Salinity Buildup And Membrane Fouling In Forward Osmosis Wastewater Treatment System:Mechanisms And Regulation

Wastewater treatment and reuse are essential for water resource sustainability and environmental protection.As a promising wastewater treatment technology,forward osmosis(FO)has attracted wide attention due to its good effluent quality and low energy consumption.However,salinity buildup and membrane fouling,which result in poor process efficiency and stability,are the remaining obstacles to its pratical application.A deep understanding on the funmemtal characterics of salinity buildup and membrane fouling in such systems,and their interaction mechaninsm as well as effective control methods are still lacking.In this dissertation,the interactions between membrane fouling and salinity buildup and control strategies are systematically investigated,which may lay a basis for optimization and practical applicatioin of FO wastewater treatment systems.The primary contents and results of this dissertation are as follows:1.The impacts of salinity on microbial metabolism pathways in anaerobic digestion process and on FO membrane fouling were investigated by combining experiments and mathemathcal modeling.The results showed a significant influence of salinity stress on microbial storage and metabolic pathways.High salinity triggered the production of more compatible solutes to counteract the extracellular osmotic pressure and meanwhile impaired the microbial metabolic activity(such as uptake of volatile fatty acids)and intracellular storage,and lowered the methane production.These processes were well captured by a salinity-response anaerobic digestion model,which incorporated the intracellular polymers production and salinity influences into the existing anaerobic digestion model for the first time.In addition,a higher extracellular polymer substrate concentration was detacted in supernatant at raised salinity,resulting in aggravated fouling and flux decline of FO membrane.2.The reverse salt diffusion in response to membrane fouling was investigated.An osmotic-resistance model was developed to analyze dynamic evolution of the driving force and diffusion resistance for FO reverse salt duffusion.Due to fouling-induced compensation of internal concentration polarization in FO system,the effective driving force increased more than the diffusion resistance upon increased foulant depositon,resulting in higher reverse salt diffusion flux.That means,the higher the hydraulic resistance of foulant layer,the fast revrese salt diffusion would occur due to fouling-induced compensation effect.Thus,an optimized fouling control strategy is need to simultaneously alleviate water flux reduction and reverse salt diffusion.3.For fouling control,a high-performance conductive FO membrane was fabricated and its feasibility for biofouling removal through in-situ electrooxidation was explored.Introducing a conductive network of carbon nanotube(CNT)into the selective layer resulted in a conductive FO membrane with higher water flux and lower salinity buildup than most commercial FO membranes.The incorporation of CNT layer endowed the membrane with good electrical conductivity(up to 631 S/m)and good electrochemical activity,achieving remarkable biofouling remoal(95.3%of tatal foulants)and water flux revovery(85.0%of unfouled water flux)after applying+2.5 V dirtect current volatage for 0.5 h.Our results demonstrate that in-situ electrooxidation may offer an effective,convenient method for cleaning FO membrane biofoulant.4.To further address the salinity buildup problem,a salt-permeable,fouling-resistant porous FO was fabricated and its performance for wastewater treatment was evaluated.This FO system,based on ultrafiltration-like polyacrylonitrile membrane and polyacrylic acid sodium draw solution,sustained higher water flux and lower salinity buildup than dense FO system during synthetic wastewater treatment.In addition,it showed significantly mitigated membrane fouling(lower flux decline and foulant deposition amount)than dense FO and ultrafiltration membranes during 12 h alginate-CaCl2 fouling.Thus,the porous FO system could be potentially used as pretreatment to lower reverse osmosis membrane fouling or used as low-fouling and low salinity buildup membrane module in membrane bioreactor system.