Resumen de Masstransport and fouling of novel forward osmosisthin- film composite membranes
Forward osmosis (FO) is known to be particularly efficient at treating impaired water sources with a high fouling potential. In the context of water reuse FO is being introduced as a robust pre-treatment process, usually as a first barrier prior to a reverse osmosis (RO) step. The commercialisation of FO however hinges on the development higher permeability membranes and a recent breakthrough came with the introduction of thin-film composite (TFC) membranes. While promising state-of-the-art TFC membranes have been successfully fabricated and tested at the pilot-scale, their performance in FO is yet to be determined by comprehensive characterisation under the changed, elevated transmembrane water flux conditions.
Despite FO’s apparent lower fouling propensity compared to RO, membrane fouling remains a major concern, limiting the long term efficiency of the process. Numerous studies have been carried out to determine the factors and complex mechanisms governing the fouling behaviour in FO membranes. However, since most of the fouling studies have been performed with the same benchmark cellulose triacetate (CTA) membrane, there is a need to investigate the fouling phenomena at the changed operating conditions with novel TFC membranes. Another challenge associated with the reclamation of impaired water for potable use is the presence of emerging trace organic contaminants (TrOCs) in wastewater. Seeing that a main benefit of FO is the high rejection of a number of organic pollutants, FO has received increasing attention as a potential barrier for these compounds. In this respect, the removal mechanisms of organic compounds are still not fully understood, particularly regarding the role of reverse draw salt diffusion and the influence of sorption saturation behaviour on the rejection of TrOCs by TFC membranes.
To fill these gaps, in this thesis mass transport through novel TFC membranes was initially studied and modelled using two FO-only characterisation methods. Improvements in membrane water permeability were clearly observed for the two selected TFC membranes, with water fluxes exceeding 20 L m-2 h-1 even at low draw solution concentrations, and these were attributed to a greater active layer free volume and thinner support layers. Although the applied characterisation methods successfully predicted the transport and structural parameters of conventional membranes, novel TFC membranes were found to be more sensitive to experimental errors during characterization and therefore require a more rigorous approach to be able to compare their performance. Greater water permeability of the TFC xi membranes was accompanied by a lower rejection of neutral and positively charged organic compounds. Whilst negatively charged solutes were rejected above 90%, some neutral and positively charged solutes had rejections of less than 60%. It was found that electrostatic interactions between these solutes and the strong membrane surface charge of novel TFC membranes played a crucial role in the rejection of trace organic contaminants. Moreover, the use of draw solution concentrations typical in FO operation was found to alter solute-membrane interactions, indirectly affecting the forward transport of trace organic compounds.
Also in this study, alginate fouling in novel TFC membranes was thoroughly examined under different driving forces. Significantly more fouling occurred when the osmotic pressure difference was maintained constant, with more than 3 times the alginate surface density deposited on the membrane, contrasting the premise of low fouling tendency in the FO process. On the other hand, the thinner but denser foulant cake layer observed at an applied hydraulic pressure of 1 bar was partly attributed to the commonly reported pressure-induced compaction along with the different response of the membrane to hydraulic pressure and the presence of membrane defects. In quantifying the cake layer structural parameter, the current model based on the water flux decline was found to lack more accurate structural-related parameters of alginate gels to allow a better description of fouling behaviour.
Overall, the knowledge obtained from this thesis on the complex mass transport phenomena through novel TFC FO membranes is highly relevant for membrane developers who aim to enhance membrane performance whilst maintaining a high rejection of feed solutes and improve anti-fouling properties. In advancing the application of FO in wastewater treatment and reuse more efficient system designs and membrane modules are needed. These will depend on accurate water flux and fouling models that take into account the impacts of hydraulic pressure and higher permeability on membrane performance.
