Research On The Application Of Forward Osmosis Technology In The Desalination Of Brackish Water And The Advanced Treatment Of Municipal Reclaimed Water

In forward osmosis (FO) process, water naturally permeates across a semi-permeate membrane from low concentration solution (feed solution, FS) side to concentrated solution (draw solution, DS) side driven by an osmotic pressure gradient. As an environmental-friendly membrane technology with low energy needed, low fouling propensity and high recovery rate, FO could be widely applied to a range of fields concerning industry and people’s daily life, such as, seawater desalination, power generation, industrial wastewater treatment, aerospace industry and food and pharmaceutical processing. In order to relieve the fresh water and energy crisis of China, it is necessary to investigate the feasibility of application of forward osmosis technology in brackish water desalination and advanced treatment of municipal reclaimed water.A laboratory-scaled plate-and-frame FO setup with TFC FO membrane was used in this study. DSs formed by NH4HCO3 and eight soluble inorganic salts (K2SO4, NaCl, KCl, KNO3, NH4Cl, NH4NO3, Urea and (NH4)2HPO4), respectively and TFC membrane were used to desalinate simulated brackish water (SBW). And three blended solutions (KCl, NH4Cl and (NH4)2HPO4 mixed with NH4HCO3, separately) were selected as DSs and CTA membrane was used for advanced treatment of secondary effluent from WWTP. The basic properties of DSs, pure water flux, reverse draw solute flux, variation of the concentration of N, P, DOC in the FS, quantitative analysis for fluorescence excitation-emission matrix fluorescence spectroscopy (EEMs) of dissolved organic matter in FS and the membrane fouling were comprehensively investigated. The main conclusions are as follows:1. Compared with single salt solution, the characteristics of blended solution were improved a lot. After single salt was mixed with NH4HCO3, the pure water flux of FO process achieved a notable increase. NH4Cl+NH4HCO3 mixture showed the highest water flux using both DI water and BW as feed. Since the osmotic gradient gradually decreased along with time and the negative influence caused by ICP, pure water flux declined as time increased and the variation curve of water flux showed nonlinear.2. The osmotic pressure driving force of every draw solution was not utilized effectively using both DI water and BW as FS. For the salts whose PR was less than that of NH4HCO3, blending with NH4HCO3 was beneficial to improve their PR.3. Blended solution achieved higher pure water flux than corresponding single salt solution in FO process for treating secondary effluent from WWTP, while its the reverse draw solute flux was also higher than that of single salt solution. The water fluxes gained by three blended draw solutions was in sequence of:KCl+NH4HCO3> NH4Cl+NH4HCO3> (NH4)2HPO4+NH4HCO3. Whereas, their reverse draw solute flux was in order of:NH4Cl+NH4HCO3> KCl+NH4HCO3> (NH4)2HPO4+NH4HCO3.4. As the FS was concentrated in FO process, the concentration of NH3-N, TN and DOC in the concentrated FS increased. Especially, except for KCl, the concentration of NH3-N and TN in FSs treated by other five DSs increased dramatically, which was caused by the reversely permeation of NH4+. However, the P in FS could diffuse from FS to DS, so the concentration of TP in concentrated FS declined. For the rejection of TP in FS, (NH4)2HPO4 performed better than any other selected salts no matter whether blended with NH4HCO3 as DS.5. Three kinds of fluorescence DOM including aromatic proteins Ⅱ, fulvic-like acids and soluble microbial-like products were abundant components of concentrated FS and secondary effluent from WWTP. Meanwhile, the contents of aromatic proteins Ⅰ and humic-like acids were less in those samples. Total cumulative fluorescence intensity of concentrated FS was less than initial FS. In concentrated FS, the cumulative fluorescence intensity of region Ⅰ, Ⅱ and Ⅲ all decreased, but that of region Ⅳ and Ⅴ kept steadily.6. In both brackish water desalination process and advanced treatment of secondary effluent from WWTP, membrane fouling all occurred. The membrane fouling in that two processes was difference. In desalination of brackish water, the fouling mater only occurred on the support layer, the membrane fouling was caused by the accumulation of draw solutes. Whereas, membrane fouling happened on both sides of membrane. And the fouling on the support layer was similar with that formed in desalination of brackish water, but the fouling on the active layer caused by the inorganic salts like reactive silicate and organic maters in FS.