Pressure-assisted forward osmosis (PAFO) has been considered an alternative of forward osmosis (FO) process to reduce capital expenditure (CAPEX) of FO. This is mainly attributed to the enhanced water flux through the FO membranes which leads to the improved dilution of draw streams. In this regard, it has been expected that employing PAFO in the FO-reverse osmosis (RO) hybrid system for seawater desalination can reduce the overall economics of the hybrid process. However, replacing FO with PAFO causes an additional energy cost in the seawater dilution step which inevitably leads to a question that PAFO-RO hybrid is truly an economically beneficial option over FO-RO and stand-alone RO desalination. More importantly, though serial connection of FO elements (SE) improves the dilution of initial draw water which can induce enhanced energy cost saving in the following RO step, this economic benefit is also compensated with the additional membrane cost in the PAFO unit process. To rationalize its overall performance and economic benefit, the thorough performance and economic evaluation were conducted based on actual pilot-scale PAFO operations for serial connection of three 8040 FO elements(SE1, SE2 and SE3). A scenario was assumed that PAFO is implemented as an additional pretreatment step of existing RO process. The results showed that the FO-RO hybrid process is not an economically feasible option unless a significant unit FO element cost cut-down is guaranteed. On the other hand, PAFO-RO hybrid showed some benefits with regards to a higher range of target RO recovery and unit FO element cost, particularly when two FO elements are serially connected (SE2). However, consideration on overall plant construction and operating cost components led to a conclusion that the skepticism on the hybrid process has to be maintained. This skepticism can be alleviated when a new PAFO-RO hybrid desalination plant is commissioned primarily due to significant potential CAPEX and OPEX savings in the RO process originated from the significant reduction of Highlights  Significant impact of initial draw flowrate and hydraulic pressure on the economics of PAFO in hybridization with RO  Hydraulic pressure dependence of FO membrane element performance  Drastic pressure-drop in the draw channel of the last element in serial configuration  Target RO recovery and FO element cost in PAFO-RO hybrid are important for economics

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Membrane fouling is closely related to the concept of critical flux. Therefore, a fouling control stategy for forward osmosis (FO) membranes that is based on the critical flux is necessary. This study systematically investigated the critical flux behavior of FO membranes (CTA and PA-TFC) in the short-term using a stepping method (draw solution (DS) concentration stepping). In addition, to test the reliability of this method, long-term experiments were conducted to evaluate the influences of operational critical flux on the fouling behavior (sustainable operation and fouling reversibility/irreversibility), thereby determining the critical flux for reversibility. Our results showed that the DS concentration stepping could be applied for critical flux determination in FO. Both membranes exhibited higher critical flux values for alginate fouling compared to other single foulants such as colloidal silica or gypsum. The values were 15.9 LMH for a cellulose triacetate membrane (CTA) and 20.5 LMH for the polyamide thin-film composite (PA-TFC). Whilst these values should be adequate in FO applications they were determined for single foulants. The presence of multispecies of foulants caused a significant decline in the critical flux values. This study found 5.4 LMH for the CTA membrane and 8.3 LMH for the PA-TFC membrane for the combined foulants of alginate + gypsum. This indicates that the critical flux behavior in FO was dependent on the foulant type and membrane type. Importantly, 98-100% restoration of water flux was achieved with the PA-TFC membrane at an operation either close to or below critical flux (i.e., in case of negligible fouling), except for the combination of alginate-combined colloidal silica. The critical fluxes for reversibility obtained in this study will aid the efficient operation of practical FO processes.

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Research on membrane technology to provide fresh water while considering inextricably linked energy issues has resulted in remarkable accomplishments in the production of membranes, such as thin film composite (TFC) membranes, for relatively lowenergy desalination and wastewater reclamation via the forward osmosis (FO) process. Exhaustive and continuous efforts in the enlargement of TFC membranes to achieve an excellent combination of flux and selectivity have revealed a considerable need to fabricate an appropriate substrate. Electrospinning, as a cheap, scalable, and simple technique, is capable of producing electrospun mats with distinctive features. These features make electrospun nanofibers (ENs) a promising substrate for TFC-FO membranes, resulting in tremendous achievements in enhancing membrane performance. Since 2011, rapid progress has been made in applying electrospinning to fabricate ENs substrates for TFC-FO membranes. This paper reviews progress in the fabrication and modification of TFC membranes supported by ENs substrates for FO applications. The theoretical background of FO, discussing the main problems associated with the use of conventional substrates, progress in applying electrospinning to overcome these problems, including breakthrough achievements in ENs substrates for FO, the synthesis and characterization of such substrates, and a comparison of energy consumption between FO and other desalination techniques were covered.