Nanofiltration For Dissolved Organic Matter (DOM) Removal And Its Membrane Flux Decline Mechanisms

Due mainly to its own merits, nanofiltration(NF) has being considered as one ofthe most promising technologies, not only for wastewater reuse and reclamation, butalso for drinking water advanced treatment. However, membrane fouling is the majorbottleneck for its widespread application. In the present work, NF flux decline profileswere found to be related closely with the source of dissolved organic matter(DOM),which include water resource from reservoir water(RW) and wastewater treatment planteffluent water(EfW). From comprehensive analysis of DOM in terms of molecular sizedistribution, hydrophibicity, and fluorescence characteristics, the correlation ofmembrane fouling in NF with the DOM was estimated. Accordingly, the pre-treatmentmethods for NF fouling minimization were evaluated based on the pollutantsinterception effectiveness and fouling alleviation rate. The experimental results hereinwould be very useful for understanding the NF process employed in water andwastewater treatment, and give rise to the guideline for elevation of itscost-effectiveness.Series of NF tests focus on the DOM interception performance and therewith fluxdecline profile during RW and EfW filtration were conducted. Two kinds of NF, markedas TW30-1812-50and NF90-2540, were selected to evaluate the pollutants interceptionefficiencies. It was observed that TW30-1812-50have consistently higher TOC removalefficiencies compared with NF90-2540did, resulted to a rather lower bacteria count.Besides, TW30-1812-50possess slower flux decline rate while the same operationalconditions kept. With the same TOC feeding, NF could retain much more DOM in RWthan those in EfW, which may correlate with its slow fouling rate and low bacteria countthat was important for the biofouling development.Afterwards, the NF flux decline mechanisms caused by different DOMs wereinvestigated from the viewpoints of organic and biological fouling, as well asconcentration polarization(CP). It is believed that the DOM components and specificfraction are the major reasons for NF flux decline discrepancy. For RW, the organicmatters with large molecular weight(<100kDa) would take main responsible for theflux decline in the initial filtration, after which, the organic fraction that has smallmolecular weight（<3kDa) seems as the major foulant in the filtration later stage. Incontrast, as for EfW, the small molecular weight organic fraction is the determiningsubstances for NF flux decline. Besides, coexisting metal ions, such as calcium andaluminum, have positive impact onto the flux decline rate. Apart from organicadsorption or accumulation on the membrane surface, the biofouling caused by bacteria growth during EfW filtration is more serious than those in RW filtration, reflected bythat the former one could cultivate more than two magnitudes order higher of bacteriacount compared with the later one did. The DOM in RW and EfW gives rise to thedifferent CP development in NF surface even with the same TOC feeding andoperational conditions. CP in membrane surface for EfW is more serious than RW.Thirdly, the NF fouling mechanisms correlated with the DOC fraction andcharacteristics, including molecular weight distribution, hydrophilicity and fluorescenceproperties. The results demonstrated that RW contained more than32%of largemolecular weight ranged from5000kDa to100kDa, and about40.8%small molecularweight organic below3kDa. The organic fraction in RW was more hydrophilic andmainly comprised with humic and fulvic-like. However, the EfW have up to68.2%small molecular weight organic fraction, majority which was hydrophobic, and wasconstituted by humic-like substances and soluble microbial products.Finally, based on the DOM characteristic and correlated NF filtration flux declineprofile, pre-treatment methods such as sand filtration, activated carbon filtration,microfiltration, coagulation and ozonation for DOM removal and NF fouling mitigationwere evaluated and optimized for different DOMs. The experimental results showed: forRW, membrane flux increases about17%only by microfiltration. For EfW, singlepretreatment can not effectively increase the membrane flux. It is deduced thatPACl+MF could effectively removal DOM from EfW, resulting to more than19%offouling mitigation efficiency.