| Recently, nanoparticles have been widely applied in many fields for their unique physicochemical properties. As one of the most promising metals of various nanoparticles with the properties of antiviral and antimicrobial, silver nanoparticles(Ag NPs) with trace amounts are toxic to the biology. With the increase of Ag NPs application, the released Ag NPs from Ag NPs-containing products will finally discharged into sewage treatment plant. Taking into account the potential hazards of Ag NPs to the environment, effective removal of Ag NPs from wastewater treatment process is significant for wastewater treating technologies. However, the conventional wastewater treatment process is difficult to effectively remove the nanoparticles. Recently, Forward osmosis(FO) process and its combination with biological technologies such as osmotic membrane bioreactors(OMBRs) have attracted growing interests in sea/brackish water desalination and wastewater reuse due to its high rejection, low fouling tendency and energy requirement. Considering the high rejection performance of FO membrane, FO and its combined process can be considered effectively removed Ag NPs from wastewater. However, the removal efficiency of Ag NPs, influencing factors during the process of removing and membrane fouling mechanism have not been reported until now. Thus, it is necessary to investigate the removal efficiency of Ag NPs by FO membrane, Ag NPs fouling and its combination with organic fouling of FO membrane in order to better understand the membrane fouling mechanisms during FO process. Furthermore, systematically investigating their effects on system performance, FO membrane fouling and microbial community dynamics in OMBRs is also helpful.The main research contents and results as follows:(1) The short-term FO process was firstly established, and then was applied for analyzing the rejection efficiency of Ag NPs by different FO membranes. The results showed that the rejection efficiency of Ag NPs were all above 99.4% for both CTA and TFC membranes treating 1.0 mg/L and 5.0 mg/L of Ag NPs. The fluxes of TFC and CTA membranes have identical trend for 1.0 mg/L Ag NPs as feed solution. However, when the concentration of Ag NPs increased up to 5.0 mg/L, the fluxes of TFC and CTA membranes were both lower, and kept decline until the end of experiment. Furthermore, the flux decline of TFC membrane was more serious than that of the CTA membrane.(2) Single and combined fouling of nanoparticles and organic macromolecules in FO process were studied using Ag NPs and bovine serum albumin(BSA) as model foulants for organic and particulate fouling, respectively. The findings demonstrated that water flux declined appreciably even at the beginning of experiment during single Ag NPs fouling, and more remarkable flux decline and larger amount of deposited Ag NPs were observed with an increase of the concentration of Ag NPs. However, the addition of BSA could effectively alleviate the FO membrane fouling induced by Ag NPs with the evidences of less flux decline and less amount of particles deposited on membrane surface. The phenomenon of the mitigation of Ag NPs fouling by BSA was mainly due to the formation of nanoparticle-protein corona, which can prevent nanoparticles from aggregation due to the steric stabilization mechanism. Meanwhile, the CLSM results showed that the deposited BSA on FO membrane surface was less abundant and appeared only in some spots for combined foulants compared to the single BSA as the foulant. It was attributed to the destruction of the structure of BSA by Ag NPs as a result of surface adsorption, which were demonstrated by the FTIR analyses. It was also found that the interaction between BSA and Ag NPs occurred not only in solution but also on FO membrane surface.(3) Impacts of Ag NPs on system performance, TFC-FO membrane fouling and microbial community dynamics in OMBRs combined with MF members(MF-OMBR) were systematically investigated with the addition of 1.0 mg/L Ag NPs into municipal wastewater. Our findings indicated that the MF membrane could effectively mitigate salt accumulation in OMBRs. The addition of Ag NPs led to an increase of NH4+-N concentration in sludge supernatant and EPS content in the bioreactor. Nevertheless, the effluent quality of MF and FO membranes kept stable. Ag NPs could alleviate the fouling of FO membrane in MF-OMBR and correspondingly enhance the water flux during the operation. Furthermore, the organic matters and biofoulants including total cells, proteins, α-D-glucopyranose polysaccharides, and β-D-glucopyranose polysaccharides on the fouled membrane significantly decreased after adding Ag NPs. Further observation on microbial community dynamics showed that the species richness decreased while the diversity of bacterial community increased in the reactor, and both the species richness and diversity of bacterial community decreased on membrane surface after the addition of Ag NPs. With regard to the distribution of Ag NPs in MF-OMBR, no obvious accumulation was found in supanatant, and most of Ag NPs were adsorbed in activated sludge. |
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