Study On Forward Osmosis Based Separation Of Microalgae From Water

In recent years, due to water environmental pollution and global warming, more attention has been put on utilizing energy microalgae for advanced wastewater treatment and CO2 absorbtion from air in order to generate the algal lipid. However, commercial extended applications of such promising technologies are being hindered by the high cost and secondary pollution in the separation of microalgae from water. Therefore, it is of great significance to achieve the separation of algae from water efficiently and economically as well as to ensure wastewater effluent quality in the advanced wastewater treatment. Although forward osmosis (FO) has been described as low energy consumption, the relatively low water flux, membrane fouling, and the difficult recovery of traditional draw solution and low separation efficiency still exist in the process of FO-based separation of algae from water. To solve these problems, this research constructes the forward osmosis system focusing on the selection of ion species and its concentration of draw solution and membrane material. The Chlorella vulgaris was used as the model algae, and the capability of forward osmosis in simulated and real algae-water seperation was explored. Additionally, in order to improve the permeate flux and to recycle the draw solution, the amino-functionalized magnetic nanoparticles Fe3O4-CMC@SiO2-NH2 was further prepared as draw solution. The main contents included the following four aspects:(1) Deveplop a forward osmosis system and contrast the salt rejection rate and permeate flux of the forward osmosis membrane of different materials. Results show the initial permeate flux of thin-film composite membrane (TFC) is slightly higher than that of cellulose triacetate membrane (CTA), while the permeate flux of CTA membrane shows a little decline compared with TFC membrane 4 hours later due to the support layer of CTA membrane, which could effectively decrease the internal concentration polarization (ICP) by preventing the solute molecules penetrating into the support layer. This research contrastes the results of the capabilities of metal and metal inorganic salt which is used as draw solution and furtherly varies the ion species, ionic valence condition and their concentration (objects:Na+、K+,、Ca2+、Mg2+、Fe3+ Al3+), showing the initial permeate flux of the FO process using nonmetal inorganic salt (NH/(+) as draw solution is the highest up to 16.5 L/(m2h) but the attenuation reached approximately 25% after 4 hours, while the permeate flux of metal inorganic salt with the same valence almost remains unchanged during the process. The permeate flux shows a positive but nonlinear response to the ascending range of concentration of metal inorganic salts. Furthermore, to improve the valence of metal inorganic salt could increase the permeate flux significantly, but the trivalent metal inorganic salt is not suitable for separation of algae from water during the FO process.(2) Utilize the selected superior ions as draw solution for simulated algae-water separation in the FO process. The concentration of the draw solution is 1 mol/L of both monovalent and divalent metal salts. Results exhibit that the initial permeate flux of the FO using divalent metal inorganic salt (CaCl2, MgCl2) as draw solution is up to 20 L/(m2h), far higher than that of the monovalent metal inorganic salts (NaCl, KCl). The reversely penetrating mass of divalent metal inorganic salts, and the subsequent amount of which is absorbed or uptaken by algae, is less than that of the monovalent metal salts. However, the usage of the divalent metal inorganic salt as draw solution shows obvious decrease in the permeate flux and will cause severe membrane fouling, and hence the monovalent metal salt solution is more suitable as draw solution than the divalent metal inorganic salt solution for performance and stability reasons. The monovalent metal inorganic salt is selected to be used as draw solution according to the results of simulated system.(3) The performance of the real algae water system in forward osmosis separation is further investigated by using monovalent metal inorganic salt solution as the draw solution. The confocal laser scanning microscopy, scanning electron microscopy, inverted fluorescence microscope, infrared spectroscopy, Zetasizer Nano ZS and Zeta plus are further used to characterize the algae biomass, intracellular components, Zeta potential, extracellular polymeric substances (EPS), and algae osmotic pressure at different growth cycles, and then the effect of biomass concentration and culture cycle on the water flux of forward osmosis is investigated. The experiment shows, as the algae entered into exponential growth phase, the Zeta potential of algae descreased from-21 mV to-26.5 mV. The phenomenon is beneficial to the reduction of membrane fouling as a result of electrostatic repulsion. Through the entire growth period, the proportion of intracellular polysaccharide shows a rising trend, while the proportion of protein is declining. The production rate of EPS in the exponential growth period is rising faster than that in almost any other period, and organics in bound EPS (BEPS) are higher than those in soluble EPS (SEPS),thus the membrane fouling and decrease of permeate flux is relatively slow when the FO process was conducted in the exponential growth period. Furthermore, the concentration of algae has significantly influenced permeate flux in the FO process. Under the same conditions, the permeate flux of 0.1 g/L algae culture is 5.5 L/(m2·h) while that of 0.8 g/L algae culture is 4 L/(m2·h).(4) The amino-functionalized and silica-coated magnetic nanoparticles can obviously improve their osmotic pressures during functional preparation. Furthermore sodium carboxymethyl cellulose (CMC) and starch could also improve the suspension of magnetic nanoparticles. Our testing results of infrared spectroscopy and the permeate flux show the functional groups of magnetic nanoparticles are stably attached during FO process. However, the permeate flux significantly decreases from about 3 L/(m2·h) to only 0.6 L/(m2·h) just after 30 minutes. It might be involved with the iron dissolution from magnetic nanoparticles which might cause serious membrane fouling and how to prevent and control it still need to be studied.