Fouling of nanofiltration membranes : mechanisms and implications for trace organic rejection

Subtitle:Vervuiling van nanofiltratie-membranen : mechanismen en implicaties voor de retentie van organische micropolluenten

The use of pressure driven membrane processes (such as nanofiltration (NF) and reverse osmosis (RO)) has increased in recent years. NF/RO processes are mainly applied in production of drinking water from contaminated water sources such as seawater, brackish water and wastewater effluent. NF/RO membranes remove most emerging trace organic compounds. However the application of NF/RO membranes in water treatment is challenged by fouling. Fouling causes membrane performance decline and increases operational costs. Most studies have shown that fouling results in decrease in membrane permeate flux and also in rejection of trace organic pollutants. However, other studies have reported increases in rejection due to fouling. Although membrane fouling has been widely investigated, understanding fouling mechanisms is still a challenge due to variations in feed water chemistries. As a result, factors influencing the rejection of trace organic solutes by fouled membranes have not been fully characterized. The aim of this thesis was to synthesise and apply novel membranes testing for organic pollutants rejection and water permeability. Specific objectives were to: 1) tailor properties of NF membranes for high rejection of organics and low salt retention efficiency; 2) investigate solute transport mechanisms through NF/RO membranes taking into account the effects of membrane and solute parameters; 3) determine factors influencing combined fouling of NF membranes; 4) understand better the influence of membrane-foulant as well as foulant-foulant interactions in fouling; ; 5) investigate the effects of fouling on rejection of trace organic compounds and 6) study the influence of membrane-solute and solute-foulant affinity on solute transport in virgin and fouled membranes. Combined fouling mechanisms in cross-flow nanofiltration Organic, colloidal and combined fouling of NF membranes was extensively investigated in cross-flow nanofiltration. The effects of feed composition (such as calcium concentration, type of foulant and organic-colloid foulant concentration ratio) on fouling behaviour and fouling rate were studied. Fouling rates/trends were related to foulant zeta potential and size, as well as membrane zeta potential. Most importantly, fouling was also related to specific membrane-foulant as well as foulant-foulant affinity interactions (computed based on the surface tension component approach). Novel fouling experiments (termed sequential fouling experiments) were designed to further gain insight on fouling mechanisms in combined fouling. In these experiments, the membrane was fouled with organics and colloids (in the presence or absence of calcium) in alternating sequence. It was clear that organic, colloidal and combined fouling resulted in flux decline due to cake filtration and increased hydraulic resistance to permeate flow. Fouling was mostly aggravated in the presence of calcium due to organic-calcium complexation which resulted in formation of dense cake layers. However, when the calcium concentration reached a certain limit (depending on the type of foulant), fouling was less severe due to formation of large aggregates which result in cake layers with low hydraulic resistance to permeate flow In combined fouling, it was found that, depending on the organic-colloid concentration ratio, the flux may decline more than that of fouling by individual foulants or not. Therefore flux decline in combined fouling is not always due to synergistic effects of the different foulants. From the novel sequential fouling experiments, it was found that flux decline in combined fouling was due to entrapment of colloids in the organic cake layer. Initial membrane fouling appeared to be controlled by membrane-foulant affinity interactions but there was less influence of colloid-organic affinity interactions in combined fouling. This showed that other mechanisms played a role. Effects of fouling on transport of trace organic solutes. The effect of membrane fouling on rejection of trace organic compounds (trace organics) was investigated. The cake-enhanced concentration polarisation (CECP), the solution-diffusion model and the surface tension component approach were used to determine the influence of cake-enhanced concentration polarization effects, flux effects and non-electrostatic membrane-solute interactions on trace organic rejection by fouled membranes. Carbamazepine was used as model organic trace contaminant, while sodium alginate (SA), latex and Al2O3 were used as model foulants. When rejection by a virgin membrane was compared to that of fouled membranes at similar fluxes, it was found that fouling by alginate + Ca2+, latex and Al2O3 + alginate (regardless of the presence of calcium) improved rejection of carbamazepine whereas fouling by alginate, latex + alginate and Al2O3 (all without the addition of calcium) lowered carbamazepine rejection. The following conclusions were made based on experimental results: 1) the decline in trace organic rejection for membranes fouled with alginate and Al2O3 was due to decrease in permeate flux, increase in solute affinity for the membrane surface and cake-enhanced concentration polarisation effects; 2) cake-enhanced concentration polarisation is not the only governing parameter explaining decline in trace organic rejection for fouled membranes 3) it is important to compare solute rejection values after fouling to that of the clean membrane at similar fluxes. Influence of membrane-solute affinity interactions on solute transport. This study aimed at further investigating the role of various factors in nanofiltration rejection of a wide range of trace organic compounds by fouled membranes. For a virgin NF-270 membrane, membrane-solute affinity interactions were repulsive for all trace organics. This however does not mean that rejection will be complete, as there is a convective driving force of solutes towards the membrane surface. All the compounds were therefore rejected. Upon fouling, rejection increased for some compounds (due to improvement in sieving mechanisms and increase in electrostatic membrane-solute repulsions), decreased for some compounds (due to cake-enhanced concentration polarisation effects and increase in non-electrostatic membrane-solute attractive interactions) and remained unchanged or followed rejection as predicted by the solution-diffusion model (showing that rejection was influenced by flux effects). It was concluded that various mechanisms influence the rejection of a single compound. Novel anti-fouling UF and/or NF PES membranes for wastewater reclamation. Novel polyethersulfone (PES) membranes incorporated with graphene oxide-zinc oxide (GO-ZnO) were synthesized with the aim of improving solute rejection and membrane anti-fouling properties by making membrane surfaces hydrophilic and membrane-solute as well as membrane-foulant non-electrostatic interactions repulsive. The membranes were synthesised following the single casting phase inversion (SCPI) and double casting phase inversion (DCPI) methods. These methods were compared for reproducibility of membrane properties and filtration test results. From the results of the SCPI and DCPI membranes, the following conclusions were made: 1) the DCPI method produced membranes with more reproducible results compared to membranes synthesised using the SCPI approach; 2) DCPI membranes had higher fluxes and rejected more trace organics than SCPI membranes and 3) DCPI membranes suffered less flux decline due to organic fouling compared to SCPI membranes. DCPI membranes are therefore a promising cost-effective technology for water and wastewater reclamation.

ISBN:9789059898196

Publication year:2015

Accessibility:Open