with the increase of the inlet flowrate. However, when the water inlet flowrate was increased higher than 1500 L/h, which means that after the average velocity upon the membrane surface reached 0.075m/s, the increase extent of water flux was not quite severe any more. Two processes, in terms of the gradual pressurization process and the discontinuous water dilution one, were utilized to mitigate the effects of concentration polarization. Both of these processes showed a certain extent of inhibiting effect on the decline of membrane flux. Specially, the operation process of diluting the inlet intermittently has a higher continuous operability.The flux of NF2 membrane did not increase in a linear trend with the operating pressure when processing the brine. The phenomenon indicated that the scale formation happened apparently upon the membrane surface, which was consistent with the theoretical prediction. However, NF2 membrane reflected a good resistance to fouling. Under the operating pressure of 3.0MPa, the flux of NF2 membrane could reach 54L·m2·h-1 when it was used to process the brine which TDS was 75000. Under the high-salinity circumstance, the retention rates of the mono-valence ions were 10%-20%, Ca2+ at around 50%, Mg2+ between 75%-80%, and the retention rate of SO42- was higher than 95%. In order to reduce the scaling tendency, crystal seeds were introduced into the feed solution to induce crystallization of the bulk solution. Compared to blank experiment, this kind of process could slow down the downturn of the membrane flux. Moreover, it could also reduce the concentration of SO42- and Ca2+In conclusion, a pilot-scale plant was used to process brine softened from the real seawater. It could provide some parameters for the applications of the NF technology in dealing with high-salinity brine. As a result, these parameters consequently enhanced the recovery rate of brine and reduced the emissions of seawater, and finally could save the cost.