Forward osmosis for desalination and water recovery

In recent years, there has been an increased interest in forward osmosis (FO) from academic research and industry with a rising number of FO academic publications in the last decade. The common perception of FO as a low energy process compared to reverse osmosis (RO) sparked interest in this area. Nevertheless, there are some major challenges that need to be addressed before FO can be successfully implemented as an effective technology. Some of these challenges are addressed in this dissertation, starting with the assessment of FO as a low energy process. A modelling approach was used to assess the energy consumption of various FO hybrid processes and provide a detailed comparison with RO for desalination, in an effort to answer the critical question: Is FO truly a low energy process compared to RO? Results showed that there was practically no difference in specific energy consumption (SEC) between standalone RO, and FO with nanofiltration (NF) draw solution (DS) recovery; this can be generalised for any pressure-driven membrane process used for the DS recovery stage in a hybrid FO process, such as UF or RO. It was also found that even if any or all of the membranes considered, FO, RO or NF, were perfect (i.e. had infinite permeance and 100% rejection), it would not improve the SEC significantly. Furthermore, in order to reduce the higher membrane footprint required by FO hybrid processes, internal concentration polarisation (ICP) within the support has to be greatly reduced or eliminated. Hence, any advantage possessed by the FO hybrid process derives from the lower fouling propensity of FO, lower pretreatment costs arising from reduced fouling, use of draw solutes which can be recovered with low cost thermal energy sources and specific applications where RO cannot compete. Inspired by this insight, subsequent work was performed to study the multifaceted interactions alongside membrane and process parameters involved in the fouling of FO membranes, specifically the HTI TFC and CTA membrane. The chapter on organic fouling behaviour of structurally and chemically different FO membranes revealed that fouling on the HTI TFC membrane was more significant compared to HTI CTA in both membrane orientations, arising from a variety of factors associated with surface chemistry, membrane morphology and structural properties. Interestingly, it was observed that in FO mode, membrane surface properties dominated over fouling layer properties in determining fouling behaviour, with some surface properties (e.g. surface roughness) having a greater effect on fouling than others (e.g. surface hydrophilicity). In PRO mode, structural properties of the support played a more dominant role whereby fouling mechanism was specific to the foulant size and aggregation as well as the support pore size relative to the foulant. Whilst pore clogging was observed in the TFC membrane due to its highly asymmetric and porous support structure, fouling occurred as a surface phenomenon on the CTA membrane support layer, indicating that the latter’s structure was more symmetric in relation to the foulant (alginate) studied. Besides pore clogging, the severe fouling observed on the TFC membrane in PRO mode was due to a high specific mass of foulant adsorbed in its porous support. A new method was successfully introduced to quantify the density of the fouling layer and correlate it with hydrodynamic conditions and fouling behaviour of the membranes studied. It was observed that a trade-off between enhanced membrane performance and fouling mitigation is apparent in these membranes, with both membranes providing improvement in one aspect at the expense of the other. Hence, significant development in their surface and structural properties are needed to achieve good anti-fouling properties without compromising flux performance. Measured fouling densities on the studied surfaces suggest that there is not a strong correlation between foulant-membrane interaction and fouling density. Cleaning results suggest that physical cleaning was more efficient on the CTA membrane compared to the TFC membrane. Further, they implied that despite different mechanisms of fouling and quantities of foulant adsorbed in FO membranes, FO is a resilient process with high cleaning efficiencies and fouling reversibility. Finally, to address the challenge of ICP, a novel method of fabricating FO membranes was developed by interfacially polymerising a free-standing, salt rejecting polyamide (PA) film using a floating technique and directly depositing this layer onto an open mesh fabric. By doing this, the need for a phase inversion support was entirely eliminated. The fabrication method resulted in the successful formation of a defect-free, salt-rejecting FO membrane with significantly reduced or eliminated ICP, attributed to large open mesh sizes and straight channels in the fabric support. Interestingly, it was observed that even in the absence of ICP, flux was limited by the support layer at lower effective open areas of the mesh fabric. At higher mesh sizes and effective open areas, the effect of the fabric support became less significant and FO performance was likely governed by diffusion through the PA film, limited by its structure and transport properties. A trade-off between surface roughness and thickness of the PA film was observed, which is linked to the mechanism of film formation at the bulk interface. It was proposed that the design of FO membranes with ideal supports should also include tailoring the PA film properties in order to achieve superior FO performance. Additionally, the use of supports with higher percentage open areas or porosities should be considered.Open Acces