Forward Osmosis-based Membrane Processes For Wastewater Resource And Membrane Fouling Mechanism

Water shortages and energy crisis have become global challenges.To address these challenges,robust effective,low-cost and low-energy technologies are needed to purify water and at the same time minimize the use of chemicals and environmental impact.To promote sustainable development,new technologies for realizing wastewater reuse and seawater desalination are also needed to increase the supply of the fresh water and energy.Forward osmosis?FO? is an emerging membrane technology,which takes advantage of the osmotic pressure gradient to drive water across the semipermeable membrane from the feed solution to the draw solution.Compared to pressure-driven membrane processes,FO delivers many potential advantages such as lower energy consumption,lower fouling tendency and high quality water recovery because hydraulic pressures is avoided.FO is a promising technology for wastewater treatment,seawater desalination and power generation.Currently,FO-related researches for wastewater resources are mainly conducted in lab-scale,and its practical application still faces challenges suboptimal operation,unavoidable membrane fouling,and limited membrane mass transfer.Therefore,it is necessary to improve our understanding about FO processes in order to provide important information and technical support for FO application in water treatment and energy recovery.Microbial fuel cells?MFC? is an emerging technology for wastewater treatment and energy recovery.It combines the microbial oxidation of organic or inorganic substrates with electricity generation.However,the important limit factor of MFC application is high efficient,low cost and fouling-tolerant separator.In this work,forward osmosis-microbial fuel cell?FO-MFC?has been reported as an efficient system to treat wastewater and meanwhile recover energy.This study compared the electrochemical performances of MFCs with FO membrane,CEM and anionic exchange membrane?AEM?as separators,and explored the underlying mechanisms of its superior performance.Meanwhile,we also investigated the effect of membrane fouling on FO-MFC performance during long-term operation.The underlying mechanisms of the fouling-induced superior performance were revealed.FO processes have attracted attention as efficient and low energy methods for nutrients recovery.In this work,we systematically investigated the effects of pH,water flux levels and membrane orientation on nutrients N,P and VFA rejection.The mass transfer properties of N,P and VFA across FO membrane were revealed.In addition,FO membrane can effectively reject emerging pollutants in natural and engineered aqueous systems such as nanoparticles?NP?.So far,the behavior and mechanism of FO membrane fouling induced by NP and organics remains unclear.To understand this process.SA was chosen as the model organics and TiO₂ was used as the NP in this study.The role of TiO₂ in the membrane fouling by NOM in FO process was revealed.The main contents and results are listed below:1.Integrating FO-MFC is an efficient system for wastewater treatment and energy recovery,and exhibited better electrochemical performance than conventional MFCs with CEM and AEM.The FO-MFC showed significantly lower internal resistance,faster proton transport,and hence generated higher voltage than other systems?almost two-fold higher than that of CEM-MFC?.Unlike the CEM that encountered severe pH splitting due to suppressed proton diffusion by other competitive cations,the FO membrane favored an improved transport of protons in relative to other larger-sized cations attributed to its unique size-selectivity and the water flux as further driven force.The present work may have implications for development and application of more efficient bioelectrochemical processes.2.Impacts of membrane fouling on electrochemical performance of FO-MFC were investigated.The current generation of FO-MFC was increased by 34%upon membrane fouling despite of the loss of water flux.The diffusion fluxes of several representative ions across the different membranes were measured in an abiotic electrochemical system.The flux of net positive charges of fouled membrane was increased by over 3.5 times.The fouled FO membrane showed significantly higher fluxes of proton and other ions than the pristine membrane control,thereby resulting lower internal resistance.The accelerated ion diffusion was likely caused by fouling enhanced ion diffusion and ion exchange/neutralization mechanisms.The results of this study suggest a high promise to utilize FO membranes as a fouling-tolerant separator for MFC and other bioelectrochemical applications.3.Rejection of N,P nutrients and VFA by FO membrane was investigated.This study showed that VFA rejection was greatly decreased under fermentation acidic conditions compared to neutral conditions.The rejection of ammonia was sensitive to pH,which gradually decreased with increasing pH.In contrast to ammonia,the rejection of phosphate was less sensitive to pH,with only slight increase in the rejection at increased pH.Increasing the draw solution concentration led to higher water flux and significantly increased rejection of N,P nutrients and VFA.In addition,the results showed that the nutrients N,P and VFA were rejected at a much lower rate when the membrane active layer was facing draw solution?PRO?compared to the opposite,due to more severe concentrative internal concentration polarization?ICP?.The present work may provide useful information for FO application in water resources recovery.4.The roles of TiO₂ on the SA-induced FO membrane fouling were explored by combined use of attenuated total reflectance-fourier transform infrared?ATR-FTIR?,zeta potential and hydrodynamtic diameter and isothermal titration calorimetry?ITC?.The results show that TiO₂ was more effective in reducing SA-related fouling compared to single SA fouling.The interaction between SA and TiO₂ increased the negative charge of membrane surface,resulting in higher electrostatic repulsion to foulants.Meanwhile,the co-presence of TiO₂ and SA enhanced the stability and dispersion state of SA and thus reduced the foulants deposition on the membrane surface compared to single SA or TiO₂.The presence of Ca²⁺ in the feed solution promoted SA fouling on the membrane surface because Ca²⁺ bridged SA to form gel network.ITC results showed that the SA-TiO₂ binding had lower heat release and exhibited lower binding capacity with Ca²⁺ compared to SA.As a result,the present of TiO₂ can reduce the Ca²⁺ binding with SA,thus resulting in less SA-Ca²⁺complexes and mitigated membrane fouling.