Forward Osmosis（FO）:Definition,Evaluation And Application Of Mass Transmission Coefficient-η_MT

Forward Osmosis（FO）usually has a measured water flux(J_w,exp)much smaller than its theoretical water flux(J_{w,theoretical});the discrepancy between J_w,exp and J_w,th,th increases at a higher theoretical osmotic pressure different(?π_theoretical).Such discrepancy has been explained with concentration polarization（CP）and membrane resistance,together with an osmotic reflection coefficient（σ）.However,it is not clear how to link the discrepancy orσwith FO performance and process optimization in different FO systems.In this study,for the first time,a FO mass transmission coefficient was defined asη_MT=J_w,exp/J_{w,theoretical}.The procedure was developed for determiningη_MT and A（water permeability coefficient）with a static FO system.Results showed thatη_MT decreased with an increase in concentration difference between the bulk of draw solution(C_DB)and feed solution(C_FB)as per logη_MT=a·log(C_DB–C_FB)+b for different FO systems.It can also evaluated the difference betweenσandη_MT.η_MT and the related equations can be used to evaluate the FO performance,predict the impacts of different test conditions onη_MT,and interpret implications related toη_MT and osmotic efficiency of the FO systems.In the past,forward osmosis（FO）models were developed to evaluate the effects of concentration polarization（CP）including external CP（ECP）and internal CP（ICP）on FO performance.However,these models are not clear about the reverse salt flux of FO membrane because they often assume osmosis reflection coefficientσto be 1.In addition,these models need some empirical parameters that are difficult to obtain via experiments.In this study,a new set of forward osmosis（FO）models was developed,for the first time,on the basis of mass transmission coefficient（ηMT）andσ.These models were used to evaluate ECP and ICP on FO performance under the influence of different parameters and conditions.Data in previous studies were used to verify the models based on experimental and model calculatedη_MT.Sensitivity analysis was conducted in these models in order to find the most sensitive parameters to ECP,ICP,ηMT and FO water flux Jv.Results indicate thatσis the most relatively influential parameter to concentrative ECP or concentrative ICP,revealing FO membrane’s reverse salt flux.